- Ruhr-Universität Bochum

The microhardness distribution in the different zones of a friction stir welded reduced activation ferritic/martensitic steel has been investigated and correlated to the hierarchical martensitic microstructure in the respective zones, characterized by electron backscatter diffraction orientation analysis. It is found that the variation of prior austenite grain size, packet size, and block width in different subzones is influenced by the peak temperature and effective strain rate during the friction stir welding process. The distribution of the microhardness correlates directly with the geometrically necessary dislocation density observed in the different zones. © 2021

We study the effect of laser shock peening (LSP) without protective coating on the surface mechanical property of NiTi alloy. The Vickers microhardness and wear resistance are measured to determine the mechanical property of NiTi samples treated with different LSP parameters (3 J with 10 ns and 5 J with 20 ns). From the electron backscatter diffraction (EBSD) analysis, it can be found that the laser shock peening does not induce obvious grain refinement in the surface region of NiTi alloy. Both compressive and tensile residual stress in the top layer are determined using the hole drilling method. The results show that the LSP treatment without a protective coating increases the roughness and enhances the surface mechanical properties of NiTi alloy. © 2021 Elsevier B.V.

Since its discovery in 1954, the omega (ω) phase in titanium and its alloys has attracted substantial attention from researchers. The β-to-ω and ω-to-α phase transformations are central to β-titanium alloy design, but the transformation mechanisms have been a subject of debate. With new generations of aberration-corrected transmission electron microscopy and atom probe tomography, both the spatial resolution and compositional sensitivity of phase transformation analysis have been rapidly improving. This review provides a detailed assessment of the new understanding gained and related debates in this field enabled by advanced characterization methods. Specifically, new insights into the possibility of a coupled diffusional-displacive component in the β-to-ω transformation and key nucleation driving forces for the ω-assisted α phase formation are discussed. Additionally, the influence of ω phase on the mechanical properties of β-titanium alloys is also reviewed. Finally, a perspective on open questions and future direction for research is discussed. © This material is authored by Battelle Memorial Institute with the US Department of Energy under Contract No. DE-AC05-76RL01830. The US Government retains for itself, and others acting on its behalf, a paid-up, non-exclusive, and irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

The Al0.75CoCrFeNi alloy (Al16Co21Cr21Fe21Ni21 in at.%) presents a lamellar microstructure in the as-cast state consisting of a spinodally-decomposed B2/BCC matrix and Widmanstätten-type FCC plates. In this study, to retain the lamellar microstructure and improve tensile strength, Al16Co21Cr21-xFe21Ni21Mox alloys with x ≤ 10 at.% were investigated. For x = 2 at.%, the Widmanstätten microstructure changed into a vermicular one due to the stabilization of the BCC phase. With increasing the Mo/Cr ratio, the BCC phase transformed into topologically close-packed (TCP) phases, i.e., σ phase for x = 4 at.% and R phase for x ≥ 6 at.%, whose volume fractions increases with x. The as-cast alloys with x = 10 and 4 at.% presented the largest microhardness of ~600 HV0.5. The former had the highest volume fraction in TCP phases, which are hard and brittle while the latter presented the finest microstructure (enhanced phase boundary strengthening). While the alloys with x > 4 at.% were too brittle to machine tensile specimens, the others were tested between 20 and 700 °C. The ultimate tensile strength increased with increasing x up to ~1460 MPa for x = 4 at.% at 400 °C. At 700 °C, the strength of all alloys significantly decreased due to the softening of the B2 phase. Most of them had limited ductility and showed intergranular fracture except for x = 4 at.% presenting pronounced necking with ~38% ductility. The latter effect was attributed to the occurrence of interfacial sliding resulting in cavitation at grain boundaries and interphase boundaries. © 2021 The Author(s)

The main focus of this work is to correlate the basic plasma properties with morphological, structural and mechanical properties of thin films to bridge the gap between the energy source, the plasma and materials. For this purpose, the deposition and growth of thin titanium films deposited by high power impulse magnetron sputtering (HiPIMS) at various discharge power densities, from 0.17 kW/cm2 to 3.5 kW/cm2 were studied, both experimentally and by kinetic Monte Carlo simulation. Simulations of film growth were performed with a three-dimensional kinetic Monte Carlo code (NASCAM) with ion fraction and species energy determined experimentally by mass spectroscopy. Our approach, which is not purely empirically driven, intends to reveal some insights of the mechanisms underlying the synthesis process, which determine the intrinsic material properties. In order to link HiPIMS plasma parameters and Ti film properties, we used different techniques to analyse Ti films. TEM, X-ray diffraction and AFM were used to evaluate the structural and morphological properties of the films, and nano indention was used to evaluate their mechanical properties. We observed that the orientation of micro-crystals, which constitute the films, changes when the discharge power density increases. At the same time, we show that the films nano hardness changes non-monotonically with the increase of the discharge power density; it decreases first, then increases. The surface roughness behaviour is also non-monotonic; first increasing, then decreasing with the further increase of the discharge power density. 3D modelling helped to reveal that these non-monotonic evolutions are due to a transition between thermally-driven to ballistically-driven Ti atom mobility. © 2021 Elsevier B.V.

Achieving high mechanical strength and ductility in age-hardenable Al7000 series (Al–Zn–Mg) alloys fabricated by selective laser melting (SLM) remains challenging. Here, we show that crack-free AlZnMgCuScZr alloys with an unprecedented strength–ductility synergy can be fabricated via SLM and heat treatment. The as-built samples had an architectured microstructure consisting of a multimodal grain structure and a hierarchical phase morphology. It consisted of primary Al3(Scx,Zr1−x) particles which act as inoculants for ultrafine grains, preventing crack formation. The metastable Mg-, Zn-, and Cu-rich icosahedral quasicrystals (I-phase) ubiquitously dispersed inside the grains and aligned as a filigree skeleton along the grain boundaries. The heat treated SLM-produced AlZnMgCuScZr alloy exhibited tunable mechanical behaviors through trade-off among the hierarchical features, including the dual-nanoprecipitation, viz, η′ phase, and secondary (Al,Zn)3(Sc9Zr), and grain coarsening. Less coarsening of grains and (Al,Zn)3(Sc9Zr) particles, due to a reduced solution treatment temperature and time, could overwhelm the more complete dissolution of I-phase (triggering more η′ phase), resulting in higher yield strength. Optimal combination of the hierarchical features yields the highest yield strength (∼647 MPa) among all reported SLM-produced Al alloys to date with appreciable ductility (∼11.6%). The successful fabrication of high-strength Al7000 series alloys with an adjustable hierarchical microstructure paves the way for designing and fine-tuning SLM-produced aluminum engineering components exposed to high mechanical loads. © 2021 Elsevier Ltd

An equiatomic CrFeNi medium-entropy alloy (MEA) that constitutes a cornerstone of austenitic stainless steels and Fe-based superalloys is investigated. Anneals at various temperatures revealed that CrFeNi forms a stable face-centered cubic (FCC) solid solution above ~1223 K. Based on this result, this alloy was cold-worked and recrystallized between 1273 K and 1473 K to produce different grain sizes. Compression tests were carried out at 293 K to investigate grain boundary strengthening (Hall-Petch slope: 966 MPa µm1/2) and this contribution was then subtracted from the overall strength to reveal the intrinsic uniaxial lattice strength (80 MPa). Additional compression and tensile tests were performed between 77 K and 873 K to study the effect of temperature on mechanical properties and deformation mechanisms. Ductility, yield and ultimate tensile strengths increased with decreasing temperature. To reveal the active deformation mechanisms in CrFeNi with the coarsest grain size (160 µm), tensile tests at 77 K and 293 K were interrupted at different strains followed by transmission electron microscopy analyses. In all cases, the deformation was accommodated by dislocation glide at low strains, while twinning additionally occurred above a critical resolved shear stress of 165 MPa, which was roughly temperature independent. This value compares well with predictions (180 MPa) based on the Kibey's model for twin nucleation. Moreover, the fact that this value is roughly temperature-independent is also consistent with the Kibey's model since the twin nucleation barrier (unstable twin stacking fault energy) of FCC metals and alloys does not vary significantly with temperature. © 2020 Acta Materialia Inc.

Heavy plate steels with bainitic microstructures are widely used in industry due to their good combination of strength and toughness. However, obtaining optimal mechanical properties is often challenging due to the complex bainitic microstructures and multiple phase constitutions caused by different cooling rates through the plate thickness. Here, both conventional and advanced microstructural characterization techniques which bridge the meso- and atomic-scales were applied to investigate how microstructure/mechanical property-relationships of a low-carbon low-alloyed steel are affected by phase transformations during continuous cooling. Mechanical tests show that the yield strength increases monotonically when cooling rates increase up to 90 K/s. The present study shows that this is associated with a decrease in the volume fraction of polygonal ferrite (PF) and a refinement of the substructure of degenerated upper bainite (DUB). The fine DUB substructures feature C-rich retained austenite/martensite-austenite (RA/M-A) constitutes which decorate the elongated micrograin boundaries in ferrite. A further increase in strength is observed when needle-shaped cementite precipitates form during water quenching within elongated micrograins. Pure martensite islands on the elongated micrograin boundaries lead to a decreased ductility. The implications for thick section plate processing are discussed based on the findings of the present work. © 2021, The Author(s).

This work examines the processing of a hot-work tool steel using laser-based powder bed fusion of metals (PBF-LB/M). The hot-work tool steel was produced using a low-cost powder mixture consisting of pure iron and other elemental powders as well as ferroalloys. Furthermore, a prealloyed starting powder with the same nominal chemical composition as the powder mixture was produced by inert-gas atomization. Besides, a reference steel was produced by casting to compare the microstructures and mechanical properties resulting from the different processing routes. The first step examined the application of a chemically homogeneous and dense layer of the powder mixture prior to PBF-LB/M densification. In addition to evaluate suitable process parameters for PBF-LB/M processing of the starting materials, the microstructure formation was comprehensively examined using electron microscopy and the processes adapted to it. To eliminate defects (cracks, pores) and chemical inhomogeneities, thermal posttreatments, namely supersolidus liquid phase heat-treatment (SLPHT) and hot isostatic pressing (HIP) were performed. Suitable heat-treatment parameters were evaluated. Finally, the obtained microstructures and the associated properties of the post-processed PBF-LB/M samples were compared with those in the reference states. As a main result, it was possible to achieve full redensification and simultaneous chemical homogenization of the PBF-LB/M-processed powder mixture by SLPHT post-processing. The hardness of the additively manufactured and SLPHT-post-processed specimens exceeds that of the cast reference. © 2021 Elsevier B.V.

This study investigates the high strain-rate tensile properties of a cold-rolled medium-Mn steel (Fe–12Mn–3Al-0.05C % in mass fraction) designed to have a multi-phase microstructure and positive strain-rate sensitivity. At the intercritical annealing temperature of 585 °C, increasing the annealing time from 0.5 h to 8 h increased the phase volume fraction of ultrafine-grained (UFG) austenite from 2% to 35% by reversion. The remainder of the microstructure was composed of UFG ferrite and recovered α′-martensite (the latter resembles the cold-rolled state). Servo hydraulic tension testing and Kolsky-bar tension testing were used to measure the tensile properties from quasi-static strain rates to dynamic strain rates (ε˙ = 10-4 s-1 to ε˙ = 103 s-1). The strain-rate sensitivities of the yield strength (YS) and ultimate tensile strength (UTS) were positive for both annealing times. Tensile properties and all non-contact imaging modalities (infrared imaging and digital image correlation) indicated an advantageous suppression of Lüders bands and Portevin Le Chatelier (PLC) bands (a critical challenge in multi-phase medium-Mn steel design) due to the unique combination of microstructural constituents and overall composition. Fracture surfaces of specimens annealed for 0.5 h showed some instances of localized cleavage fracture (approximately 30 μm wide areas and lath-like ridges). Specimens annealed for 8 h maintained a greater product of strength and elongation by at least 2.5 GPa % (on average for each strain rate). The relevant processing-structure-property relationships are discussed in the context of recommendations for design strategies concerning multi-phase steels such that homogeneous deformation behavior and positive strain-rate sensitivities can be achieved. © 2020

The use of interstitial elements has been a key factor for the development of different kinds of steels. However, this aspect has been little explored in the field of high entropy alloys (HEAs). In this investigation, the effect of carbon and nitrogen in a near-equiatomic CrMnFeCoNi HEA is studied, analyzing their impact on the microstructure, and mechanical properties from 77K to 673K, as well as wear, and corrosion resistance. Carbon and nitrogen are part of the FCC solid solution and contribute to the formation of precipitates. An increase in the yield and ultimate tensile strength accompanied with a decrease in the ductility are the main effects of C and N. The impact toughness of the interstitial-free material is higher than that of C and C+N alloyed systems. Compared to CrNi and CrMn austenitic steels, the wear resistance of the alloys at room temperature is rather low. The surface corrosion resistance of HEAs is comparable to austenitic steels; nevertheless HEAs are more susceptible to pitting in chloride containing solutions. © 2021, The Author(s).

We use an analytical model to propose candidate compositionally complex alloys of the Mo-Nb-Ta-W family with optimal yield stress. We then introduce a computationally tractable method based on first-principles calculations to model phase equilibria in complex alloys at arbitrary concentrations. We utilize this method to predict the phase diagram at the optimized compositions and observe a tendency towards ordering for some of the proposed alloys. By combining yield stress data and thermodynamic equilibria, we suggest two alloy compositions with optimal mechanical properties and a strong solid solution forming ability for further experimental validation. © 2021 American Physical Society.

It is widely accepted that the different types of crystalline imperfections, such as vacancies or dislocations, greatly influence a material's physical and mechanical properties. However, imaging individual vacancies in solids and revealing their atomic neighborhood remains one of the frontiers of microscopy and microanalysis. Here, we study a creep-deformed binary Ni-2 at.% Ta alloy. Atom probe tomography reveals a random distribution of Ta. Field ion microscopy, with contrast interpretation supported by density-functional theory and time-of-flight mass spectrometry, evidences a positive correlation of Ta with vacancies, supporting positive solute-vacancy interactions previously predicted by atomistic simulations. © 2021

In future fusion reactors tungsten coatings shall protect First Wall components, made of reduced activation ferritic martensitic steel, against the plasma, because of tungsten's favourable thermo-mechanical properties and low sputtering yield. Functionally graded material layers implemented between the coating and the steel substrate, compensate the difference in the coefficient of thermal expansion. By using the vacuum plasma spraying technique several layer systems were successfully produced and tested, among other aspects, in regard to their thermal fatigue behaviour up to 500 thermal cycles in a vacuum furnace. However, higher numbers of thermal cycles are anticipated for future fusion reactors and, therefore, a less time consuming approach for thermal fatigue testing is required. Hence, a new testing apparatus with induction heating and inert gas cooling was built and first thermal fatigue experiments with up to 5000 cycles were carried out on different functionally graded tungsten/steel layers systems. The subsequent investigations of these samples show that the layer systems are stable for the applied number of thermal cycles and their properties are solely determined during their respective coating processes. © 2020

The high-entropy alloy (HEA) concept was based on the idea that high mixing entropy can promote formation of stable single-phase microstructures. During the past 15 years, various alloy systems have been explored to identify HEA systems with improved property combinations, leading to an extraordinary growth of this field. In the large pool of alloys with varying characteristics, the first single-phase HEA with good tensile properties, the equiatomic CrMnFeCoNi alloy has become the benchmark material, and it forms the basis of much of our current fundamental understanding of HEA mechanical behavior. As the field is evolving to the more broadly defined complex concentrated alloys (CCAs) and the available data in the literature increase exponentially, a fundamental question remains unchanged: how special are these new materials? In the first part of this review, select mechanical properties of HEAs and CCAs are compared with those of conventional engineering alloys. This task is difficult because of the limited tensile data available for HEAs and CCAs. Additionally, the wider suite of mechanical properties needed to assess structural materials is woefully lacking. Nonetheless, our evaluations have not revealed many HEAs or CCAs with properties far exceeding those of conventional engineering alloys, although specific alloys can show notable enhancements in specific properties. Consequently, it is reasonable to first approach the understanding of HEAs and CCAs through the assessment of how the well-established deformation mechanisms in conventional alloys operate or are modified in the presence of the high local complexity of the HEAs and CCAs. The second part of the paper provides a detailed review of the deformation mechanisms of HEAs with the FCC and BCC structures. For the former, we chose the CrMnFeCoNi (Cantor) alloy because it is the alloy on which the most rigorous and thorough investigations have been performed and, for the latter, we chose the TiZrHfNbTa (Senkov) alloy because this is one of the few refractory HEAs that exhibits any tensile ductility at room temperature. As expected, our review shows that the fundamental deformation mechanisms in these systems, and their dependence on basic physical properties, are broadly similar to those of conventional FCC and BCC metals. The third part of this review examines the theoretical and modeling efforts to date that seek to provide either qualitative or quantitative understanding of the mechanical performance of FCC and BCC HEAs. Since experiments reveal no fundamentally new mechanisms of deformation, this section starts with an overview of modeling perspectives and fundamental considerations. The review then turns to the evolution of modeling and predictions as compared to recent experiments, highlighting both successes and limitations. Finally, in spite of some significant successes, important directions for further theory development are discussed. Overall, while the individual deformation mechanisms or properties of the HEAs and CCAs are not, by and large, “special” relative to conventional alloys, the present HEA rush remains valuable because the compositional freedom that comes from the multi-element space will allow exploration of whether multiple mechanisms can operate sequentially or simultaneously, which may yet lead to the creation of new alloys with a spectrum of mechanical properties that are significantly superior to those of current engineering alloys. © 2019 Acta Materialia Inc.

We report on a multilayered structure comprising of rock-salt (rs) structured CrN layers of constant thickness and AlN layers of varying thicknesses, which surprisingly enables the growth of metastable zinc-blende (zb) AlN layers for certain layer-thickness combinations. The multilayer exhibits an atomic and electronic structure gradient as revealed using advanced electron microscopy and electron spectroscopy. Gradient structures are also accompanied by a modulation of the chemical compositions. A combined experimental analysis based on valence electrons and inner shell electrons allowed mapping the mechanical properties of the multilayer at the nanometer scale and further unveiled the effect of oxygen impurities on the bulk modulus. We found that the presence of oxygen impurities causes a remarkable reduction of the bulk modulus of rs-CrN while having no significant effect on the bulk modulus of the stable wurtzite structure wz-AlN layers. The findings are unambiguously validated by theoretical calculations using density functional theory. © 2020 Acta Materialia Inc.

Aerosol deposition (AD) is a novel deposition process for the fabrication of dense and rather thick oxide films at room temperature. The bonding of the deposited ceramic particles is based on a shock-loading consolidation, resulting from the impact of the ceramic particles on the substrate. However, the deposition mechanism is not fully understood. In addition, many technical challenges have been observed for achieving a successful deposition of the oxides with higher efficiency. In this work, the influence of different processing parameters on the properties of the deposited layer is studied. Proof of concept was done using 8 mol.% yttria-stabilized zirconia (8YSZ) powder as starting material. The window of deposition with respect to carrier gas flows for successful deposition was identified. The influence of this carrier gas flow, the substrate materials and the carrier gas species on the coating thickness, interface quality and coating microstructure was systematically investigated. The derived mechanical characteristics revealed an unexpected behavior related to a gradient microstructure. This study supports understanding of the mechanism of room-temperature impact consolidation and its effect on the mechanical properties of the deposited layer. © 2020, ASM International.

A photolithographic process for the rapid fabrication of thin-film tensile-test structures is presented. The process is applicable to various physical vapor deposition techniques and can be used for the combinatorial fabrication of thin-film tensile-test structure materials libraries for the high-throughput characterization of mechanical properties. The functionality of the fabrication process and the feasibility of performing high-quality measurements with these structures are demonstrated with Cu tensile-test structures. In addition, the scalability from unary structures to libraries with compositional variations is demonstrated. Copyright © 2020 American Chemical Society.

Micromechanical modeling is one of the prominent numerical tools for the prediction of mechanical properties and the understanding of deformation mechanisms of metals. As input parameters, it uses data obtained from microstructure characterization techniques, among which the electron backscatter diffraction (EBSD) technique allows us to understand the nature of microstructural features, that are usually described by statistics. Because of these advantages, the EBSD dataset is widely used for synthetic microstructure generation. However, for the statistical description of microstructural features, the population of input data must be considered. Preferably, the EBSD measurement area must be sufficiently large to cover an adequate number of grains. However, a comprehensive study of this measurement area with a crystal plasticity finite element method (CPFEM) framework is still missing although it would considerably facilitate information exchange between experimentalists and simulation experts. Herein, the influence of the EBSD measurement area and the number of grains on the statistical description of the microstructural features and studying the corresponding micromechanical simulation results for 316L stainless steel samples produced by selective laser melting is investigated. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The degradation of moulds, dies and tools employed in plastic, food and chemical processing industries has necessitated the development of suitable wear and corrosion-resistant materials. As improving the wear and corrosion resistance of iron base alloys tend to have opposing demands regarding chemical composition and heat treatment, optimisation of both parameters has to be kept in mind. One alloying element that is known to improve both corrosion and wear resistance of steels is nitrogen. Hence, an investigation into the densification of high chromium X190CrVMo20-4-1 cold work tool steel in a vacuum and under a nitrogen atmosphere at different pressures via supersolidus liquid-phase sintering (SLPS) process is reported in this paper. The investigation aimed to elucidate the influence of different atmospheres and nitrogen partial pressures employed during densification on the microstructure, optimal heat treatment parameters and micromechanical properties of the steel. Experimental findings were supplemented by computational thermodynamics calculations. The results revealed that increasing nitrogen pressure promoted the diffusion of vanadium from Cr-rich carbides (M7C3) to form V-rich carbonitrides, M(C,N). Optimum quench-hardening temperature was strongly influenced by the matrix chemistry. Upon tempering, the nitrogen-sintered samples had higher secondary hardening potential than the vacuum-sintered at a higher temperature, but a low-temperature tempering is beneficial to the corrosion resistance of the steel. The mechanical properties of the carbides in the densified steels in different atmospheres were influenced by their chemical composition. Experimental observations are in good agreement with computational thermodynamic evaluations. © 2020 Elsevier B.V.

Titanium-niobium (Ti–Nb) alloys have great potential for biomedical applications due to their superior biocompatibility and mechanical properties that match closely to human bone. Powder metallurgy is an ideal technology for efficient manufacture of titanium alloys to generate net-shape, intricately featured and porous components. This work reports on the effects of Nb concentrations on sintered Ti-xNb alloys with the aim to establish an optimal composition in respect to mechanical and biological performances. Ti-xNb alloys with 33, 40, 56 and 66 wt% Nb were fabricated from elemental powders and the sintering response, mechanical properties, microstructures and biocompatibility assessed and compared to conventional commercial purity titanium (CPTi). The sintered densities for all Ti-xNb compositions were around 95%, reducing slightly with increasing Nb due to increasing open porosity. Higher Nb levels retarded sintering leading to more inhomogeneous phase and pore distributions. The compressive strength decreased with increasing Nb, while all Ti-xNb alloys displayed higher strengths than CPTi except the Ti–66Nb alloy. The Young's moduli of the Ti-xNb alloys with ≥40 wt% Nb were substantially lower (30–50%) than CPTi. In-vitro cell culture testing revealed excellent biocompatibility for all Ti-xNb alloys comparable or better than tissue culture plate and CPTi controls, with the Ti–40Nb alloy exhibiting superior cell-material interactions. In view of its mechanical and biological performance, the Ti–40Nb composition is most promising for hard tissue engineering applications. © 2020

Extended diffusion layers of the cubic C15 and hexagonal C14 and C36 NbCo2 Laves phases with concentration gradients covering their entire homogeneity ranges were produced by the diffusion couple technique. Single-phase and single-crystalline micropillars of the cubic and hexagonal NbCo2 Laves phases were prepared in the diffusion layers by focused ion beam (FIB) milling. The influence of chemical composition, structure type, orientation and pillar size on the deformation behavior and the critical resolved shear stress (CRSS) was studied by micropillar compression tests. The pillar orientation influences the activated slip systems, but the deformation behavior and the CRSS are independent of orientation. The deformation of the smallest NbCo2 micropillars (0.8 µm in top diameter) appears to be dislocation nucleation controlled and the CRSS approaches the theoretical shear stress for dislocation nucleation. The CRSS of the 0.8 µm-sized NbCo2 micropillars is nearly constant from 26 to 34 at.% Nb where the C15 structure is stable. It decreases as the composition approaches the Co-rich and Nb-rich boundaries of the homogeneity range where the C15 structure transforms to the C36 and the C14 structure, respectively. The decrease in the CRSS at these compositions is related to the reduction of shear modulus and stacking fault energy. As the pillar size increases, stochastic deformation behavior and large scatter in the CRSS values occur and obscure the composition effect on the CRSS. © 2019

The experiment study presents the influence of femtosecond laser shock peening (FsLSP) without a protective layer in the air on the surface hardness and surface mechanical property of NiTi shape memory alloy. Femtosecond laser shock peening is a new possibility of direct laser ablation without any protective layer under atmospheric conditions, which can produce intense shock waves with low pulse energy in the air. The average surface roughness values of the NiTi alloy samples were measured, because the surface roughness may affect its friction resistance. The results showed that the surface roughness of NiTi increased after femtosecond laser shock peening treatment. In comparison with the initial state, the coefficient of friction decreased and surface microhardness increased after femtosecond laser shock peening treatment with different FsLSP parameters. This improvement of wear properties may be attributed to the enhancement of surface microhardness and surface titanium oxide layer induced by the shock wave and laser ablation during FsLSP treatment. © 2020 SPIE.

The Cr-Co-Ni system was studied by combining experimental and computational methods to investigate phase stability and mechanical properties. Thin-film materials libraries were prepared and quenched from high temperatures up to 700 °C using a novel quenching technique. It could be shown that a wide A1 solid solution region exists in the Cr-Co-Ni system. To validate the results obtained using thin-film materials libraries, bulk samples of selected compositions were prepared by arc melting, and the experimental data were additionally compared to results from DFT calculations. The computational results are in good agreement with the measured lattice parameters and elastic moduli. The lattice parameters increase with the addition of Co and Cr, with a more pronounced effect for the latter. The addition of ∼20 atom % Cr results in a similar hardening effect to that of the addition of ∼40 atom % Co. Copyright © 2020 American Chemical Society.

Designing structured materials with optimized mechanical properties generally focuses on engineering microstructures, which are closely determined by the processing routes, such as phase transformations. However, the direct connection between phase transformations and mechanical properties remains largely unexplored. Here, we propose a new concept of generalized stability (GS) to correlate phase transformations with plastic deformations in terms of the trade-off relationship that exists between thermodynamics and kinetics. We then suggest that, to achieve structured materials with excellent strength–plasticity combinations, phase transformations and/or plastic deformations with high GS, thermodynamic driving force (ΔG), and kinetic activation energy (Q), are highly expected. We verify the GS concept against a phase transformation-modulated nanostructured Fe alloy, for which an ultrahigh yield strength of 2.61 GPa and an ultimate compressive strength of 3.32 GPa while having a total strain to failure of 35% are achieved via multiple strengthening and hardening mechanisms. A theoretical analysis, in combination with microstructural characterization, indicates that the desired thermo-kinetic parameter triplets (i.e., high GS-high ΔG-high Q) could be inherited from the phase transformation to the plastic deformation, which ultimately yields good mechanical performance. The proposed concept can be regarded as the first theoretical criterion or a general rule that correlates phase transformation with plastic deformation, and can assist in the rapid selection of phase transformations to facilitate superior mechanical properties. © 2020

We applied two types of hot-rolling direct quenching and partitioning (HDQ&P) schemes to a low-C low-Si Al-added steel and obtained two ferrite-containing TRIP-assisted steels with different hard matrix structures, viz, martensite or bainite. Using quasi in-situ tensile tests combined with high-resolution electron back-scattered diffraction (EBSD) and microscopic digital image correlation (µ-DIC) analysis, we quantitatively investigated the TRIP effect and strain partitioning in the two steels and explored the influence of the strain partitioning between the soft and hard matrix structures on the TRIP effect. We also performed an atomic-scale analysis of the carbon partitioning among the different phases using atom probe tomography (APT). The results show that the strain mainly localizes in the ferrite in both types of materials. For the steel with a martensitic hard-matrix, a strong strain contrast exists between ferrite and martensite, with the local strain difference reaching up to about 75% at a global strain of 12.5%. Strain localization bands initiated in the ferrite rarely cross the ferrite/martensite interfaces. The low local strain (2%–10%) in the martensite regions leads to a slight TRIP effect with a transformation ratio of the retained austenite of about 7.5%. However, for the steel with bainitic matrix, the ferrite and bainite undergo more homogeneous strain partitioning, with an average local strain in ferrite and bainite of 15% and 8%, respectively, at a global strain of 12.5%. The strain localization bands originating in the ferrite can cross the ferrite/bainite (F/B) interfaces and increase the local strain in the bainite regions, resulting in an efficient TRIP effect. In that case the transformation ratio of the retained austenite is about 41%. The lower hardness difference between the ferrite and bainite of about 178 HV, compared with that between the ferrite and martensite of about 256 HV, leads to a lower strain contrast at the ferrite/bainite interfaces, thus retarding interfacial fracture. Further microstructure design for TRIP effect optimization should particularly focus on adjusting the strength contrast among the matrix structures and tuning strain partitioning to enhance the local strain partitioning into the retained austenite. © 2020 Acta Materialia Inc.

