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The indentation of bearing raceways frequently contributes to bearing failures, as in gearboxes. The dents act as surface stress raisers through the Rolling Contact Fatigue (RCF) cycles, leading to crack initiation and spalling. The microstructural evolution mechanism occurring is not completely understood. Hence, to enhance bearing lifetime, it is crucial to identify microstructural parameters that influence crack initiation and propagation. Therefore, this study presents a multi-scale characterization of martensitic 100Cr6 bearings before and after RCF. It includes SEM, SEM/FIB, EBSD, TKD and ACOM-ASTAR observations. It showed that, before RCF, the bearing raceway features a 0.5 µm thick surface layer consisting in refined martensite and spread primary carbides, resulting from finishing operations. After RCF, no further grain refinement was observed along the raceway, except beneath the dent shoulder where a material plastic flow, nano-grains and ultra-fine grains of martensite, and sheared and spread primary carbides were detected up to 1.4 µm deep. Below and up to 3 µm deep, a fine grain layer is observed. The martensite morphology, size and disorientation suggest that it undergoes refinement through continuous Dynamic Recrystallisation and twinning. Crack initiation was preferentially observed in the nano-grains layer, at primary carbides/matrix interfaces. Above the crack, another refined region is observed resulting from high deformation and stresses through crack propagation. Finally, even in severely deformed areas, nanosized retained austenite islands were detected, indicating that it is not completely transformed. [Display omitted] [ABSTRACT FROM AUTHOR]

Digital holographic microscopy (DHM) was employed to investigate the formation process and shape evolution of the most frequently observed bainite variant pairs in low-carbon alloy steel. Through in-situ observation coupled with theoretical predictions, the continuous changes in surface relief corresponding to bainite and adjacent austenite were measured. Additionally, the crystal orientations of the observed variant pairs were determined using electron backscattered diffraction (EBSD). The results indicated that at the onset of the transformation, the bainite variant shape agreed with theoretical predictions, while the adjacent austenite experienced plastic deformation. However, as the transformation proceeded towards completion, the shape of some bainite variants deviated from the theoretical predictions, while their adjacent austenite remained undeformed. This shift of deformation strain between austenite and bainite suggests a change in strain accommodation mechanism. Furthermore, the observed 'tent-shaped' surface relief was attributed to back-to-back mutually accommodating variant pairs, resulting in reduction of the total shape strain, compared to the theoretical prediction. [Display omitted] [ABSTRACT FROM AUTHOR]

During the thermoforming of TC18 titanium alloy multi-cavity components, different deformation modes always exist and change, such as uniaxial compression (UC), shear-compression deformation (SCD), uniaxial tension (UT) and shear-tension deformation (STD). Dynamic recrystallization (DRX) of β grains occurs during the single-phase field deformation and has a great influence on the performance of components. In this study, the types and mechanisms of DRX in TC18 titanium alloy as well as transition and correlation under different deformation modes are investigated. It is found that discontinuous dynamic recrystallization (DDRX) initiates through grain boundary bulging at a low strain and dominates in different modes of deformation. Continuous dynamic recrystallization (CDRX) initiates at different strains depending on deformation modes, and the mechanisms vary, subgrain rotation within grains and lattice rotation near GBs under UC and STD, while only subgrain rotation under UT, in addition to these two mechanisms, grain fragmentation is also involved under SCD. Secondary dynamic recrystallization (SDRX) only occurs at a high strain under SCD and STD. Deformation modes lead to differences in orientation, slip and rotation of grains, further result in different dislocation density, distribution and accumulation, which contribute to the occurrence and transition of different types of DRX. Meanwhile deformation modes result in differences in the difficulty of GB migration and lattice rotation, ultimately in the initiation and degree of DDRX and CDRX. The shear stress in SCD and STD promotes the occurrence of CDRX. The present results can provide a guidance for obtaining good performance of the titanium alloy components. [Display omitted] [ABSTRACT FROM AUTHOR]

A single-phase or simple-structured alloy does not always possess outstanding combinations of strength and ductility over a wide range of temperatures and strain rates for engineering applications. In the present work, a high-entropy alloy with multi-heterogeneous microstructures was in-situ fabricated via powder-plasma-arc additive manufacturing. The compressive behavior of the additive manufactured high-entropy alloy over a wide range of temperatures and strain rates was studied, using an improved split Hopkinson bar system and electronic universal testing machine. It shows exceptional combination of strength and ductility within the selected temperature and strain rate ranges. Microstructural evolution was characterized at various temperatures and strain rates, providing insight into the intricate relationship between microstructure and property. The multicomponent Laves phase is hard yet deformable, while the multicomponent FCC phase is soft and ductile. The deformation twins observed all over the selected temperature and strain rate ranges and dynamic recrystallization appearing at high temperatures in the FCC phase enhance the ductility of the FCC phase and rise the crack-arresting capability. The third-type strain aging occurs at different strain rates, which shifts to a higher temperature range as strain rate increases. Ta and impurity atom, Si, acting as "solute atoms" form atom atmosphere and silicide, pinning the moving dislocations in the FCC phase. Finally, a deformation mechanism map was proposed over a wide temperature and strain rate range. The study explored a potentially new avenue to design alloys with exceptional combinations of strength and ductility over a wide range of temperatures and strain rates. [Display omitted] [ABSTRACT FROM AUTHOR]

Hierarchical microstructures spanning from micro-sized eutectic structure to nano-sized precipitates are promisingly engineered in lightweight Al alloys to improve the high-temperature creep resistance that is increasingly required for rapid industrial development. However, the intrinsically-brittle eutectic phase is ready to fracture upon applied loading, which, dramatically reducing room-temperature ductility and fracture toughness, greatly hampers practical applications of the creep-resistant Al alloys. Here, through the combination of Sc microalloying with sub-rapid solidification, we observe the ductilization of Al 11 Ce 3 eutectic phase in cast heat-resistant Al-Ce-Sc alloys due to the formation of atomic-scale compositional complexity. High-concentration Sc atoms are frozen within the Al 11 Ce 3 intermetallic phase by the sub-rapid solidification, which then assemble into unusual atomic-scale compositional dipoles with the Sc atoms enriched at one pole and the Al atoms at the opposite during subsequent heat treatment. The dispersed Sc-Al compositional dipoles induce local lattice distortions that stimulate dislocation activities, as temporally and spatially visualized by in-situ neutron diffraction tensile test and microstructural characterizations. The unexpected plastic deformation triggered in Al 11 Ce 3 improves the deformation compatibility between the eutectic phases, enabling the sub-rapidly-solidified Al-Ce-Sc alloy to reach a room-temperature tensile elongation 3 times and fracture toughness over 8 times of its counterpart derived from traditional solidification. In addition, the sub-rapidly-solidified Al-Ce-Sc alloy exhibits an excellent creep resistance at 300 °C, achieving a tensile creep stress threshold of ∼ 70 MPa. These findings provide new perspectives on the design of ductile intermetallic phases and the development of creep-resistant Al alloys with application-level ductility. [Display omitted] [ABSTRACT FROM AUTHOR]