This study analyzes the local deformation behavior of austenitic stainless steel 316L, manufactured conventionally by casting and additively by laser metal deposition (LMD). We produced directionally solidified 316L specimens with most grains showing (001) orientations parallel to the longitudinal specimen axis. We conducted nanoindentation and scratch experiments for local mechanical characterization and topography measurements (atomic force microscopy and confocal laser scanning microscopy) of indentation imprints and residual scratch grooves for the analysis of the deformation behavior and, in particular, of the pile-up behavior. The local mechanical properties and deformation behavior were correlated to the local microstructure investigated by scanning electron microscopy with energy dispersive X-ray spectroscopy and electron backscatter diffraction analysis. The results show that the local mechanical properties, deformation behavior, and scratch resistance strongly depend on the crystallographic orientation. Nearly (001)-oriented grains parallel to the surface show the lowest hardness, followed by an increasing hardness of nearly (101)-and (111)-oriented grains. Consequently, scratch depth is the greatest for nearly (001)-oriented grains followed by (101) and (111) orientations. This tendency is seen independently of the analyzed manufacturing route, namely Bridgman solidification and laser metal deposition. In general, the laser metal deposition process leads to a higher strength and hardness, which is mainly attributed to a higher dislocation density. Under the investigated loading conditions, the cellular segregation substructure is not found to significantly and directly change the local deformation behavior during indentation and scratch testing. © 2020 by the authors.

This brief paper contains raw data of X-ray diffraction (XRD) measurements, microstructural characterization, chemical compositions, and mechanical properties describing the influence of Al, Ti, and C on as-cast Al0.6CoCrFeNi compositionally complex alloys (CCAs). The presented data are related to the research article in reference [1] and therefore this article can be referred to as for the interpretation of the data. X-ray diffraction data presented in this paper are measurements of 2θ versus intensities for each studied alloy. A Table lists the obtained lattice parameters of each identified phase determined by Rietveld analysis. Microstructural-characterization data reported here include backscattered electron (BSE) micrographs taken at different magnifications in a scanning electron microscope (SEM) of Widmanstätten and dendritic microstructures and microstructural parameters such as phase volume fractions, thickness of face-centered cubic (FCC) plates, and prior grain sizes. The compositions of the identified individual phases determined by energy-dispersive X-ray spectroscopy (EDX) in the transmission electron microscope (TEM) are listed as well. Finally, mechanical data including engineering stress-strain curves obtained at different temperatures (room temperature, 400 °C, and 700 °C) for all CCAs are reported. © 2019 The Authors

This study investigates the evolution of the microstructure and mechanical properties of mortar. Mortar samples consisting of Portland cement CEM I42.5 R (~ 60 vol% of quartz sand 0/2 mm, w/c-ratio of 0.5) were prepared and stored according to EN 1015. After 1, 2, 7, 14 and 28 days, the samples were oven-dried until constant weight as well as vacuum-dried. The microstructure of the mortar samples was investigated using scanning electron microscopy. Phase analysis was performed using X-ray diffraction, allowing the description of the crystalline phase evolution during hardening. Mechanical properties were evaluated using nanoindentation. Based on the nanoindentation results, the effective Young’s modulus was calculated using the model by Hashin and Shtrikman. The moduli calculated based on the values of the nanoindentation experiments were compared to the Young’s modulus determined in compression experiments. The results show that the Young’s modulus determined by the nanoindentation and compression test describes a degressive curve progression. The studies show a correlation between the results from nanoindentation tests and the mechanical properties obtained from the compression tests. Therefore, the microstructural evolution of mortar, including the influence of pores on Young’s modulus, must be taken into account to estimate the macroproperties from the nanoindentation tests. © 2018, Iran University of Science and Technology.

The study of mechanical properties of materials at high temperatures at the microstructural length regime requires dedicated setups for testing. Despite the advances in the instrumentation in these setups over the last decade, further optimization is required in order to extend the temperature range well-beyond 600 °C. Particularly, an improvement of the contact temperature measurement is required. A design with a novel approach of temperature measurement with independent tip and sample heating is developed to characterize materials at high temperatures. This design is realized by modifying a displacement controlled room temperature microstraining rig with the addition of two miniature hot stages, one each carrying the sample and indenter tip. The sample reaches temperatures of >600 °C with a 50 W diode laser system. The stages have slots for the working sample as well as a reference sample on both ends for precise temperature measurements, relying on the symmetry of the stage toward the ends. The whole setup is placed inside a custom-made steel chamber, capable of attaining a vacuum of 10-4 Pa. Alternatively, the apparatus can be operated under environmental conditions by applying various gases. Here, the unique design and its high temperature capabilities will be presented together with the first results of microtension experiments on freestanding copper thin films at 400 °C. © 2019 Author(s).

Thermomechanical processing has been used to control the grain size/shape of the equiatomic CrCoNi medium-entropy alloy (MEA) and obtain excellent strength and ductility. However, in the cast state, the alloy has coarse columnar grains with average widths and lengths of approximately 120 and 1000 μm, respectively, resulting in inferior mechanical properties. To overcome this deficiency, here we microalloyed with Ti and C and successfully changed the grain shape (from columnar to equiaxed) and refined the grain size. The degree to which the microstructure changes depends on the amount of Ti and C added, with the best results obtained at 0.4 at.% each. In the optimal alloy [(CrCoNi)99.2Ti0.4C0.4], the as-cast grains were nearly equiaxed with a uniform size of ∼75 μm. Associated with this change in grain shape/size was a significant improvement of yield strength, ultimate tensile strength and elongation to fracture at both 293 and 77 K. The columnar to equiaxed transition is attributed to the strong mutual affinity of C and Ti, which leads to their build-up ahead of the solid-liquid interface and, in turn, to enhanced constitutional undercooling. © 2018 Elsevier B.V.

Multiple-principal element alloys known as high-entropy alloys have rapidly been gaining attention for the vast variety of compositions and potential combinations of properties that remain to be explored. Of these alloys, one of the earliest, the ‘Cantor alloy’ CrMnFeCoNi, displays excellent damage-tolerance with tensile strengths of ∼1 GPa and fracture toughness values in excess of 200 MPa√m; moreover, these mechanical properties tend to further improve at cryogenic temperatures. However, few studies have explored its corresponding fatigue properties. Here we expand on our previous study to examine the mechanics and mechanisms of fatigue-crack propagation in the CrMnFeCoNi alloy (∼7 μm grain size), with emphasis on long-life, near-threshold fatigue behavior, specifically as a function of load ratio at temperatures between ambient and liquid-nitrogen temperatures (293 K–77 K). We find that ΔKth fatigue thresholds are decreased with increasing positive load ratios, R between 0.1 and 0.7, but are increased at decreasing temperature. These effects can be attributed to the role of roughness-induced crack closure, which was estimated using compliance measurements. Evidence of deformation twinning at the crack tip during fatigue-crack advance was not apparent at ambient temperatures but seen at higher stress intensities (ΔK ∼ 20 MPa√m) at 77 K by post mortem microstructural analysis for tests at R = 0.1 and particularly at 0.7. Overall, the fatigue behavior of this alloy was found to be superior, or at least comparable, to conventional cryogenic and TWIP steels such as 304 L or 316 L steels and Fe-Mn steels; these results coupled with the remarkable strength and fracture toughness of the Cantor alloy at low temperatures indicate significant promise for the utility of this material for applications at cryogenic environments. © 2019

The important role of grain boundaries for the mechanical properties of polycrystalline materials has been recognized for many decades. Up to now, the underlying deformation mechanisms at the nano- and micro scale are not understood quantitatively. An overview of the synthesis and subsequent mechanical testing of specific grain boundaries at the micro and sub-micro scale is discussed in the present contribution, including various methods for producing one or multiple specific, crystallographically well-defined grain boundaries. Furthermore, established micromachining methods for isolating and measuring local dislocation-grain boundary interactions are portrayed. Examples of the techniques described are shown with to the aid of copper grain boundaries. © Carl Hanser Verlag, München

Increased demand for light-weighting in passenger vehicles has created a need for strong, light, ductile materials to be used in body-in-white applications. The AA6xxx-series of aluminum alloys are suitable candidates meeting most requirements but can fall short of the formability demands of designers, necessitating an understanding of what controls the formability in this alloy series. This work examines the effect of copper alloying in AA6xxx on the pre-ageing and natural ageing responses of the microstructure and mechanical properties. The changes in microstructure observed by differential scanning calorimetry and hardness testing are related to the work-hardening and strain-rate sensitivity parameters for these alloys measured by tensile testing. An observed asymmetry in the measured strain-rate sensitivity associated with increasing versus decreasing strain rate changes suggests that a different mechanism operates for the two conditions. It is postulated how this asymmetry in strain-rate sensitivity will impact the necking and ductility behaviour of these alloys. © 2019, The Minerals, Metals & Materials Society.

We demonstrate a novel approach of utilizing a hierarchical microstructure design to improve the mechanical properties of an interstitial carbon doped high-entropy alloy (HEA) by cold rolling and subsequent tempering and annealing. Bimodal microstructures were produced in the tempered specimens consisting of nano-grains (∼50 nm) in the vicinity of shear bands and recovered parent grains (10–35 μm) with pre-existing nano-twins. Upon annealing, partial recrystallization led to trimodal microstructures characterized by small recrystallized grains (<1 μm) associated with shear bands, medium-sized grains (1–6 μm) recrystallized through subgrain rotation or coalescence of parent grains and retained large un-recrystallized grains. To reveal the influence of these hierarchical microstructures on the strength-ductility synergy, the underlying deformation mechanisms and the resultant strain hardening were investigated. A superior yield strength of 1.3 GPa was achieved in the bimodal microstructure, more than two times higher than that of the fully recrystallized microstructure, owing to the presence of nano-sized grains and nano-twins. The ductility was dramatically improved from 14% to 60% in the trimodal structure compared to the bimodal structure due to the appearance of a multi-stage work hardening behavior. This important strain hardening sequence was attributed to the sequential activation of transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects as a result of the wide variation in phase stability promoted by the grain size hierarchy. These findings open a broader window for achieving a wide spectrum of mechanical properties for HEAs, making better use of not only compositional variations but also microstructure and phase stability tuning. © 2018 Acta Materialia Inc.

Technologically important mechanical properties of engineering materials often degrade at low temperatures. One class of materials that defy this trend are CrCoNi-based medium- and high-entropy alloys, as they display enhanced strength, ductility, and toughness with decreasing temperature. Here we show, using in situ straining in the transmission electron microscope at 93 K (−180 °C)that their exceptional damage tolerance involves a synergy of deformation mechanisms, including twinning, glide of partials and full dislocations, extensive cross-slip, and multiple slip activated by dislocation and grain-boundary interactions. In particular, massive cross-slip occurs at the early stages of plastic deformation, thereby promoting multiple slip and dislocation interactions. These results indicate that the reduced intensity of thermal activation of defects at low temperatures and the required increase of applied stress for continued plastic flow, together with high lattice resistance, play a pivotal role in promoting the concurrent operation of multiple deformation mechanisms, which collectively enable the outstanding mechanical properties of these alloys. © 2019 Elsevier Ltd

Low-alloyed steels with body-centered cubic crystal structure are a material class that is widely used for sheet metal forming applications. When having an adequate crystallographic texture and microstructure, their mechanical behavior is characterized by an isotropic in-plane flow behavior in combination with a low yield strength. The decisive processing steps for obtaining these beneficial mechanical properties are cold rolling and subsequent annealing. While for the former the number of passes, the deformation rates, and the total thickness reduction are the main processing parameters, the latter is described mainly by the heating rate and the holding temperature and time. Primary static recrystallization during annealing subsequent to the cold rolling process alters mainly two aspects of the material state: It firstly replaces the elongated and heavily deformed grains of the cold rolled microstructure by small, globular grains with low dislocation density and secondly it changes the crystallographic texture insofar as it typically diminishes the α- and strengthens the γ-fiber texture components. In the present work, the recrystallization behavior of a commercial non-alloyed low carbon steel is studied. A quasi in situ setup that enables site-specific characterization is employed to gain a local picture of the nucleation and recrystallization process. From the Kernel Average Misorientation (KAM) values of the deformation structure, the tendency to be consumed by new grains can be predicted. Crystallographic analysis shows that the most deformed regions have either a γ-fiber orientation or belong to heavily fragmented regions. New grains nucleate especially in such highly deformed regions and inherit often the orientation from the deformation microstructure. © 2019 Elsevier B.V.

During the last decade, Suspension Plasma Spraying (SPS) attracted a lot of interest as an alternative process to produce columnar Thermal Barrier Coatings (TBCs). In this study, columnar TBCs were deposited with SPS. After spraying, samples were isothermally annealed at 1373 K for 1 h, 3 h, 10 h and 50 h, respectively. Microstructures and mechanical properties of the ceramic coatings were investigated as a function of annealing time. Annealing resulted in healing of micro-cracks, coarsening of pores, growth of domain size, companied with a decrease of porosity within columns. The change of coating microstructure led to change of mechanical properties. In addition, residual stress in SPS coatings was also investigated. Furthermore, as-sprayed coatings and pre-annealed coatings were subjected to burner rig tests. Short time pre-annealing allowed to enhance thermal cycling lifetime of such SPS coatings. The thermal cycling results were related to microstructure modifications of coatings. © 2018 Elsevier Ltd

Cr-Al-N thin film materials libraries were synthesized by combinatorial reactive high power impulse magnetron sputtering (HiPIMS). Different HiPIMS repetition frequencies and peak power densities were applied altering the ion to growth flux ratio. Moreover, time-resolved ion energy distribution functions were measured with a retarding field energy analyzer (RFEA). The plasma properties were measured during the growth of films with different compositions within the materials library and correlated to the resulting film properties such as phase, grain size, texture, indentation modulus, indentation hardness, and residual stress. The influence of the ion to growth flux ratio on the film properties was most significant for films with high Al-content (xAl = 50 at. %). X-ray diffraction with a 2D detector revealed hcp-AlN precipitation starting from Al-concentration xAl ≥ 50 at. %. This precipitation might be related to the kinetically enhanced adatom mobility for a high ratio of ions per deposited atoms, leading to strong intermixing of the deposited species. A structure zone transition, induced by composition and flux ratio JI/JG, from zone T to zone Ic structure was observed which hints toward the conclusion that the combination of increasing flux ratio and Al-concentration lead to opposing trends regarding the increase in homologous temperature. © 2019 American Chemical Society.

Tungsten (W) and various composites are being considered as the primary plasma-facing materials for fusion reactors. Like all engineering materials, they contain certain levels of impurities, which can have an important impact on mechanical properties. In the present work, oxygen was identified as a major impurity in our starting tungsten powder. At elevated temperatures, the presence of interstitial elements such as oxygen leads to the formation of an oxide-rich tungsten phase at the tungsten grain boundaries. In this study, we determined the capacity of tungsten carbide (WC) nanoparticles to remove the oxide impurities from a tungsten body. Tungsten composites with 0.05, 0.25 and 0.51 wt. % carbon (C) in the form of WC were sintered using a field-assisted sintering technique (FAST) at 1900 °C for 5 min. The sintered samples were characterized using field-emission scanning and transmission electron microscopy. Thermodynamic and kinetic considerations allowed us to determine the optimum theoretical amount of WC to prevent the in-situ formation of WO2. © 2019 Andreja Šestan, Janez Zavašnik, Marjeta Maček Kržmanc, Matej Kocen, Petra Jenuš, Saša Novak, Miran Čeh, Gerhard Dehm

The cast microstructure of the Al0.6CoCrFeNi compositionally complex alloy was successfully refined with small additions of Al, Ti and C and its mechanical properties were optimized. In the as-cast state, this alloy has a Widmanstätten microstructure with coarse grains (∼110 μm) of a strong BCC/B2 matrix and soft FCC plates (∼65 vol.%) with large widths (∼1.3 μm). The addition of 0.25 at.% C to Al0.6CoCrFeNi stabilizes the FCC phase and favors the formation of a coarse dendritic microstructure making this alloy unsuitable for structural applications. In contrast, alloying of either 3 at.% Al, Ti, or 3% Ti and 0.25% C to Al0.6CoCrFeNi refined its Widmanstätten microstructure, i.e. the thickness of the FCC plates and/or the size of the prior BCC/B2 grains were significantly reduced. As a result of these microstructural changes, Al and Ti containing alloys show an outstanding strength (twice higher than that of Al0.6CoCrFeNi) and ductilities ≤5% at 20 °C. These properties are retained at 400 °C but at 700 °C, the strength and ductility of almost all alloys decrease. However, Ti containing alloys exhibit much larger ductilities (∼50%) at 700 °C due to their high density of grain boundaries which accommodate plastic deformation through grain boundary sliding. © 2019 The Authors

Coefficients of elastic stiffnesses and thermal expansion of hot isostatically pressed, reaction-sintered and technical fused-mullite ceramics were measured between 100 and 1673 K in comparison with single crystal mullite employing resonant ultrasound spectroscopy and dilatometry, respectively. Additionally, chemical and phase compositions and the microstructure of the ceramics were studied using X-ray diffraction techniques and scanning electron microscopy. Our studies revealed that despite polycrystallinity and slight porosity of up to 1.6%, the elastic behavior of the hot isostatically pressed ceramics is near to ideal aggregate elastic properties of mullite single crystal, for example, their bulk moduli fit within 0.7% to B = 170.0 GPa of single crystal mullite. On the other hand, with B = 155 GPa, the reaction-sintered mullite behaves significantly softer. The difference can be explained with more tight grain to grain contacts in hot isostatically pressed ceramics as compared to reaction-sintered materials. The thermal expansion of both types of ceramics almost coincides with the corresponding averaged behavior of single crystal mullite. For example, between 573 and 1273 K, the volume expansion coefficients of all these materials are (18.0 ± 0.3)·10−6 K−1. Obviously, the microstructural features are less important for the macroscopic thermal expansion. Due to heterogeneous microstructure and high α-alumina and zirconia contents, the corresponding properties of fused-mullite refractory deviate strongly from those of the other mullite materials. © 2018 The American Ceramic Society

Additive manufacturing (AM) has recently become one of the key manufacturing processes in the era of Industry 4.0 because of its highly flexible production scheme. Due to complex thermal cycles during the manufacturing process itself and special solidification conditions, the microstructure of AM components often exhibits elongated grains together with a pronounced texture. These microstructural features significantly contribute to an anisotropic mechanical behavior. In this work, the microstructure and mechanical properties of additively manufactured samples of 316L stainless steel are characterized experimentally and a micromechanical modeling approach is employed to predict the macroscopic properties. The objective of this work is to study the effects of texture and microstructural morphology on yield strength and strain hardening behavior of face-centered cubic additively manufactured metallic components. To incorporate the texture in synthetic representative volume elements (RVE), the proposed approach considers both the crystallographic and grain boundary textures. The mechanical behavior of these RVEs is modeled using crystal plasticity finite element method, which incorporates size effects through the implementation of strain gradients. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Mechanical properties of crystalline materials strongly correlate with deformation behaviour at the grain level. In the present work, we establish a 3D crystal plasticity finite element model of nanoscratching of single crystalline copper using a Berkovich probe, which is capable of addressing the crystallography influence. In particular, nanoindentation experiments and high resolution electron back-scatter diffraction characterization are jointly carried out to precisely calibrate parameters used in the crystal plasticity finite element model. Subsequent finite element simulations of nanoscratching are performed to reveal fundamental deformation behaviour of single crystalline copper in terms of mechanical response and surface pile-up topography, as well as their dependence on crystallographic orientation. Furthermore, nanoscratching experiments with the same parameters used in the finite element simulations are carried out, the results of which are further compared with predication results by the finite element simulations. Simulation data and experimental results jointly demonstrate the strong anisotropic characteristics of single crystalline copper under nanoscratching, due to the crystallographic orientation dependent coupled effects of intrinsic dislocation slip and extrinsic discrete stress distribution by probe geometry. © 2019 Elsevier B.V.

In the current work we investigate the room temperature tensile properties of a medium-Mn twinning- and transformation-induced plasticity (TWIP-TRIP) steel from quasi-static to low-dynamic strain rates (ε˙ = 10−4 s−1 to ε˙ = 102 s−1). The multi-phase microstructure consists of coarse-grained recovered α'-martensite (inherited from the cold-rolled microstructure), multiple morphologies of ultrafine-grained (UFG) austenite (equiaxed, rod-like and plate-like), and equiaxed UFG ferrite. The multi-phase material exhibits a positive strain-rate sensitivity for yield and ultimate tensile strengths. Thermal imaging and digital image correlation allow for in situ measurements of temperature and local strain in the gauge length during tensile testing, but Lüders bands and Portevin Le Chatelier bands are not observed. A finite-element model uses empirical evidence from electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM), plus constitutive equations to dissect the microstructural influences of grain size, dislocation density and TWIP-TRIP driving forces on tensile properties. Calibration of tensile properties not only captures the strain rate sensitivity of the multi-phase TWIP-TRIP steel, but also provides opportunity for a complete parametric analysis by changing one variable at a time (phase fraction, grain size, strain-induced twin fraction and strain-induced ε-martensite fraction). An equivalent set of high-rate mechanical properties can be matched by changing either the austenite phase fraction or the ratio of twinning vs. transformation to ε-martensite. This experimental-computational framework enables the prediction of mechanical properties in multi-phase steels beyond the experimental regime by tuning variables that are relevant to the alloy design process. © 2019 Acta Materialia Inc.

The microstructure-mechanical property relationships of a non-equiatomic FeMnCoCr high entropy alloy (HEA), which shows a single face-centered cubic (fcc) structure in the undeformed state, have been systematically investigated at room and cryogenic temperatures. Both strength and ductility increase significantly when reducing the probing temperature from 293 K to 77 K. During tensile deformation at 293 K, dislocation slip and mechanical twinning prevail. At 173 K deformation-driven athermal transformation from the fcc phase to the hexagonal close-packed (hcp) martensite is the dominant mechanism while mechanical twinning occurs in grains with high Schmid factors. At 77 K athermal martensitic transformation continues to prevail in addition to dislocation slip and twinning. The reduction in the mean free path for dislocation slip through the fine martensite bundles and deformation twins leads to the further increased strength. The joint activation of transformation and twinning under cryogenic conditions is attributed to the decreased stacking fault energy and the enhanced flow stress of the fcc matrix with decreasing temperature. These mechanisms lead to an elevated strain hardening capacity and an enhanced strength-ductility combination. The temperature-dependent synergy effects of martensite formation, twinning and dislocation plasticity originate from the metastability alloy design concept. This is realized by relaxing the equiatomic HEA constraints towards reduced Ni and increased Mn contents, enabling a non-equiatomic material with low stacking fault energy. These insights are important for designing strong and ductile Ni-saving alloys for cryogenic applications. © 2019 Elsevier B.V.

Despite numerous studies and decades of industrial application, there is still a lack of understanding about the formation and the impact of aluminum titanate (Al2TiO5) in Al2O3-TiO2 thermal spray coatings. Especially the influence of the feedstock powder characteristics on the phase composition has only crudely been investigated so far. Therefore, in this work we have characterized commercial fused and crushed Al2O3-TiO2 feedstock powders: Three of them containing 13 wt.% TiO2 and three containing 40 wt.% TiO2. The effect of the varying phase compositions of the powders and their relevance on the deposition efficiency, the phase compositions, the porosity, and the hardness of the respective APS coatings is described in detail. While detrimental to the mechanical properties of 40 wt.% TiO2 coatings, we have found an enhancement of the hardness for 13 wt.% TiO2 coatings with a high Al2TiO5/Al6Ti2O13 content in the feedstock powder. Furthermore, it was found that Al2TiO5 may reform during APS when sprayed from an Al2TiO5-free powder. © Published under licence by IOP Publishing Ltd.

We investigate the cryogenic deformation response and underlying mechanisms of a carbon-doped interstitial high-entropy alloy (iHEA) with a nominal composition of Fe49.5Mn30Co10Cr10C0.5 (at. %). Extraordinary strain hardening of the iHEA at 77 K leads to a substantial increase in ultimate tensile strength (∼1300 MPa) with excellent ductility (∼50%) compared to that at room temperature. Prior to loading, iHEAs with coarse (∼100 μm) and fine (∼6 μm) grain sizes show nearly single face-centered cubic (FCC) structure, while the fraction of hexagonal close-packed (HCP) phase reaches up to ∼70% in the cryogenically tensile-fractured iHEAs. Such an unusually high fraction of deformation-induced phase transformation and the associated plasticity (TRIP effect) is caused by the strong driving force supported by the reduced stacking fault energy and increased flow stress at 77 K. The transformation mechanism from the FCC matrix to the HCP phase is revealed by transmission electron microscopy (TEM) observations. In addition to the deformation-induced phase transformation, stacking faults and dislocation slip contribute to the deformation of the FCC matrix phase at low strains and of the HCP phase at medium and large strains, suggesting dynamic strain partitioning among these two phases. The combination of TRIP and dynamic strain partitioning explain the striking strain hardening capability and resulting excellent combination of strength and ductility of iHEAs under cryogenic conditions. The current investigation thus offers guidance for the design of high-performance HEAs for cryogenic applications. © 2018 Elsevier B.V.

We study the microstructure formation and mechanical properties of Ti-1Fe (wt%) and Ti-3Fe (wt%) alloys for different heat treatments in the β-phase and α + β-phase regions. By applying different heat treatment routes, we observe different microstructure formation mechanisms causing a wide range of mechanical properties from high strength (1.3 GPa) and low ductility (2%) to intermediate strength (700 MPa) and high ductility (30%) in these simple binary alloys. We performed microstructure characterizsation using scanning electron microscopy, transmission electron microscopy and atom probe tomography to show that the alloying content and heat treatment significantly affect the local martensitic and / or diffusional phase transformations causing the substantial changes in the mechanical behavior. © 2018

Atmospheric plasma sprayed (APS) Al2O3-TiO2 coatings have found a wide range of industrial application due to their favorable properties, combined with low costs and a high availability. However, the detailed effect of the phase composition and the element distribution of the feedstock powders on the coating properties and the spraying process have only crudely been investigated so far. Here the impact of aluminum titanate (Al2TiO5) on the microstructural features and mechanical properties of Al2O3-40 wt.% TiO2 APS coatings is demonstrated by investigating the detailed phase composition and the distribution of aluminum and titanium in three fused and crushed feedstock powders and the respective coatings. Thereby, a direct influence of Al2TiO5 content on the deposition efficiency, the porosity, the elastic modulus, and the hardness of the coatings is revealed. The results emphasize the need for a more detailed specification of commercial Al2O3-TiO2 feedstock powders to ensure a high reliability of the coating properties. © 2019 Elsevier Ltd

The degradation of mechanical properties due to sintering is one of the major issues during high temperature service of thermal barrier coating system for advanced gas turbines. In this study, a constitutive model was developed by the variational principle, based on the experimentally observed microstructure features of suspension plasma-sprayed thermal barrier coatings. The constitutive model was further implemented in finite element analysis software, in order to investigate the effect of vertical cracks. The evolution of microstructure during sintering, coating shrinkage and mechanical degradation were predicted. The numerical predictions of Young's modulus were generally in agreement with experimental results. Furthermore, the effect of vertical cracks on the strain tolerance and sintering resistance were discussed. It was confirmed that the introduction of vertical cracks contributed to the improvement of both properties. © 2019 The American Ceramic Society

The mechanical properties of investment casted open-pore metal foams have been investigated on the example of the binary alloy Al–11Zn. The samples were subjected to different cooling conditions subsequent to casting and to different homogenization and ageing treatments. Variation in cooling was done either by quenching the mold in water or slowly cooling it in air. Homogenization and ageing varied in terms of temperature and time. The effects of the different treatments were investigated through microstructural and mechanical characterization methods. Using TEM, we found that the presence of GP zones and their morphological arrangement are the main factors dominating the mechanical performance. Micro- and nanoindentation testing of single foam struts reveal maximum hardness H when room temperature ageing was applied. Ageing at a temperature of 150 °C results in the lowest H in the present study; that is approximately 2/3 of the hardness achieved when ageing at room temperature. This can also be confirmed by the strength of non-porous bulk material obtained by tensile tests, which further show an increase in ductility up to a factor of 5 due to ageing at elevated temperatures. By compression testing of open-pore Al–11Zn foams, we notice that the presence of the microstructural effects varies in extent as a function of the strain ε. At low strains, we observe differences in mechanical performance to a high extent, becoming less with increasing compaction of the samples until they behave as non-porous bulk material. Based on these findings, we deduce a strong interaction of the structural morphology of the foam and its microstructure that determines the mechanical properties dominated by strength and ductility of the base material. © 2019 Elsevier B.V.

The iron aluminides discussed here are Fe-Al-based alloys, in which the matrix consists of the disordered bcc (Fe,Al) solid solution (A2) or the ordered intermetallic phases FeAl (B2) and Fe3Al (D03). These alloys possess outstanding corrosion resistance and high wear resistance and are lightweight materials relative to steels and nickel-based superalloys. These materials are evoking new interest for industrial applications because they are an economic alternative to other materials, and substantial progress in strengthening these alloys at high temperatures has recently been achieved by applying new alloy concepts. Research on iron aluminides started more than a century ago and has led to many fundamental findings. This article summarizes the current knowledge of this field in continuation of previous reviews. © 2019 by Annual Reviews. All rights reserved.

Large single crystals of GdCa4O(BO3)3 (space group Cm) were grown by the Czochralski method. Dielectric, piezoelectric and elastic coefficients at room temperature as well as specific heat capacity, thermal expansion and cation disorder were studied employing a variety of methods including resonant ultrasound spectroscopy, differential scanning calorimetry, dilatometry and X-ray diffraction techniques. The electromechanical parameters (4 dielectric, 10 piezoelectric and 13 elastic stiffness coefficients) obtained on different samples are in excellent agreement indicating high internal consistency of our approach, whereas the values reported in literature differ significantly. The elastic behaviour of GdCa4O(BO3)3 resembles the one of structurally related fluorapatite, i.e. the elastic anisotropy is relatively small and the longitudinal effect of the deviations from Cauchy-relations exhibit a pronounced minimum along the direction of the dominating chains of cation polyhedra. GdCa4O(BO3)3 exhibits a maximum longitudinal piezoelectric effect of 7.67 × 10-12 CN-10, a value in the order of that of langasite-type materials. Significant changes of the calcium/gadolinium distribution on the 3 independent cation sites accompanied by characteristic anomalies of heat capacity and thermal expansion suggest processes of nonconvergent cation ordering above about 900 K in GdCa4O(BO3)3. © 2019 Walter de Gruyter GmbH, Berlin/Boston.

The mechanical properties of low-pressure plasma sprayed (LPPS) MCrAlY (M = Ni, Co) bond coats, Amdry 386, Amdry 9954 and oxide dispersion strengthened (ODS) Amdry 9954 (named Amdry 9954 + ODS) were investigated after annealing in three atmospheres: Ar–O2, Ar–H2 O, and Ar–H2 –H2 O. Freestanding bond coats were investigated to avoid any influence from the substrate. Miniaturized cylindrical tensile specimens were produced by a special grinding process and then tested in a thermomechanical analyzer (TMA) within a temperature range of 900–950◦ C. Grain size and phase fraction of all bond coats were investigated by EBSD before testing and no difference in microstructure was revealed due to annealing in various atmospheres. The influence of annealing in different atmospheres on the creep strength was not very pronounced for the Co-based bond coats Amdry 9954 and Amdry 9954 + ODS in the tested conditions. The ODS bond coats revealed significantly higher creep strength but a lower strain to failure than the ODS-free Amdry 9954. The Ni-based bond coat Amdry 386 showed higher creep strength than Amdry 9954 due to the higher fraction of the β-NiAl phase. Additionally, its creep properties at 900◦ C were much more affected by annealing in different atmospheres. The bond coat Amdry 386 annealed in an Ar–H2 O atmosphere showed a significantly lower creep rate than the bond coat annealed in Ar–O2 and Ar–H2 –H2 O atmospheres. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.

Despite having otherwise outstanding mechanical properties, many single-phase medium and high entropy alloys are limited by modest yield strengths. Although grain refinement offers one opportunity for additional strengthening, it requires significant and undesirable compromises to ductility. This work therefore explores an alternative, simple processing route to achieve strength by cold-rolling and annealing an equiatomic CrCoNi alloy to produce heterogeneous, partially recrystallized microstructures. Tensile tests reveal that our approach dramatically increases the yield strength (to ∼1100 MPa) while retaining good ductility (total elongation ∼23%) in the single-phase CrCoNi alloy. Scanning and transmission electron microscopy indicate that the strengthening is due to the non-recrystallized grains retaining their deformation-induced twins and very high dislocation densities. Load-unload-reload tests and grain-scale digital image correlation are also used to study the accumulation of plastic deformation in our highly heterogeneous microstructures. © 2018 Acta Materialia Inc.

The investigation of hard coatings under thermal load is crucial in order to obtain information on the thermal stability and possible changes of microstructure and mechanical properties. In addition, advanced heating studies may also provide feedback for the grain growth mechanism occurring during annealing and thus, help to predict optimum post-growth annealing conditions for producing high-performance hard coatings. Here, we investigate the thermal response of Mo2BC, deposited by bipolar pulsed direct current magnetron sputtering in an industrial chamber on a silicon substrate at a substrate temperature of 380 °C. Ex-situ and in-situ X-ray diffraction and transmission electron microscopy studies are performed at elevated temperatures to track changes in the structure. Whereas the as-deposited nanocomposite coating exhibits small spherical nanocrystals (1.2 nm in diameter) embedded in an amorphous matrix, a fully crystalline structure, mainly consisting of elongated and interconnected crystals with lengths of up to 1 μm, is obtained at elevated annealing temperatures. Hardness and Young's modulus increase by ~8% and ~47%, respectively, compared to the as-deposited coating. Delamination from the silicon substrate only occurs at temperatures above 840 °C. Thus, our detailed study of the micro- and nanostructure evolution upon thermal annealing suggests that heat treatments below 840 °C are a suitable method to improve the crystallinity and mechanical properties of nanocomposite Mo2BC coatings. © 2018

A computational method is proposed for the construction of statistically similar representative volume elements (SSRVEs) for discontinuous fiber composites (DFCs) in order to enable an efficient calculation of material properties based on computational homogenization. The SSRVEs are obtained by solving an optimization problem which minimizes the difference between the power spectral density of a target microstructure and its simplified one. The SSRVEs are constructed for target microstructures serving as examples for DFCs, which are validated by means of comparing the mechanical properties of the target microstructures with the ones of the SSRVEs. The results show that the mechanical properties of the SSRVEs agree with the target microstructures and that the SSRVEs can extremely reduce the computational costs of finite element analyses to derive macroscopic material properties of DFCs. © 2018 Elsevier Ltd

A new class of materials called high-entropy alloys (HEAs) constitutes multiple principal elements in similar compositional fractions. The equiatomic Fe20Mn20Cr20Co20Ni20 (at%) HEA shows attractive mechanical properties, particularly under cryogenic conditions. Yet, it lacks sufficient yield and ultimate tensile strengths at room temperature. To strengthen these materials, various strategies have been proposed mainly by tuning the composition of the bulk material while no efforts have been made to decorate and strengthen the grain boundaries. Here, we introduce a new HEA design approach that is based on compositionally conditioning the grain boundaries instead of the bulk. We found that as little as 30 ppm of boron doping in single-phase HEAs, more specific in an equiatomic FeMnCrCoNi and in a non-equiatomic Fe40Mn40Cr10Co10 (at%), improves dramatically their mechanical properties, increasing their yield strength by more than 100% and ultimate tensile strength by ∼40% at comparable or even better ductility. Boron decorates the grain boundaries and acts twofold, through interface strengthening and grain size reduction. These effects enhance grain boundary cohesion and retard capillary driven grain coarsening, thereby qualifying boron-induced grain boundary engineering as an ideal strategy for the development of advanced HEAs. © 2018 Acta Materialia Inc.

Currently, additive manufacturing (AM) experiences significant attention in nearly all industrial sectors. AM is already well established in fields such as medicine or spare part production. Nevertheless, processing of high-performance nickel-based superalloys and especially single crystalline alloys such as CMSX-4® is challenging due to the difficulty of intense crack formation. Selective electron beam melting (SEBM) takes place at high process temperatures (~ 1000 °C) and under vacuum conditions. Current work has demonstrated processing of CMSX-4® without crack formation. In addition, by using appropriate AM scan strategies, even single crystals (SX SEBM CMSX-4®) develop directly from the powder bed. In this contribution, we investigate the mechanical properties of SX SEBM CMSX-4® prepared by SEBM in the as-built condition and after heat treatment. The focus is on hardness, strength, low cycle fatigue, and creep properties. These properties are compared with conventional cast and heat-treated material. © 2018 The Author(s)

The high-entropy-alloy Al0.7CoCrFeNi (molar ratio) was prepared by vacuum arc melting followed by directional solidification (DS) with <001> oriented seed. The unique lamellar-dendrite microstructure was obtained over a wide cooling rate range. During solidification, Fe and Co are prone to segregate to the dendrite, while Cr and Al segregate to interdendrite. The solute pile-up of Cr and Al at the solid/liquid interface leads to the dendritic solidification. During the following cooling process, the BCC phase precipitates from the FCC dendrite to form the lamellar structure, while the ordered B2 phase precipitates from the interdendrite. Moreover, the lamellar spacing is significantly refined with increasing cooling rate, resulting in the higher hardness and compressive yield strength. Directional solidification is proved to be an efficient way to improve the mechanical properties of multi-phases high-entropy alloys. © 2017 Elsevier Ltd

A composition-spread Cu-Zr thin film library with Zr contents from 2.5 up to 6.5 at.% was synthesized by magnetron sputtering on a thermally oxidized Si wafer. The compositional and microstructural variations of the Cu-Zr thin film across the composition gradient were examined using energy dispersive X-ray spectroscopy, X-ray diffraction, and high-resolution scanning and transmission electron microscopy of cross-sections fabricated by focused ion beam milling. Composition-dependent hardness and elastic modulus values were obtained by nanoindentation for measurement areas with discrete Zr contents along the composition gradient. Similarly, the electrical resistivity was investigated by 4-point resistivity measurements to study the influence of Zr composition and microstructural changes in the thin film. Both, the mechanical and electrical properties reveal a significant increase in hardness and resistivity with increasing Zr content. The trends of the mechanical and functional properties are discussed with respect to the local microstructure and composition of the thin film library. © 2017

The accumulation of plastic strain as an essential element of the compression behavior of metal foams is investigated by analyzing effective stress-strain curves which were recorded during testing. By applying loading/unloading cycles within the low-strain region until reaching the stress plateau, it is studied how reversible elastic deformation is gradually transformed into irreversible plastic deformation and it is shown that both, elastic and plastic strains, contribute to the total strain ε. This behavior is found to be independent on the investigated mesostructural foam morphologies. Furthermore, a method is derived which can be used to determine a proof stress σϕPl=0.5 at which yielding dominates the deformation of a metal foam. © 2018 Elsevier B.V.

Aluminum profiles—for instance, profiles made of precipitation-hardenable alloys—are increasingly used for decorative details in the automotive industry. Typically, after hot extrusion and at least two to three days of natural aging (NA), the aluminum profiles are artificially aged. A commercial EN AW-6060 alloy of high purity was used for this investigation. Tensile tests were used as the main measurement method. This article focuses on the effect of short-term heat treatment on the point in time at which a significant increase of the ultimate tensile strength (UTS) during NA can be measured. Short-term heat treatment is shown to delay this point in time by almost four days, but it increases the variation of UTS. A heterogeneous temperature profile during short-term heat treatment was identified as one reason for this result. Finally, a strategy for minimizing variations in mechanical properties of artificially-aged aluminum alloys was developed, based on the experimental results of this study. © 2018 by the authors. Licensee MDPI, Basel, Switzerland.

In high-strength carbon steels suitable for use in the automotive industry, hydrogen embrittlement (HE) is a potential barrier to the widespread application of these materials. The behaviour of hydrogen within the most prevalent carbide, namely cementite, has been investigated via ab initio simulation. In order to examine possible decohesion effects of hydrogen on carbon steels, the binding and diffusion of hydrogen at the interface between ferrite and cementite has been examined. In order to understand the effect of hydrogen on the mechanical properties of carbon steels, simulated ab initio tensile tests have been performed on the ferrite-cementite bicrystal. The results of the tensile tests can be combined with thermodynamic considerations in order to obtain the expected hydrogen concentrations at such ferrite-cementite phase boundaries. We find that the effect of hydrogen on the cohesion of the phase boundary may be significant, even when the bulk hydrogen concentration is low. © 2018 Acta Materialia Inc.

The effects of quasi-static and low-dynamic strain rate (ε̇ = 10−4 /s to ε̇ = 102 /s) on tensile properties and deformation mechanisms were studied in a Fe-25Mn-3Al-3Si (wt%) twinning and transformation-induced plasticity [TWIP-TRIP] steel. The fully austenitic microstructure deforms primarily by dislocation glide but due to the room temperature stacking fault energy [SFE] of 21 ± 3 mJ/m2 for this alloy, secondary deformation mechanisms such as mechanical twinning (TWIP) and epsilon martensite formation (TRIP) also play an important role in the deformation behavior. The mechanical twins and epsilon-martensite platelets act as planar obstacles to subsequent dislocation motion on non-coplanar glide planes and reduce the dislocation mean free path. A high-speed thermal camera was used to measure the increase in specimen temperature as a function of strain, which enabled the use of a thermodynamic model to predict the increase in SFE. The influence of strain rate and strain on microstructural parameters such as the thickness and spacing of mechanical twins and epsilon-martensite laths was quantified using dark field transmission electron microscopy, electron channeling contrast imaging, and electron backscattered diffraction. The effect of sheet thickness on mechanical properties was also investigated. Increasing the tensile specimen thickness increased the product of ultimate tensile strength and total elongation, but had no significant effect on uniform elongation or yield strength. The yield strength exhibited a significant increase with increasing strain rate, indicating that dislocation glide becomes more difficult with increasing strain rate due to thermally-activated short-range barriers. A modest increase in ultimate tensile strength and minimal decrease in uniform elongation were noted at higher strain rates, suggesting adiabatic heating, slight changes in strain-hardening rate and observed strain localizations as root causes, rather than a significant change in the underlying TWIP-TRIP mechanisms at low values of strain. © 2017 Elsevier B.V.

A set of advanced single crystalline γ′ strengthened Co-base superalloys with at least nine alloying elements (Co, Ni, Al, W, Ti, Ta, Cr, Si, Hf, Re) has been developed and investigated. The objective was to generate multinary Co-base superalloys with significantly improved properties compared to the original Co-Al-W-based alloys. All alloys show the typical γ/γ′ two-phase microstructure. A γ′ solvus temperature up to 1174 °C and γ′ volume fractions between 40 and 60 pct at 1050 °C could be achieved, which is significantly higher compared to most other Co-Al-W-based superalloys. However, higher contents of Ti, Ta, and the addition of Re decrease the long-term stability. Atom probe tomography revealed that Re does not partition to the γ phase as strongly as in Ni-base superalloys. Compression creep properties were investigated at 1050 °C and 125 MPa in 〈001〉 direction. The creep resistance is close to that of first generation Ni-base superalloys. The creep mechanisms of the Re-containing alloy was further investigated and it was found that the deformation is located preferentially in the γ channels although some precipitates are sheared during early stages of creep. The addition of Re did not improve the mechanical properties and is therefore not considered as a crucial element in the design of future Co-base superalloys for high temperature applications. Thermodynamic calculations describe well how the alloying elements influence the transformation temperatures although there is still an offset in the actual values. Furthermore, a full set of elastic constants of one of the multinary alloys is presented, showing increased elastic stiffness leading to a higher Young’s modulus for the investigated alloy, compared to conventional Ni-base superalloys. The oxidation resistance is significantly improved compared to the ternary Co-Al-W compound. A complete thermal barrier coating system was applied successfully. © 2018, The Minerals, Metals & Materials Society and ASM International.

Micro- and nanomechanical testing has seen a rapid development over the last decade with miniaturized test rigs and MEMS-based devices providing access to the mechanical properties and performance of materials from the micrometer down to the tenths of nanometer length scale. In this overview, we summarize firstly the different testing concepts with excursions into recent imaging and diffraction developments, which turn micro- and nanomechanical testing into “quantitative mechanical microscopy” by resolving the underlying material physics and simultaneously providing mechanical properties. A special focus is laid on the pitfalls of micro-compression testing with its stringent boundary conditions often hampering reliable experiments. Additionally, the challenges of instrumented micro- and nanomechanical testing at elevated temperature are summarized. From the wide variety of research topics employing micro- and nanomechanical testing of materials we focus here on miniaturized samples and test rigs and provide three examples to elucidate the state-of-the-art of the field: (i) probing the “strength” of individual grain boundaries in metals, (ii) temperature dependent deformation mechanisms in metallic nanolayered and -alloyed structures, and (iii) the prospects and challenges of fracture studies employing micro- and nanomechanical testing of brittle and ductile monolithic materials, and materials containing interfaces. Proven concepts and new endeavors are reported for the topics discussed in this overview. © 2017 Acta Materialia Inc.

Hydrogen exposure has been found to result in metal embrittlement. In this work, we use nanoindentation to study the mechanical properties of polycrystalline tungsten subjected to deuterium plasma exposure. For the purpose of comparison, nanoindentation tests on exposed and unexposed reference tungsten were carried out. The results exhibit a decrease in the pop-in load and an increase in hardness on the exposed tungsten sample after deuterium exposure. No significant influence of grain orientation on the pop-in load was observed. After a desorption time of td ≥ 168 h, both the pop-in load and hardness exhibit a recovering trend toward the reference state without deuterium exposure. The decrease of pop-in load is explained using the defactant theory, which suggests that the presence of deuterium facilitates the dislocation nucleation. The increase of hardness is discussed based on two possible mechanisms of the defactant theory and hydrogen pinning of dislocations. © 2018 Materials Research Society.

A reduction in synthesis temperature is favorable for hard coatings, which are designed for industrial applications, as manufacturing costs can be saved and technologically relevant substrate materials are often temperature-sensitive. In this study, we analyzed Mo2BC hard coatings deposited by direct current magnetron sputtering at different substrate temperatures, ranging from 380 °C to 630 °C. Transmission electron microscopy investigations revealed that a dense structure of columnar grains, which formed at a substrate temperature of 630 °C, continuously diminishes with decreasing substrate temperature. It almost vanishes in the coating deposited at 380 °C, which shows nanocrystals of ~1 nm in diameter embedded in an amorphous matrix. Moreover, Argon from the deposition process is incorporated in the film and its amount increases with decreasing substrate temperature. Nanoindentation experiments provided evidence that hardness and Young's modulus are modified by the nanostructure of the analyzed Mo2BC coatings. A substrate temperature rise from 380 °C to 630 °C resulted in an increase in hardness (21 GPa to 28 GPa) and Young's modulus (259 GPa to 462 GPa). We conclude that the substrate temperature determines the nanostructure and the associated changes in bond strength and stiffness and thus, influences hardness and Young's modulus of the coatings. © 2018 The Authors

This paper concerns the use of a recently-developed methodology for inferring stress-strain curves from indentation data, based on iterative FEM simulation of the procedure. A relatively large indenter (2 mm diameter) is used, with deep penetration (to about 25% of the indenter radius). This has been carried out on (polished) free surfaces of sprayed superalloy overlayers on single crystal superalloy substrates. Both load-displacement data and residual indent profiles were obtained, with the overlayers being in two different conditions (as-sprayed and annealed). The overlayers were relatively thick (∼2.5 mm), so it was also possible to carry out uniaxial compression tests on them (in the through-thickness direction). The inferred stress-strain curves were similar in each case when derived from load-displacement data and indent profiles, and also close to the plots obtained by conventional uniaxial testing. The yield stress levels in both cases were around 1000 MPa, but the work hardening rate was significantly higher for the as-sprayed material. This kind of information is of considerable potential value when attempting to optimize the properties of such overlayers. The procedure can be employed, with some confidence, to cases for which uniaxial testing is difficult or impossible. © 2018 Acta Materialia Inc.

This work provides a comparative and comprehensive study of the indentation hardness and indentation modulus of iron-rich borides and carboborides of types Fe2B, Fe3(C,B), and Fe23(C,B)6. In addition, the hardness and elastic modulus of Cr-rich M7C are investigated for comparative purposes. We investigated the impact of increasing B content and indentation size effect (ISE). The phases of interest were stabilized in cast Fe-C-B alloys that varied with respect to the B / (B + C) ratio and heat treatment. The resulting microstructures were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and wavelength X-ray spectroscopy (WDS). Dynamic in-situ nanoindentation experiments based on the method of continuous stiffness measurement (CSM) were coupled to SEM and EBSD investigations to determine the mechanical properties of the individual borides and carboborides as a function of the indentation depth. The results were compared to values obtained for the Cr-rich M7C3 carbide. It was found that the hardness of the B-rich Fe3(C,B) phase is considerably higher than pure Fe3C and increases with increasing B content. The ISE was present in all investigated phases, and the hardness decreased as a function of indentation depth. The hardness at infinite indentation depth H0 was estimated according to the model of Nix and Gao. The Fe2B phase was found to be the hardest phase (H0 = 19.04 GPa), followed by M7C3 (H0 = 16.43 GPa), Fe3(C,B) (H0 = 11.18 to 12.24 GPa), and Fe23(C,B)6 (H0 = 10.39 GPa). © 2017 Elsevier Inc.

To investigate the effect of laser shock peening (LSP) with different LSP impacts on the mechanical properties in artificial seawater and corrosion resistance of shipbuilding 5083Al alloy in 3.5% NaCl solution, wear property and electrochemical corrosion resistance tests were performed by a ball-on-disk sliding wear tester and electrochemical workstation. The wear mass losses of the samples treated by 1 and 3 LSP impacts are much lower, by 55.22% and 65.94%, respectively, than those of untreated specimens in artificial seawater. Compared with the untreated sample, the electrochemical corrosion rate of the treated samples decreased by 74.91% and 95.03% after being treated by 1 and 3 LSP impacts, respectively. The reasons for the enhancement of the wear properties and electrochemical corrosion behavior were caused by the increased residual stress and microhardness after the LSP treatment. © 2018 Optical Society of America.

In the atmospheric plasma spray (APS) process, micro-sized ceramic powder is injected into a thermal plasma where it is rapidly heated and propelled toward the substrate. The coating formation is characterized by the subsequent impingement of a large number of more or less molten particles forming the so-called splats and eventually the coating. In this study, a systematic investigation on the influence of selected spray parameters on the coating microstructure and the coating properties was conducted. The investigation thereby comprised the coating porosity, the elastic modulus, and the residual stress evolution within the coating. The melting status of the particles at the impingement on the substrate in combination with the substrate surface condition is crucial for the coating formation. Single splats were collected on mirror-polished substrates for selected spray conditions and evaluated by identifying different types of splats (ideal, distorted, weakly bonded, and partially molten) and their relative fractions. In a previous study, these splat types were evaluated in terms of their effect on the above-mentioned coating properties. The particle melting status, which serves as a measure for the particle spreading behavior, was determined by in-flight particle temperature measurements and correlated to the coating properties. It was found that the gun power and the spray distance have a strong effect on the investigated coating properties, whereas the feed rate and the cooling show minor influence. © 2018 ASM International

Tools used for mineral processing applications are affected by strong abrasive wear and high dynamic loads. This results in opposing demands on the mechanical properties of these tools. Therefore, modern concepts for the manufacturing of mineral processing tools include a composite tool concept consisting of a low-alloyed substrate and a high-alloyed, wear-resistant cladding material. These coatings can be applied using different production processes such as composite casting, deposit welding, and HIP cladding. During the deposition of the cladding, interdiffusion between the substrate and cladding material occurs. This interdiffusion may have a negative impact on the compound, since characteristics such as wear resistance, mechanical properties, and the local microstructure are influenced. This article is focused on the investigation and simulation of interdiffusion processes in supersolidus-sintered compounds using computational thermodynamics, diffusion calculations, optical emission spectrometry, hardness profiles, and microstructural investigations. It is shown that the interdiffusion processes between the solid substrate and the semisolid cladding can be simulated using a dispersed phase model to give results with a close concordance to optical emission spectrometry measurements. © 2018, The Minerals, Metals & Materials Society and ASM International.

Boron is an essential solute element for improving the grain boundary strength in several high temperature metallic alloys especially in Ni- and Co-base superalloys although the detailed strengthening mechanisms are still not well understood. In superalloys, boron leads to the formation of borides and precipitate depleted zones around the grain boundaries and alters the bond strength among the grains directly. In this paper, we explore in detail the role of the boron content in ternary γ/γ′ Co-9Al-9W alloys. Local as well as bulk mechanical properties were evaluated using nanoindentation and compression testing and correlated to near-atomic scale microstructure and compositions obtained from electron microscopy and atom probe tomography. The alloy variant with low B content (0.005 at.% B) reveals an increase in yield strength at room temperature and 600 °C and atom probe tomography investigations show that solute B segregates to the grain boundaries. However, in the bulk B exclusively partitions to the γ′ phase. Additionally, the γ′/γ′ grain boundaries are depleted in W and Al with the concentration locally shifted towards the γ composition forming a very thin γ layer at the γ′/γ′ grain boundaries, which supports dislocation mobility in the γ′/γ′ grain boundary region during deformation. Higher content of B (0.04 at.% B) promotes formation of W-rich borides at the grain boundaries that leads to undesirable precipitate depleted zones adjacent to these borides that decrease the strength of the alloy drastically. However, it was also found that a subsequent annealing heat treatment eliminates these detrimental zones by re-precipitating γ′ and thus elevating the strength of the alloy. This result shows that, if a precipitate depleted zone can be avoided, B significantly improves the mechanical properties of polycrystalline Co-base superalloys by strengthening the γ′ phase and by improving grain boundary cohesion. © 2017 Acta Materialia Inc.

We present a novel alloy design strategy for cost-efficient high modulus steels with an increased stiffness / mass density ratio. The concept is based on the liquid metallurgy synthesis of Fe – Cr – B based alloys, straightforward processability, and well tuneable mechanical properties via plain heat treatments. The base alloy Fe – 18 Cr – 1.6 B (wt%) contained 14–17 vol% of (Cr,Fe)2B particles of ellipsoidal morphology in a ferritic matrix. Hot rolled materials revealed a specific modulus of 32.8 GPa g−1 cm3, exceeding that of conventional Fe-Cr steels by almost 30%. Mechanical properties obtained are comparable to TiB2 based high modulus steels. Addition of 1 wt% Cu to the base alloy did not interact with the formation, fraction, size and morphology of (Cr,Fe)2B particles, and allowed to mildly increase the strength values by ageing treatments, however at the price of a reduction of the specific modulus. C additions of 0.2 wt% did not affect the (Cr,Fe)2B particle microstructure greatly, but free C dissolved in the matrix enables for the first time to utilize the wide range of microstructures and mechanical properties of established C-containing high strength steels also in high modulus steels. © 2018 Elsevier B.V.

Surface pile-up topography is very significant for property extraction in nanoindentation tests. In the present work, we perform crystal plasticity finite element simulations of Berkovich nanoindentation of single crystalline copper with different crystallographic orientations, which derive quantitatively comparable mechanical properties and surface pile-up topographies with experimental data. Simulation results demonstrate that there is a coupled effect of crystallographic orientation of indented material and indenter geometry on surface pile-up behavior, due to the interaction between intrinsic dislocation slip events and extrinsic discrete stress distribution patterns. Based on the relative spatial orientation between crystallographic orientation of indented material and indenter geometry, a surface pile-up density factor mp is proposed to qualitatively characterize the propensity of surface pile-up behavior in nanoindentation tests of single crystalline copper. © 2018 Elsevier Ltd

The determination of biomechanical properties of the cornea by a non-contact tonometry (NCT) examination requires a precise knowledge of the air puff generated in the device, which is applied to the cornea surface. In this study, a method is proposed to identify the resulting stress profile on the surface, which may be used to numerically solve an inverse problem to obtain the material properties. This method is based on an experimental characterization of the air puff created by the Corvis ST in combination with computational fluid dynamic (CFD) simulations, which are adjusted to the experimental data. The identified nozzle inlet pressure of approximately 25 kPa (188.5mmHg) is then used for a numerical influence study of the interaction between the air puff and the cornea deformation. Therefore, eleven cornea deformation states based on measurements are implemented in the CFD model. A more realistic model is also analyzed by the geometrical reproduction of the human face, which is used for a further influence study. The outcomes showed a dependence between the cornea deformation and the pressure as well as the shear stress distribution. However, quantitatively, the shear stress component can be considered of minor importance being approximately one hundred times smaller than the pressure. The examination with consideration of the human face demonstrates that the pressure and shear stress distributions are not rotationally symmetric in measurements on real humans, which indicates the requirement to include more complex stress distributions on the eye. We present the detailed stress distribution on the cornea during a non-contact tonometry examination, which is made accessible for further investigations in the future by analytical nonlinear functions. © 2018, Society for Experimental Mechanics.

We report the enhancement of fracture toughness and strength of a cobalt‑tantalum-based metallic glass thin film with increasing boron content. The improvement of the mechanical performance is attributed to the formation of a compositionally lamellar compared to uniform glass microstructure, which becomes thinner with increasing boron content as revealed by transmission electron microscopy. Compositional variations across the lamellar structure are revealed by atom probe tomography. Cobalt- and boron-rich regions alternate sequentially, whereas tantalum exhibits slight variations across the lamellae. Our results can be utilized in future design efforts for metallic glass thin films with outstanding mechanical performance. © 2018 Acta Materialia Inc.

In this study we present and discuss the influence of compositional inhomogeneity on the mechanical behavior of an interstitially alloyed dual-phase non-equiatomic high-entropy alloy (Fe49.5Mn30Co10Cr10C0.5). Various processing routes including hot-rolling, homogenization, cold-rolling and recrystallization annealing were performed on the cast alloys to obtain samples in different compositional homogeneity states. Grain sizes of the alloys were also considered. Tensile testing and microstructural investigations reveal that the deformation behavior of the interstitial dual-phase high-entropy alloy samples varied significantly depending on the compositional homogeneity of the specimens probed. In the case of coarse-grains (∼300 μm) obtained for cast alloys without homogenization treatment, ductility and strain-hardening of the material was significantly reduced due to its compositional inhomogeneity. This detrimental effect was attributed to preferred deformation-driven phase transformation occurring in the Fe enriched regions with lower stacking fault energy, promoting early stress-strain localization. The grain-refined alloy (∼4 μm) with compositional heterogeneity which was obtained for recrystallization annealed alloys without homogenization treatment was characterized by almost total loss in work-hardening. This effect was attributed to large local shear strains due to the inhomogeneous planar slip. These insights demonstrate the essential role of compositional homogeneity through applying corresponding processing steps for the development of advanced high-entropy alloys. © 2017 Elsevier B.V.

The tensile properties of CrCoNi, a medium-entropy alloy, have been shown to be significantly better than those of CrMnFeCoNi, a high-entropy alloy. To understand the deformation mechanisms responsible for its superiority, tensile tests were performed on CrCoNi at liquid nitrogen temperature (77 K) and room temperature (293 K) and interrupted at different strains. Microstructural analyses by transmission electron microscopy showed that, during the early stage of plasticity, deformation occurs by the glide of 1/2<110> dislocations dissociated into 1/6<112> Shockley partials on {111} planes, similar to the behavior of CrMnFeCoNi. Measurements of the partial separations yielded a stacking fault energy of 22 ± 4 mJ m−2, which is ∼25% lower than that of CrMnFeCoNi. With increasing strain, nanotwinning appears as an additional deformation mechanism in CrCoNi. The critical resolved shear stress for twinning in CrCoNi with 16 μm grain size is 260 ± 30 MPa, roughly independent of temperature, and comparable to that of CrMnFeCoNi having similar grain size. However, the yield strength and work hardening rate of CrCoNi are higher than those of CrMnFeCoNi. Consequently, the twinning stress is reached earlier (at lower strains) in CrCoNi. This in turn results in an extended strain range where nanotwinning can provide high, steady work hardening, leading to the superior mechanical properties (ultimate strength, ductility, and toughness) of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi. © 2017 Acta Materialia Inc.