The effects of Ni additions on microstructure and microhardness of near-eutectic Al-9Ce-xNi-0.75Mn-0.18Sc-0.12Zr (x = 2.5, 3.2, and 3.9 wt.%) alloys are investigated for various cooling rates. The as-cast microstructure consists of fine, micron-scale Al 11 Ce 3 and Ni-rich phases formed during eutectic solidification. Two Ni-rich phases are observed: (i) Al 27 Ce 3 Ni 6 at higher Ni contents and slower solidification rates, and (ii) Al 9 (Ni,Mn,Fe) 2 at lower Ni contents and faster solidification rates. While these Ni-containing alloys have higher microhardness than a Ni-free control alloy, varying the Ni concentration in the 2.5–3.9 wt.% range does not significantly affect the microhardness, indicative of competing strengthening and weakening effects from adding Ni. The alloy with intermediate Ni content (3.2 wt.%) is selected for further microstructural and mechanical characterization. It contains four coarsening-resistant strengthening constituents: (i) micron-scale Al 11 Ce 3 and (ii) Al 27 Ce 3 Ni 6 or Al 9 (Ni,Mn,Fe) 2 platelets, all formed during eutectic solidification, (iii) L1 2 -Al 3 (Sc,Zr) nanoprecipitates formed on aging, and (iv) Mn atoms in solid solution in the α-Al matrix. The combined strengthening mechanisms impart high strength at ambient and elevated temperatures, as measured by microhardness (as a function of aging time), by compression and tensile experiments, and by compressive creep measurements. Alloys previously reported in the literature - with various combinations of one, two or three of these four strengthening phases - show lower hardness and creep resistance, indicating that cumulative strengthening can be achieved when combining mechanisms; the present alloy containing all four phases shows a remarkably high creep threshold stress of 62 MPa at 300 °C. [Display omitted] [ABSTRACT FROM AUTHOR]

We have investigated the influence of 101 mass ppm hydrogen content on the room temperature deformation structure and mechanical behavior of an austenitic Fe30Mn6.5Al0.3C (wt.%) low-density steel by several electron microscopy techniques, such as electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and scanning transmission electron microscopy (STEM). The steel exhibits a high hydrogen embrittlement resistance associated with a moderated increase in strength (yield stress increase of 10%) and ductility (increase in the elongation to fracture of ∼ 8%). Analysis of the deformation structure reveals that hydrogen influences the deformation behavior by promoting deformation mechanisms associated with inhomogeneous plasticity (hydrogen-enhanced deformation banding (HEDB)) and strain localization (hydrogen-enhanced microbanding (HEMB)). These deformation mechanisms are ascribed to hydrogen-induced effects on dislocation plasticity, resulting in macroscopic kink bands, sub-micron localized strain gradients, and localized shear at cell blocks. We find that HEMB plays a relevant role in the deformation behavior of sub-micron localized strain gradients by promoting plastic relaxation and the enhanced storage of geometrically necessary dislocations within them. These effects mitigate the activation of damage mechanisms and enhance the strain-hardening capacity, contributing to the high HE resistance of the steel, comparable to that of high HE-resistant fcc alloys and steels. [Display omitted] [ABSTRACT FROM AUTHOR]

A 2D bicrystal atomistic model of dislocation transmission through a Σ11<101>{131} symmetric-tilt grain boundary reveals details of Hall-Petch breakdown in the single dislocation regime in Al, Ni, and Cu. Partly based on a previous study [1] , this research determines the stress required for a single dislocation to be transmitted through the Σ11 boundary and finds that single dislocation transmission typically deviates from Hall-Petch behavior because the leading partial dislocation becomes trapped in the boundary and emits glissile grain boundary disconnections that cause boundary sliding deformation at stresses well below boundary transmission stresses. Thus, mechanisms of inverse Hall-Petch, namely grain boundary shear and sliding, are shown to operate in lieu of grain boundary transmission in the single dislocation regime. However, by controlling the applied stresses, inverse Hall-Petch can be reversed and typical Hall-Petch grain boundary transmission regained. This, in turn, allows the direct computation of Hall-Petch coefficients. A second dislocation on the same slip plane preserves typical Hall-Petch behavior for Al and Ni and does not lead to boundary sliding events, thus implying a critical grain size for Hall-Petch breakdown based on the pileup model. [Display omitted] [ABSTRACT FROM AUTHOR]

The present work proposes a new elementary deformation mechanism which governs high temperature and low stress creep of single crystal superalloys (SXs), where the misfit between the γ- and the γ′-phase plays a central role. In the coherent two phase SX microstructure, there is a tendency to minimize the overall elastic strain energy. This is accomplished by the formation of dislocation networks in the γ-phase close to the γ/γ'-interfaces. The stress fields of the network dislocations and misfit stresses accommodate each other to keep the overall strain energy of the system at a minimum. Previous work has shown that dynamic recovery is associated with knitting-out reactions, where dislocations from the network shear the γ'-phase and annihilate with dislocations of opposite sign on the other side of the γ'-phase region. Due to the presence of the misfit this results in an increase of elastic strain energy which is counteracted by coupled knitting-in reactions where newly arriving γ-channel dislocations re-establish the minimum energy configuration. Here we provide microstructural evidence for knitting-out and knitting-in reactions and show that while these reactions clearly occur, dislocation network spacings stay constant. The misfit between the γ- and the γ′-phase accounts for a constant network spacing. Dislocation networks are not static but represent dynamic steady state equilibrium structures in an evolving microstructure. Knitting regular networks requires climb processes which are suggested to be rate controlling. This new view of high temperature and low stress creep mechanism of SXs allows to rationalize previous results published in the literature. [Display omitted] [ABSTRACT FROM AUTHOR]

We have investigated the influence of the deformation temperature from 25 °C (RT) to -196 °C on the dislocation structures associated with strain localization phenomena in an austenitic Fe-30Mn-6.5Al-0.3C (wt.%) low-density steel by electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and bright-field transmitted forescattered electron imaging ((BF) t-FSEI) techniques. The characteristics of the dislocation structures were evaluated on the main texture components, i.e. <111>//tensile axis, <112>//tensile axis, and <001>//tensile axis directions. The inhomogeneous character of the plastic behavior is promoted upon cryogenic deformation due to the formation of dislocation structures associated with strain localization, namely microbands (MBs) and deformation bands (DBs). ECCI analysis of the dislocation structure reveals that the deformation temperature has a strong influence on the thermal-assisted dislocation processes controlling the dislocation configurations and MB formation mechanisms. The MB nucleation mechanism evolves from a cross-slip-assisted mechanism at RT deformation conditions to a slip band-assisted mechanism at cryogenic deformation temperatures. This effect has a profound effect on the grain orientation dependence of the MB structure but not on its crystallographic alignment. On the other hand, cryogenic deformation temperatures (-196 °C) enhance the material's mechanical strength and ductility due to the activation of deformation twinning, which is associated with the reduction of the stacking fault energy. We find that MBs have a small contribution to strain-hardening and ductility due to the small mechanical resistance of these dislocation structures against the advance of deformation twins and dense dislocation layers, and the comparatively small plastic strain accommodated by them, respectively. These findings provide new insights into the microband-induced plasticity (MBIP) effect. [Display omitted] [ABSTRACT FROM AUTHOR]

Elemental powders of Al, Ti, Sc, and Zr are blended to create binary Al-0.5Ti and Al-0.25Ti-0.25Zr (at%) alloys via laser powder-bed fusion (L-PBF), with Al-1Ti and Al-0.25Sc-0.25Zr alloys produced for comparison. The Al-Ti alloys formed primary D0 23 -Al 3 Ti precipitates upon solidification, whereas the Al-(Sc/Ti)-Zr alloys formed primary L1 2 -Al 3 (Sc,Ti,Zr) precipitates which nucleated bands of fine grains. Upon aging, the Al-Ti alloys do not harden (consistent with most Ti remaining in solid solution), unlike the Al-(Sc,Ti)-Zr alloys, as expected from precipitation of secondary L1 2 -Al 3 (Sc,Ti,Zr) nanoprecipitates. Upon compressive creep testing at 300–400 °C, Al-0.25Ti-0.25Zr exhibits steady-state strain rates comparable to those of Al-0.5Zr at low stresses (up to ∼14 MPa), but about an order of magnitude faster at higher stresses. The effects of bands of fine grains (formed by L1 2 -Al 3 (Sc,Ti,Zr) primary precipitates at the bottom of the melt pools) on hardness and creep resistance is investigated via finite element modeling of simple slab models, with varying geometries approximating experimental melt pool geometries. The degree of connectivity of coarse- and fine-grain regions is a greater factor in the resulting creep strain rates than the volume fraction or spatial orientations of these two regions. This suggests that reducing the concentration of Zr (in Al-0.5Zr), or partially substituting with a less expensive element such as Ti (as for Al-0.25Ti-0.25Zr), may maintain the added l -PBF processability benefits of Zr and reduce the volume fraction of fine grains without sacrificing creep resistance, allowing for further tailoring of custom microstructures. [Display omitted] [ABSTRACT FROM AUTHOR]