We present a brief overview on recent developments in the field of strong and ductile non-equiatomic high-entropy alloys (HEAs). The materials reviewed are mainly based on massive transition-metal solute solutions and exhibit a broad spectrum of microstructures and mechanical properties. Three relevant aspects of such non-equiatomic HEAs with excellent strength–ductility combination are addressed in detail, namely phase stability-guided design, controlled and inexpensive bulk metallurgical processing routes for appropriate microstructure and compositional homogeneity, and the resultant microstructure–property relations. In addition to the multiple principal substitutional elements used in these alloys, minor interstitial alloying elements are also considered. We show that various groups of strong and ductile HEAs can be obtained by shifting the alloy design strategy from single-phase equiatomic to dual- or multiphase non-equiatomic compositional configurations with carefully designed phase instability. This design direction provides ample possibilities for joint activation of a number of strengthening and toughening mechanisms. Some potential research efforts which can be conducted in the future are also proposed. © 2017, The Author(s).

We introduce a new class of high-entropy alloys (HEAs), i.e., quinary (five-component) dual-phase (DP) HEAs revealing transformation-induced plasticity (TRIP), designed by using a quantum mechanically based and experimentally validated approach. Ab initio simulations of thermodynamic phase stabilities of Co20Cr20Fe40-xMn20Nix (x = 0–20 at. %) HEAs were performed to screen for promising compositions showing the TRIP-DP effect. The theoretical predictions reveal several promising alloys, which have been cast and systematically characterized with respect to their room temperature phase constituents, microstructures, element distributions and compositional homogeneity, tensile properties and deformation mechanisms. The study demonstrates the strength of ab initio calculations to predict the behavior of multi-component HEAs on the macroscopic scale from the atomistic level. As a prototype example a non-equiatomic Co20Cr20Fe34Mn20Ni6 HEA, selected based on our ab initio simulations, reveals the TRIP-DP effect and hence exhibits higher tensile strength and strain-hardening ability compared to the corresponding equiatomic CoCrFeMnNi alloy. © 2017 Acta Materialia Inc.

We present a systematic microstructure oriented mechanical property investigation for a newly developed class of transformation-induced plasticity-assisted dual-phase high-entropy alloys (TRIP-DP-HEAs) with varying grain sizes and phase fractions. The DP-HEAs in both, as-homogenized and recrystallized states consist of a face-centered cubic (FCC) matrix containing a high-density of stacking faults and a laminate hexagonal close-packed (HCP) phase. No elemental segregation was observed in grain interiors or at interfaces even down to near-atomic resolution, as confirmed by energy-dispersive X-ray spectroscopy and atom probe tomography. The strength-ductility combinations of the recrystallized DP-HEAs (Fe50Mn30Co10Cr10) with varying FCC grain sizes and HCP phase fractions prior to deformation are superior to those of the recrystallized equiatomic single-phase Cantor reference HEA (Fe20Mn20Ni20Co20Cr20). The multiple deformation micro-mechanisms (including strain-induced transformation from FCC to HCP phase) and dynamic strain partitioning behavior among the two phases are revealed in detail. Both, strength and ductility of the DP-HEAs increase with decreasing the average FCC matrix grain size and increasing the HCP phase fraction prior to loading (in the range of 10–35%) due to the resulting enhanced stability of the FCC matrix. These insights are used to project some future directions for designing advanced TRIP-HEAs through the adjustment of the matrix phase's stability by alloy tuning and grain size effects. © 2017 Acta Materialia Inc.

Polycrystalline samples with the Al0.693Cu0.201Fe0.106 composition, corresponding to the tetragonal P4/mnc ω-Al7Cu2Fe crystallographic structure, were synthesised by spark plasma sintering and deformed in compression under constant strain-rate conditions, ε = 2 × 10-4 s-1, over the temperature range 650 K-1000 K. A brittle-to-ductile transition is evidenced between 700 K and 750 K. The stress-strain curves exhibit a yield point followed by softening or steady state conditions only. The upper yield stress, σUYS, shows a strong temperature dependence suggesting that the rate controlling deformation mechanisms are highly thermally activated. The strain-rate sensitivity of stress characterised either by stress exponents, nexp, or by activation volumes, Vexp, was measured by the load relaxation technique. High nexp values, i.e., larger than 7, associated with low Vexp, typically smaller than 1 nm3, are measured. The Gibbs free activation energy, ΔG, deduced by integrating Vexp with respect to stress varies from nearly 2 eV at 790 K to 4 eV at 1000 K. Because plasticity of the ω-Al7Cu2Fe phase takes place at temperatures at which diffusion processes are considered as dominant, the results are interpreted in the frame of dislocation climb models proposed to account for high temperature plasticity of crystalline phases. © 2016 Published by Elsevier B.V.

Low-alloyed TRIP steels are often used in the automotive industry due to their favorable mechanical properties such as high ductility and strength and their moderate production costs. These steels possess a heterogeneous multiphase microstructure, initially consisting of ferrite, bainite and retained austenite which is responsible for the mechanical properties. Upon deformation, a diffusionless, stress-induced, martensitic phase transformation from austenite to martensite is observed, enhancing ductility and strength. We focus on multi-scale methods in the sense of FE2 to describe the macroscopic behavior of low-alloyed TRIP-steels, because this approach allows for a straightforward inclusion of various influencing factors such as residual stress distribution, graded material properties which can hardly included in phenomenological descriptions of these heterogeneous multiphase materials. In order to allow for efficient computations, a simplified microstructure is used in an illustrative direct micro-macro simulation. The inelastic processes in the austenitic inclusions involve the phase transformation from austenite to martensite and the inelastic deformation of these two phases. The isotropic, rate-independent, hyperelastic-plastic material model of Hallberg et al. (IJP, 23, pp. 1213-1239, 2007), originally proposed for high-alloyed TRIP steel, is adopted here for the inclusion phase. Minor modifications of the model are proposed to improve its implementation and performance. The influence of various material parameters associated with the phase transformation on the evolution of retained austenite is studied for different homogeneous deformation states. The non-monotonic stress-state dependence observed in experiments is clearly captured by the model. A numerical two-scale calculation is carried out to enlighten the ductility enhancement in low-alloyed TRIP-steels due to the martensitic phase transformation.

We investigated novel pathways to improve the mechanical properties of liquid metallurgy produced Fe – TiB2 based high modulus steels (HMS) by controlled solidification kinetics and subsequent thermo-mechanical treatments. The solidification rate was varied by casting of hyper-eutectic alloys (20 vol% TiB2) into moulds with differing internal thickness. Ingots between 5 and 40 mm thickness exhibited irregular particle microstructure consisting of sharp-edged coarse primary particles (increasingly clustered with slower solidification) and closely spaced irregular lamellae. Casting defects can be alleviated by hot rolling, but the mechanical properties remain unsatisfactory. Increasing the solidification rate results only at mould thicknesses of 4 mm and below in a significant refinement of the particle microstructure, necessitating liquid metal deposition techniques to utilise it for obtained improved mechanical performance of HMS. Decreasing the solidification rate causes density-induced floatation of the primary particles, which can be used in block-casting for the production of alloys consisting of small and spheroidised eutectic particles, exhibiting high ductility and superior toughness. Annealing just above the solidus-temperature allows the eutectic zones to liquefy and sink, leaving only primary TiB2 particles behind in the top zone of the alloy. Despite the increased particle fraction up to 24 vol%, both strength, specific modulus and ductility are improved over standard processed HMS alloys with 20 vol% TiB2. © 2016 Acta Materialia Inc.

We systematically study the morphology, size and dispersion of TiB2 particles formed in-situ from Fe-Ti-B based melts, as well as their chemical composition, crystal structure and mechanical properties. The effects of 5 wt.% additions of Cr, Ni, Co, Mo, W, Mn, Al, Si, V, Ta, Nb and Zr, respectively, as well as additional annealing treatments, were investigated in order to derive guidelines for the knowledge based alloy design of steels with an increased stiffness/density ratio and sufficiently high ductility. All alloying elements were found to increase the size of the coarse primary TiB2 particles, while Co led to the most homogeneous size distribution. The size of the eutectic TiB2 constituents was decreased by all alloying additions except Ni, while their aspect ratio was little affected. No clear relation between chemical composition, crystal structure and mechanical properties of the particles could be observed. Annealing of the as-cast alloys slightly increased the size of the primary particles, but at the same time strongly spheroidised the eutectics. Additions of Co and Cr appear thus as the best starting point for designing novel in-situ high modulus metal matrix composite steels, while using Mn in concert with thermo-mechanical processing is most suited to adapt the matrix' microstructure and optimise the particle/matrix co-deformation processes. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Here, we applied hot-rolling in conjunction with direct quenching and partitioning (HDQ&P) processes with different rolling schedules to a low-C low-Si Al-added steel. Ferrite was introduced into the steel by intercritical rolling and air cooling after hot-rolling. The effect of intercritcal deformation on the microstructure evolution and mechanical properties was investigated. The promotion of austenite stabilization and the optimization of the TRIP effect due to a moderate degree of intercritical deformation were systematically explored. The results show that the addition of 1.46 wt% of Al can effectively promote ferrite formation. An intercritical deformation above 800 °C can result in a pronounced bimodal grain size distribution of ferrite and some elongated ferrite grains containing sub-grains. The residual strain states of both austenite and ferrite and the occurrence of bainite transformation jointly increase the retained austenite fraction due to its mechanical stabilization and the enhanced carbon partitioning into austenite from its surrounding phases. An intercritical deformation below 800 °C can profoundly increase the ferrite fraction and promote the recrystallization of deformed ferrite. The formation of this large fraction of ferrite enhances the carbon enrichment in the untransformed austenite and retards the bainite transformation during the partitioning process and finally enhances martensite transformation and decreases the retained austenite fraction. The efficient TRIP effect of retained austenite and the possible strain partitioning of bainite jointly improve the work hardening and formability of the steel and lead to the excellent mechanical properties with relatively high tensile strength (905 MPa), low yield ratio (0.60) and high total elongation (25.2%). © 2016 Elsevier B.V.

We apply the exact-muffin-tin-orbitals (EMTO) method to investigate structural properties, formation enthalpies, mechanical stability and polycrystalline moduli in Ni-Re, Ni-Cr and Cr-Re disordered fcc, bcc and hcp phases. Substitutional disorder is treated by using the coherent potential approximation (CPA). We predict the alloy lattice parameters in good agreement with the experiment. We find a continuous softening, as a function of Cr composition, of the tetragonal shear modulus C′ in fcc Ni-Cr phase indicating mechanical instability in Cr-rich Ni-Cr alloys. On the other hand, we show that the mechanical stability of fcc Ni-Re alloys persists through the whole composition range. We observe an intriguing behaviour of the Young's modulus vs. the intrinsic ductility curve in Ni-rich Ni-Re fcc phase. © 2016 Elsevier B.V. All rights reserved.

We studied the effects of Mn additions from 0 to 30 wt.% on microstructure, mechanical and physical properties of liquid metallurgy synthesised high modulus steels in hypo- and hyper-eutectic TiB2 concentrations. While Mn has little effect on density, both Young's modulus and mechanical properties were strongly affected by the achieved wide spectrum of matrix microstructures, ranging from ferrite to martensite, reverted austenite, ε-martensite and austenite. Mn additions of 20 and 30 wt.% did not translate into enhanced mechanical performance despite the higher inherent ductility of the predominantly austenitic matrix, and instead eliminate the intended weight saving potential by significantly reducing the Young's modulus. Martensitic matrices of Mn concentrations of 10 wt.%, on the other hand, are favourable for improved matrix/particle co-deformation without sacrificing too much of the composites' stiffness. In hypo-eutectic Fe – TiB2 based steels, mechanical properties on the level of high strength dual phase steels could be achieved (~ 900 MPa UTS and 20% tensile elongation) but with an enhanced Young's modulus of 217 GPa and reduced density of 7460 kg m− 3. These significantly improved physical and mechanical properties render Mn alloyed high modulus steels promising candidate materials for next generation lightweight structural applications. © 2016 Elsevier Ltd

Release of Ni1+ ions from NiTi alloy into tissue environment, biological response on the surface of NiTi and the allergic reaction of atopic people towards Ni are challengeable issues for biomedical application. In this study, composite coatings of hydroxyapatite-silicon multi walled carbon nano-tubes with 20 wt% Silicon and 1 wt% multi walled carbon nano-tubes of HA were deposited on a NiTi substrate using electrophoretic methods. The SEM images of coated samples exhibit a continuous and compact morphology for hydroxyapatite-silicon and hydroxyapatite-silicon-multi walled carbon nano-tubes coatings. Nano-indentation analysis on different locations of coatings represents the highest elastic modulus (45.8 GPa) for HA-Si-MWCNTs which is between the elastic modulus of NiTi substrate (66.5 GPa) and bone tissue (≈30 GPa). This results in decrease of stress gradient on coating-substrate-bone interfaces during performance. The results of nano-scratch analysis show the highest critical distance of delamination (2.5 mm) and normal load before failure (837 mN) as well as highest critical contact pressure for hydroxyapatite-silicon-multi walled carbon nano-tubes coating. The cell culture results show that human mesenchymal stem cells are able to adhere and proliferate on the pure hydroxyapatite and composite coatings. The presence of both silicon and multi walled carbon nano-tubes (CS3) in the hydroxyapatite coating induce more adherence of viable human mesenchymal stem cells in contrast to the HA coated samples with only silicon (CS2). These results make hydroxyapatite-silicon-multi walled carbon nano-tubes a promising composite coating for future bone implant application. © 2016 Elsevier Ltd.

The microstructure of dual-phase steels consisting of a ferrite matrix with embedded martensite inclusions is the main contributor to the mechanical properties such as high ultimate tensile strength, high work hardening rate, and good ductility. Due to the composite structure and the wide field of applications of this steel type, a wide interest exists in corresponding virtual computational experiments. For a reliable modeling, the microstructure should be included. For that reason, in this paper we follow a computational strategy based on the definition of a representative volume element (RVE). These RVEs will be constructed by a set of tomographic measurements and mechanical tests. In order to arrive at more efficient numerical schemes, we also construct statistically similar RVEs, which are characterized by a lower complexity compared with the real microstructure but which represent the overall material behavior accurately. In addition to the morphology of the microstructure, the austenite–martensite transformation during the steel production has a relevant influence on the mechanical properties and is considered in this contribution. This transformation induces a volume expansion of the martensite phase. A further effect is determined in nanoindentation test, where it turns out that the hardness in the ferrite phase increases exponentially when approaching the martensitic inclusion. To capture these gradient properties in the computational model, the volumetric expansion is applied to the martensite phase, and the arising equivalent plastic strain distribution in the ferrite phase serves as basis for a locally graded modification of the ferritic yield curve. Good accordance of the model considering the gradient yield behavior in the ferrite phase is observed in the numerical simulations with experimental data. © 2015, Springer-Verlag Berlin Heidelberg.

We have adapted our genetic algorithm based optimization approach, originally developed to generate force field parameters from quantum mechanic reference data, to derive a first coarse grained force field for a MOF, taking the atomistic MOF-FF as a reference. On the example of the copper paddle-wheel based HKUST-1, a maximally coarse grained model, using a single bead for each three and four coordinated vertex, was developed as a proof of concept. By adding non-bonded interactions with a modified Buckingham potential, the resulting MOF-FF-CGNB is able to predict local deformation energies of the building blocks as well as bulk properties like the tbo vs. pto energy difference or elastic constants in a semi-quantitative way. As expected, the negative thermal expansion of HKUST-1 is not reproduced by the maximally coarse grained model. At the expense of atomic resolution, substantially larger systems (up to tens of nanometers in size) can be simulated with respect to structural and mechanical properties, bridging the gap to the mesoscale. As an example the deformation of the [111] surface of HKUST-1 by a "tip" could be computed without artifacts from periodic images. © The Royal Society of Chemistry 2016.

Quench processing is widely used in industry to impart the desired structural and mechanical properties by controlling microstructure and compositional gradients, e.g. to obtain supersaturated solid solutions in aluminium alloys, or to achieve martensitic hardening in steels. Rapid cooling, also referred to as quenching or tempering, is also the principal production route for bulk metallic glasses that exhibit high hardness and strength due to their amorphous structure that precludes plastic deformation by easy crystal slip. Importantly, rapid cooling is accompanied by the creation of residual stresses that also have a strong effect on the deformation behaviour. The present study aims to obtain insight into the residual stresses in cylindrical samples of Zr-based bulk metallic glass (BMG) by combining analytical modelling of thermal and mechanical problems with experimental measurements using Focused Ion Beam–Digital Image Correlation (FIB-DIC) ring-core milling. The results show good agreement between the two approaches, providing improved confidence in the validity of the two approaches considered here. © 2016 Elsevier Ltd

The microstructure evolution, i.e. Reorientation of martensite variants, is an important deformation mechanism in shape-memory alloys. This microstructure evolution occurs by the motion of twin boundaries and the nucleation and annihilation of twins in the hierarchical microstructure. An appropriate discrete disclination model for the description of the internal elastic fields and microstructure evolution is introduced for representative volume elements. The model is applied to an experimentally characterized microstructure, i.e. Conjugation boundary, and the predicted mechanical response is verified by comparison to experimental measurements. The influence of the twin microstructure on the homogenized stress-strain curve is studied. It is found that regular twinned microstructures have a low strain energy and a high resistance against deformation. These simulations also reason the origin of the microstructural stability of conjugation boundaries. © 2016 Acta Materialia Inc.Published by Elsevier Ltd. All rights reserved.

The solubility of zirconium (Zr) in the Nb4AlC3 host lattice was investigated by combining the experimental synthesis of (Nbx, Zr1-x)4AlC3 solid solutions with density functional theory calculations. High-purity solid solutions were prepared by reactive hot pressing of NbH0.89, ZrH2, Al, and C starting powder mixtures. The crystal structure of the produced solid solutions was determined using X-ray and neutron diffraction. The limited Zr solubility (maximum of 18.5% of the Nb content in the host lattice) in Nb4AlC3 observed experimentally is consistent with the calculated minimum in the energy of mixing. The lattice parameters and microstructure were evaluated over the entire solubility range, while the chemical composition of (Nb0.85, Zr0.15)4AlC3 was mapped using atom probe tomography. The hardness, Young's modulus, and fracture toughness at room temperature as well as the high-temperature flexural strength and E-modulus of (Nb0.85, Zr0.15)4AlC3 were investigated and compared to those of pure Nb4AlC3. Quite remarkably, an appreciable increase in fracture toughness was observed from 6.6 ± 0.1 MPa/m1/2 for pure Nb4AlC3 to 10.1 ± 0.3 MPa/m1/2 for the (Nb0.85, Zr0.15)4AlC3 solid solution. © 2016 American Chemical Society.

This article focuses on four topics that demonstrate the importance of atom probe tomography for obtaining nanostructural information that provides deep insights into the structures of metallic alloys, leading to a better understanding of their properties. First, we discuss the microstructure-coercivity relationship of Nd-Fe-B permanent magnets, essential for developing a higher coercivity magnet. Second, we address equilibrium segregation at grain boundaries with the aim of manipulating their interfacial structure, energies, compositions, and properties, thereby enabling beneficial material behavior. Third, recent progress in the search to extend the performance and practicality of the next generation of advanced high-strength steels is discussed. Finally, a study of the temporal evolution of a Ni-Al-Cr alloy through the stages of nucleation, growth, and coarsening (Ostwald ripening) and its relationship with the predictions of a model for quasi-stationary coarsening is described. This information is critical for understanding high-Temperature mechanical properties of the material. © Copyright Materials Research Society 2016.

An efficient way to study the relationship between chemical composition and mechanical properties of thin films is to utilize the combinatorial approach, where spatially resolved mechanical property measurements are conducted along a concentration gradient. However, for thin film glasses many properties including the mechanical response are affected by chemical topology. Here a novel method is introduced which enables spatially resolved short range order analysis along concentration gradients of combinatorially synthesized metallic glass thin films. For this purpose a CoZrTaB metallic glass film of 3 μm thickness is deposited on a polyimide foil, which is investigated by high energy X-ray diffraction in transmission mode. Through the correlative chemistry-topology-stiffness investigation, we observe that an increase in metalloid concentration from 26.4 to 32.7 at% and the associated formation of localized (hybridized) metal - metalloid bonds induce a 10% increase in stiffness. Concomitantly, along the same composition gradient, a metalloid-concentration-induced increase in first order metal - metal bond distances of 1% is observed, which infers itinerant (metallic) bond weakening. Hence, the metalloid concentration induced increase in hybridized bonding dominates the corresponding weakening of metallic bonds. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Vulcanized fiber is a macromolecular cellulose-based composite material manufactured using the parchmentizing process. The cellulose is produced from the chemical digestion of plant-based raw materials (wood, cotton) or textile waste. Chemical additives used during manufacturing are completely removed. After the process, vulcanized fiber possesses improved properties concerning mechanical strength and abrasion as well as corrosion resistance in comparison to its raw materials. Concerning its economic life cycle assessment, low density, electrical insulating capability and balanced properties, vulcanized fiber has a potential, up to now unused, as a light and renewable structural material for applications in automotive or civil engineering industries. Research activities concerning the mechanical properties are insufficient and existing standards are out-of-date. In this work, for the first time a direction-dependent characterization of the process-related anisotropic mechanical properties of the material is realized with the aim to formulate an adequate material model for numerical simulations in the next step.

The effect of tempering temperature on microstructure and mechanical properties was studied in a low-nitrogen, high-boron, 9%Cr steel. After normalizing and low-temperature tempering, cementite platelets precipitated within the martensitic matrix. This phase transformation has no distinct effect on mechanical properties. After tempering at 500 °C, M23C6 carbides appeared in the form of layers and particles with irregular shapes along the high-angle boundaries. Approximately, 6% of the retained austenite was observed after normalizing, which reduced to 2% after tempering at 550 °C. This is accompanied by reduction in toughness from 40 J/cm2 to 8.5 J/cm2. Further increase of the tempering temperature led to spheroidization and coagulation of M23C6 particles that is followed by a significant increase in toughness to 250 J/cm2 at 750 °C. Three-phase separations of M(C,N) carbonitrides to particles enriched with V, Nb and Ti were detected after high-temperature tempering. © 2016 Elsevier B.V.

Iridium alloys have been utilized as structural materials for certain high-temperature applications due to their superior strength and ductility at elevated temperatures. In some applications where the iridium alloys are subjected to high-temperature and high-speed impact simultaneously, the high-temperature high-strain-rate mechanical properties of the iridium alloys must be fully characterized to understand the mechanical response of the components in these severe applications. In this study, the room-temperature Kolsky tension bar was modified to characterize a DOP-26 iridium alloy in tension at elevated strain rates and temperatures. The modifications include (1) a unique cooling system to cool down the bars while the specimen was heated to high temperatures with an induction heater; (2) a small-force pre-tension system to compensate for the effect of thermal expansion in the high-temperature tensile specimen; (3) a laser system to directly measure the displacements at both ends of the tensile specimen independently; and (4) a pair of high-sensitivity semiconductor strain gages to measure the weak transmitted force. The dynamic high-temperature tensile stress-strain curves of the iridium alloy were experimentally obtained with the modified high-temperature Kolsky tension bar techniques at two different strain rates (~1000 and 3000 s-1) and temperatures (~750 and 1030 °C). © The Society for Experimental Mechanics, Inc. 2016.

Abstract We study the effects of different heat treatment routes on microstructure engineering and the resulting mechanical response in a plain binary Ti-4Mo (wt%) model alloy. We observe a broad variety of microstructure formation mechanisms including diffusion driven allotropic phase transformations as well as shear and/or diffusion dominated modes of martensitic transformations, enabling a wealth of effective microstructure design options even in such a simple binary Ti alloy. This wide variety of microstructures allows tailoring the mechanical properties ranging from low yield strength (350 MPa) and high ductility (30-35% tensile elongation) to very high yield strength (1100 MPa) and medium ductility (10-15% tensile elongation) as well as a variety of intermediate states. Mechanical testing and microstructure characterization using optical microscopy, scanning electron microscopy based techniques, transmission electron microscopy and atom probe tomography were performed revealing that minor variations in the heat treatment cause significant changes in the resulting microstructures (e.g. structural refinement, transition between diffusive and martensitic transformations). The experimental results on microstructure evolution during the applied different heat treatment routes are discussed with respect to the mechanical properties. © 2015 Acta Materialia Inc.

In the present work, we show how conventional and advanced mechanical, chemical, and microstructural methods can be used to characterize cast single crystal Ni-base superalloy (SX) plates across multiple length scales. Two types of microstructural heterogeneities are important, associated with the castmicrostructure (dendrites (D) and interdendritic (ID) regions - large scale heterogeneity) and with the well-known γ/γ′ microstructure (small scale heterogeneity). Using electron probe microanalysis (EPMA), we can showthat elements such as Re, Co, andCr partition to the dendrites while ID regions contain more Al, Ta, and Ti. Analytical transmission electron microscopy and atom probe tomography (APT) show that Al, Ta, and Ti partition to the γ′ cubes while g channels show higher concentrations of Co, Cr, Re, andW.We can combine large scale (EPMA) and small-scale analytical methods (APT) to obtain reasonable estimates for γ′ volume fractions in the dendrites and in the ID regions. The chemical and mechanical properties of the SX plates studied in the present work are homogeneous, when they are determined from volumes with dimensions, which are significantly larger than the dendrite spacing. For the SX plates (140mm x 100mm x 20mm) studied in the present work this holds for the average chemical composition as well as for elastic behavior and local creep properties. We highlight the potential of HRTEM and APT to contribute to a better understanding of the role of dislocations during coarsening of the γ′ phase and the effect of cooling rates after high temperature exposure on the microstructure. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.

Precipitates of topologically close-packed (TCP) phases play an important role in hardening mechanisms of high-performance steels. We analyze the influence of atomic size, electron count, magnetism and external stress on TCP phase stability in Fe-based binary transition metal alloys. Our density-functional theory calculations of structural stability are complemented by an analysis with an empirical structure map for TCP phases. The structural stability and lattice parameters of the Fe-Nb/Mo/V compounds are in good agreement with experiment. The average magnetic moments follow the Slater-Pauling relation to the average number of valence-electrons and can be rationalized in terms of the electronic density of states. The stabilizing effect of the magnetic energy, estimated by additional non-magnetic calculations, increases as the magnetic moment increases with band filling for the binary systems of Fe and early transition metals. For the case of Fe2Nb, we demonstrate that the influence of magnetism and external stress is sufficiently large to alter the energetic ordering of the closely competing Laves phases C14, C15 and C36. We find that the A15 phase is not stabilized by atomic-size differences, while the stability of C14 is increasing with increasing difference in atomic size. © 2014 Elsevier Ltd. All rights reserved.

Equiatomic multi-component alloys, referred to variously as high-entropy alloys, multi-component alloys, or compositionally complex alloys in the literature, have recently received significant attention in the materials science community. Some of these alloys can display a good combination of mechanical properties. Here, we review recent work on the processing, microstructure and mechanical properties of one of the first and most studied high-entropy alloys, namely the single-phase, face-centered cubic alloy CrMnFeCoNi, with emphasis on its excellent damage tolerance (strength with toughness) in the temperature range from room temperature down to liquid nitrogen temperature. © 2015, The Minerals, Metals & Materials Society.

The mechanical properties of TiAl alloys are very sensitive to the inherent microstructure. For an in-depth understanding of microstructural influences on mechanical properties a duplex type TNB (Nb-containing TiAl) alloy has been investigated. For varying the microstructure of this alloy controlled heat treatments (HT) have been performed with eight distinct maximum temperatures, ranging from 1230. °C to 1300. °C with a 10. °C temperature increment. The series of annealing processes resulted in duplex microstructures with a gradual change of the ratio of globular grains and lamellar colonies, keeping the global chemical composition unchanged. Microstructure of each sample was characterized using SEM and TEM before and after mechanical testing to correlate the morphology and microstructure features to the tensile properties. Quantitative data analysis from these results revealed how the evolution of duplex microstructures influences the room temperature tensile properties: i.e. the elastic stiffness, room temperature ductility, work hardening, fracture stress, and fracture strain. The results are discussed with respect to deformation mechanisms as understood from the tensile test results and fracture surface investigations. From the observed correlations between microstructure and properties an optimized constellation of globular and lamellar microstructure for relevant properties can be predicted. Furthermore, the required heat-treatment window for properties targeted can be defined. © 2015 Elsevier B.V.

Abstract The processing parameters which govern the evolution of microstructure and texture during rotary swaging and subsequent heat treatments were studied in an equiatomic single-phase CoCrFeMnNi high-entropy alloy. After vacuum induction melting and casting, the diameter of the 40 mm cast ingot was reduced at room temperature to a final diameter of 16.5 mm by rotary swaging (diameter reduction of 60%/area reduction of 80%) and the alloy was then annealed at different temperatures for 1 h. The resulting microstructures were analyzed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron backscatter diffraction and correlated with results of microhardness measurements. It was found that the microhardness first increases slightly upon annealing below the recrystallization temperature but then drops steeply at higher annealing temperatures due to the onset of recrystallization. Special emphasis was placed on how the microstructure evolves with respect to the radial and longitudinal position in the rod. Finally, a combination of swaging and heat treatment parameters were identified that can produce CoCrFeMnNi high-entropy alloys with a homogeneous composition and grain size and almost no texture. © 2015 Published by Elsevier B.V.

New γ'-strengthened Co-base superalloys show an interesting potential for high temperature applications. However, protective coatings are needed as for Ni-base superalloys to ensure sufficient oxidation and corrosion resistance. Therefore the properties of a commercial coating on a multinary new γ'-strengthened Co-base superalloy have been studied. Especially the influence of the coating process on the substrate also after long term annealing is discussed. It was found that the highly deformed areas at the coating-substrate interface indicated by a high local misorientation and caused by the sandblasting process led to a recrystallization of the interdiffusion zone during the age hardening heat treatment. A chemical gradient of γ and γ' promoting elements was found in the interdiffusion zone causing a change in hardness as measured by nanoindentation. Depending on the composition two separate recrystallized regions formed in the interdiffusion zone, one with single phase γ-(Co,Ni) and the other with a cellular two phase microstructure of discontinuously grown γ and γ'. © 2015 Elsevier B.V.

Copper microcantilevers were produced by focused ion beam milling and tested in situ using a scanning electron microscope. To provide different interfaces for piling up dislocations, cantilevers were fabricated to be single crystalline, bicrystalline, or single crystalline with a slit in the region of the neutral axis. The aim of the experiment was to study the influence of dislocation pile-ups on (i) strength and (ii) Bauschinger effects in micrometer-sized, focused ion beam milled bending cantilevers. The samples were loaded monotonically for several times under displacement control. Even though the cantilevers exhibited the same nominal strain gradient the strength varied by 34% within the three cantilever geometries. The Bauschinger effect can be promoted and prohibited by the insertion of different interfaces. © 2015 Materials Research Society.

Carbon-supersaturated nanocrystalline hypereutectoid steels with a tensile strength of 6.35 GPa were produced from severely cold-drawn pearlite. The nanocrystalline material undergoes softening upon annealing at temperatures between 200 and 450°C. The ductility in terms of elongation to failure exhibits a non-monotonic dependence on temperature. Here, the microstructural mechanisms responsible for changes in the mechanical properties were studied using transmission electron microscopy (TEM), TEM-based automated scanning nanobeam diffraction and atom probe tomography (APT). TEM and APT investigations of the nanocrystalline hypereutectoid steel show subgrain coarsening upon annealing, which leads to strength reduction following a Hall-Petch law. APT analyzes of the Mn distribution near subgrain boundaries and in the cementite give strong evidence of capillary-driven subgrain coarsening occurring through subgrain boundary migration. The pronounced deterioration of ductility after annealing at temperatures above 350°C is attributed to the formation of cementite at subgrain boundaries. The overall segregation of carbon atoms at ferrite subgrain boundaries gives the nanocrystalline material excellent thermal stability upon annealing. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Four types of resorcinol–formaldehyde (RF) aerogels, stiff, brittle, low-flexible, and super-flexible are studied in this work. Despite several studies on mechanical properties on RF aerogels their response when exposed to compressive loading and their fracture behavior are not well investigated. Here, we cover aerogels with a very broad density range of 0.08–0.3 g cm−3 and compressive moduli from 0.12 to 28 MPa. We relate the microstructure of the synthesized aerogels and their behavior under uniaxial compression. Additionally, this work is the first, to our knowledge, to implement the usage of digital image correlation for characterizing the deformation of RF aerogels. The comparison of surface strain distribution of four types of aerogels provides an insight to their reaction on compressive loading. © 2015, Springer Science+Business Media New York.

In many industrial processes, the resulting mechanical properties of produced steel parts are directly influenced by the thermo-physical properties, which affect the heat treatment significantly. The quality of application-oriented simulations is strongly dependent on the input quantities, which are often generated by regression analysis or simple extrapolations. The aim of this paper is to demonstrate the influence of the thermo-physical properties on such a process simulation referring to the hot stamping. Hot stamping is an established process in the automotive industry to produce ultra-high strength parts. A typical material used for this application is the low-alloyed steel 22MnB5. The thermal conductivity of this steel was investigated referring to the temperature-dependent microstructural changes during the hot-stamping process, particularly the γ to α′ transformation. In terms of the dynamic measuring method, the specific heat capacity, the thermal expansion coefficient, the density and the thermal diffusivity for the different temperature-dependent microstructures of the steel 22MnB5 were determined. The thermal conductivity for the complete temperature range of the hot-stamping process was generated, referring to measured and extrapolated data. To account for the fast γ–α′ transformation kinetics, a novel characterization and extrapolation method was applied. The heat capacity and the thermal diffusivity have a major impact on the thermal conductivity compared to the subordinated influence of the density. The metastable austenitic condition (T ≥ 900 °C) was compared to the martensitic condition (T ≤ 400 °C). The dependent thermal conductivity is significantly dependent on the crystallographic orientation of the lattice. The face-centred cubic lattice (austenite) has referring to the body-centred cubic lattice (martensite), a proportionally low thermal conductivity. During the transformation from austenite to martensite, the development is not linear but based on complex interactions. The results reveal that the temperature-dependent thermal conductivity has to be considered for reliable process simulations. © 2015, Springer Science+Business Media New York.

We present a novel method to locally control the constitution, morphology, dispersion and transformation behavior of multiphase materials. The approach is based on the targeted, site-specific formation and confined dissolution of precipitated carbides or intermetallic phases. These dispersoids act as "vessels" or "containers" for specific alloying elements forming controlled chemical gradients within the microstructure upon precipitation and subsequent (partial) dissolution at elevated temperatures. The basic processing sequence consists of three subsequent steps, namely: (i) matrix homogenization (conditioning step); (ii) nucleation and growth of the vessel phases (accumulation step); and (iii) (partial) vessel dissolution (dissolution step). The vessel phase method offers multiple pathways to create dispersed microstructures by the variation of plain thermomechanical parameters such as time, temperature and deformation. This local microstructure design enables us to optimize the mechanical property profiles of advanced structural materials such as high strength steels at comparatively lean alloy compositions. The approach is demonstrated on a 11.6Cr-0.32C (wt.%) steel, where by using M23C6 carbides as a vessel phase, Cr and C can be locally enriched so that the thus-lowered martensite start temperature allows the formation of a significant quantity of retained austenite (up to 14 vol.%) of fine dispersion and controlled morphology. The effects of processing parameters on the obtained microstructures are investigated, with a focus on the dissolution kinetics of the vessel carbides. The approach is referred to as vessel microstructure design. © 2014 Acta Materialia Inc.

An equiatomic CoCrFeMnNi high-entropy alloy (HEA), produced by arc melting and drop casting, was subjected to severe plastic deformation (SPD) using high-pressure torsion. This process induced substantial grain refinement in the coarse-grained casting leading to a grain size of approximately 50 nm. As a result, strength increased significantly to 1950 MPa, and hardness to ∼520 HV. Analyses using transmission electron microscopy (TEM) and 3-dimensional atom probe tomography (3D-APT) showed that, after SPD, the alloy remained a true single-phase solid solution down to the atomic scale. Subsequent investigations characterized the evolution of mechanical properties and microstructure of this nanocrystalline HEA upon annealing. Isochronal (for 1 h) and isothermal heat treatments were performed followed by microhardness and tensile tests. The isochronal anneals led to a marked hardness increase with a maximum hardness of ∼630 HV at about 450 °C before softening set in at higher temperatures. The isothermal anneals, performed at this peak hardness temperature, revealed an additional hardness rise to a maximum of about 910 HV after 100 h. To clarify this unexpected annealing response, comprehensive microstructural analyses were performed using TEM and 3D-APT. New nano-scale phases were observed to form in the originally single-phase HEA. After times as short as 5 min at 450 °C, a NiMn phase and Cr-rich phase formed. With increasing annealing time, their volume fractions increased and a third phase, FeCo, also formed. It appears that the surfeit of grain boundaries in the nanocrystalline HEA offer many fast diffusion pathways and nucleation sites to facilitate this phase decomposition. The hardness increase, especially for the longer annealing times, can be attributed to these nano-scaled phases embedded in the HEA matrix. The present results give new valuable insights into the phase stability of single-phase high-entropy alloys as well as the mechanisms controlling the mechanical properties of nanostructured multiphase composites. © 2015 Acta Materialia Inc. Published by Elsevier Ltd.

To improve the fundamental understanding of the multi-scale characteristics of martensitic microstructures and their micro-mechanical properties, a multi-probe methodology is developed and applied to low-carbon lath martensitic model alloys. The approach is based on the joint employment of electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), atom probe tomography (APT) and nanoindentation, in conjunction with high precision and large field-of-view 3D serial sectioning. This methodology enabled us to resolve (i) size variations of martensite sub-units, (ii) associated dislocation sub-structures, (iii) chemical heterogeneities, and (iv) the resulting local mechanical properties. The identified interrelated microstructure heterogeneity is discussed and related to the martensitic transformation sequence, which is proposed to intrinsically lead to formation of a nano-composite structure in low-carbon martensitic steels. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Polycrystalline material generally exhibits degradation in its mechanical properties and shows more tendency for intergranular fracture due to segregation and diffusion of hydrogen on the grain boundaries (GBs). Understanding the parameters affecting the diffusion and binding of hydrogen within GBs will allow enhancing the mechanical properties of the commercial engineering materials and developing interface dominant materials. In practice during forming processes, the coincidence site lattice (CSL) GBs are experiencing deviations from their ideal configurations. Consequently, this will change the atomic structural integrity by superposition of sub-boundary dislocation networks on the ideal CSL interfaces. For this study, the ideal ∑ 3 111 [11 0] GB structure and its angular deviations in BCC iron within the range of Brandon criterion will be studied comprehensively using molecular statics (MS) simulations. The clean GB energy will be quantified, followed by the GB and free surface segregation energies calculations for hydrogen atoms. Rice-Wang model will be used to assess the embrittlement impact variation over the deviation angles. The results showed that the ideal GB structure is having the greatest resistance to embrittlement prior GB hydrogen saturation, while the 3° deviated GB is showing the highest susceptibility to embrittlement. Upon saturation, the 5° deviated GB appears to have the highest resistance instead due to the lowest stability of hydrogen atoms observed in the free surfaces of its simulation cell. Molecular dynamics (MD) simulations are then applied to calculate hydrogen diffusivity within the ideal and deviated GB structure. It is shown that hydrogen diffusivity decreases significantly in the deviated GB models. In addition, the 5° deviated GB is representing the local minimum for diffusivity results suggesting the existence of the highest atomic disorder and excessive secondary dislocation accommodation within this interface. Copyright © 2015 by ASME.

The solid electrolyte interphase (SEI) film formed at the surface of negative electrodes strongly affects the performance of a Li-ion battery. The mechanical properties of the SEI are of special importance for Si electrodes due to the large volumetric changes of Si upon (de)insertion of Li ions. This manuscript reports the careful determination of the Young's modulus of the SEI formed on a sputtered Si electrode using wet atomic force microscopy (AFM)-nanoindentation. Several key parameters in the determination of the Young's modulus are considered and discussed, e.g., wetness and roughness-thickness ratio of the film and the shape of a nanoindenter. The values of the Young's modulus were determined to be 0.5-10 MPa under the investigated conditions which are in the lower range of those previously reported, i.e., 1 MPa to 10 GPa, pointing out the importance of the conditions of its determination. After multiple electrochemical cycles, the polymeric deposits formed on the surface of the SEI are revealed, by force-volume mapping in liquid using colloidal probes, to extend up to 300 nm into bulk solution. © 2015 American Chemical Society.

In dual-phase (DP) steels, inherited microstructures and elemental distributions affect the kinetics and morphology of phase transformation phenomena and the mechanical properties of the final material. In order to study the inheritance process, we selected two model materials with the same average DP steel composition but with different initial microstructures, created by coiling at different temperatures after hot rolling. These samples were submitted to a DP-steel heat treatment consisting of a short isothermal annealing in the pure austenite region and a quenching process. The evolution of microstructure, chemical composition and mechanical properties (hardness) during this treatment was investigated. The initial samples had a bainitic-martensitic (B + M) microstructure for the material coiled at lower temperature and a ferritic-pearlitic (P + F) microstructure for that coiled at higher temperature. The P + F microstructure had a much more inhomogeneous distribution of substitutional elements (in particular of Mn) and of carbon. After complete heat treatment, both materials showed a typical DP microstructure (martensite islands embedded in ferrite) but the P + F material showed lower hardness compared to the B + M material. It was found that the inhomogeneous elemental distribution prevailed in the P + F material. The inheritance process was studied by combining measurements of the elemental distribution by Wavelength-Dispersive X-ray spectroscopy (WDX), simulations of the evolution of the elemental composition via the DICTRA (diffusion-controlled reactions) computer programme, dilatometry to observe the kinetics of phase transformation, and observation and quantification of the microstructures by Electron Backscatter Diffraction (EBSD) measurements. For the P + F material it was found that the α-γ transformation during annealing is slowed down in regions of lower Mn content and is therefore not completed. During the subsequent cooling the incompletely autenitized material does not require ferrite nucleation and the γ-α transformation starts at relative high temperatures. For B + M, in contrast, nucleation of ferrite is needed and the transformation starts at lower temperatures. As a result the B + M material develops a higher martensite content as well as a higher density of geometrically necessary dislocations (GNDs). It is speculated that for the B + M material the γ-α transformation occurs through a bainitic (i.e. partly displacive) process while the transformation at higher temperatures in the P + F material proceeds exclusively in a diffusive way. © 2015 Acta Materialia Inc.

We study an Fe-18Al (at.%) alloy after various thermal treatments at different times (24-336 h) and temperatures (250-1100 °C) to determine the nature of the so-called 'komplex' phase state (or "K-state"), which is common to other alloy systems having compositions at the boundaries of known order-disorder transitions and is characterised by heterogeneous short-range-ordering (SRO). This has been done by direct observation using atom probe tomography (APT), which reveals that nano-sized, ordered regions/particles do not exist. Also, by employing shell-based analysis of the three-dimensional atomic positions, we have determined chemically sensitive, generalised multicomponent short-range order (GM-SRO) parameters, which are compared with published pairwise SRO parameters derived from bulk, volume-averaged measurement techniques (e.g. X-ray and neutron scattering, Mössbauer spectroscopy) and combined ab-initio and Monte Carlo simulations. This analysis procedure has general relevance for other alloy systems where quantitative chemical-structure evaluation of local atomic environments is required to understand ordering and partial ordering phenomena that affect physical and mechanical properties. © 2015 Elsevier Ltd. All rights reserved.

In this study, tension experiments on Cu micro-samples at temperatures between 143 and 873 K were performed in order to analyze the influence of grain size, temperature and strain rate on the mechanical properties and fracture mode. The activation energy and evolution of the dislocation density have been analyzed to identify the deformation mechanisms. A transition from bulk-like to stochastic, small-scale behavior has been found with increasing grain size. Furthermore, dependent on the grain size and temperature a change from dislocation based plasticity to diffusion controlled deformation was observed. © 2015 Acta Materialia Inc.

The phase transformations and compositional changes occurring during thermo-mechanical processing and subsequent high temperature ageing of Ti-5Al-5Mo-5V-2Cr-1Fe (wt.%) were investigated using scanning transmission electron microscopy (STEM) and atom probe tomography (APT). High resolution STEM revealed nano-sized α (< 10 nm) and athermal ω (∼1-3 nm) formed during accelerated cooling from 800°C and slow heating to an ageing temperature of 650°C. Nuclei of α were found to form heterogeneously in the β matrix as well as at the ω phase. APT revealed pronounced Mo compositional fluctuations in the β matrix. No direct connection was established between Mo-rich or Mo-lean regions and α or ω nuclei. APT also failed to detect the ω phase, which supports theories that it forms by a shuffle mechanism, without any compositional difference from the β phase. Very small α particles, after initial ageing, showed only a minute change in composition with respect to the β matrix, indicative of a displacive-diffusional transformation. With further ageing, growth of the α lamellae was accompanied by compositional changes according to the diffusion rates of β-stabilising elements. Pile-up of the slowest diffusing solutes (Mo, V) at the α/β interface were pronounced in the initial stages of ageing. The best combination of mechanical properties (1200 MPa ultimate tensile strength with 15% total elongation) was recorded after 3.6 ks of ageing. © 2015 Elsevier B.V. All rights reserved.

In austenitic stainless steel nitrogen stabilizes the austenitic phase improves the mechanical properties and increases the corrosion resistance. Nitrogen alloying enables to produce austenitic steels without the element nickel which is high priced and classified as allergy inducing. A novel production route is nitrogen alloying of CrMn-prealloyed steel powder via the gas phase. This is beneficial as the nitrogen content can be adjusted above the amount that is reached during conventional casting. A problem which has to be overcome is the oxide layer present on the powder surface which impedes both the sintering process and the uptake of nitrogen. This study focuses on whether heat treatment under pure nitrogen is an appropriate procedure to enable sintering and nitrogen uptake by reduction of surface oxides. X-ray photoelectron spectroscopy (XPS) in combination with scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) are used to investigate the surface of powdered FeMn19Cr17C0.4N heat treated under nitrogen atmosphere. The analyses showed reduction of iron oxides already at 500 °C leading to oxide-free metallic surface zones. Mn and Cr oxides are reduced at higher temperatures. Distinct nitrogen uptake was registered, and successful subsequent sintering was reached. Copyright © 2014 John Wiley & Sons, Ltd.

Addition of Re in nickel-based superalloys results in an increase of the creep life. However, Re is also known to segregate to the dendrite core and to promote the formation of topologically closed packed (TCP) phases. In the present work, the local segregation of Re was studied in the heat treated and creep deformed state of a nickel-based superalloy. Tensile creep deformation at 1050°C resulted in the formation of TCP phases in the dendritic regions. Characterization using TEM confirmed the presence of μ phase that grows on {111} planes. Measurements with a nanoindenting AFM show that the μ phase is harder and shows less work hardening than both the γ and the γ' phases. Furthermore, in the creep deformed state the hardness of the matrix phase is very similar in the dendrite core and in interdendritic areas, although Re is still enriched in the dendrite core. It is shown that Re is consumed in the dendrite core by the TCP phases. © 2015 Elsevier B.V.

Molecular dynamics simulations were performed to investigate dislocation network formations in a coherent twin boundary in Cu. Depending on the activated glide system, the initial flawless twin boundary can be heavily or sparsely decorated by a dislocation network. The dislocation mechanism leading to a heavy dislocation network at the twin boundary and its consequence on mechanical properties will be discussed. © 2015 Acta Materialia Inc.

Calcium Silicate Hydrates (C-S-H) are the major hydration products of portland cement paste. The accurate description of acid-base reactions at the surface of C-S-H particles is essential for both understanding the ion sorption equilibrium in cement and prediction of mechanical properties of the hardened cement paste. Ab initio molecular dynamics simulations at the density functional level of theory were applied to calculate intrinsic acidity constants (pK a's) of the relevant -SiOH and -CaOH2 groups on the C-S-H surfaces using a thermodynamic integration technique. Ion sorption equilibrium in C-S-H was modeled applying ab initio calculated pKa's in titrating Grand Canonical Monte Carlo simulations using a coarse-grained model for C-S-H/solution interface in the framework of the Primitive Model for electrolytes. The modeling results were compared with available data from electrophoretic measurements. The model predictions were found to satisfactorily reproduce available experimental data. © 2014 American Chemical Society.

The I1 intrinsic stacking fault energy (I1 SFE) serves as an alloy design parameter for ductilizing Mg alloys. In view of this effect we have conducted quantum-mechanical calculations for Mg15X solid-solution crystals (X = Dy, Er, Gd, Ho, Lu, Sc, Tb, Tm, Nd, Pr, Be, Ti, Zr, Zn, Tc, Re, Co, Ru, Os, Tl). We find that Y, Sc and all studied lanthanides reduce the I1 SFE and render hexagonal closed-packed (hcp) and double hcp phases thermodynamically, structurally and elastically similar. Synthesis, experimental testing and characterization of some of the predicted key alloys (Mg-3Ho, Mg-3Er, Mg-3Tb, Mg-3Dy) indeed confirm reduced I1 SFEs and significantly improved room-temperature ductility by up to 4-5 times relative to pure Mg, a finding that is attributed to the higher activity of non-basal dislocation slip. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Compared to decades-old theories of strengthening in dilute solid solutions, the mechanical behavior of concentrated solid solutions is relatively poorly understood. A special subset of these materials includes alloys in which the constituent elements are present in equal atomic proportions, including the high-entropy alloys of recent interest. A unique characteristic of equiatomic alloys is the absence of "solvent" and "solute" atoms, resulting in a breakdown of the textbook picture of dislocations moving through a solvent lattice and encountering discrete solute obstacles. To clarify the mechanical behavior of this interesting new class of materials, we investigate here a family of equiatomic binary, ternary and quaternary alloys based on the elements Fe, Ni, Co, Cr and Mn that were previously shown to be single-phase face-centered cubic (fcc) solid solutions. The alloys were arc-melted, drop-cast, homogenized, cold-rolled and recrystallized to produce equiaxed microstructures with comparable grain sizes. Tensile tests were performed at an engineering strain rate of 10-3 s-1 at temperatures in the range 77-673 K. Unalloyed fcc Ni was processed similarly and tested for comparison. The flow stresses depend to varying degrees on temperature, with some (e.g. NiCoCr, NiCoCrMn and FeNiCoCr) exhibiting yield and ultimate strengths that increase strongly with decreasing temperature, while others (e.g. NiCo and Ni) exhibit very weak temperature dependencies. To better understand this behavior, the temperature dependencies of the yield strength and strain hardening were analyzed separately. Lattice friction appears to be the predominant component of the temperature-dependent yield stress, possibly because the Peierls barrier height decreases with increasing temperature due to a thermally induced increase of dislocation width. In the early stages of plastic flow (5-13% strain, depending on material), the temperature dependence of strain hardening is due mainly to the temperature dependence of the shear modulus. In all the equiatomic alloys, ductility and strength increase with decreasing temperature down to 77 K. © 2014 Acta Materialia Inc.

Due to the existence of constituents with strong distinction in mechanical properties, dual phase steels exhibit remarkably high-energy absorption along with excellent combination of strength and ductility. Furthermore, these constituents also affect deformation and microcrack formation in which various mechanisms can be observed. Thus, a reliable microstructure-based simulation approach for describing these deformations and microcrack initiation is needed. Under this framework of modeling scheme development, several work packages have been carried out. These work packages includes algorithm to generate the artificial microstructure model, a procedure to derive plasticity parameters for each constituent, and characterization of the microcrack formation and initiation criteria determination. However, due to the complexity of topic and in order to describe each work package in detail, this paper focused only on the approach to generate the artificial microstructure model and its application to predict the strain hardening behavior. The approach was based on the quantitative results of metallographic microstructure analysis and their statistical representation. The dual phase steel was first characterized by EBSD analysis to identify individual phase grain size distribution functions. The results were then input into a multiplicatively weighted Voronoi tessellation based algorithm to generate artificial microstructure geometry models. Afterwards, nanoindentation was performed to calibrate crystal plasticity parameters of ferrite and empirical approach based on local chemical composition was used to approximate flow curve of martensite. By assigning the artificial microstructure model with plasticity description of each constituent, strain-hardening behavior of DP-steel was then predicted. © 2014 Elsevier Ltd. All rights reserved.

Emerging new applications and growing demands of plasma-sprayed coatings initiate the development of new materials. Regarding ceramics, often complex compositions are employed to achieve advanced material properties, e.g., high thermal stability, low thermal conductivity, high electronic and ionic conductivity as well as specific thermo-mechanical properties and microstructures. Such materials however, often involve particular difficulties in processing by plasma spraying. The inhomogeneous dissociation and evaporation behavior of individual constituents can lead to changes of the chemical composition and the formation of secondary phases in the deposited coatings. Hence, undesired effects on the coating characteristics are encountered. In this work, examples of such challenging materials are investigated, namely pyrochlores applied for thermal barrier coatings as well as perovskites for gas separation membranes. In particular, new plasma spray processes like suspension plasma spraying and plasma spray-physical vapor deposition are considered. In some cases, plasma diagnostics are applied to analyze the processing conditions. © 2014, ASM International.

Plasma spray physical vapor deposition (PS-PVD) is a very promising route to manufacture ceramic coatings, combining the efficiency of thermal spray processes and characteristic features of thin PVD coatings. Recently, this technique has been investigated to effectively deposit dense thin films of perovskites particularly with the composition of La0.58Sr 0.4Co0.2Fe0.8O3-δ (LSCF) for application in gas separation membranes. Furthermore, asymmetric type of membranes with porous metallic supports has also attracted research attention due to the advantage of good mechanical properties suitable for use at high temperatures and high permeation rates. In this work, both approaches are combined to manufacture oxygen transport membranes made of gastight LSCF thin film by PS-PVD on porous NiCoCrAlY metallic supports. The deposition of homogenous dense thin film is challenged by the tendency of LSCF to decompose during thermal spray processes, irregular surface profile of the porous metallic substrate and crack and pore-formation in typical ceramic thermal spray coatings. Microstructure formation and coating build-up during PS-PVD as well as the annealing behavior at different temperatures of LSCF thin films were investigated. Finally, measurements of leak rates and oxygen permeation rates at elevated temperatures show promising results for the optimized membranes. © 2013 ASM International.

Present investigation deals with the effectiveness of the chemically synthesized nano cement in controlling physical and mechanical performances of concrete. In this investigation, concrete samples were fabricated using variable amounts of aggregates and alkali activator content w.r.t. weight of nano cement. Based on the mechanical properties analyses, it is assessed that chemically synthesized cement is able to produce 43 MPa compressive strength of concrete after 14 days curing instead of 28 days at an optimized amount of aggregates content as well as alkali activator content. Finally, a model has been proposed to explain the overall performances of nano cement based concrete. © 2014 Elsevier Ltd. All rights reserved.

A fine and uniform distribution of α phase at grain boundaries is expected to improve the mechanical properties and thermal stability of beta Ti alloys. To design high strength alloys, a key factor is the volume fraction of α, which is related to the concentration of the α phase. In this study, α-phase precipitates were characterized in an ultrafine-grained Ti-15Nb-2Mo-2Zr-1Sn (at%) alloy processed by severe plastic deformation in two different ways (hot drawing and cold rolling in conjunction with annealing). A combination of transmission Kikuchi diffraction, transmission electron microscopy and atom-probe tomography revealed that ultra-fine α precipitates precipitate at grain boundaries in hot-drawn material or at sub-grain boundaries in the cold-rolled samples. The Nb concentrations of α phases formed were not those expected for an equilibrium state, which highlights the importance of understanding the chemistry of the α precipitates for engineering microstructures in advanced Ti alloys. © 2014 Elsevier B.V.

Superelastic Shape Memory Alloys (SMA) are typically used in applications where the martensitic phase transformation is exploited for its reversible, large deformation such as medical applications (e.g. stents). In this work, we focus on the mechanical and thermal behavior of a Nickel-Titanium SMA strip in bending mode. One possible application of this mode is to provide a restoring force when used in joints of SMA wire actuator systems making the need for an antagonistic SMA actuator redundant. In these applications mentioned above, typically only the mechanical properties are of interest while the temperature is considered constant, even though the martensitic phase transformation in SMA is a thermomechanically coupled process. As a part of the DFG (German Research Association) Priority Programme SPP1599 "Ferroic Cooling" which aims at advancing the development of solid state cooling devices, we have an equally large interest for the thermal evolution of Nickel-Titanium SMA during deformation and its induced phase transformation. In this paper we investigate the thermal and the mechanical response of a SMA beam during bending experiments in which the deformation is induced by holding one end of a SMA strip fixed while the other end is subject to a prescribed deflection. Sensors and high speed thermal cameras are used to capture reaction forces, deformations and temperature changes. We compare these experimental results with numerical simulation results obtained from Finite Element simulations where a thermo-mechanically coupled SMA model is implemented into a finite deformation framework. © 2014 by ASME.

The outer part of shark teeth is formed by the hard and mineral-rich enameloid that has excellent mechanical properties, which makes it a very interesting model system for the development of new bio-inspired dental materials. We characterized the microstructure, chemical composition and resulting local mechanical properties of the enameloid from teeth of Isurus oxyrinchus (shortfin mako shark) by performing an in-depth analysis using various high-resolution analytical techniques, including scanning electron microscopy, qualitative energy-dispersive X-ray spectroscopy and nanoindentation. Shark tooth enameloid reveals an intricate hierarchical arrangement of thin (50-80 nm) and long (>1 μm) crystallites of fluoroapatite with a high degree of structural anisotropy, which leads to exceptional mechanical properties. Both stiffness and hardness are surprisingly homogeneous in the shiny layer as well as in the enameloid: although both tooth phases differ in structure and composition, they show almost no orientation dependence with respect to the loading direction of the enameloid crystallites. The results were used to determine the structural hierarchy of shark teeth, which can be used as a base for establishing design criteria for synthetic bio-inspired and biomimetic dental composites. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The physical and mechanical properties of a σ5 (310) [001] symmetrical tilt grain boundary (STGB) in body centred cubic (bcc) Fe are investigated by means of ab initio calculations with respect to the effect of a varying number of C and H atoms at the grain boundary. The obtained results show that with increasing number of C atoms the grain boundary energy is lowered, and the segregation energy remains negative up to a full coverage of the grain boundary with C. Thus, in a bcc Fe-C system with a sufficient amount of interstitial C, the C segregated state should be considered as the ground state of this interface. Ab initio uni-axial tensile tests of the grain boundary reveal that the work of separation as well as the theoretical strength of the σ5 (310) [001] STGB increases significantly with increasing C content. The improved cohesion due to C is mainly a chemical effect, but the mechanical contribution is also cohesion enhancing. The presence of hydrogen changes the cohesion enhancing mechanical contribution of C to an embrittling contribution, and also reduces the beneficial chemical contribution to the cohesion. When hydrogen is present together with C at the grain boundary, the reduction in strength amounts to almost 20% for the co-segregated case and to more than 25% if C is completely replaced by H. Compared to the strength of the STGB in pure iron, however, the influence of H is negligible. Hence, H embrittlement can only be understood in the three component Fe-C-H system. © 2014 Elsevier B.V.