Determination of strain path across martensitic transformation in materials with two transformation orientation relationships (ORs) is challenging due to the temporal and spatial limitation of the modern techniques. In this work, an analyzing strategy, i.e. , the determination of transformation path via the variant organization feature of martensite, was suggested and applied for the Ni–Mn-based alloys as an example, based on the consideration that the different crystallographic symmetries of transformation systems will result in distinct organizations of martensite variants. For the selected Ni–Mn-based alloys, the orientation examination revealed that both the K−S and the Pitsch OR are respected. Further analyses in terms of the strain and stress compatibility condition and the minimum energy criteria showed that, theoretically, the K−S path produces 2 variants as a self-accommodated variant group, whereas the Pitsch path produces 4 variants as a self-accommodated variant group. Compared with the experimental results, the Pitsch path is the energetically favorable transformation path and actually occurs in stress-free austenite of the Ni−Mn based alloys. The significance of this work is multi-fold. It first resolves the long puzzling issue of transformation strain path of Ni–Mn-based alloys. Moreover, the analyzing scheme can be generalized to determine the transformation characters for martensitic transformation in other systems with multiple possible transformation orientation relationships. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Triple junctions, intersections of three or more grains in polycrystalline solids, are known locations of potential stress concentration and strain localization. In this work, intragranular lattice curvatures and elastic strains are measured at triple junctions via a novel zoom-in style combination of synchrotron X-ray techniques. Additionally, this work revisits the impact of including elastic strain gradients while calculating the dislocation density within embedded grains via the Nye dislocation tensor. Without elastic strain gradients, the dislocation density is underestimated at a hotspot by 27% of the maximum dislocation density value. Spatial regions near triple junctions are found to contain both the highest values of the intragranular misorientations metrics and generally higher values than grain boundaries (between two grains). A heterogenous distribution of intragranular misorientation, elastic strain, and dislocation density is measured within a grain along a single triple junction line, and the triple junction line end points (quad points) exhibit substantially different micromechanical fields. This work provides a unique 3D view of triple junctions within individual grains and highlights their spatially heterogenous intragranular micromechanical response along triple junction lines, especially when compared to relatively less complex behavior along grain boundaries. While a grain averaged example is provided of intergranular strain across grains, several examples of intragranular strain were visualized near a triple junction. The combined analysis of intragranular lattice curvature (associated with strain metrics) and elastic strain (associated with stress) provides a more complete characterization of the 3D sub-grain fields than previously reported. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract Domain behavior of (111)- oriented perovskite ferroelectric films is significantly different from (001)-/(101)- oriented ones, resulting in enhancing property responses such as a superior susceptibility and a reduced coercive field. However, the domain structures and evolutions with the thickness of (111)-oriented ferroelectric films, which are crucial to further understand the distinctive properties, are still obscure. In this study, the ferroelectric domains of (111)- oriented PbTiO 3 films are investigated by transmission electronic microscopy (TEM) and piezoresponse force microscopy (PFM). We identify the domain evolution with the film thicknesses under anisotropic strains imposed by the orthorhombic GdScO 3 (101) O substrates. Contrast analysis and electron diffraction patterns reveal that only four ferroelectric variants evolve in PTO films: d 2 +, d 2 -, d 3 + and d 3 -, with the polarization directions along [010], [0 1 ¯ 0], [001] and [00 1 ¯ ], respectively. Two kinds of domain walls are formed: "inclined" (011) domain walls for d 2 -/d 3 + (d 2 +/d 3 -) domains, "normal" (01 1 ¯) domain walls for d 2 -/d 3 + (d 2 +/d 3 -) domains. The width of periodically distributed d 2 +/d 3 + (d 2 -/d 3 -) domains increases with film thickness following the square root rule. Aberration-corrected scanning transmission electronic microscopy demonstrates the lattice characteristics of domains in (111)- oriented PbTiO 3 films are consistent with tetragonal ferroelectric domains. PFM studies reveal that both the out-of-plane and in-plane polarization components of d 2 -/d 3 + (d 2 +/d 3 -) domains are non-identical, whereas the d 2 +/d 3 + (d 2 -/d 3 -) domains possess uniform out-of-plane polarization component and non-uniform in-plane polarization component. This study discloses the domain structure in (111)- oriented tetragonal ferroelectric films under anisotropic strains and the association with the ferroelectric properties. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract A modification to the empirical Olson-Cohen strain-induced austenite to martensite transformation kinetic model is proposed. The proposed kinetic model accounts for the stress state at the grain level and the crystallography of the transformation mechanism. Two transformation mechanisms sensitive to the local stress state are incorporated in the model. First, the resolved shear stress on a slip plane in the direction perpendicular to the Burgers vector determines the stacking fault width (SFW) which in turn determines the potential nucleation sites. Second, the stress triaxiality governs the probability of the structural α ′-martensite formation at a nucleation site. The kinetic model is implemented in the elasto-plastic self-consistent (EPSC) crystal plasticity model to study the stress state and texture dependence of the strain-induced α ′-martensite transformation and the mechanical response of metastable austenitic steels. The simulations are compared with experimental mechanical and phase fraction data from different austenitic steels subjected to simple tension, plane strain tension, equibiaxial tension, simple compression, and torsion. It is demonstrated that the appropriate modeling of α ′-martensite phase fractions allows capturing the experimentally measured mechanical response. The implementation and insights from these predictions, including the role of texture evolution on martensite transformation, are discussed in this paper. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract The effects of strain on the ionic conductivity of rare-earth substituted CeO 2 have been extensively studied, but the results have been inconsistent and focused upon the 'optimised' conductors such as Gd or Sm substituted CeO 2 where defect association is minimised. By thermally annealing epitaxial films deposited by pulsed laser deposition, we varied the strain systematically, whilst avoiding any influence from interfacial or grain boundary effects. The activation energy of the in-plane conductivity was found to increase with increasing compressive biaxial strain, which was quantitatively in excellent agreement with previous computational and experimental studies. These results provide a much needed quantitative consensus on the effects of lattice strain on ionic transport. Furthermore, we demonstrate that the change in the activation energy for Yb-substituted CeO 2 is around three times that for Gd or La substitutions for the same applied strain, indicating the important role played by defect association. These results have significant implications for ionic transport at reduced or ambient temperatures, where changes in conductivity due to strain may be several orders of magnitude larger for 'non-optimised' conductors compared with 'optimised' conductors. We rationalise our results by considering the defect-defect interactions in these materials and through force-field calculations. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Alloying Mg with rare-earth elements, such as Nd, Gd, or Y, enables hardening from the precipitation of metastable coherent phases during aging at low temperatures. While the aging potential of binary Mg-Nd alloys is relatively limited due to the nucleation of coarse β 1 and β precipitates at the expense of the strengthening β‴ precipitates, binary Mg-Y alloys exhibit exceptional stability of the strengthening coherent β S ′ phase when aged at 200 °C. Through combination of high-resolution characterization, density functional theory calculations, and finite-element elasticity studies, we demonstrate that the strengthening β S ' phase is thermodynamically stable at low temperatures (as opposed to metastable as in other Mg-RE binaries) and that misfit strains play a key role in not only controlling precipitate structure and morphology but also their unusual evolution into an interconnected network. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract We applied a hot rolling direct quenching and partitioning (HDQ&P) process to a low-C low-Si Al-added steel and obtained a Q&P steel containing 40 vol % of ferrite. Microstructure characterization was performed by means of SEM, EBSD, TEM and XRD. Atomic-scale characterization of carbon partitioning among the phases was carried out by atom probe tomography (APT). The carbon distribution in the retained austenite and near the martensite/retained austenite interfaces was quantitatively analyzed to study its partitioning behavior. The macroscopic strain distribution evolution across the tensile sample surface was investigated using macro digital image correlation (DIC) analysis. Combining these results with joint micro-DIC and EBSD analysis during quasi in-situ tensile testing, we investigated the strain partitioning among the different phases and the TRIP effect. Coupling of these results enabled us to reveal the relation among carbon partitioning, strain partitioning and the TRIP effect. The large blocky retained austenite with a side length of about 300–600 nm located near the ferrite/martensite (F/M) interfaces has low stability and transforms to martensite during the early deformation stages, i.e. at average strain below 21%. The retained austenite films in the centers of the martensite regions are more stable. The carbon distribution in both, the martensite and the retained austenite are inhomogeneous, with 0.5–2.0 at. % in the martensite and 4.0–7.5 at. % in the retained austenite. Strong carbon concentration gradients of up to 1.1 at. %/nm were observed near the martensite/retained austenite interfaces. The large blocky retained austenite (300–600 nm in side length) near the F/M interfaces has 1.5–2.0 at. % lower carbon content than that in the narrow retained austenite films (20–150 nm in thickness). The ferrite is soft and deforms prior to the martensite. The strain distribution in ferrite and martensite is inhomogeneous, varying by up to 20% within the same phase at an average strain of about 20%. Ferrite deformation is the main origin of ductility of the material. The balance between ferrite fraction and martensite morphology controls the TRIP effect and its efficiency in reaching a suited combination of strength and ductility. Reducing the ferrite volume fraction and softening the martensite by coarsening and polygonization can enhance the strain carried by the martensite, thus promoting more retained austenite in the martensite regions enabling a TRIP effect. The enhancement of the TRIP effect and the decrease of the strain contrast between ferrite and martensite jointly optimize the micromechanical deformation compatibility of the adjacent phases, thus improving the material's ductility. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Recent studies on hydrogen enhancement of the strain-induced generation of vacancies, i.e. the HESIV mechanism, and its role in hydrogen embrittlement (HE) of steels are overviewed. A high density of vacancies and their clustering have been detected by newly developed low temperature hydrogen thermal desorption spectroscopy and positron lifetime measurements. The promotion of crack nucleation and growth is ascribed to the evolution of damage associated with intense strain localization, and characteristic features of HE such as dependence on the microstructures of steels and test conditions are consistent with the HESIV mechanism. The predominant and supplementary roles of vacancies and hydrogen, respectively, in premature fracture are demonstrated by experiments devised to separate each contribution. A constitutive relation for porous materials is applicable to describe mechanistic effects of the HESIV mechanism on crack growth resistance and strain localization. The sequential void nucleation, growth and link mechanism in nanoscale are suggested. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract The creation of structural defects in low-mismatched GaAs/GaAs 0.92 Sb 0.08 /GaAs(001) heterostructures and their evolution during strain relaxation have been studied using transmission electron microscopy as well as high-resolution x-ray diffraction and atomic force microscopy. These GaAsSb films had thicknesses in the range of 50–4000 nm with 50-nm-thick capping layers and were grown using molecular beam epitaxy. The strain relaxation had three distinct phases as the film thickness was increased, whereas the thin GaAs capping layers exhibited only the initial sluggish stage of relaxation in heterostructures with thick GaAsSb films. The character of the misfit dislocations at the two interfaces was determined using g.b analysis, and atomic-scale structural information was obtained using aberration-corrected electron microscopy. Stage-I relaxation took place primarily by glide of dissociated 60° dislocations. Although the films were mostly free of threading dislocations, many curved dislocations extended into the substrate side for heterostructures that had undergone Stage-II and Stage-III relaxation. Investigation of dislocation density evolution at the cap/film interface and morphological evolution of the growth surface revealed a strong correlation. The smoother growth surface in the heterostructure with 4000-nm-thick film resulted in a reduced areal density of surface troughs that acted as nucleation sites for dislocations, which explained the decreased dislocation density at the cap/film interface. Overall, these results prove that heterogeneously nucleated surface half-loops are the primary source of threading dislocations in low-mismatched heterostructures. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract A coherent core-shell structure in 0.24BiFeO 3 -0.56K 0.5 Bi 0.5 TiO 3 -0.20PbTiO 3 ceramics, with T m around 385 °C, was revealed using a combination of characterisation methods including scanning and transmission electron microscopy. The core regions consist of a cubic matrix with minor monoclinic phase with limited coherence length, while the shell regions are a tetragonal phase with characteristic long-range ordered ferroelectric domain structures. The electric field-induced strains for both core and shell regions were investigated by conducting an in-situ poling study using high energy synchrotron X-ray diffraction (XRD). A methodology is developed to evaluate both intrinsic (lattice strain) and extrinsic (domain switching) contributions for each phase based on diffraction peak profile fitting for multiple reflections. It is shown that there were no significant variations in the cubic and tetragonal phase fractions under an electric field in the range from −6 to +6 kV mm−1. An irreversible strain ∼0.04% was attributed mainly to ferroelectric domain switching in the tetragonal shell regions, while a reversible lattice strain ∼0.12% was identified in the pseudo-cubic core, leading to significant levels of intra-granular stress. Strain anisotropy, dependent on the crystallographic orientation of different grain families, was also evident. Macroscopic strain measurements, conducted using a novel digital image correlation (DIC) method, demonstrated the formation of typical 'butterfly'-type strain-electric field loops and were consistent with the total strain values calculated from the XRD results. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Interfaces play a crucial role in mechanical behaviors of laminated materials. In this study, a series of hard/soft nanotwinned Cu laminates with varying interface spacing from 200 to 33 μm are prepared by means of direct current electrodeposition. Simultaneous improvement of strength and work hardening with decreasing interface spacing is found in tensile tests. Extra strengthening and geometrically necessary dislocations (GNDs), but without any strain concentration, are found in the vicinity of interfaces. An obvious strain discrepancy appears across the interface and decreases with decreasing interface spacing. Most importantly, a highest plastic strain gradient appearing in the vicinity of interfaces, regardless of interface spacing, indicates a significant deformation compatibility across the interfaces. The spatially-distributed strengthening mediated by interface contributes for increasing strength and decreasing strain discrepancy by decreasing the interface spacing. [Display omitted] [ABSTRACT FROM AUTHOR]