The mechanical properties of amorphous Si thin films, lithiated electrochemically to different Si£Li compositions are studied by ex situ nanoindentation. The compositions of the films are adjusted using an electrochemical routine that corrects for the Li consumed by SEI layer growth during initial lithiation. The mechanical properties such as Young's modulus and hardness are derived from nanoindentation. For compositions between Si and SiLi2.5 the Young's modulus decreases with increasing Li content from ∼160 GPa to ∼8 GPa and the hardness decreases from ∼14 GPa to ∼0.1 GPa. The yield strength values, as deduced from hardness measurements, decrease from ∼5 GPa to 0.05 GPa. AFM imaging is used on the electrochemically cycled films to assess the SEIs impact on the nanomechanical measurements. XPS depth-profiling of the electrochemically cycled sample indicated a Li concentration gradient across the film thickness. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The deformability and strength of lamellar two-phase (γ and α2) TiAl alloys strongly depends on the mechanical properties of the different interfaces in such microstructures. We carried out ab-initio density functional theory calculations of interface energy and strength for all known interface variants as well as the corresponding single crystal slip/cleavage planes to obtain a comprehensive database of key mechanical quantities. This data collection can be used for meso-scale simulations of deformation and fracture in TiAl. In spite of the different atomic configurations of the lamellar interfaces and the single crystal planes, the calculated values for the tensile strength are in the same range and can be considered as equal in a meso-scale model. Analysis of generalized stacking fault energy surfaces showed that the shear strength is directional dependent, however, the [112̄] direction is an invariant easy gliding direction in all investigated systems. The probability of different dislocation dissociation reactions as part of a shear deformation mechanism are discussed as well. © 2014 Elsevier Ltd. All rights reserved.

A precise geometrical method employing optical profilometry for green density measurements of thick films is presented that provides a typical reproducibility of 0.1-0.2% theoretical density (TD) and a measurement uncertainty of 0.2-0.4% TD for layer thicknesses of around 50. μm. The procedure can be applied for all thick films with a dried thickness of 10. μm or greater. In a case study, the green densities of screen-printed zirconia layers were investigated as a function of the starting powders (grain sizes from 0.1 to 0.4. μm), the solid content, the chain length of ethyl cellulose as binder and its concentration, and two different dispersants and their concentration. Rheological ink properties, surface roughness, drying stresses from deflection measurements, the mechanical properties of green films, and the equivalent compaction pressure were measured and correlated with the green density data. Compressive binder forces and lubrication effects dominated the packing of the particles. © 2014 Elsevier Ltd.

Metal-organic (MO)CVD of ZrO2 thin films is performed using the precursor [Zr(NMe2)2(guan)2] (guan=η2-(iPrN)2CNMe2) as the Zr source, together with oxygen. Film deposition is carried out on both Si(100) and glass substrates at various deposition temperatures. The resulting films are characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM) for investigating the crystallinity and morphology, respectively. Optical properties are measured by ellipsometry and UV-vis on Si substrates and glass substrates, respectively, showing a high average refractive index of 2.14 and transmittance of more than 80% in visible light for the film deposited at 500°C. The potential of ZrO2 thin films as gate dielectrics is verified by carrying out capacitance-voltage (C-V) and current-voltage (I-V) measurements. Dielectric constants are estimated from the accumulation capacitance, and found to be in the range 12 - 19 at an AC frequency of 1MHz, and a leakage current of the order of 10-6 A cm-2 at the applied field of 1 to 2 MV cm-1 is measured for the films deposited at temperatures from 500 to 700°C. The low leakage current and high dielectric constant implies the good quality of the film, relevant for high-k applications. The hardness of the film ranges from 4.2 to 6.3GPa for the 400nm thick film, as determined by nano-indentation measurements. The optimum dielectric and hardness is found for the film deposited at 600°C, while the highest refractive index is found to be 2.14 for the film deposited at 500°C, due to higher density of the layers. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The mechanical properties of the ω-Al7Cu2Fe crystalline phase have been investigated over a large temperature range (650-1000 K). Despite of its antinomic structure with the icosahedral Al-Cu-Fe quasicrystalline phase, i.e. periodic vs non-periodic, its mechanical properties are very similar to those of the quasicrystalline phase, which strongly suggest similar deformation mechanisms. Consequently, as for the quasicrystalline structure, we propose that dislocation climb might control the plastic deformation of the ω-phase. However, in the present case, the specificities of the quasicrystalline structure cannot be invoked to justify the predominance of dislocation climb, which questions the role of quasiperiodicity on dislocation mobility. We suggest that this deformation mode certainly results from specific non-planar extensions of the dislocation core. © 2014 Elsevier Ltd. All rights reserved.

A key requirement of modern steels - the combination of high strength and high deformability - can best be achieved by enabling a local adaptation of the microstructure during deformation. A local hardening is most efficiently obtained by a modification of the stacking sequence of atomic layers, resulting in the formation of twins or martensite. Combining ab initio calculations with in situ transmission electron microscopy, we show that the ability of a material to incorporate such stacking faults depends on its overall chemical composition and, importantly, the local composition near the defect, which is controlled by nanodiffusion. Specifically, the role of carbon for the stacking fault energy in high-Mn steels is investigated. Consequences for the long-term mechanical properties and the characterisation of these materials are discussed. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Nanolaminate coatings based on transition metal nitrides such as CrN, AlN and TiN deposited via physical vapor deposition (PVD) have shown great advantage as protective coatings on tools and components subject to high loads in tribological applications. By varying the individual layer materials and their thicknesses it is possible to optimize the coating properties, e.g. hardness, Young's modulus and thermal stability. One way for further improvement of coating properties is the use of advanced PVD technologies. High power pulsed magnetron sputtering (HPPMS) is an advancement of pulsed magnetron sputtering (MS). The use of HPPMS allows a better control of the energetic bombardment of the substrate due to the higher ionization degree of metallic species. It provides an opportunity to influence chemical and mechanical properties by varying the process parameters. The present work deals with the development of CrN/AlN nanolaminate coatings in an industrial scale unit by using two different PVD technologies. Therefore, HPPMS and mfMS (middle frequency magnetron sputtering) technologies were used. The bilayer period Λ, i.e. the thickness of a CrN/AlN double layer, was varied between 6.2nm and 47.8 nm by varying the rotational speed of the substrate holders. In a second step the highest rotational speed was chosen and further HPPMS CrN/AlN coatings were deposited applying different HPPMS pulse lengths (40, 80, 200 μs) at the same mean cathode power and frequency. Thickness, morphology, roughness and phase composition of the coatings were analyzed by means of scanning electron microscopy (SEM), confocal laser microscopy, and X-ray diffraction (XRD), respectively. The chemical composition was determined using glow discharge optical emission spectroscopy (GDOES). Detailed characterization of the nanolaminate was conducted by transmission electron microscopy (TEM). The hardness and the Young's modulus were analyzed by nanoindentation measurements. The residual stress was determined via Si microcantilever curvature measurements. The phase analysis revealed the formation of h-Cr2N, c-CrN and c-AlN mixed phases for the mfMS CrN/AlN coatings, whereas the HPPMS coatings exhibited only cubic phases (c-CrN, c-AlN). A hardness of 31.0 GPa was measured for the HPPMS coating with a bilayer period of 6.2 nm. The decrease of the HPPMS pulse length at constant mean power leads to a considerable increase of the cathode current on the Cr and Al target associated with an increased ion flux towards the substrate. Furthermore, it was observed that the deposition rate of HPPMS CrN/AlN decreases with shorter pulse lengths, so that a CrN/AlN coating with a bilayer period of 2.9 nm, a high hardness of 40.8 GPa and a high compressive stress (- 4.37 GPa) was achieved using a short pulse length of 40 μs. © 2014 Elsevier B.V. All rights reserved.

B2 NiMn and Ni2MnAl Heusler nanoprecipitates are designed via elastic misfit stabilization in Fe-Mn maraging steels by combining transmission electron microscopy (TEM) correlated atom probe tomography (APT) with ab initio simulations. Guided by these predictions, the Al content of the alloys is systematically varied, and the influence of the Al concentration on structure stability, size and distribution of precipitates formed during ageing at 450 °C is studied using scanning electron microscopy-electron backscatter diffraction, TEM and APT. Specifically, the Ni2MnAl Heusler nanoprecipitates exhibit the finest sizes and highest dispersion and hence lead to significant strengthening. The formation of the different types of precipitates and their structure, size, dispersion and effect on the mechanical properties of the alloys are discussed. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Three newly designed heat-resistant ferritic alloys containing the intermetallic Laves phase were investigated with respect to an annealing dwell time of up to 1440 h at 900C and were compared with commercially available steels. A detailed characterization of the microstructure evolution in dependence of the annealing dwell time was performed. In order to estimate the influence of Laves phase formation and coarsening on the strength, ductility and toughness, the results of the microstructural analysis were correlated with tensile tests at room temperature and with Charpy-V impact tests. Precipitates of the Laves phase were observed in the recrystallized state with a mean particle diameter about 0.25 μm. The Laves phase in all investigated alloys showed rapid growth and coarsening with increasing annealing time. In spite of this behavior, the strength and ductility of the newly designed alloys were conserved, even after annealing for 1440 h. However, the toughness decreased with coarsening of the Laves phase, which is expressed by a shift of the ductile-to-brittle transition temperature to a higher temperature. Overall, it was shown that the influence of grain growth on the mechanical properties is more significant than the presence of the Laves phase. Precipitation of Laves phase lowers the mobility of the grain boundaries so that grain growth can be avoided. To improve mechanical properties of ferritic heat-resistant steels at high temperature, steels were developed that contain a certain amount of Laves phase. In the present work, the effect of its precipitation and coarsening on mechanical properties at room temperature was investigated. It was found, that the Laves phase rises DBTT, but retards grain coarsening and therefore stabilizes mechanical properties during annealing. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Elemental partitioning and hardness in Ti- and Ta-containing Co-base superalloys, strengthened by γ′-Co3(Al, W) precipitates, have been studied by local measurements. Using atom probe tomography, we detect strong partitioning of W (partitioning coefficients from 2.4 to 3.4) and only slight partitioning of Al (partitioning coefficients ≤1.1) to the γ′-Co3(Al, W) phase. Al segregates to the γ/γ′ phase boundaries, whereas W is depleted at the γ side of the boundaries after aging at 900 °C and slow air cooling. This kind of Al segregation and W depletion is much less pronounced when water quenching is applied. As a result, these effects are considered to be absent at high temperatures and therefore should not influence the creep properties. Ti and Ta additions are found to strongly partition to the γ′ phase and greatly increase the γ′ volume fraction. Our results indicate that the alloying elements Al, W, Ti and Ta all occupy the B sublattice of the A 3B structure (L12 type) and affect the partitioning behavior of each other. Nanoindentation measurements show that Ta also increases the hardness of the γ′ phase, while the hardness of the γ channels remains nearly constant in all alloys. The change in hardness of the γ′ phase can be ascribed to the substitution of Al and W atoms by Ti and/or Ta. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The proposal of configurational entropy maximization to produce massive solid-solution (SS)-strengthened, single-phase high-entropy alloy (HEA) systems has gained much scientific interest. Although most of this interest focuses on the basic role of configurational entropy in SS formability, setting future research directions also requires the overall property benefits of massive SS strengthening to be carefully investigated. To this end, taking the most promising CoCrFeMnNi HEA system as the starting point, we investigate SS formability, deformation mechanisms, and the achievable mechanical property ranges of different compositions and microstructural states. A comparative assessment of the results with respect to room temperature behavior of binary Fe-Mn alloys reveals only limited benefits of massive SS formation. Nevertheless, the results also clarify that the compositional requirements in this alloy system to stabilize the face-centered cubic (fcc) SS are sufficiently relaxed to allow considering nonequiatomic compositions and exploring improved strength–ductility combinations at reduced alloying costs. © 2014, The Minerals, Metals & Materials Society.

Ni-free austenitic steels alloyed with Cr and Mn are an alternative to conventional Ni-containing steels. Nitrogen alloying of these steel grades is beneficial for several reasons such as increased strength and corrosion resistance. Low solubility in liquid and δ-ferrite restricts the maximal N-content that can be achieved via conventional metallurgy. Higher contents can be alloyed by powder-metallurgical (PM) production via gas–solid interaction. The performance of sintered parts is determined by appropriate sintering parameters. Three major PM-processing routes, hot isostatic pressing, supersolidus liquid phase sintering (SLPS), and solid-state sintering, were performed to study the influence of PM-processing route and N-content on densification, fracture, and mechanical properties. Sintering routes are designed with the assistance of thermodynamic calculations, differential thermal analysis, and residual gas analysis. Fracture surfaces were studied by X-ray photoelectron spectroscopy, secondary electron microscopy, and energy dispersive X-ray spectroscopy. Tensile tests and X-ray diffraction were performed to study mechanical properties and austenite stability. This study demonstrates that SLPS process reaches high densification of the high-Mn-containing powder material while the desired N-contents were successfully alloyed via gas–solid interaction. Produced specimens show tensile strengths >1000 MPa combined with strain to fracture of 60 pct and thus overcome the other tested production routes as well as conventional stainless austenitic or martensitic grades. © 2014, The Author(s).

In this paper, we contribute to the methodology of material modeling by presenting a potential-based approach for non-isothermal inelastic processes. It is based on the principle of the minimum of the dissipation potential which was used previously only in the isothermal context. In contrast to the principle of maximum dissipation, the presented procedure results in mathematically simplified equations. Due to its variational character, the inclusion of constraints is very simple. After derivation of our method, we use the examples of non-isothermal perfect plasticity and shape memory alloys for demonstration of the validity and performance of the concept. © 2013 Springer-Verlag Berlin Heidelberg.

We use a nanoindenter with a Berkovich tip to study local mechanical properties of two polycrystalline intermetallics with a B2 crystal structure, NiAl and NiTi. We use orientation imaging scanning electron microscopy to select a relevant number of grains with appropriate sizes and surface normals parallel to 〈0 0 1〉, 〈1 0 1〉 and 〈1 1 1〉. As a striking new result, we find a strong crystallographic orientation dependence for NiTi. This anisotropy is less pronounced in the case of NiAl. For NiTi, the indentation force required to impose a specific indentation depth is highest for indentation experiments performed in the 〈0 0 1〉 direction and lowest along the 〈1 1 1〉 direction. We consider transmission electron microscopy results from cross-sections below the indents and use molecular dynamics simulations and resolved shear stress calculations to discuss how this difference can be accounted for in terms of elementary deformation and transformation processes, related to dislocation plasticity (NiAl and NiTi), and in terms of the stress-induced formation and growth of martensite (NiTi). Our results show that the crystallographic anisotropy during nanoindentation of NiTi is governed by the orientation dependence of the martensitic transformation; dislocation plasticity appears to be less important. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Ferritic heat-resistant steels are commonly used for automotive exhaust systems and have replaced cast iron, the traditional material for this application. Efforts to improve the efficiency of engines, reduce weight, and minimize toxic ingredients by increasing the gas temperature have shifted the requirement for ferritic heat-resistant steels to a higher hot strength. Methods of improving the high-temperature strength are solid-solution strengthening, precipitation hardening, and grain refinement. In this work, the influence of MX precipitates on the high-temperature mechanical properties of three different ferritic Fe-Cr stainless steels was investigated and compared to a reference material. Investigations were performed with uniaxial compression tests of samples aged isothermally at 900 °C for up to 1440 h. The most effective method of increasing the high-temperature strength is to alloy the steel with 2 mass% tungsten. Grain growth during annealing at 900 °C was decelerated by solid-state formation of MX carbonitrides. Microstructural investigations also revealed a slow coarsening rate of the MX precipitates. © [2013] by Walter de Gruyter Berlin Boston 2013.

Functionalized metal-organic frameworks (fu-MOFs) of general formula [Zn2(fu-L)2dabco]n show unprecedentedly large uniaxial positive and negative thermal expansion (fu-L = alkoxy functionalized 1,4-benzenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane). The magnitude of the volumetric thermal expansion is more comparable to property of liquid water rather than any crystalline solid-state material. The alkoxy side chains of fu-L are connected to the framework skeleton but nevertheless exhibit large conformational flexibility. Thermally induced motion of these side chains induces extremely large anisotropic framework expansion and eventually triggers reversible solid state phase transitions to drastically expanded structures. The thermo-responsive properties of these hybrid solid-liquid materials are precisely controlled by the choice and combination of fu-Ls and depend on functional moieties and chain lengths. In principle, this combinatorial approach allows for a targeted design of extreme thermo-mechanical properties of MOFs addressing the regime between crystalline solid matter and the liquid state. Extremely large thermal expansion is shown by pillared-layered metal-organic frameworks (MOFs) exhibiting alkoxy-functionalized 1,4-benzenedicarboxylate linkers. At a certain threshold temperature the materials reversibly switch from a narrow pore to large pore form. This unprecedented thermo-mechanical behavior is an intrinsic property of the materials and can be modulated substantially by mixing differently functionalized linkers to obtain mixed linker MOF solid solutions. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We have demonstrated that the physical characteristics and mechanical properties of cement mortar are significantly improved by the jute fibre reinforcement. Three different processes methodologies were adopted to mix the jute fibre homogeneously in the mortar matrix. By optimising the processing conditions and fibre loading; the cold crushing strength and flexural strength, flexural toughness and the toughness index of the mortar has significantly been increased. Based on the Fourier transformed infrared spectroscopy and thermo-gravimetric analyses a plausible mechanism of the effect of jute reinforcement controlling the physical and mechanical properties of cement mortar have been proposed. © 2012 Elsevier Ltd. All rights reserved.

Segregation and precipitation of second phases in metals and metallic alloys is an important phenomenon that has a strong influence on the mechanical properties of the material. Models exist that describe the growth of coherent, semi-coherent and incoherent precipitates. One important parameter of these models is the energy of the interface between matrix and precipitate. In this work we apply ab initio density functional theory calculations to obtain this parameter and to understand how it depends on chemical composition and mechanical strain at the interface. Our example is a metastable Mo-C phase, the body-centred tetragonal structure, which exists as a semi-coherent precipitate in body-centred cubic molybdenum. The interface of this precipitate is supposed to change from coherent to semi-coherent during the growth of the precipitate. We predict the critical thickness of the precipitate by calculating the different contributions to a semi-coherent interface energy by means of ab initio density functional theory calculations. The parameters in our model include the elastic strain energy stored in the precipitate, as well as a misfit dislocation energy that depends on the dislocation core width and the dislocation spacing. Our predicted critical thickness agrees well with experimental observations. © 2013 IOP Publishing Ltd.

An equiatomic CoCrFeMnNi high-entropy alloy, which crystallizes in the face-centered cubic (fcc) crystal structure, was produced by arc melting and drop casting. The drop-cast ingots were homogenized, cold rolled and recrystallized to obtain single-phase microstructures with three different grain sizes in the range 4-160 μm. Quasi-static tensile tests at an engineering strain rate of 10-3 s-1 were then performed at temperatures between 77 and 1073 K. Yield strength, ultimate tensile strength and elongation to fracture all increased with decreasing temperature. During the initial stages of plasticity (up to ∼2% strain), deformation occurs by planar dislocation glide on the normal fcc slip system, {1 1 1}〈1 1 0〉, at all the temperatures and grain sizes investigated. Undissociated 1/2〈1 1 0〉 dislocations were observed, as were numerous stacking faults, which imply the dissociation of several of these dislocations into 1/6〈1 1 2〉 Shockley partials. At later stages (∼20% strain), nanoscale deformation twins were observed after interrupted tests at 77 K, but not in specimens tested at room temperature, where plasticity occurred exclusively by the aforementioned dislocations which organized into cells. Deformation twinning, by continually introducing new interfaces and decreasing the mean free path of dislocations during tensile testing ("dynamic Hall-Petch"), produces a high degree of work hardening and a significant increase in the ultimate tensile strength. This increased work hardening prevents the early onset of necking instability and is a reason for the enhanced ductility observed at 77 K. A second reason is that twinning can provide an additional deformation mode to accommodate plasticity. However, twinning cannot explain the increase in yield strength with decreasing temperature in our high-entropy alloy since it was not observed in the early stages of plastic deformation. Since strong temperature dependencies of yield strength are also seen in binary fcc solid solution alloys, it may be an inherent solute effect, which needs further study. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We investigated the hydrogen embrittlement of a Fe-18Mn-1.2%C (wt.%) twinning-induced plasticity steel, focusing on the influence of deformation twins on hydrogen-assisted cracking. A tensile test under ongoing hydrogen charging was performed at low strain rate (1.7 × 10-6 s -1) to observe hydrogen-assisted cracking and crack propagation. Hydrogen-stimulated cracks and deformation twins were observed by electron channeling contrast imaging. We made the surprising observation that hydrogen-assisted cracking was initiated both at grain boundaries and also at deformation twins. Also, crack propagation occurred along both types of interfaces. Deformation twins were shown to assist intergranular cracking and crack propagation. The stress concentration at the tip of the deformation twins is suggested to play an important role in the hydrogen embrittlement of the Fe-Mn-C twining-induced plasticity steel. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We have determined the influence of carbon on mechanical properties such as grain boundary energy, work of separation (WoS) and fracture strength of the Σ5(3 1 0)[0 0 1] symmetrical tilt grain boundary (STGB) in molybdenum with ab initio methods. From our ab initio results, we derived traction-separation laws that can be used in continuum simulations of fracture employing cohesive zones. Our results show that with an increasing number of C atoms at the grain boundary, the energy of the grain boundary is lowered, indicating a strong driving force for segregation. Uni-axial tensile tests of the grain boundary reveal that there is only a small effect of segregated C atoms on the cohesive energy or WoS of the grain boundary, while the strength of the Σ5(3 1 0)[0 0 1] STGB increases by almost 30% for a complete monolayer of C. This increase in strength is accompanied by an increase in grain boundary stiffness and a decrease of the interface excess volume. The characteristic parameters are combined in the concentration-dependent traction-separation laws. A study of the scaling behaviour of the different investigated systems shows that the energy-displacement curves can be well described by the universal binding energy relationship even for different C concentrations. These findings open the way for significant simplification of the calculation of ab initio traction separation laws for grain boundaries with and without impurities. © 2013 IOP Publishing Ltd.

Composite materials based on 8 wt% yttria partially stabilized zirconia, with additions of gadolinium zirconate, lanthanum lithium hexaaluminate, yttrium aluminum garnet and strontium zirconate were characterized. Samples were fabricated by hot-press sintering at 1550° C. The effect of the secondary phase content on the mechanical properties of the composites was evaluated. Hardness, elastic modulus and fracture toughness of the fabricated composites were determined by means of depth-sensitive indentation testing. The fracture toughness of the samples as determined by the indentation method was found to increase with increasing YSZ content, reaching 3 MPa·m0.5 for samples with 80 wt% YSZ. The fracture toughness appeared to be affected by thermal expansion coefficient mismatch, crack bridging and crack deflection. © 2013 Elsevier Ltd and Techna Group S.r.l.

In this paper, we investigate the effects of Re additions on the microstructure and mechanical properties of a ternary alloy with the composition Mo-12.5Si-8.5B (at.%). This alloy has a three-phase microstructure consisting of Mo solid-solution (MoSS), Mo3Si, and Mo 5SiB2 and our results show that up to 8.4 at.% Re can be added to it without changing its microstructure or forming any brittle σ phase at 1600 °C. Three-point bend tests using chevron-notched specimens showed that Re did not improve fracture toughness of the three-phase alloy. Nanoindentation performed on the MoSS phase in the three-phase alloy showed that Re increases Young's modulus, but does not lower hardness as in some Mo solid solution alloys. Based on our thermodynamic calculations and microstructural analyses, the lack of a Re softening effect is attributed to the increased Si levels in the Re-containing MoSS phase since Si is known to increase its hardness. This lack of softening is possibly why there is no Re-induced improvement in fracture toughness. © 2012 Elsevier B.V. All rights reserved.

Structure and composition of teeth of the saltwater crocodile Crocodylus porosus were characterized by several high-resolution analytical techniques. X-ray diffraction in combination with elemental analysis and infrared spectroscopy showed that the mineral phase of the teeth is a carbonated calcium-deficient nanocrystalline hydroxyapatite in all three tooth-constituting tissues: Dentin, enamel, and cementum. The fluoride content in the three tissues is very low (<0.1. wt.%) and comparable to that in human teeth. The mineral content of dentin, enamel, and cementum as determined by thermogravimetry is 71.3, 80.5, and 66.8. wt.%, respectively. Synchrotron X-ray microtomography showed the internal structure and allowed to visualize the degree of mineralization in dentin, enamel, and cementum. Virtual sections through the tooth and scanning electron micrographs showed that the enamel layer is comparably thin (100-200 μm). The crystallites in the enamel are oriented perpendicularly to the tooth surface. At the dentin-enamel-junction, the packing density of crystallites decreases, and the crystallites do not display an ordered structure as in the enamel. The microhardness was 0.60 ± 0.05. GPa for dentin, 3.15 ± 0.15. GPa for enamel, 0.26 ± 0.08. GPa for cementum close to the crown, and 0.31 ± 0.04. GPa for cementum close to the root margin. This can be explained with the different degree of mineralization of the different tissue types and is comparable with human teeth. © 2013 Elsevier Inc.

Polymer modified alkali treated jute fibre as a reinforcing agent, substantially improves the physical and mechanical properties of cement mortar with a mix design cement:sand:fibre:water::1:3:0.01:0.6. The workability of the mortar is found to increase systematically from 155 ± 5 mm (control mortar) to 167 ± 8 mm (0.2050% polymer modified mortar). The density of the mortar is increased from 2092 kg/m3 to 2136 kg/m3 with a concomitant reduction of both water absorption and apparent porosity. Optimal polymer content in emulsion (0.0513%) is found to increase the compressive strength, modulus of rupture and flexural toughness 25%, 28%, 387% respectively as compared to control mortar. Based on the X-ray diffraction and infra-red spectroscopy analyses of the mortar samples a plausible mechanism of the effect of modified jute fibre controlling the physical and mechanical properties of cement mortar has been proposed. © 2013 Elsevier Inc. All rights reserved.

Novel composites based on borassus fruit fine fiber (BFF) and polypropylene (PP) were fabricated with variable fiber composition (5, 10, 15 and 20 wt%) by injection molding. Maleated PP (MAPP) was also used as compatibilizer at 5 wt% for effective fiber-matrix adhesion. FTIR analysis confirms the evidence of a chemical bonding between the fiber and polymeric matrix through esterification in presence of MAPP. The tensile and flexural properties were found to increase with 15 and 10 wt% fiber loadings respectively, and decreased thereafter. Coir, jute and sisal fiber composites were also fabricated with 15 wt% fiber loading under the same conditions as used for BFF/PP composites. It was found that the mechanical properties of BFF (15 wt%)/PP composites were equivalent to jute/PP, sisal/PP and superior to coir/PP composites. Jute/PP and sisal/PP composites showed higher water absorption than BFF/PP and coir/PP composites. These results have demonstrated that the BFF/PP composites can also be an alternative material for composites applications. © 2013 Elsevier Ltd. All rights reserved.

The CALPHAD method was employed to assess the austenite stability of model alloys based on the Cr-Mn-Ni-Cu system. Stability was evaluated as the difference in Gibbs free energy between the austenite and ferrite phases. This energy difference represents the chemical driving force for the martensitic transformation and is employed as a design criterion. Six novel alloys featuring a lower driving force compared to the reference material AISI 316L were produced in laboratory. The susceptibility of all alloys to hydrogen gas embrittlement was evaluated by slow strain-rate tensile testing in air and hydrogen gas at 40 MPa and -50 C. The mechanical properties and ductility response of four of the six alloys exhibited an equivalent performance in air and hydrogen. Thermodynamic calculations were in agreement with the amount of α′-martensite formed during testing. Furthermore, a 4.5 wt.% reduction in the nickel content in comparison to 316L promises a cost benefit for the novel materials. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

A NiAl-Mo eutectic having a nominal composition of Ni-45Al-10Mo was directionally solidified in a floating-zone furnace at two different growth rates, 20 and 80 mm/h. At the slower growth rate, the Mo fibers in the composite are well-aligned with the growth direction, whereas at the higher growth rate cellular microstructures are observed. Creep testing at 900°C showed that the minimum creep rate is much higher for cellular than for well-aligned structures. In the cellular case a "soft" cell boundary consisting primarily of binary NiAl surrounding "hard" eutectic cell interiors seems to facilitate cell boundary sliding and therefore results in low creep strength. A microstructure-based composite model is used to explain the effects of fiber alignment on creep resistance. © 2013 Elsevier Ltd. All rights reserved.

The effect of local martensite-to-austenite reversion on microstructure and mechanical properties was studied with the aim of designing ductile martensitic steels. Following a combinatorial screening with tensile and hardness testing on a matrix of six alloys (0-5. wt% Mn, 0-2. wt% Si, constant 13.5. wt% Cr and 0.45. wt% C) and seven martensite tempering conditions (300-500. °C, 0-30. min), investigations were focussed on martensite-to-austenite reversion during tempering as function of chemical composition and its correlation with the mechanical properties. While Mn additions promoted austenite formation (up to 35. vol%) leading to a martensitic-austenitic TRIP steel with optimum mechanical properties (1.5. GPa ultimate tensile strength and 18% elongation), Si led to brittle behaviour despite even larger austenite contents. Combined additions of Mn and Si broadened the temperature range of austenite reversion, but also significantly lowered hardness and yield strength at limited ductility. These drastically diverging mechanical properties of the probed steels are discussed in light of microstructure morphology, dispersion and transformation kinetics of the austenite, as a result of the composition effects on austenite retention and reversion. © 2013 Elsevier B.V.

At room temperature the macroscopic tensile behavior of TiAl alloys is extremely microstructure sensitive. In general the microstructures of TiAl alloys are heterogeneous at micro and meso scale. The materials micromechanisms that occur at different length scale have to be linked for a proper understanding of the macroscopic response. In order to explore those micromechanisms, methodologies combining advanced experimental and computational analysis have been proposed. Linking microstructure and properties using a two-scale numerical model we are able to explain the stress-strain and hardening behavior of this alloy. © (2013) Trans Tech Publications, Switzerland.

To improve the load bearing capacity of underground sewage pipe, we have formulated a concrete mix using chemically-modified jute fibre (reinforcing agent), polymer latex (surface modifier both for fibre and matrix) and tannin (water reducing admixture). As compared to commercial non-pressure grade pipes (NP3 type), significant strength improvement, under three-edge-bearing test (∼129.4%), is achieved in the pipes made using the modified concrete mix. NP3 pipes made using this modified concrete exhibit similar mechanical properties to that of NP4 pipes resulting an effective reduction of 31.6 wt% of steel used for reinforcement in NP4 pipes. © 2012 Elsevier Ltd. All rights reserved.

The temperature dependent mechanical properties of the metallization of electronic power devices are studied in tensile tests on micron-sized freestanding copper beams at temperatures up to 400 °C. The experiments are performed in situ in a scanning electron microscope. This allows studying the micromechanical processes during the deformation and failure of the sample at different temperatures. © 2012 American Institute of Physics.

The exoskeleton of crustaceans is formed by the cuticle, a chitin-protein-based nano-composite with hierarchical organization over at least eight levels. On the molecular level, it consists of chitin associated with proteins forming fibres, which are organized in the form of twisted plywood. On the higher levels, the twisted plywood organization is modified and forms skeletal elements with elaborate functions. The load-bearing parts of crustacean cuticle are reinforced with both crystalline and amorphous biominerals. During evolution, all parts of the exoskeleton were optimized to fulfill different functions according to different ecophysiological strains faced by the animals. This is achieved by modifications in microstructure and chemical composition. In order to understand the relationship between structure, composition, mechanical properties and function we structurally characterized cuticle from the dorsal carapace of the edible crab Cancer pagurus using light and scanning electron microscopy (SEM). The local chemical composition was investigated using energy dispersive X-ray spectroscopy (EDX) and confocal m-Raman spectroscopy. Nanoindentation tests were performed to study the resulting local mechanical properties. The results show local differences in structure on several levels of the structural hierarchy in combination with a very heterogeneous mineralization. The distal exocuticle is mineralized with calcite, followed by a layer containing a magnesium, phosphate and carbonate rich phase and ACC in the proximal part. The endocuticle contains magnesian calcite and ACC in special regions below the exocuticle. Structure and mineral phase are reflected in the local stiffness and hardness of the respective cuticle regions. The heterogeneity of structural organization and mechanical properties suggests remarkable consequences for the mechanical behaviour of the bulk material.

To investigate the mechanical shear properties of interfaces in metals, we have determined the γ-surfaces of different special tilt and twist grain boundaries in aluminum by means of ab initio calculations. From the γ-surfaces, we obtained minimum energy paths and barriers, as well as the theoretical shear strength. For the [110] tilt grain boundaries, there is a pronounced easy-sliding direction along the tilt axis. The theoretical shear strength scales with the height of the slip barrier and exhibits a relation with the misorientation angle: the closer the angle to 90°, the higher the shear stress. There is no simple relationship with the periodicity of the grain boundary, i.e., the Σ value or the grain boundary energy. © 2012 American Institute of Physics.

Chemically modified jute fibres are potentially useful as natural reinforcement in composite materials. Jute fibres were treated with 0.25%-1.0% sodium hydroxide (NaOH) solution for 0.5-48. h. The hydrophilicity, surface morphology, crystallinity index, thermal and mechanical characteristics of untreated and alkali treated fibres were studied.The two-parameter Weibull distribution model was applied to deal with the variation in mechanical properties of the natural fibres. Alkali treatment enhanced the tensile strength and elongation at break by 82% and 45%, respectively but decreased the hydrophilicity by 50.5% and the diameter of the fibres by 37%. © 2011 Elsevier Ltd.

Using atomistic simulations, this work indicates that cement nanotubes can exist. The chemically compatible nanotubes are constructed from the two main minerals in ordinary Portland cement pastes, namely calcium hydroxide and a calcium silicate hydrate called tobermorite. These results show that such nanotubes are stable and have outstanding mechanical properties, unique characteristics that make them ideally suitable for nanoscale reinforcements of cements. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The use of light-weight materials for industrial applications is a driving force for the development of joining techniques. Friction stir welding (FSW) inspired joints of dissimilar materials because it does not involve bulk melting of the basic components. Here, two different grades of high strength steel (HSS), with different microstructures and strengths, were joined to AA6181-T4 Al alloy by FSW. The purpose of this study is to clarify the influence of the distinct HSS base material on the joint efficiency. The joints were produced using the same welding parameter/setup and characterised regarding microstructure and mechanical properties. Both joints could be produced without any defects. Microstructure investigations reveal similar microstructure developments in both joints, although there are differences e.g. in the size and amount of detached steel particles in the aluminium alloy (heat and thermomechanical affected zone). The weld strengths are similar, showing that the joint efficiency depends foremost on the mechanical properties of the heat and the thermomechanical affected zone of the aluminium alloy. © 2012 Elsevier B.V.

Mechanical properties of metal films on polymer substrates are normally studied in terms of the fracture and adhesion of the film, while the properties of the polymer substrate and testing conditions are overlooked. Substrate orientation and thickness, as well as strain rate and temperature effects, are examined using Cr films deposited onto polyethylene terephthalate substrates. A faster strain rate affects only the initial fracture strain of the Cr film and not the crack and buckle spacings in the high strain condition. The substrate orientation slightly changes the average crack spacing while the substrate thickness has little effect on the cracking and buckling behaviour. Straining experiments at high temperature increased the average crack spacing and led to a change in buckling mode. The lack of sizeable changes in the mechanical behaviour over the large range of testing procedures leads to a resilient material system for flexible applications. © 2012 Taylor & Francis.

Raw pineapple leaf fibers (PALFs) were chemically modified by scouring, NaOH treatment, and bleaching (NaClO2). The graft copolymerization of synthetic acrylonitrile monomer onto bleached PALFs was carried out in aqueous medium using potassium persulfate (K2S2O8/FeSO4) as a redox initiator. The maximum grafting level at optimum conditions, namely, monomer concentration, initiator concentration, catalyst concentration, reaction time, and temperature have been determined. The main objective of this study is to decrease the amorphous region of lignocellulose in PALFs and improve its hydrophobic nature by incorporation of synthetic polymer of polyacrylonitrile and mechanical properties. The modified and grafted fibers were characterized by Fourier transform infrared spectroscopy, scanning electron microscope, thermogravimetric analysis, and X-ray diffraction study techniques. The moisture content and tensile strength properties were also evaluated for their environmental and mechanical performances. © The Author(s) 2011.

New developed (20-30)Mn12Cr(0.56-0.7)CN TWIP steels developed from thermodynamic calculations exhibit great mechanical properties, such as high strength (1800MPa UTS), deformability (80-100% elongation), toughness (300 J ISO-V), and impact wear resistance equivalent to that of Hadfield steel. In addition, they exhibit corrosion resistance by passivation in aqueous acidic media. Microstructure examination by SEM and EBSD at different degrees of deformation reveals that twinning takes place and is responsible for the high cold-work hardening of the steels. Stacking fault energy measurement of three different developed steels locates them in the range of 30-40mJm -2, being highly dependent on the N and Mn contents. Measurements carried out with digital image correlation indicate that at room temperature dynamic strain aging or Portevin-LeChatelier effect takes place. Measurements of impact toughness indicate that the steels have ductile to brittle transition at cryogenic temperatures as a consequence of the effect of nitrogen on the deformation mechanisms, resulting in a quasi-cleavage fracture along the {111} planes at -196°C. New Fe-Cr-Mn-C-N TWIP steels developed from thermodynamic calculations exhibit great mechanical properties, such as high strength (1800MPa UTS), deformability (80-100% elongation), toughness (300J ISO-V), high impact wear resistance, and corrosion resistance by passivation in aqueous acidic media. This work examines the microstructure, stacking fault energy, and dynamic strain aging to understand the tensile behavior and toughness of these materials. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Bi 2M 4O 9 (M = Al 3+, Ga 3+, Fe 3+) belongs to the family of mullite-type crystal structures. The phases are orthorhombic with the space group Pbam. The backbones of the isostructural phases are edge-connected, mullite-type octahedral chains. The octahedral chains are linked by dimers of M 2O 7 tetrahedral groups and by BiO polyhedra. The Bi 3+ cations in Bi 2M 4O 9 contain stereo-chemically active 6s 2 lone electron pairs (LEPs) which are essential for the stabilization of the structure. Although the octahedral chains of the closely related Bi 2Mn 4O 10 are similar to those of Bi 2M 4O 9, Bi 2 Mn 4O 10 contains dimers of edge-connected, five-fold coordinated pyramids instead of four-fold coordinated tetrahedra. Also the 6s 2 LEPs of Bi 3+ in Bi 2Mn 4O 10 are not stereo-chemically active. Complete and continuous solid solutions exist for Bi 2(Al 1-xFe x) 4O 9 and Bi 2(Ga 1-x Fe x) 4O 9 (x = 0 - 1). Things are more complex in the case of the Bi 2(Fe 1-xMn x) 4O 9+y mixed crystals, where a miscibility gap occurs between x = 0.25 - 0.75. In the Fe-rich mixed crystals most Mn atoms enter the octahedra as Mn 4+, with part of the tetrahedral dimers being replaced by fivefold coordinated polyhedra, whereas in the Mn-rich compound Fe 3+ favorably replaces Mn 3+ in the pyramids. The crystal structure of Bi 2M 4O 9 directly controls its mechanical properties. The stiffnesses of phases are highest parallel to the strongly bonded octahedral chains running parallel to the crystallographic c-axis. Perpendicular to the octahedral chains little anisotropy is observed. The temperature- induced expansion perpendicular to the octahedral chains is probably superimposed by contractions. As a result the c-axis expansion appears as relatively high and does not display its lowest value parallel to c, as could be inferred. Maximally 6% of Bi 3+ is substituted by Sr 2+ in Bi 2Al 4O 9 corresponding to a composition of (Bi 0.94Sr 0.06) 2Al 4O 8.94. Sr 2+ for Bi 3+ substitution is probably associated with formation of vacancies of oxygen atoms bridging the tetrahedral dimers. Hopping of oxygen atoms towards the vacancies should strongly enhance the oxygen conductivity. Actually the conductivity is rather low (σ = 7 . 10 -2 S m -1 at 1073 K, 800 °C). An explanation could be the low thermal stability of Sr-doped Bi 2Al 4O 9, especially in coexistence with liquid Bi 2O 3. Therefore, Bi 2Al 4O 9 single crystals and polycrystalline ceramics both with significant amounts of M2+ doping (M = Ca 2+, Sr 2+) have not been produced yet. Thus the question whether or not M 2+-doped Bi 2M 4O 9 is an oxygen conducting material is still open. © 2012 Carl Hanser Verlag.

Downsizing trends in the design of internal combustion engines require ferritic steels with greater strength at elevated temperatures. One method of improving the high-temperature strength is precipitation hardening with intermetallic phases such as the Laves phase. Thermodynamic calculations show, that the elements Nb and Si contribute to the Laves phase formation strongly. In this work, the influence of intermetallic precipitates on the mechanical properties of three different ferritic FeCr stainless steels was investigated and compared to a reference material. The three main hardening mechanisms - precipitation-hardening, grain refinement, and solid-solution strengthening - were studied with appropriate alloy compositions and thermo mechanical treatment. Investigations were performed with uniaxial compression tests of samples aged isothermally at 900°C for up to 1440h. It is shown that, the solid solution effect of Mo and W increases the high-temperature strength about 40%, also after long-term annealing. The contribution of the Laves phase precipitates on the high-temperature strength is rather small due to their rapid coarsening. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Understanding alloying and thermal processing at an atomic scale is essential for the optimal design of high-carbon (0.71 wt.%) bainitic-austenitic transformation-induced plasticity (TRIP) steels. We investigate the influence of the austempering temperature, chemical composition (especially the Si:Al ratio) and partitioning on the nanostructure and mechanical behavior of these steels by atom probe tomography. The effects of the austempering temperature and of Si and Al on the compositional gradients across the phase boundaries between retained austenite and bainitic ferrite are studied. We observe that controlling these parameters (i.e. Si, Al content and austempering temperature) can be used to tune the stability of the retained austenite and hence the mechanical behavior of these steels. We also study the atomic scale redistribution of Mn and Si at the bainitic ferrite/austenite interface. The observations suggest that either para-equilibrium or local equilibrium-negligible partitioning conditions prevail depending on the Si:Al ratio during bainite transformation. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We introduce a new experimental approach to the compositional and thermo-mechanical design and rapid maturation of bulk structural materials. This method, termed rapid alloy prototyping (RAP), is based on semi-continuous high throughput bulk casting, rolling, heat treatment and sample preparation techniques. 45 Material conditions, i.e. 5 alloys with systematically varied compositions, each modified by 9 different ageing treatments, were produced and investigated within 35 h. This accelerated screening of the tensile, hardness and microstructural properties as a function of chemical and thermo-mechanical parameters allows the highly efficient and knowledge-based design of bulk structural alloys. The efficiency of the approach was demonstrated on a group of Fe-30Mn-1.2C-xAl steels which exhibit a wide spectrum of structural and mechanical characteristics, depending on the respective Al concentration. High amounts of Al addition (>8 wt.%) resulted in pronounced strengthening, while low concentrations (<2 wt.%) led to embrittlement of the material during ageing. © 2012 Acta Materialia Inc. Published by Elsevier Ltd.

Hydrogen environment embrittlement of metastable austenitic stainless steels is a well-known phenomenon partially related to the formation of straininduced martensite. In the literature, hydrogen environment embrittlement is often discussed on the basis of nominal chemical compositions only and neglects effects of metallurgical production and processing. The aim of this study is to investigate the influence of the d-ferrite volume fraction and grain size on the mechanical properties of a standard grade 1.4307 (AISI 304L) tested in high-pressure hydrogen gas. A negligible influence was found for dferrite volume fractions between 2 and 10 %. This result is explained by the dominating influence of machininginduced a-martensite on the surface of the tensile samples. In contrast, the grain size was found to have a significant effect on hydrogen environment embrittlement. In particular, grain sizes smaller than 50 lm were found to have a higher ductility. The results are discussed with respect to stacking fault energy, formation of strain-induced a-martensite, trapping of hydrogen and microsegregations. The results are of particular interest for the materials selection and development of materials for hydrogen applications. © Springer Science+Business Media, LLC 2012.

The mechanical behavior of a Mo-TiC 30 vol.% ceramic-metal composite was investigated over a wide temperature range (25-700 °C). High-energy X-ray tomography was used to reveal percolation of the hard titanium carbide phase through the composite. Using a polycrystal approach for a two-phase material, finite-element simulations were performed on a real three-dimensional (3-D) aggregate of the material. The 3-D microstructure, used as the starting configuration for the predictions, was obtained by serial sectioning in a dual beam focused ion beam scanning electron microscope coupled to an electron backscattered diffraction system. The 3-D aggregate consists of a molybdenum matrix and a percolating TiC skeleton. As for most body-centered cubic (bcc) metals, the molybdenum matrix phase is characterized by a change in plasticity mechanism with temperature. We used a polycrystal model for bcc materials which was extended to two phases (TiC and Mo). The model parameters of the matrix were determined from experiments on pure molydenum. For all temperatures investigated the TiC particles were considered to be brittle. Gradual damage to the TiC particles was treated, based on an accumulative failure law that is approximated by evolution of the apparent particle elastic stiffness. The model enabled us to determine the evolution of the local mechanical fields with deformation and temperature. We showed that a 3-D aggregate representing the actual microstructure of the composite is required to understand the local and global mechanical properties of the composite studied. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

As a result of their microstructures being made up by constituents with strong distinctions in mechanical properties, multiphase steels exhibit high energy absorption as well as an excellent combination of strength and ductility. Furthermore, the microstructural composition influences the failure behaviour of such kind of steels because of the occurrence of different fracture mechanisms in parallel. When the failure behaviour of dual phase (DP) steels is investigated, several types of failures are typically observed, such as the ductile failure of ferrite, the brittle failure of martensite and the interface debonding between phases. Hence, a reliable microstructure-based simulation approach must be developed that describes material deformation and failure under any given loading condition. In this work, two different damage mechanics methods were employed to study the interaction between failure modes in DP steels by means of a representative volume element (RVE). In order to consider the characteristics of a real microstructure, all involved phases were modelled with a precise volume fraction. Firstly, the extended finite element method (XFEM) was used to study the damage onset and progression in martensitic regions without prescribing the crack path. Secondly, a damage curve was derived and employed for the ductile ferritic phase. By combining these two damage models in the RVE model on microscopic scale, development of different failures modes in DP steels could be investigated. © 2011 Elsevier B.V. All rights reserved.

The study of mechanical properties in micron- and submicron-sized metal crystals raises fundamental questions about the influence of size on different aspects of plasticity. In situ characterization of the microstructure evolution during loading is necessary to understand the physics underlying crystal deformation. In situ μLaue diffraction is able to provide unique statistical information on the evolution of type and density of stored dislocations. Here we show macroscopically expected and unexpected plastic behavior at low strains, observed during in situ μLaue tensile tests on micron-sized, single slip oriented Cu samples. Regardless of the initial behavior, a steady state is reached which qualifies a technical yield criterion at the micron scale. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The teeth of two different shark species (Isurus oxyrinchus and Galeocerdo cuvier) and a geological fluoroapatite single crystal were structurally and chemically characterized. In contrast to dentin, enameloid showed sharp diffraction peaks which indicated a high crystallinity of the enameloid. The lattice parameters of enameloid were close to those of the geological fluoroapatite single crystal. The inorganic part of shark teeth consisted of fluoroapatite with a fluoride content in the enameloid of 3.1 wt.%, i.e., close to the fluoride content of the geological fluoroapatite single crystal (3.64 wt.%). Scanning electron micrographs showed that the crystals in enameloid were highly ordered with a special topological orientation (perpendicular towards the outside surface and parallel towards the center). By thermogravimetry, water, organic matrix, and biomineral in dentin and enameloid of both shark species were determined. Dentin had a higher content of water, organic matrix, and carbonate than enameloid but contained less fluoride. Nanoindentation and Vicker's microhardness tests showed that the enameloid of the shark teeth was approximately six times harder than the dentin. The hardness of shark teeth and human teeth was comparable, both for dentin and enamel/enameloid. In contrast, the geological fluoroapatite single crystal was much harder than both kinds of teeth due to the absence of an organic matrix. In summary, the different biological functions of the shark teeth (" tearing" for Isurus and "cutting" for Galeocerdo) are controlled by the different geometry and not by the chemical or crystallographic composition. © 2012 Elsevier Inc.

Intermetallic γ-TiAl based alloys with a chemical composition of Ti-(42-45)Al-(3-5)Nb-(0.1-2)Mo-(0.1-0.2)B (in atom percent) are termed TNM ™ alloys. They exhibit several distinct characteristics, including excellent hot-workability and balanced mechanical properties. In this study, the relationship between microstructure and mechanical behavior in a Ti-43.5Al-4Nb-1Mo-0.1B alloy after two different heat treatments was investigated. One of the analyzed microstructures consisted of lamellar γ-TiAl/α2-Ti3Al colonies with a small volume fraction of globular γ-TiAl and β0-TiAl grains at their grain boundaries, whereas the second microstructure basically exhibited the same arrangement of the microstructural constituents, but a fraction of the lamellar colonies was altered by a cellular reaction. The prevailing microstructures have been analyzed by means of scanning electron microscopy and transmission electron microscopy. Macro-and micro-hardness measurements as well as room temperature tensile tests have revealed that the sample with both cellular and lamellar features show lower yield stress and hardness than the ones exhibiting undisturbed lamellar microstructures. The strength and hardness properties are primarily connected to the lamellar spacing within the colonies, where strength increases with decreasing lamellar spacing. The appearance of a cellular reaction leads to a refinement of the lamellar colonies which in turn influences positively the plastic fracture strain at room temperature. © Hanser Verlag GmbH & Co. KG.

Ultrafine-grained (UFG) materials processed by severe plastic deformation are known to exhibit good mechanical properties. Much about the annealing behavior of such materials is still unknown, and this work aims to provide a better understanding of the thermal properties of UFG materials. For this purpose a Cu-0.17 wt%Zr alloy was subjected to high pressure torsion (HPT) with a maximal pressure of 4.8GPa at room temperature. The microstructures of the specimens were characterized using electron back scatter (EBSD) measurements, transmission electron microscopy (TEM), and hardness measurements. During annealing of the samples, dispersoids were formed which improved the thermal stability of the alloy. At higher strain levels the fraction of high angle grain boundaries (HAGBs) increased above 70% of the total grain boundaries. Ultrafine-grained materials processed by severe plastic deformation are known to exhibit good mechanical properties. Much about the annealing behavior of such materials is still unknown, and this work aims to provide a better understanding of the thermal properties of such materials. For this purpose a Cu-0.17 wt%Zr alloy was subjected to high pressure torsion. The microstructures of the specimens were characterized in the deformed state as well as after annealing using EBSD and hardness measurements. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA.

Austenitic steels exhibiting twinning induced plasticity (TWIP) are materials with exceptional mechanical properties. In this work, the development of new grades of TWIP steels exhibiting corrosion resistance is presented. The alloy development was supported by thermodynamic and diffusion calculations within the (Fe-Mn-Cr)-(C-N) alloy system. For the calculations ambient pressure and primary austenitic solidification were considered as necessary to avoid nitrogen degassing in all processing steps. Manganese is used as an austenite stabilizer, chromium to increase nitrogen solubility and provide corrosion resistance, while carbon and nitrogen provide mechanical strength. Diffusion calculations were used in order to predict the extent of micro segregations and additionally to evaluate the effect of diffusion annealing treatments. The material was cast in a laboratory scale with a nominal composition of Fe-20Mn-12Cr-0.25C-0.3N. Diffusion annealing was followed by hot rolling and solution annealing resulting in a fully austenitic microstructure. Tensile tests at room temperature were performed, exhibiting yield strengths of 430 MPa and elongation to fracture of 93%. In addition, not only the mechanical properties but also the weldability was studied, focussing on the characterization of the microstructure of bead on plate welds obtained by laser and TIG welding. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Ultra-fine grain sizes have been shown to enhance some key mechanical and functional properties of engineering materials, including shape memory alloys. While the effect of ultra-fine and nanocrystalline grain sizes on pseudoelastic shape memory materials is well-appreciated in medical device engineering, the effect of such microstructures on actuators has not been sufficiently characterized. In the present work, it is demonstrated that NiTi spring actuators with ultra-fine grained microstructures can be obtained by conventional wire drawing in combination with heat treatments and that the final grain size can be controlled by varying the final annealing temperature. Annealing at 400deg;C for 600s allows for the evolution of microstructures with median grain sizes of about 34nm, while annealing at 600deg;C for the same length of time results in median grain sizes of about 5 μm. It is observed that the grain size strongly affects the elementary processes of the martensitic phase transformation. Small austenite grain sizes inhibit twinning accommodation of transformation strains, such that a higher driving force is required to nucleate martensite. This increase in the martensite nucleation barrier decreases the martensite transformation temperatures such that only partial transformation to martensite is possible upon cooling to room temperature. The incomplete martensitic transformation reduces the exploitable actuator stroke; however, a reduction in grain size is shown to improve the functional stability of the material during thermal and thermomechanical cycling by reducing the irreversible effects of dislocation plasticity. NiTi spring actuators with ultra-fine grained and nanocrystalline microstructures can be obtained by conventional wire drawing in combination with heat treatments. Grain size refinements into this range improve the functional stability during thermal and thermomechanical cycling by reducing the irreversible effects of dislocation plasticity. The improvement in functional stability comes at the cost of exploitable actuator stroke, however, because very fine grain sizes result in only a partial transformation to martensite upon cooling to room temperature. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA.

Quantum-mechanical (so-called ab initio) calculations have achieved considerable reliability in predicting physical and chemical properties and phenomena. Due to their reliability they are becoming increasingly useful when designing new alloys or revealing the origin of phenomena in existing materials, also because these calculations are able to accurately predict basic material properties without experimental input. Due to the universal validity of fundamental quantum mechanics, not only ground-state properties, but also materials responses to external parameters can reliably be determined. The focus of the present paper is on ab initio approaches to the elasticity of materials. First, the methodology to determine single-crystalline elastic constants and polycrystalline moduli of ordered compounds as well as disordered alloys is introduced. In a second part, the methodology is applied on α-Fe, with a main focus on (i) investigating the influence of magnetism on its elasticity and phase stability and (ii) simulating extreme loading conditions that go up to the theoretical tensile strength limits and beyond. Copyright © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The plastic deformation of lamellar microstructures composed of the two phases γ-TiAl and α2-Ti3Al is highly orientation dependent. In this paper we present a homogenized model that takes into account the micromechanical effect of the plate-like morphologies that are often observed in two-phase titanium aluminide alloys. The model is based on crystal elasto-viscoplasticity and 18 deformation systems were implemented that have been identified to govern the plastic flow of the lamellar microstructures. The model is validated against experiments on polysynthetically twinned (PST) crystals and shows good agreement with the data. On a larger length scale, the model is applied to a 64-grain aggregate to investigate the mechanical response of two different kinds of microstructures. Different magnitudes of the kinematic constraints exerted by the densely spaced and highly aligned interfaces are shown to affect the macroscopic flow behavior of the microstructures. The phenomenon of pronounced microplasticity of fully lamellar material as well as the stress variation inside two-phase microstructures are studied quantitatively. © 2011 Elsevier Ltd. All rights reserved.

Hardness and Young's moduli values for TixNi90-xCu10 (37at.%< x< 67at.%) thin films from a continuous composition spread type materials library, annealed at 500°C for 1h, were determined at room temperature (martensitic state) and 80°C (austenitic state) using high-throughput nanoindentation experiments. These values are found to increase as the compositions deviate from Ti contents close to 50at.%. The increases in hardness is correlated to the presence of Ti-rich and (Ni,Cu)-rich precipitates resulting in precipitate hardening and grain size refinement (Hall-Petch effect). The increase of the Young's moduli is rationalized by considering the significantly higher Young's moduli of the different precipitate phases and applying the rule of mixtures. The contributions of the precipitate phases and the matrix to the combined Young's modulus were estimated by evaluating the load-displacement curves in detail. The obtained results are in good agreement with the Young's moduli determined from thin film curvature measurements [R. Zarnetta et al., Smart Mater. Struct. 19 (2010) 65032]. Thus, the experimental restrictions for nanoindentation experiments at elevated temperatures are concluded to not adversely affect the validity of the results. © 2011 Elsevier B.V.

The biomechanics of the optic nerve head is assumed to play an important role in ganglion cell loss in glaucoma. Organized collagen fibrils form complex networks that introduce strong anisotropic and nonlinear attributes into the constitutive response of the peripapillary sclera (PPS) and lamina cribrosa (LC) dominating the biomechanics of the optic nerve head. The recently presented computational remodeling approach (Grytz and Meschke in Biomech Model Mechanobiol 9:225-235, 2010) was used to predict the micro-architecture in the LC and PPS, and to investigate its impact on intraocular pressure-related deformations. The mechanical properties of the LC and PPS were derived from a microstructure-oriented constitutive model that included the stretch-dependent stiffening and the statistically distributed orientations of the collagen fibrils. Biomechanically induced adaptation of the local micro-architecture was captured by allowing collagen fibrils to be reoriented in response to the intraocular pressure-related loading conditions. In agreement with experimental observations, the remodeling algorithm predicted the existence of an annulus of fibrils around the scleral canal in the PPS, and a predominant radial orientation of fibrils in the periphery of the LC. The peripapillary annulus significantly reduced the intraocular pressure-related expansion of the scleral canal and shielded the LC from high tensile stresses. The radial oriented fibrils in the LC periphery reinforced the LC against transversal shear stresses and reduced LC bending deformations. The numerical approach presents a novel and reasonable biomechanical explanation of the spatial orientation of fibrillar collagen in the optic nerve head. © 2010 Springer-Verlag.

Recently, we proposed a hierarchical model for the elastic properties of mineralized lobster cuticle using (i) ab initio calculations for the chitin properties and (ii) hierarchical homogenization performed in a bottom-up order through all length scales. It has been found that the cuticle possesses nearly extremal, excellent mechanical properties in terms of stiffness that strongly depend on the overall mineral content and the specific microstructure of the mineral-protein matrix. In this study, we investigated how the overall cuticle properties changed when there are significant variations in the properties of the constituents (chitin, amorphous calcium carbonate (ACC), proteins), and the volume fractions of key structural elements such as chitin-protein fibers. It was found that the cuticle performance is very robust with respect to variations in the elastic properties of chitin and fiber proteins at a lower hierarchy level. At higher structural levels, variations of design parameters such as the volume fraction of the chitin-protein fibers have a significant influence on the cuticle performance. Furthermore, we observed that among the possible variations in the cuticle ingredients and volume fractions, the experimental data reflect an optimal use of the structural variations regarding the best possible performance for a given composition due to the smart hierarchical organization of the cuticle design. © 2010 Elsevier Ltd.