The superior strength and toughness balance prevails in the face-centered-cubic-structured high/medium-entropy alloys (H/MEAs) of low strength, while a gain in yield strength is normally accompanied by a sacrifice in toughness, leading to the strength and toughness trade-off particularly when yield strength increases to the gigapascal levels. Here, we showed the superior fracture toughness by heterostructuring an N-doped CrCoNi MEA having different levels of yield strength higher than 1 GPa. The fracture toughness was 91 MPa · m 1 / 2 at yield strength of 1.3 GPa in the heterogeneous lamella structure and 168 MPa · m 1 / 2 at yield strength of 1.0 GPa in the heterogeneous grain structure. The fracture toughness was attributed to forest hardening plus an extra hetero-deformation induced hardening. The pile-ups of geometrically necessary dislocations were observed to be formed at the domain boundaries to accommodate strain gradient at the plastic zone of the crack tip. The chemical short-range orders were found for the enhanced strain hardening near the crack tip by the interaction with dislocations. Moreover, a new parameter was proposed to characterize the work hardening capacity at the crack tip by the integral of hardness increment in the plastic zone which shows a linear relation-ship with the J -integral value during the crack initiation. [Display omitted] [ABSTRACT FROM AUTHOR]

An ultrafine grained Al-0.3 wt %Cu has been produced by cold rolling to a thickness reduction of 98% (ε vM  = 4.5). The deformed structure is a typical lamellar structure with a boundary spacing of 200 nm as characterized by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Coarsening of the deformed structure to recrystallization is achieved by heat treatment in the range of 100–300 °C. Good thermal stability has been observed up to 175 °C with some segregation of Cu to the boundaries as observed by 3D atom probe characterization. Tensile tests have shown a flow stress (0.2% offset) of 198 MPa with continuous flow with no yield drop and Lüders elongation. To quantify the contribution of boundary strengthening to the flow stress, dislocation strengthening and solid solution hardening have been calculated and subtracted from the flow stress. It has been found that boundary strengthening can be expressed by a Hall-Petch relationship and that the constants in this equation are in very good agreement with previous observation of recrystallized pure polycrystalline aluminium with a grain size in the tens of micrometer range. Thereby the Hall-Petch relationship of aluminium can be extended an order of magnitude from the micrometer to the sub-micrometer range, which is of both scientific and technical importance. [ABSTRACT FROM AUTHOR]

The twinning behavior and deformation mechanisms of a pure magnesium (Mg) single crystal were investigated via Erichsen test at room temperature (RT). In order to establish the unique twinning behaviors according to the position of a deformed specimen, microtexture analyses were performed on two cross-sections of a deformed specimen via the electron backscatter diffraction (EBSD) technique. The EBSD results revealed that thin twin bands with different types of twin variants were developed throughout the deformed specimen. The crystal plasticity finite element method (CPFEM), in relation to both the crystallographic slip and deformation twinning, was used to explain the heterogeneous evolution of the twin bands throughout the deformed specimen during the Erichsen test at RT. CPFEM results such as strain components, relative activity of deformation modes, and accumulated volume fraction of twin variants can effectively explain the experimentally observed heterogeneity of twin bands. [ABSTRACT FROM AUTHOR]

At elevated temperatures, heavy liquid metals and their alloys are known to create a highly corrosive environment that causes irreversible degradation of most iron-based materials. It has been found that an appropriate concentration of oxygen in the liquid alloy can significantly reduce this issue by creating a passivating oxide scale that controls diffusion, especially if Al is present in Fe-based materials (by Al-oxide formation). However, the increase of the temperature and of oxygen content in liquid phase leads to the increase of oxygen diffusion into bulk, and to promotion of the internal Al oxidation. This can cause a strain in bulk near the oxide layer, due either to mismatch between the thermal expansion coefficients of the oxides and bulk material, or to misfit of the crystal lattices (bulk vs. oxides). This work investigates the strain induced into proximal bulk of a Fe-Cr-Al alloy by oxide layers formation in liquid lead-bismuth eutectic utilizing synchrotron X-ray Laue microdiffraction. It is found that internal oxidation is the most likely cause for the strain in the metal rather than thermal expansion mismatch as a two-layer problem. [ABSTRACT FROM AUTHOR]

We investigated and thoroughly analyzed compressive response of ice-templated ceramics in quasistatic and dynamic regimes of strain rate. In the high pore volume regime, sintered scaffolds exhibited highly lamellar pore morphology and pore architecture transitioned to dendritic with the decreasing pore volume. Mechanical property measurements in the quasistatic regime of strain rate revealed that with increasing density, compressive response transitioned from a damageable, cellular-like failure to a brittle-like failure. We rationalized the measured results in terms of the propensity of the lamella walls to undergo buckling. Our conjecture is that in the high pore volume regime, lamella walls of the scaffolds are prone to buckling-induced elastic instability, which leads to a compressive response that manifests a gradual decrease of stress beyond peak stress. In contrast, we suggest that in the low pore volume regime thick lamella walls and extensive transverse bridging exhibit marked resistance to buckling-induced instability and scaffolds undergo a global failure, which is manifested in the measured quasistatic compressive response as a sharp drop of stress beyond peak stress. Dynamic compressive response of the scaffolds exhibited measurable differences relative to the quasistatic compressive response. Scaffolds exhibited a relatively gradual decrease of dynamic compressive stress beyond peak stress, and an overall improvement of compressive response and energy absorption capacity was measured under the dynamic loading conditions. We rationalized the measured differences in between the strain rate regimes in terms of the micro-inertia and other effects. This study is of critical significance for applications of ice-templated structures in dynamic environments. [ABSTRACT FROM AUTHOR]

The CrCoNi-based medium and high entropy alloys (MHEAs) have drawn much attention due to their exceptional mechanical properties at cryogenic temperatures. The twinning critical resolved shear stress (CRSS) is a fundamental parameter for evaluating the strength-ductility properties of MHEAs. Here we construct and apply an extended twinning nucleation Peierls-Nabarro (P-N) model to predict the twinning CRSSes of face-centered cubic (FCC) CrCoNi-based MHEAs. The order of the twinning CRSSes of the selected alloys is CrCoNi > CrCoNiMn > CrCoNiFe > CrCoNiFeMn and the values are 291, 277, 274 and 236 MPa, respectively. These theoretical predictions agree very well with the experimental twinning CRSSes of CrCoNi and CrCoNiFeMn accounting for 260 ± 30 and 235 ± 10  MPa, respectively and are perfectly consistent with the strength-ductility properties including yield stress, ultimate tensile stress and uniform elongation for fracture of the FCC CrCoNi-based MHEAs obtained at cryogenic temperatures. The present method offers a first-principle quantum-mechanical tool for optimizing and designing new MHEAs with exceptional mechanical properties. [ABSTRACT FROM AUTHOR]

To meet the 30 years (∼263,000 h) design lifetime of a typical thermal power plant, understanding and modeling of the long-term creep behaviors of the power plant structural steels are critical to prevent premature structural failure due to creep. The extremely long exposure to high operation temperature results in evolving microstructure during service, which cannot be observed in standard short-term (<1 year) creep tests. In fact, creep rupture life predictions based on short-term creep testing data often overestimate the creep rupture times for long period of time, and are therefore insufficient to provide a reliable failure time prediction. In this article, a microstructure-sensitive, long-term creep model is developed and validated against existing long-term creep experimental data (∼80,000 h) for ferritic steel Grade 91 (Fe-8.7Cr-0.9Mo-0.22V-0.072Vb-0.28Ni in wt.%). The mechanistic creep model is based on fundamental dislocation creep mechanisms – the particle bypass model based on dislocation climb from Arzt and Rösler [1] - that describe the steady state creep strain rate as a function of stress, temperature, and microstructure. In particular, the model incorporates the evolution of the particle size via coarsening into the dislocation climb theory, dislocation detachment mechanism and back-stress generated by subgrain dislocation structures. Model inputs were obtained based on the microstructure information obtained from published literature from the National Institute of Material Science (NIMS, Japan) creep database. The model, which can be used on many types of alloys, shows excellent agreement with existing long-term creep experimental data (∼80,000 h) of Grade 91 ferritic steel. [ABSTRACT FROM AUTHOR]

In order to rationalize the homologous nature of the Peierls stress reported in a previous paper (Y. Kamimura et al., Acta Mater. 61 (2013) 294–309), we have calculated numerically the Peierls stresses via the Peierls-Nabarro model (P-N model) of the planar core dislocation, using calculated ab-initio γ -surface for a variety of crystals: fcc metals, bcc metals, NaCl type crystals, CsCl type ionic crystals and various tetrahedrally coordinated crystals. The obtained γ- surfaces for the same group of crystals are found to have similar shape and height when normalized by materials parameters, indicating homologous nature. Except for some very soft crystals ( τ P exp / G ≪ 1 × 10 −4 ), the calculated Peierls stresses for majority of crystals ( τ P exp / G > 1 × 10 −4 ) are shown, for the first time, to have a correlation with experimental ones reflecting the homologous nature of the γ -surface, although the former are generally larger than the latter by half an order on the average, with an exception of bcc metal in which the strain field of the controlling screw dislocation is known to be non-planar. For very soft crystals, the calculated Peierls stresses are orders of magnitude larger than experimental ones; possible origins of this large discrepancy are discussed. [ABSTRACT FROM AUTHOR]

Microstructures and mechanical properties of Al-Ni-Co-Fe-Cr high-entropy alloys (HEAs) were investigated by systematically varying transition metals instead of Al, within the chemical formula of Al 2 M 14 (M represents different mutations of transition metals). The formation of different crystal structures (FCC, BCC, or FCC+BCC mixture) and its effects on the resulting mechanical properties of this series of HEAs, both in tension and compression, were evaluated. It was found that, in the BCC-dominated HEAs, ordered B2 precipitates were always coherently dispersed in the BCC solid-solution matrix. The shape of these B2 precipitates was strongly affected by the lattice misfit between the disordered BCC and ordered B2. A uniform distribution of cuboidal B2 particles could be obtained by properly adjusting M, thus the lattice misfit, in a manner similar to that in Ni-based superalloys . Strengthening effects caused by different BCC/B2 morphologies were also estimated and compared with experimental measurements. The optimal strengthening as a function of the shape and size of the coherent precipitates was discussed in light of the lattice misfit in these HEAs. [ABSTRACT FROM AUTHOR]

Allotropy is a fundamental concept that has been frequently studied since the mid-1800s. Although the bulk allotropy of elemental solids is fairly well understood, it remains challenging to reliably produce an allotrope at the nanoscale that has a different crystal structure and accompanies a change in physical properties for specific applications. Here, we demonstrate a "heterostructure" approach to produce allotrope-like bismuth nanowires, where it utilizes the lattice constant difference between bismuth and tellurium in core/shell structure. We find that the resultant strain of [100]-grown Bi nanowires increases the atomic linear density along the c-axis that has been predicted from theoretical considerations, enabling us to establish a design rule for strain-induced allotropic transformation. With our >400-nm-diameter nanowires, we measure a thermoelectric figure of merit ZT of 0.5 at room temperature with reduced thermal conductivity and enhanced Seebeck coefficient, which are primarily a result of the rough interface and the reduced band overlap according to our density-functional calculations. [ABSTRACT FROM AUTHOR]

In order to investigate their microcracking behaviour, the microstructures of several β-eucryptite ceramics, obtained from glass precursor and cerammed to yield different grain sizes and microcrack densities, were characterized by laboratory and synchrotron x-ray refraction and tomography. Results were compared with those obtained from scanning electron microscopy (SEM). In SEM images, the characterized materials appeared fully dense but computed tomography showed the presence of pore clusters. Uniaxial tensile testing was performed on specimens while strain maps were recorded and analyzed by Digital Image Correlation (DIC). X-ray refraction techniques were applied on specimens before and after tensile testing to measure the amount of the internal specific surface ( i.e. , area per unit volume). X-ray refraction revealed that (a) the small grain size (SGS) material contained a large specific surface, originating from the grain boundaries and the interfaces of TiO 2 precipitates; (b) the medium (MGS) and large grain size (LGS) materials possessed higher amounts of specific surface compared to SGS material due to microcracks, which decreased after tensile loading; (c) the precursor glass had negligible internal surface. The unexpected decrease in the internal surface of MGS and LGS after tensile testing is explained by the presence of compressive regions in the DIC strain maps and further by theoretical arguments. It is suggested that while some microcracks merge via propagation, more close mechanically, thereby explaining the observed X-ray refraction results. The mechanisms proposed would allow the development of a strain hardening route in ceramics. [ABSTRACT FROM AUTHOR]

Strain accumulation was studied by digital image correlation technique (DIC) during microstructurally small fatigue crack propagation in polycrystalline 18%Cr ferritic stainless steel. Load-controlled fatigue testing was performed with R-ratio of 0.1 and frequency 10 Hz. The maximum applied stress was well below the yield stress of the studied material. The effect of the observed strain field on crack growth rate variation is discussed. Fracture surfaces were studied by scanning electron microscopy (SEM) evidencing the connection between the mechanism of the fatigue crack growth, accumulated strain and crack growth rate. Detailed study of fracture surface morphology was carried out by atomic force microscopy (AFM). Results indicate two processes of material damage accumulation and failure during cyclic loading: 1) local shear strain zones form successively ahead of the crack tip, and 2) fatigue crack growth occurs by both single- and multiple-slip mechanisms. The place and intensity of shear strain localization zones vary during the crack growth that is related closely to the local variation of crack growth rate. [ABSTRACT FROM AUTHOR]

The full three-dimensional stress state of 172 individual bulk grains in austenitic stainless steel 316 L at 0.1 and 1% sample elongation has been determined with sufficient accuracy to allow comparison with the theoretical Bishop-Hill stress states for plastically deforming grains as well as calculation of the resolved shear stresses on the individual slip systems. At 0.1%, the resolved shear stresses exhibit quite large variations between grains of similar orientation. When averaging over similarly oriented grains, the resolved shear stresses correspond to the Schmid factors for uniaxial tension. At 1%, only about half of the grains were close to a Bishop-Hill stress state. The stress state of the other half of the grains was closer to the applied uniaxial stress, in between Bishop-Hill states, or in some cases none of these. The orientation dependence of the assigned stress states deviate somewhat from the theoretical expectation. These deviations are found to originate from a larger tensile stress component than in the theoretical Bishop-Hill stress states and to be associated also with deviations from axisymmetric plastic strain. This conclusion was supported by finite-element crystal plasticity simulations. [ABSTRACT FROM AUTHOR]

In this work, a data-driven reduced-order model is presented to predict the microscale spatial distribution of the plastic strain rate tensor in an isotropic two-phase composite subjected to an arbitrary macroscopically imposed strain rate tensor. This model was built using the framework of localization linkages called Material Knowledge Systems (MKS), which has been demonstrated to exhibit a remarkable combination of accuracy and low computational cost. In prior work, the MKS framework was successfully used to predict the local strain rate fields in multiphase composites subjected to a selected macroscale strain rate tensor. In this work, the MKS framework is extended to include the complete set of all macroscale strain rate tensors that could be applied. This is accomplished by developing novel representations that allow a parametrization of the localization kernel over the complete space of unit symmetric traceless second-rank tensors and implementing them with the required fast computational strategies. The MKS localization linkage produced in this work was calibrated and validated to results from microscale finite element models. [ABSTRACT FROM AUTHOR]

We have investigated the dynamic (or high strain rate) response of nanostructured face-centered-cubic alloys with different stacking fault energies after severe plastic deformation under uniaxial compression. It is found that adiabatic shear bands were easier to appear in those alloys with lower stacking fault energy, even though under quasi-static loading more obvious hardening behavior was observed with decreasing stacking fault energy. To understand the formation mechanism of adiabatic shear bands, firstly thermal softening during dynamic loading was considered. Dynamic tests within a wide range of temperature and interrupted loading revealed that adiabatic temperature rise prior to plastic instability has only minor effect on the onset of shear localization. Unlike medium/high stacking fault energy materials, strong textures have always been reported after severe plastic deformation in the metals with low stacking fault energy. Hence, influence of texture on the propensity to adiabatic shear bands was taken into account. The dynamic anisotropic behaviors show that the pre-existing texture formed during severe plastic deformation is responsible for the initiation of localized deformation, and the adiabatic temperature rise in the localized region promotes the formation of adiabatic shear bands. To verify the effect of texture on the formation of adiabatic shear bands we have incorporated preferred orientation of grains into finite element modeling based on crystal plasticity. An elastic-viscoplastic continuum slip constitutive relation was adopted to describe the mechanical response of materials, in which the dependence of slip systems' resistance on the temperature evolution was also considered. The simulation results are in good accordance with the experimental results. As such a more comprehensive picture of the formation of adiabatic shear bands in such materials has been uncovered along with the underlying mechanism. [ABSTRACT FROM AUTHOR]

Martensitic and intermartensitic transformation induced by changing temperature and/or applying mechanical stress was investigated systematically in Ni 50 Mn 30 Ga 20 and Ni 46 Mn 24 Ga 22 Co 4 Cu 4 polycrystalline shape memory microwire. In particular, intermartensitic transformation induced by stress can be obtained in single crystal but difficult in polycrystalline materials that are usually intrinsically brittle due to the weak grain boundary cohesion. In this paper, two kinds of oligocrystalline microwires, containing free surface and bamboo-like grains, with austenite and martensite at room temperature exhibit multistep superelastic behavior with larger than 10% fully reversible strain from intermartensitic transformation. The two alloys undergo four kinds of phase transition sequence under the condition of stress-temperature coupling, and the later has smaller stress hysteresis and exhibits excellent high temperature mechanical properties during loading-unloading loops. Meanwhile, the transformation critical stress as a function of temperature phase diagram for the two kinds of alloys has been established. Utilizing the elements of Co and Cu replacing Ni and Mn in Ni 50 Mn 30 Ga 20 alloys, phase transformation temperature is broadened and increased. For Ni-Mn-Ga microwire, continuous martensitic transformation process from 228 to 298 K is investigated by in-situ TEM observation, and the structure is confirmed to be five-layered modulated martensite at low temperature. Multiple martensites coexistence, including five-layered and seven layered modulated martensite, is explored in Ni-Mn-Ga-Co-Cu microwire at room temperature. Those multistep superelastic behavior and mechanical properties can be comparable but different from single crystal. This is an effective method to obtain small size, high performance shape memory alloys, which are of great significance for device miniaturization and intelligence. [ABSTRACT FROM AUTHOR]

Although presenting attractive features in dealing with small-scale size effects, strain gradient plasticity (SGP) theories can lead to uncommon phenomena for some boundary value problems. Almost all non-incremental (Gurtin-type) SGP theories including thermodynamically-consistent higher-order dissipation predict elastic gaps under certain non-proportional loading conditions. An elastic gap is defined as a finite change in the equivalent yield stress after an infinitesimal change in the strain conditions, at the occurrence of the non-proportional loading source. The existence of such gaps in reality is largely questioned and represents a major source of uncertainty preventing the development of robust SGP theories for real small-scale applications. Using 3D discrete dislocation dynamics (3D-DDD), the present paper aims at investigating size effects within micron-scale single crystal structures under various non-proportional loading conditions, including tension–compression–passivation, bending–passivation and tension–bending. An in-depth investigation of the occurrence of elastic gaps under these conditions, which are known to entail such gaps when using classical non-incremental SGP theories, is conducted. The obtained 3D-DDD results reproduce well known experimentally confirmed size effects like Hall–Petch effect, Asaro's type III kinematic hardening and reversible plasticity. However, no evidence of the phenomenon of elastic gaps is found, which constitutes a first indication that this phenomenon may not exist in reality. The simulations are performed on face-centered cubic (FCC) Nickel single grains with cuboid shapes ranging from 2 μ m to 15 μ m and different orientations. [Display omitted] [ABSTRACT FROM AUTHOR]

While the three-dimensional (3D) shape and stress field of a twin in hexagonal close-packed (HCP) metals have attracted considerable interest in recent years due to their substantial impact on internal stress and mechanical properties, a detailed understanding of their variation with twin size is still lacking. An analytical model that is not restricted by spatial scale is developed in this work by considering the effects of anisotropic twin boundary energy, elastic strain energy and plastic relaxation to predict the 3D shape with the minimum energy, and the stress field, of an isolated ellipsoidal twin of different sizes. The model is applied to Mg with a focus on the { 10 1 ¯ 2 } twin type. The analytical calculation results show that the nucleation of the nano-sized twin embryos is facilitated by the stress field near structural defects such as dislocations. During the expansion of this nano-sized twin embryo, the interplay between the elastic strain energy and interfacial energy changes the length of the twin along the twin shear (forward) direction from being shorter to longer than that along the lateral direction. In contrast to the current understandings, the maximum shear stress on the twin plane along the twin shear direction occurs at the lateral, rather than the forward, side of the twin. At the forward side, the maximum shear stress occurs at a distance ahead of the twin tip and this distance increases with increasing twin thickness. [Display omitted] [ABSTRACT FROM AUTHOR]

Monocrystalline microwires of silver are produced by means of a novel casting process that combines precise mold production by silicon microfabrication with molten metal pressure infiltration. The wires have a shape amenable to tensile testing and a diameter from a few to several tens of micrometer. Displacement-controlled in-situ tensile tests are conducted at room-temperature, 200 °C and 400 °C. Data are compared with bulk counterparts of the same silver cast to have a diameter near 1 mm. Results show clear evidence of small-scale plasticity. The influence of size, crystal orientation, and temperature on the deformation, yield, strain burst statistics and strain hardening of these micrometric dense metal samples is analyzed. Yield stress values agree with the single arm source model provided that the density of forest dislocations comprises a scale-dependent component likely resulting from thermal mismatch stresses. Statistics of strain burst amplitudes agree with an exponentially truncated power-law complementary cumulative distribution. The power-law exponent agrees with predictions of mean-field theory, while the cutoff intensity exhibits no visible dependence on sample diameter or test temperature and agrees with predictions of current theory. [Display omitted] [ABSTRACT FROM AUTHOR]

In this work, 99.999% pure magnesium (Mg) single microcrystals have been deformed in [0001] compression (c -axis compression) and [10 1 ¯ 0] tension (c -axis contraction) conditions at room temperature and under loading rates ranging from 5 × 10−4 up to ∼590 s−1. The strain rate sensitivity and apparent activation volume of prismatic and pyramidal slip systems were evaluated. In c -axis contraction, at strain rates of 45 s−1, the formation of a new grain whose crystallographic characteristics do not correspond to those of well-known twin systems could be observed. An explanation was found but it requires the breakdown of the invariant plane strain condition, and a unit cell reconstruction via pyramidal II to basal plane transformation. This unconventional twin is at 2.1° far from a classical simple shear twin on {10 1 ¯ 5} planes. In c -axis compression, at the highest applied strain rate, no twin could be detected in the 5 μm sized pillars of 2:1 (height to width) aspect ratio. Plasticity is thus purely mediated by slip. However, the appearance of newly oriented grains was observed by lowering the sample size or by reducing the aspect ratio. Their crystallographic features suggest a mechanism of unit cell reconstruction through the transformation from pyramidal I to basal plane. The results presented in this study impose to consider twinning as a reorientation mechanism not necessarily limited to a simple shear. [Display omitted] [ABSTRACT FROM AUTHOR]

Compared to stress controlled high-cycle fatigue, enhancing strain controlled low-cycle fatigue (LCF) properties of material is much more difficult and less reported. In this study, we introduced two strategies and technologies to improve the LCF performance of Fe-Mn-C twinning induced plasticity (TWIP) steel. One is grain refining without introducing residual stress by traditional cold rolling and following recrystallization annealing (FG sample); the other is to introduce a linear gradient in grain size into TWIP steel by the original processing technology (GS sample). It is found that GS samples exhibit a higher cyclic hardening ability and cyclic saturation stress than that of as-received coarse grain (CG) samples and FG samples, which invalids the rule of mixture. Based on the dependence of the fatigue life ( N f ) on the total strain amplitude (Δε/2), GS shows the longest life at high strain amplitudes, while FG shows the longest life at low strain amplitudes. Judging from the aspect of stress (Δσ/2- N f curve), i.e. the Basquin curve, GS exhibits a better LCF performance than both FG and CG. The excellent fatigue properties of GS are believed to originate from the large generation of geometrically necessary dislocations (GNDs) and the formation of hard core and soft shell structure during cyclic loading. The significant influences of grain size and distribution on fatigue damage mechanisms may provide new and important implications for the optimized microstructural design of the high fatigue performance material. [ABSTRACT FROM AUTHOR]

Kink band formation is a common deformation mode for anisotropic materials and has been observed in polymer matrix fiber composites, single crystals, geological formations, and recently in metallic nanolaminates. While numerous studies have been devoted to kink band formation, the majority do not consider the often rapid and unstable process of kink band propagation. Here we take advantage of stable kink band formation in Cu-Nb nanolaminates to quantitatively map the local strain fields surrounding a propagating kink band during uniaxial compression. Kink bands are observed to initiate at specimen edges, propagate across the sample during a rising global stress, and induce extended strain fields in the non-kinked material surrounding the propagating kink band. It is proposed that these stress/strain fields significantly contribute to the total energy dissipated during kinking and, analogous to crack tip stress/strain fields, influence the direction of kink propagation and therefore the kink band inclination angle. [ABSTRACT FROM AUTHOR]

As the length scale of individual layers were reduced in a Cu/Nb multilayer, a face centered cubic (fcc)-to-body centered cubic (bcc) and bcc-to-fcc transformations were noted for Cu and Nb respectively. In addition, at equal fractions of Cu/Nb and at a very thin thickness, the layers vitrified. These phase transformations have been modeled using a thermodynamic phase diagram where strain, interfacial, and volumetric energy considerations were used to determine the phase as a function of the bilayer thickness and volume fraction in the multilayer. Using HRTEM, the evolution of the interface from semicoherent-to-coherent was observed as a function of phase and bilayer spacing and is discussed in terms of the interfacial energy contributions towards phase stability. The vitrification has been rationalized by the positive enthalpy of mixing with clustering quantified by atom probe tomography between these two species. All of these phase transformations have been related to real-time, intrinsic growth stress measurements. Upon phase transforming, a reduction in the compressive stress within all of the multilayers were noted. This relaxation was captured in a molecular dynamics simulation of the multilayers and is contributed to the change in interfacial stress and stress distributions within the Cu layer. This demonstrates that phase transformations provide a means to tailor film stress in nanolaminate materials. [ABSTRACT FROM AUTHOR]

In this work, a bulk gum metal (GM) was fabricated via arc melting from high purity powders. The ingots were first extruded using a conventional route followed by equal channel angular pressing (ECAP). The mechanical behavior of the extruded GM and ECAP-processed GM was studied under both quasi-static and high strain rate compression conditions to evaluate the influence of strain rate. In addition, the associated mechanical anisotropy, or the lack thereof, was investigated through loading in different orientations with respect to the extrusion or ECAP direction. Precipitous stress drops were observed under dynamic compression of both extruded and ECAP-processed GM specimens when loading perpendicular to the extrusion direction. Adiabatic shear banding (ASB) was found to be associated with the precipitous stress drops on the dynamic stress-strain curves. The details of the ASBs were characterized by optical and scanning electron microscopy, with emphasis on electron backscattered diffraction (EBSD). The mechanisms responsible for the formation of ASB were examined both from thermal softening and geometrical softening perspectives. Significant microstructure refinement within ASBs was established, and a possible grain refinement mechanism was proposed. [ABSTRACT FROM AUTHOR]

Brown's bowing and passing model for persistent slip bands (PSBs) was extended in Cu and Ni to the whole range of temperatures in which a saturation plateau is observed. Advantage was taken of the similitude relation to rewrite in dimensionless form the equations of the original model and of a more accurate revisited version. Input quantities for computing the solutions were taken from a previous study of experimental results; all unknown quantities could then be directly calculated without any assumption or approximation. The comparison between experimental results and the predictions of the revisited model confirms the basic assumptions of the bowing and passing model, according to which the thermally activated annihilation of screw dipoles is governing the channel widths, the Orowan stresses and the critical stresses in the channels. All other assumptions and numerical predictions are perfectly confirmed, save for the occurrence of small resistive stresses in the channels. In addition, a better understanding of the complex behavior of PSB walls under stress is necessary in order to accurately determine the plastic strain amplitude of PSBs. Mesoscale and atomistic simulations are needed for further modeling of the wall properties and the screw dipole annihilations. [ABSTRACT FROM AUTHOR]

The β to α phase transformation of Ti alloys progresses in a displacive-diffusive mixed-mode. The associated transformation strain has important influence on the resultant microstructure. In this work, the microstructural features of α precipitates in a metastable β Ti alloy, Ti-7333, were thoroughly investigated. Special attention was paid to the intragranular α for the advantage of a stress-free transformation environment. Results show that the constituents of each α precipitate is not single. Two kinds of nano-sized α domains exist. One is situated on the broad face of the major α precipitate (named interface α), and the other goes through the major α (termed penetrating α). The interface α is related with the β matrix by the Burgers orientation relationship (BOR) and with the major α by a 60°/ 〈 11 2 ¯ 0 〉 α rotation. The nucleation of such α particles is induced by the largest shear strain generated by the formation of the major α. They act as stress-associated sympathetic nuclei of the neighboring α precipitates and eventually contribute to the formation of the triangular α cluster. The penetrating α does not obey the BOR with the β matrix but is related with the major α by a 60° rotation around another 〈 11 2 ¯ 0 〉 α axis. The nucleation of such α is induced by the largest normal strain generated by the formation of the main α plate. This work provides comprehensive information on the displacive characters of the β to α transformation and their impact on the microstructure of metastable β Ti alloys. [ABSTRACT FROM AUTHOR]

Irradiation-induced defects have been experimentally observed either to inhibit or promote strain bursts and lead to plastic flow localization. Through three dimensional discrete dislocation dynamics simulations, this work unravels the mystery of how and why irradiation-induced defects enhance or inhibit strain bursts in submicron BCC iron and FCC Cu single crystals. It is shown that smaller strain burst amplitudes in irradiated nano- and micro-pillars are obtained under stress control conditions. However, under strain control conditions, bursts are found not to be sensitive to irradiation, despite the arresting effect of radiation defects. This feature is a result of rapid stress relaxation truncating the strain burst, compared with the influence of irradiation-induced defects. In heavily irradiated materials, dislocation avalanches and strain bursts are shown to be promoted. The influence of dislocation-defect interaction mechanisms, cross slip, irradiation dose, as well as loading mode on strain bursts is systematically investigated, and the results compared with experimental observations. [ABSTRACT FROM AUTHOR]

We experimentally studied sequential { 10 1 ¯ 2 } twinning in rolled pure titanium at room temperature and identified a new sequential twinning mechanism, i.e., { 10 1 ¯ 2 } ( T i I ) extension twin stimulated by the twin-twin junction of two primary {11 2 ¯ 1 } extension twins T i − 1 II and T i II , which holds a certain relation T i − 1 II T i II → T i I . Schmid factor analysis, as the local stress is taken into account, is able to determine the position of sequential twin variant, but cannot determine the twin variant. Displacement gradient accommodation is used to determine the twin variant. The well-known secondary {10 1 ¯ 2} ( T j I ) extension twins in primary {11 2 ¯ 2} ( C i I ) contraction twins (referred to as C i I → T j I double twinning) are also analyzed according to a generalized Schmid factor analysis. Displacement gradient accommodation and twin nucleation based on dislocations dissociation only work well for the most active twin variants, but cannot address other phenomena associated with C i I → T j I double twinning. For rarely activated twin variants, displacement gradient accommodation was not satisfied. [ABSTRACT FROM AUTHOR]