An additional coating against wear or corrosion on component parts is required for many applications. These coatings protect the substrate material against external influences, thus increasing the economic lifetime of the component. Coating processes such as build-up welding and thermal spraying are well established and commonly used. The thermal spray process, in particular, permits deposition of metals, ceramics, or cermets materials to produce near net shape coatings on complex surface geometries. However, commonly used coating materials suffer from high raw material costs, thus decreasing the cost effectiveness of the coating process. Fe based materials are low priced and possess noteworthy mechanical properties; they thus provide the possibility of substituting the expensive Ni and Co based materials commonly used for thermal spray processes. In this work, 2 mm thick high velocity oxyfuel sprayed Fe based coatings in the as sprayed and thermally sprayed and hot isostatic pressed condition were investigated with respect to their mechanical and wear properties. Additionally, the fracture surface was investigated by scanning electron microscopy to characterise the fracture behaviour. It could be demonstrated that the substrate and the heat treatment have the greatest impact on the shear strength of thermally sprayed cold work tool steel. It is shown that the substrate materials as well as the heat treatment are promoting diffusion processes across the interface between the coating and the substrate. Hence, a material integrated bond is formed. The microstructures of the thermally sprayed coatings become more important regarding the mechanisms of failure of the four point bending tests. The material strength is influenced by quenching and tempering and the specimen deflection is influenced by diffusion reactions induced by hot isostatic pressing treatment. The thermally sprayed coatings in the as sprayed condition feature the highest wear resistance due to their hardness. © 2011 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute.

The microstructure of dual phase steels can be compared with a composite composed of a matrix of ferrite reinforced by small islands of martensite. This assumption has been used in several attempts to model the mechanical properties of dual phase steels. However, recent measurements show that the properties of the ferrite phase change with distance from the martensite grains. These measurements showed that the grains of the ferrite phase are harder in the vicinity of martensite grains. As a consequence of this local hardening effect, the ferrite phase has to be considered as an inhomogeneous matrix in modeling dual phase steels. This experiment inspired the idea that local hardening is caused by geometrically necessary dislocations. The idea is investigated experimentally and numerically in the present analysis, which for the first time leads to good agreement with experimental observations of the mechanical stress-strain behavior. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Three ferrite/martensite dual-phase steels varying in the ferrite grain size (12.4, 2.4 and 1.2 μm) but with the same martensite content (∼30 vol.%) were produced by large-strain warm deformation at different deformation temperatures, followed by intercritical annealing. Their mechanical properties were compared, and the response of the ultrafine-grained steel (1.2 μm) to aging at 170 °C was investigated. The deformation and fracture mechanisms were studied based on microstructure observations using scanning electron microscopy and electron backscatter diffraction. Grain refinement leads to an increase in both yield strength and tensile strength, whereas uniform elongation and total elongation are less affected. This can be partly explained by the increase in the initial strain-hardening rate. Moreover, the stress/strain partitioning characteristics between ferrite and martensite change due to grain refinement, leading to enhanced martensite plasticity and better interface cohesion. Grain refinement further promotes ductile fracture mechanisms, which is a result of the improved fracture toughness of martensite. The aging treatment leads to a strong increase in yield strength and improves the uniform and total elongation. These effects are attributed to dislocation locking due to the formation of Cottrell atmospheres and relaxation of internal stresses, as well as to the reduction in the interstitial carbon content in ferrite and tempering effects in martensite. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

High-temperature corrosion resistant ferritic steels are commonly used in heat exchangers for auxiliary power units (APU), automotive exhaust systems and structural parts of solid oxide fuel cells (SOFC) due to their excellent thermal fatigue resistance. As the process temperatures in these applications are primarily limited by the materials high temperature strength, the main focus of this work is on the improvement of this property by adjustments in the material design. Generally, two mechanisms were used to increase the high temperature strength, solid solution strengthening and precipitation hardening. Due to their large atomic radii and high solubility in α-Fe, W and Mo were used for solid solution strengthening. Furthermore, the content of niobium, which is well known to form Laves phase precipitates, was raised. This led to a higher content of Laves phase precipitates compared to the reference material. The analyses concentrated on the effect of the Laves phase. Strength at elevated temperature was investigated in compression tests at 900°C with respect to the annealing time which was varied between 1h and 1440 h. © 2011 Published by Elsevier Ltd.

The formation of intermetallic reaction layers and their influence on mechanical properties was investigated in friction stir welded joints between a low C steel and both pure Al (99.5wt.%) and Al-5wt.% Si. Characterisation of the steel/Al interface, tensile tests and fractography analysis were performed on samples in the as-welded state and after annealing in the range of 200-600°C for 9-64min. Annealing was performed to obtain reaction layers of distinct thickness and composition. For both Al alloys, the reaction layers grew with parabolic kinetics with the η phase (Al5Fe2) as the dominant component after annealing at 450°C and above. In joints with pure Al, the tensile strength is governed by the formation of Kirkendall-porosity at the reaction layer/Al interface. The tensile strength of joints with Al-5wt.% Si is controlled by the thickness of the η phase (Al5Fe2) layer. The pre-deformation of the base materials, induced by the friction stir welding procedure, was found to have a pronounced effect on the composition and growth kinetics of the reaction layers. © 2011 Elsevier B.V.

The present investigations were undertaken to find out whether and how often cycling, processing and programming can be repeated, whether repeated programming affects the one way effect and how much irreversible strain the shape memory polymeric material accumulates at a particular temperature. The effect was investigated in dependence of different stress levels, and the effect of both recovery temperature and recovery time was considered. As a model material the commercially and industrially applicable amorphous shape memory polymer Tecoflex® was examined and subjected to 50 programming cycles. Tecoflex® is characterized by a glass transition temperature, Tg, of 74 °C, above which it looses all its strength. During tensile testing at 20 °C (T < Tg), stresses a steady increase to 26 MPa as strains approached the rupture strain of 25%. It is observed that at 60 °C (T < Tg, but near Tg) the material can be strained to more than 2500% before rupture occurs while stresses slowly increase to values less than 0.3 MPa. It turns out that programming, cooling, unloading and heating to trigger the one way effect causes an increase of irreversible strain that is associated with a corresponding decrease of the intensity of the one way effect during the first thermomechanical cycles. © The Author(s) 2011.

The formation of deformation-induced shear bands plays an important role for the room temperature deformation of both, Mg and Mg-Y alloys, but the formation and structure of shear bands is distinctively different in the two materials. Due to limited deformation modes in pure Mg, the strain is localized in few shear bands leading to an early failure of the material during cold deformation. Contrarily, Mg-RE (RE: rare earth) alloys exhibit a high density of homogeneously distributed local shear bands during deformation at room temperature. A study of the microstructure of the shear bands by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) at different strains was performed. These investigations give insight into the formation of shear bands and their effects on the mechanical behaviour of pure Mg and Mg-3Y. Since in pure Mg mainly extension twinning and basal dislocation slip are active, high stress fields at grain resp. twin boundaries in shear bands effect fast growth of the shear bands. In Mg-RE alloys additionally contraction and secondary twinning and pyramidal dislocation slip are active leading to the formation of microscopic shear bands which are limited to the boundary between two grains. The effects of shear bands on the mechanical behaviour of pure Mg and Mg-RE alloys are discussed with respect to their formation and growth. © (2011) Trans Tech Publications.

The analysis and simulation of microstructures in solids has gained crucial importance, virtue of the influence of all microstructural characteristics on a material's macroscopic, mechanical behavior. In particular, the arrangement of dislocations and other lattice defects to particular structures and patterns on the microscale as well as the resultant inhomogeneous distribution of localized strain results in a highly altered stress-strain response. Energetic models predicting the mechanical properties are commonly based on thermodynamic variational principles. Modeling the material response in finite strain crystal plasticity very often results in a non-convex variational problem so that the minimizing deformation fields are no longer continuous but exhibit small-scale fluctuations related to probability distributions of deformation gradients to be calculated via energy relaxation. This results in fine structures that can be interpreted as the observed microstructures. In this paper, we first review the underlying variational principles for inelastic materials. We then propose an analytical partial relaxation of a Neo-Hookean energy formulation, based on the assumption of a first-order laminate microstructure, thus approximating the relaxed energy by an upper bound of the rank-one-convex hull. The semi-relaxed energy can be employed to investigate elasto-plastic models with a single as well as multiple active slip systems. Based on the minimization of a Lagrange functional (consisting of the sum of energy rate and dissipation potential), we outline an incremental strategy to model the time-continuous evolution of the laminate microstructure, then present a numerical scheme by means of which the microstructure development can be computed, and show numerical results for particular examples in single- and double-slip plasticity. We discuss the influence of hardening and of slip system orientations in the present model. In contrast to many approaches before, we do not minimize a condensed energy functional. Instead, we incrementally solve the evolution equations at each time step and account for the actual microstructural changes during each time step. Results indicate a reduction in energy when compared to those theories based on a condensed energy functional. © 2010 Springer-Verlag.

The effect of solidification conditions on microstructural and mechanical properties of eutectic TiFe alloy cast under different conditions was examined. Samples exhibit different ultrafine eutectic structures (β-Ti(Fe) solid solution + TiFe). Different cooling conditions lead to the evolution of ultrafine eutectic oval-shaped colonies or elongated lamellar colonies with preferred orientation. Isotropic as well as anisotropic mechanical properties were obtained. Alloys exhibit compressive strengths between 2200 and 2700 MPa and plastic strains between 7 and 19 pct. in compression. © 2010 Elsevier Ltd. All rights reserved.

Nanoindentation is a suitable tool for characterizing the local mechanical properties of shape memory alloys (SMA) and to study their pseudoelastic behavior. There is a special interest in indenting with different indenter tips (as not all tips are associated with strain states that predominantly induce the martensitic transformation) and in indenting at different temperatures, where different phases are present. In this study, we perform nanoindentation on a ternary NiTiFe SMA with different indenter tips and at various testing temperatures. For nanoindentation with spherical tips, load-displacement hystereses clearly indicate pseudoelastic behavior, whereas indentation with Berkovich tips results in more pronounced plastic deformation. Testing at different temperatures is associated with different volume fractions of austenite, martensite, and R-phase. The corresponding nanoindentation responses differ considerably in terms of pseudoelastic behavior. Best pseudoelastic recovery is found at testing temperatures close to the R-phase start temperature, even though this temperature is below the austenite finish temperature, which is a well-known lower temperature bound for full recovery in macroscopic tests. Our results are discussed considering micromechanical aspects and the interaction between stress-induced phase transformation and dislocation plasticity. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Laves phases are the most abundant intermetallic phases. However, their mechanical properties are only poorly understood due to difficulties in producing defect-free samples which are large enough for mechanical testing. By using a modified cold crucible levitation melting technique with in situ heat treatment and subsequent defined cooling, massive and large bars of several hundreds of grams of brittle and high-melting Laves phases were produced. Metallographic investigation revealed single- or near single-phase microstructures and a homogeneous chemical composition within the cast ingots with a diameter of 15. mm and of more than 100. mm length. With these high-quality bars it is now possible to prepare large samples that can be used for obtaining mechanical data of these phases. © 2010 Elsevier B.V.

Single crystals of γ-TiAl cannot be grown in the near-stoichiometric compositions that are present inside two-phase γ / α2-microstructures with attractive mechanical properties. Therefore, the single-crystal constitutive behavior of γ-TiAl was studied by nanoindentation experiments in single-phase regions of these γ / α2-microstructures. The experiments were characterized by orientation microscopy and atomic force microscopy to quantify the orientation-dependent mechanical response during nanoindentation. Further, they were analyzed by a three-dimensional crystal plasticity finite element model that incorporated the deformation behavior of γ-TiAl. The spatially resolved activation of competing deformation mechanisms during indentation was used to assess their relative strengths. A convention was defined to unambiguously relate any indentation axis to a crystallographic orientation. Experiments and simulations were combined to study the orientation-dependent surface pile-up. The characteristic pile-up topographies were simulated throughout the unit triangle of γ-TiAl and represented graphically in the newly introduced inverse pole figure of pile-up patterns. Through this approach, easy activation of ordinary dislocation glide in stoichiometric γ-TiAl was confirmed independently from dislocation observation by transmission electron microscopy. © 2010 Acta Materialia Inc.

With the aim of investigating a laser-welded dissimilar joint of TWIP and TRIP steel sheets, the microstructure was characterized by means of OM, SEM, and EBSD to differentiate the fusion zone, heat-affected zone, and the base material. OIM was used to differentiate between ferritic, bainitic, and martensitic structures. Compositions were measured by means of optical emission spectrometry and EDX to evaluate the effect of manganese segregation. Microhardness measurements and tensile tests were performed to evaluate the mechanical properties of the joint. Residual stresses and XRD phase quantification were used to characterize the weld. Grain coarsening and martensitic areas were found in the fusion zone, and they had significant effects on the mechanical properties of the weld. The heat-affected zone of the TRIP steel and the corresponding base material showed considerable differences in the microstructure and properties. © 2009 Elsevier B.V. All rights reserved.

The present paper gives an overview of the simultaneous research work carried out by RWTH Aachen University and ThyssenKrupp Steel Europe AG. With a combination of sophisticated simulation tools and experimental techniques it is possible to predict the relations between temperature distribution in the mould, solidification velocity, chemical steel composition and, furthermore, the mechanical properties of the steel shell. Simulation results as well as experimentally observed microstructure parameters are used as input data for hot tearing criteria. A critical choice of existing hot tearing criteria based on different approaches, like critical strain and critical strain rate, are applied and developed. The new "damage model" is going to replace a basic approach to determine hot cracking susceptibility in a mechanical FEM strand model for continuous slab casting of ThyssenKrupp Steel Europe AG. Critical strains for hot cracking in continuous casting were investigated by in situ tensile tests for four steel grades with carbon contents in the range of 0.036 and 0.76 wt%. Additionally to modeling, fractography of laboratory and industrial samples was carried out by SEM and EPMA and the results are discussed. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this study, we produced an Al matrix composite material reinforced by Al-Cu-Fe particles of the icosahedral phase. The composite material was prepared using a hot isostatic pressure technique at T = 673 K and P = 180 MPa. The mechanical properties were investigated by compression tests performed at constant strain rate over the temperature range 290-823 K. The results show a vigorous strengthening effect resulting from the reinforcement particles. Strengthening is attributed to two main contributions arising from load transfer between the Al matrix and the reinforcement particles and from plastic deformation of the Al grains. The present results are compared with those obtained in a previous work on an Al-based composite reinforced by Al-Cu-Fe particles of the ω-tetragonal phase. © 2010 Materials Research Society.

Directionally solidified (DS) alloys of the eutectic systems NiAl-10Mo and NiAl-34Cr (at.%) are potential candidates for high-temperature structural applications. Here, these alloys were first arc-melted and drop-cast. Thereafter, they were directionally solidified (DS) at growth rates of 20 and 80 mm/h while rotating at a fixed rotation speed of 60 revolutions per minute. Specimens of the DS alloys were tested in three-point-bending and uniaxial compression to obtain mechanical properties, including the ductile to brittle transition temperature (DBTT). For the NiAl-Cr system DBTT was found to be around 300 °C. Microstructural observations revealed that in the section perpendicular to the growth direction a uniform distribution of fibres was observed. The expected decrease of the fibre diameter with increasing growth rate was not observed. Instead, the fibre diameter slightly increased with increasing crystal growth rates. First compression tests were performed to get insights into the creep behaviour of these fibre-reinforced microstructures. © 2010 IOP Publishing Ltd.

Large strain warm deformation at different temperatures and subsequent intercritical annealing has been applied to obtain fine grained (2.4μm) and ultrafine grained (1.2μm) ferrite/martensite dual-phase (DP) steels. Their mechanical properties were tested under tensile and impact conditions and compared to a hot deformed coarse grained (12.4μm) reference material. Both yield strength and tensile strength follow a Hall-Petch type linear relationship, whereas uniform elongation and total elongation are hardly affected by grain refinement. The initial strain hardening rate as well as the post-uniform elongation increase with decreasing grain size. Ductile fracture mechanisms are considerably promoted due to grain refinement. Grain refinement further lowers the ductile-to-brittle transition temperature and leads to higher absorbed impact energies. Besides the common correlations with the ferrite grain size, these phenomena are explained in terms of the martensite particle size, shape and distribution and the more homogeneous dislocation distribution in ultrafine ferrite grains. © 2010 Elsevier B.V.

Ab initio calculations are becoming increasingly important for designing new alloys as these calculations can accurately predict basic structural, mechanical, and functional properties using only the atomic composition as a basis. In this paper, fundamental physical properties (like formation energies and elastic constants) of a set of bcc Mg-Li and Mg-Li-based compounds are calculated using density functional theory (DFT). These DFT-determined properties are in turn used to calculate engineering parameters such as (i) specific Young's modulus (Y/p) or (ii) shear over bulk modulus ratio (G/B) differentiating between brittle and ductile behavior. These parameters are then used to identify those alloys that have optimal mechanical properties for lightweight structural applications. First, in case of the binary Mg-Li system, an Ashby map containing Y/r versus G/B shows that it is not possible to increase Y/r without simultaneously increasing G/B (i.e., brittleness) by changing only the composition of a binary alloy. In an attempt to bypass such a fundamental materials-design limitation, a set of Mg-Li-X ternaries (X=Ca, Al, Si, Cu, Zn) based on stoichiometric Mg-Li with CsCl structure was studied. It is shown that none of the studied ternary solutes is able to simultaneously improve both specific Young's modulus and ductility. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Diamond is one of the most important functional materials for film applications due to its extreme physical and mechanical properties, many of which depend on the crystallographic texture. The influence of various deposition parameters matters to the texture formation and evolution during chemical vapor deposition (CVD) of diamond films. In this overview, the texture evolutions are presented in terms of both simulations and experimental observations. The crystallographic textures in diamond are simulated based on the van der Drift growth selection mechanism. The film morphology and textures associated with the growth parameters α (proportional to the ratio of the growth rate along the 〈100〉 direction to that along the 〈111〉 direction) are presented and determined by applying the fastest growth directions. Thick films with variations in substrate temperature, methane concentration, film thickness, and nitrogen addition were analyzed using high-resolution electron back-scattering diffraction (HR-EBSD) as well as X-ray diffraction (XRD), and the fraction variations of fiber textures with these deposition parameters were explained. In conjunction with the focused ion beam (FIB) technique for specimen preparation, the grain orientations in the beginning nucleation zones were studied using HR-EBSD (50 nm step size) in another two sets of thin films deposited with variations in methane concentration and substrate material. The microstructures, textures, and grain boundary character were characterized. Based on the combination of an FIB unit for serial sectioning and HR-EBSD, diamond growth dynamics was observed using a 3D EBSD technique, with which individual diamond grains were investigated in 3D. Microscopic defects were observed in the vicinity of the high-angle grain boundaries by using the transmission electron microscopy (TEM) technique, and the advances of TEM orientation microscopy make it possible to identify the grain orientations in nano-crystalline diamond. © 2010 Higher Education Press and Springer Berlin Heidelberg.

Deep cryogenic treatment (DCT) of tool steels is used as an additive process to conventional heat treatment and usually involves cooling the material to liquid nitrogen temperature (-196 °C). This kind of treatment has been reported to improve the wear resistance of tools. In this study, the Taguchi method was used to identify the main factors of DCT that influence the mechanical properties and the wear resistance of the powder metallurgically produced cold-work tool steel X153CrVMo12 (AISI D2). Factors investigated were the austenitizing temperature, cooling rate, holding time, heating rate, and tempering temperature. In order to study the significance of these factors and the effect of possible two-factor interactions L27(313), an orthogonal array (OA) was applied to conduct several heat treatments, including a single DCT cycle directly after quenching prior to tempering. The results show that the most significant factors influencing the properties of tool steels are the austenitizing and tempering temperatures. In contrast, the parameters of deep cryogenic treatment exhibit a lower level of significance. Further investigations identified a nearly constant wear rate for holding times of up to 24 h. The wear rate reaches a minimum for a longer holding time of 36 h and increases again with further holding. © 2010 Elsevier B.V. All rights reserved.

The main strengthening precipitates of aluminum alloy 2198-T8, which are of the T1 phase, dissolve during friction stir welding, sending many Li atoms into solid solution. The stir zone precipitates are characterized using high-resolution transmission electron microscopy, energy dispersive spectroscopy, and selected area diffraction techniques to begin answering questions about the microstructural evolution and the relationship between microstructure and mechanical properties in friction stir welding of the next generation of lightweight Li-containing Al alloys. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Single-crystals of γ-TiAl cannot be grown for the compositions present inside the two-phase γ/α 2-microstructures that show good mechanical properties. Therefore the single crystal constitutive behaviour of γ-TiAl was studied by nanoindentation experiments in single phase regions of these microstructures. The experiments were extensively characterized by a combined experimental approach to clarify the orientation dependent mechanical response during nanoindentation. They further were analyzed by a three-dimensional crystal plasticity finite element model that incorporated the deformation behaviour of γ-TiAl. The spatially resolved activation of competing deformation mechanisms during indentation was used to assess their relative strengths. On the length-scale of multi-grain aggregates two kinds of microstructures were investigated. The lamellar microstructure was analyzed in terms of kinematic constraints perpendicular to densely spaced lamellar boundaries which lead to pronounced plastic anisotropy. Secondly, the mechanical behaviour of massively transformed microstructures was modelled by assuming a lower degree of kinematic constraints. This resulted in less plastic anisotropy on a single grain scale and lower compatibility stresses in a 64-grain aggregate. On the macroscopic length scale, the results could possibly explain the pre-yielding of lamellar microstructures. © 2010 IOP Publishing Ltd.

Using SEM/EBSD the substructure and texture evolution in dual phase steels in the first steps of the process chain, i.e. hot rolling, cold rolling, and following annealing were characterized. In order to obtain dual phase steels with high ductility and high tensile strength an industrial process was reproduced by cold rolling of industrially hot rolled steel sheets of a thickness of 3.75 mm with ferrite and pearlite morphology down to a thickness of 1.75 mm and finally annealing at different temperatures. Such technique allows a compilation of ferrite and martensite morphology typical for dual phase steels. Due to the competition between recovery, recrystallization and phase transformation during annealing a variety of ferrite martensite morphologies was produced by promoting one of the mechanisms through the variation of technological parameters such as heating rate, intercritical annealing temperature, annealing time, cooling rate and the final annealing temperature. Annealing induced changes of the mechanical properties were determined by hardness measurements and are discussed on the basis of the results of the substructure investigations. © (2010) Trans Tech Publications.

The effects of thermal annealing at 1000°C in air on the microstructure and the mechanical properties (Young's modulus and hardness) of thermal barrier coatings consisting of a 4mol% Y2O3 partially stabilized ZrO2 top coat and a NiCoCrAlY bond coat, deposited by electron beam physical vapour deposition on nickel-based superalloy IN 625, have been investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM), image analysis and nanoindentation. During annealing, the ceramic top coat undergoes sintering and recrystallization. These processes lead to stress relaxation, an increase of the intra-columnar porosity and the number of large pores as measured by image analysis of SEM micrographs. An increase of the grain size of the γ-phase in the bond coat, accompanied by changes in the morphology of γ-grains with annealing time, is also observed. Correlations between these microstructural changes in the top coat and the bond coat and their mechanical properties are established and discussed. © 2010 Elsevier B.V.

Body centered cubic (bcc) Mg-Li-based alloys are a promising light-weight structural material. In order to tailor the Mg-Li composition with respect to specific industrial requirements, systematic materials-design concepts need to be developed and applied. Quantum-mechanical calculations are increasingly employed when designing new alloys as they accurately predict basic thermodynamic, structural, and functional properties using only the atomic composition as input. We have therefore performed a quantum-mechanical study using density functional theory (DFT) to systematically explore fundamental physical properties of a broad set of bcc MgLi-based compounds. These DFT-determined properties are used to calculate engineering parameters such as (i) the specific Young's modulus (Y/ρ) or (ii) the bulk over shear modulus ratio (B/G) which allow differentiating between brittle and ductile behavior. As we have recently shown, it is not possible to increase both specific Young's modulus, as a measure of strength, and B/G ratio, as a proxy for ductility, by changing only the composition in the binary bcc Mg-Li system. In an attempt to bypass such fundamental materials-design limitations, a large set of MgLi-X substitutional ternaries derived from stoichiometric MgLi with CsCl structure are studied. Motivated by the fact that for Mg-Li alloys (i) 3rd row Si and Al and (ii) 4th row Zn are industrially used as alloying elements, we probe the alloying performance of the 3rd (Na, Al, Si, P, S, Cl) and 4th row transition metal (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) elements. The studied solutes offer a variety of properties but none is able to simultaneously improve both specific Young's modulus and ductility. Therefore, in order to explore the alloying performance of yet a broader set of solutes, we predict the bulk modulus of MgX and LiX B2-compounds running over 40 different elements. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.

Room-temperature deformation of a niobium-rich TiAl alloy with duplex microstructure has been numerically investigated. The model links the microstructural features at micro- and meso-scale by the two-level (FE 2) multi-scale approach. The deformation mechanisms of the considered phases were described in the micro-mechanical crystal-plasticity model. Initial material parameters for the model were taken from the literature and validated using tensile experiments at macro-scale. For the niobium-rich TiAl alloy further adaptation of the crystal plasticity parameters is proposed. Based on these model parameters, the influences of the grain orientation, grain size, and texture on the global mechanical behavior have been investigated. The contributions of crystal deformation modes (slips and dislocations in the phases) to the mechanical response are also analyzed. The results enable a quantitative prediction of relationships between microstructure and mechanical behavior on global and local scale, including an assessment of possible crack initiation sites. The model can be used for microstructure optimization to obtain better material properties. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Anodic oxides were grown on the surface of an electropolished (Ti-30at% Nb) beta-titanium (β-Ti) alloy by cyclic voltammetry. The scan rate was 100 mVs -1 between 0 and 8V in increments of lV in an acetate buffer of pH 6.0. Electrochemical impedance spectroscopy was carried out right after each anodic oxide growth increment to study the electronic properties of the oxide/electrolyte interface in a wide frequency range from 100 kHz to 10 MHz with an AC perturbation voltage of 10 mV. A film formation factor of 2.4 nm V -1 was found and a relative permittivity number (dielectric constant) of 42.4 was deter- mined for the oxide film formed. Mott-Schottky analysis on a potentiostatically formed 7 nm thick oxide film was performed to assess the semiconducting properties of the mixed anodic oxide grown on the alloy. A flat band potential of - 0.47 V (standard hydrogen electrode, SHE) was determined, connected to a donor density of 8.2 × 1017cm -3. β-Ti being highly isotropic in terms of mechanical properties should be superior to the stiffer α-Ti compound. Its application, however, requires a passivation behaviour comparable or better than α-Ti which in fact is found. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The exoskeleton of the crustacean Homarus americanus, the American lobster, is a biological multiphase composite consisting of a crystalline organic matrix (chitin), crystalline biominerals (calcite), amorphous calcium carbonate and proteins. One special structural aspect is the occurrence of pronounced crystallographic orientations and resulting directional anisotropic mechanical properties. The crystallographic textures of chitin and calcite have been measured by wide-angle Bragg diffraction, calculating the Orientation Distribution Function (ODF) from pole figures by using the series expansion method according to Bunge. A general strong relationship can be established between the crystallographic and the resulting mechanical and physical properties. © (2010) Trans Tech Publications.

In this study, we produced four composite materials with Al-based matrix reinforced by Al-Cu-Fe particles initially of the quasicrystalline (QC) phase. The processing route was a gas-pressure infiltration of QC particle preforms by molten commercial Al and Al alloys. The resulting composites were investigated by scanning electron microscopy (SEM) working in the energy dispersive spectroscopy (EDS) mode and by X-ray diffraction (XRD). It is shown that such a synthesis technique leads to the formation of various phases resulting from specific diffusion processes. Compression tests were performed at constant strain rate in the temperature range 290-770 K. The stress-strain curves look similar to those of Al-Cu-Fe poly-quasicrystals and show the yield point, the origin of which is however of very different nature. Composite deformation is recognised to occur through the rupture of a hard phase skeleton and localised plastic deformation in the matrix. © 2009 Elsevier B.V. All rights reserved.

Natural materials are hierarchically structured nanocomposites. A bottom-up multiscale approach to model the mechanical response of the chitin-based mineralized cuticle material of Homarus americanus is presented, by combining quantummechanical ab initio calculations with hierarchical homogenization. The simulations show how the mechanical properties are transferred from the atomic scale through a sequence of specifically designed microstructures to realize optimal stiffness. (Figure Presented) © 2010 WILEY-VCH Verlag GmbH & Co. KGaA,.

Organized collagen fibrils form complex networks that introduce strong anisotropic and highly nonlinear attributes into the constitutive response of human eye tissues. Physiological adaptation of the collagen network and the mechanical condition within biological tissues are complex and mutually dependent phenomena. In this contribution, a computational model is presented to investigate the interaction between the collagen fibril architecture and mechanical loading conditions in the corneo-scleral shell. The biomechanical properties of eye tissues are derived from the single crimped fibril at the micro-scale via the collagen network of distributed fibrils at themeso-scale to the incompressible and anisotropic soft tissue at the macro-scale. Biomechanically induced remodeling of the collagen network is captured on the meso-scale by allowing for a continuous re-orientation of preferred fibril orientations and a continuous adaptation of the fibril dispersion. The presented approach is applied to a numerical human eye model considering the cornea and sclera. The predicted fibril morphology correlates well with experimental observations from X-ray scattering data. © Springer-Verlag 2009.

NiTiCr is used in medicine for retriever devices, dental- and guide wires [1-3]. Very little has been published in literature about this ternary NiTi-alloy. In the present work, the mechanical properties of NiTiCr shape memory alloy (SMA) wires (0.25 wt.% Cr) in the as-received condition and after a number of different heat treatments were studied. The surface quality of the wires was examined, because it is known that the surface condition has an influence on fatigue life. Mechanical uniaxial loading-unloading tensile and cyclic tests were performed. Bending-rotation fatigue (BRF) tests at higher rpm-levels were conducted in air and in oil. This is important with regard to possible applications, where NiTiCr might be used as a structural component. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA.