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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]

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]

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]

A synchrotron technique, differential aperture X-ray microscopy (DAXM), has been applied to characterize the microstructure and analyze the local mesoscale residual elastic strain fields around graphite nodules embedded in ferrite matrix grains in ductile cast iron. Compressive residual elastic strains are measured with a maximum strain of ∼6.5–8 × 10 −4 near the graphite nodules extending into the matrix about 20 μm, where the elastic strain is near zero. The experimental data are compared with a strain gradient calculated by a finite element model, and good accord has been found but with a significant overprediction of the maximum strain. This is discussed in terms of stress relaxation during cooling or during storage by plastic deformation of the nodule, the matrix or both. Relaxation by plastic deformation of the ferrite is demonstrated by the formation of low energy dislocation cell structure also quantified by the DAXM technique. [ABSTRACT FROM AUTHOR]

Abstract: This study is motivated by discrepancies between recent experimental compression test data of nanoporus gold and the scaling laws for strength and elasticity by Gibson and Ashby. We present a systematic theoretical investigation of the relationship between microstructure and macroscopic behaviour of nanoporous metals. The microstructure is modelled by four-coordinated spherical nodes interconnected by cylindrical struts. The node positions are randomly displaced from the lattice points of a diamond lattice. We report scaling laws for Young’s modulus and yield strength, which depend on the extension of nodal connections between the ligaments and the solid fraction. A comparison with the scaling laws of Gibson and Ashby revealed a significant deviation for the yield stress. The model was applied for identifying a continuum constitutive law for the solid fraction. Matching the model’s predicted macroscopic stress–strain behaviour to experimental data for the flow stress at large compression strain requires the incorporation of work hardening in the constitutive law. Furthermore, the amount of disorder of the node positions is decisive in matching the model results to the experimental observations of an anomalously low stiffness and an almost complete lack of transverse plastic strain. [Copyright &y& Elsevier]

Abstract: Using microcompression methodology, various plastic deformation characteristics, namely the strength, strain-rate sensitivity (SRS) and activation volume, of 〈100〉-single-crystalline (SC) Cu micropillars and nanostructured multicrystalline (MC) Cu micropillars with several grains were systematically investigated at different strain rates for a wide range of external sample sizes ranging from 300 to 2000nm. The results revealed that the paradigm of “smaller is stronger” for small-scaled SC pillars holds true in nanostructured MC Cu micropillars above the strongest external size; however, below this size, a decrease in the external size triggers the reverse size effect, i.e. “smaller is weaker”. The appropriate introduction of grain boundaries into SC pillars can markedly improve the smoothness of their plastic flow and significantly enhance their SRS. Additionally, we demonstrated that the SRS of MC Cu pillars increases with decreasing size up to a certain limit and then decreases with a further decrease in the external sample size; thus, a maximum SRS emerges. A phenomenological model was proposed to rationalize the scaling behavior of the activation volume with the external sample size and to highlight the effect of the internal feature size on the rate-limiting behavior of conventional bulk nanostructured metals (Cu). [Copyright &y& Elsevier]

Abstract: Micron-scale single-crystalline materials deform plastically via large intermittent strain bursts that make the deformation process non-deterministic. Here we investigate this stochastic phenomenon by analyzing the plastic response of an ensemble of specimens differing only in the initial arrangement of dislocations. We apply discrete dislocation dynamics simulations and microcompression tests on identically fabricated Cu single-crystalline micropillars. We find that a characteristic yield stress can be defined in the average sense for a given specimen ensemble, where the average and the variance of the plastic strain start to increase considerably. In addition, in all studied cases the stress values at a given strain follow a Weibull distribution with similar Weibull exponents, which suggests that dislocation-mediated plastic yielding is characterized by an underlying weakest-link phenomenon. These results are found not to depend on fine details of the actual set-up; rather, they represent general features of micron-scale plasticity. [Copyright &y& Elsevier]

Abstract: Deformation behavior in a Fe–31.2 Pd (at.%) single crystal, which exhibits a second order-like face-centered cubic (fcc)–face-centered tetragonal martensitic transformation at 230K, is investigated by compressive tests in the [001]P direction (subscript P represents the parent phase). As a result, a large reversible strain of 6.5% with quite small stress hysteresis is obtained at 240K under a stress of 250MPa. The large elastic-like strain is essentially attributed to continuous change in the lattice parameter of a single martensite variant under stress. The yield point appears at 285MPa, at which the strain is 7.3%, and the residual strain does not recover by heating to 300K. The main reason for the residual strain is the occurrence of {111} mechanical twinning. The critical resolved shear stress of a Fe–31.2 Pd (at.%) single crystal is close to the ideal value. [Copyright &y& Elsevier]

Abstract: We performed equal channel angular pressing (ECAP) on bulk metallic glasses with the composition of (La0.5Ce0.5)65Co25Al10 and Zr52.5Cu17.9Ni14.6Al10Ti5. The analysis of the high energy X-ray diffraction patterns indicates that anisotropic strain is present in the processed samples. We also determined the nature and magnitude of strain in the sample after thermo-mechanical creep, i.e. after homogenous deformation. Comparison between diffraction patterns indicates that the strain anisotropy in the ECAP processed samples is due to an elastic strain and is not induced by bond anisotropy as in creep samples. We conclude that the apparent elastic strain is brought on by a residual stress originating from the shear bands and that the deformation in ECAP experiment is inherently inhomogeneous. [Copyright &y& Elsevier]

Abstract: The two-parameter model for strain hardening has proven to be a powerful tool in explaining the main features of dislocation storage during plastic deformation. The well-established but empirical Taylor equation relates dislocation density to yield stress. The free path for dislocations relates strain to the increase in density and appears as an empirical constant. The goal of the present paper is to address the Taylor equation from a probabilistic framework; future work will explore the calculation of the free path length. The distribution of obstacles in the slip plane will be modelled as a plane Poisson process and the relationship between its Delaunay triangulation and dislocation storage will be established. The statistical properties of this triangulation allow determining the number of segments stored after dislocation bow-out as well as their length distribution. This naturally leads to the percolation limit for dislocation flow between impenetrable obstacles; together with a differential equation which describes the evolution of dislocation density as a function of stress, this provides a mathematical foundation for the Taylor equation in the case of forest hardening. [Copyright &y& Elsevier]

Abstract: Bulk metallic glass (BMG) matrix composites with crystalline dendrites as reinforcements exhibit a wide variance in their microstructures (and thus mechanical properties), which in turn can be attributed to the processing route employed, which affects the size and distribution of the dendrites. A critical investigation on the microstructure and tensile properties of Zr/Ti-based BMG composites of the same composition, but produced by different routes, was conducted so as to identify “structure–property” connections in these materials. This was accomplished by employing four different processing methods—arc melting, suction casting, semi-solid forging and induction melting on a water-cooled copper boat—on composites with two different dendrite volume fractions, V d. The change in processing parameters only affects microstructural length scales such as the interdendritic spacing, λ, and dendrite size, δ, whereas compositions of the matrix and dendrite are unaffected. Broadly, the composite’s properties are insensitive to the microstructural length scales when V d is high (∼75%), whereas they become process dependent for relatively lower V d (∼55%). Larger δ in arc-melted and forged specimens result in higher ductility (7–9%) and lower hardening rates, whereas smaller dendrites increase the hardening rate. A bimodal distribution of dendrites offers excellent ductility at a marginal cost of yield strength. Finer λ result in marked improvements in both ductility and yield strength, due to the confinement of shear band nucleation sites in smaller volumes of the glassy phase. Forging in the semi-solid state imparts such a microstructure. [Copyright &y& Elsevier]

Abstract: Expressions for obtaining the dislocation cell size and misorientation angle evolution as functions of strain, strain rate and temperature are presented. The basis of the theory is to express the cell formation energy as a set of dislocation partials, which is equated to the energy of the dislocation forest in the non-cellular material plus the dislocation slip energy to form cellular structures. The latter is expressed in terms of the statistical entropy for dislocation slip. The Young–Laplace equation is applied to obtain the cell misorientation angle at stages III and IV of deformation. This equation is also applied to obtain an expression for the dislocation density evolution at stage IV. The theory is applied to the deformation of Cu, Al and Ni, from low to high temperature conditions and at various strain rates, describing well the cell properties and the corresponding stress–strain curves. [Copyright &y& Elsevier]

Abstract: Neutron diffraction has been used to characterize the evolution of residual elastic strain in grains with different orientations due to room temperature plastic deformation in two plate product forms of Ti–6Al–4V. The evolution of lattice strains has been rationalized using a two-phase elastic–plastic self-consistent model using only the texture difference between the two product forms. It is found that the elastic properties of both the bulk and individual orientations can be reproduced quite satisfactorily, with a C′ modulus of the β phase of 15GPa. The residual microstrains produced are generally greater in the unidirectionally rolled material than the cross-rolled, but are smaller than in Ti-834. The residual strains accumulated in the (0002) orientation are near-zero, which can only be reproduced in the modelling by assuming a critical resolved shear stress for 〈c + a〉 slip only 1.5× that for 〈a〉 slip, compared to the 3× factor found for isolated single crystals. The implications of this for our understanding of deformation in these materials are discussed. [Copyright &y& Elsevier]

Abstract: Irreversible thermodynamics is employed as a framework to describe plastic deformation in pure metals and alloys. Expressions to describe saturation stress in single crystals and nanocrystals are employed over wide ranges of temperature, strain rate and grain size. The importance of the roles played by vacancy self-diffusion in dislocation climb and in plasticity is shown. Equations to describe the stress–strain response of single crystals and ultrafine-grained metals are derived. The activation energy for dislocation annihilation plays a central role in the mechanical response of the systems. Succinct formulations for predicting hot deformation behaviour and relaxation of industrial alloys are presented; the influence of composition in the activation energy for dislocation annihilation is shown. All formulations describing stress–strain relationships can be reduced to Kocks–Mecking classical formulation, but incorporating grain size and compositional effects. The importance of the recovery term in such formulation is established, as well as the need to obtain it employing more fundamental approaches. [Copyright &y& Elsevier]

Abstract: We develop a geometry-based model from first-principles data for the interaction of solutes with a prismatic screw dislocation core, and predict the thermally activated cross-slip stress above room temperature in Mg alloys. Electronic structure methods provide data for the change in prismatic stacking fault energy for different possible fault configurations for 29 different solutes. The direct solute–dislocation interaction energies for solutes that produce stable prismatic screw dislocation cores (K, Na, Sc and Ca) is correlated with stacking fault misfits. This geometric interaction model produces similar prediction errors for all 29 solutes. The model predicts alloys with cross-slip stresses lower than pure Mg for three previously considered solutes (K, Na and Sc) and three new solutes (Ca, Y and Zr). The model also qualitatively confirms the experimental observation that Mg–Li alloys have lower cross-slip stress than pure Mg. In particular, low concentrations of Y are predicted to significantly decrease the cross-slip stress in Mg. [Copyright &y& Elsevier]

Abstract: The deformation and fracture behaviors of a Ti-based metallic glass (MG) under a multiaxial stress state were investigated using a small punch test. It was found that controlling both the initiation and propagation of a shear crack can significantly stabilize the plastic deformation of Ti-based MG by forming multiple shear bands and delaying the shear crack from reaching the critical crack length. Radial, circumferential and spiral shear band patterns and corresponding fracture modes were observed. The relationship between the shear band pattern and the stress state was established. This finding implies that a MG could be stabilized and become ductile in nature under suitable stress conditions. [Copyright &y& Elsevier]

Abstract: Previous density functional theory (DFT) studies of the 1/3 screw dislocation in titanium have shown metastable core structures depending on the initial position of the dislocation line. We investigate this problem by modeling a screw dislocation with two initial positions using both DFT and a modified embedded atom (MEAM) potential for Ti with flexible boundary conditions. Both DFT and MEAM produce initial-position-dependent core structures. The MEAM potential stacking fault energies and core structures are in good agreement with DFT. MEAM potential computes the core energies and shows the behavior of both cores under applied strain. We found that the higher-energy core always reconstructs into the lower-energy one independent of the applied strain direction. Transformation from low- to high-energy core was not observed. Therefore, at T =0K, only the low-energy core is stable under applied strain. [Copyright &y& Elsevier]

Abstract: The high strength of micro-crystals is determined in the early flow regime, where a transition from elastic to plastic flow is obscured if compared to stress–strain data from bulk single crystals. In the present work we therefore focus on the evolution of dislocation structures in Ni micro-pillars during early deformation by employing in-situ Laue micro-diffraction. It will be shown that substantial changes in the lattice fine structure, such as multiple subgrain formation and significant rotational gradients, can be resolved prior to the onset of large strain generation. The results reveal more pronounced effects for smaller sample dimensions and also suggest that most of the evolving dislocation structure is formed prior to the occurrence of large strain bursts. A clear increase in dislocation density as a function of strain is observed, which we discuss in the context of size-dependent strain hardening and exhaustion hardening. [Copyright &y& Elsevier]

Abstract: Commercial purity (99.8%) magnesium single crystals were subjected to plane strain compression (PSC) along the c-axis at 200 and 370°C and a constant strain rate of 10−3 s−1. Extension was confined to the direction and the specimens were strained up to a logarithmic true strain of −1. The initial rapid increase in flow stress was followed by significant work softening at different stresses and comparable strains of about −0.05 related to macroscopic twinning events. The microstructure of the specimen after PSC at 200°C was characterized by a high density of and compression twins, some of which were recrystallized. After PSC at 370°C, completely recrystallized twin bands were the major feature of the observed microstructure. All new grains in these bands retained the same c-axis orientation of their compression twin hosts. The basal plane in these grains was randomly rotated around the c-axis, forming a fiber texture component. The obtained results are discussed with respect to the mechanism of recrystallization, the specific character of the boundaries between new grains and the initial matrix, and the importance of the dynamically recrystallized bands for strain accommodation in these deformed magnesium single crystals. [Copyright &y& Elsevier]

Abstract: We have performed molecular dynamics simulations and first-principles calculations to investigate the first stages of plasticity in single-crystalline silicon nanostructures free of initial defects, under compressive and tensile strain along the [001] axis. In compression especially, we observe the activation of {011} planes, both in nanowires and in thin films, regardless of the temperature and the interatomic potential used. The occurrence of such an unexpected slip system can be explained by a careful investigation of the generalized stacking fault energy under different stress conditions, and the associated restoring forces. Finally, the activation of the {011} planes is shown to be an indirect consequence of the small dimensions of the nanostructures considered. [Copyright &y& Elsevier]

Abstract: This work presents an in-depth investigation of the dispersion in the compressive yield strength of a Zr-based bulk metallic glass as the specimen dimensions decrease from the micron to the submicron length scale. While the investigated alloy belongs to one of the so-called “ductile” glass families, the degree of variation in strength exhibited in the submicron size range approaches that of conventional brittle ceramics. Using a three-parameter Weibull analysis, however, we find that the engineering reliability of the glass is independent of specimen size. The glass exhibits a size-independent failure-free stress of 1825MPa, below which yielding is not expected. The three-parameter Weibull modulus is also size independent. The dispersion in yield strengths above the failure-free stress is due to the change in the sampling volume for specimens 1μm in diameter or larger, while interactions between defects and the surface actually limit the breadth of the strength distribution observed in specimens 200–330nm in diameter relative to what may be expected for that volume. Using an instability criterion for a penny-shaped crack under shear with interfacial friction, a relationship between yield strength and the corresponding defect size is established, and the defect concentration is estimated. [Copyright &y& Elsevier]

Abstract: Thin face-centered cubic metal films on substrates often have mixed (111)/(100) fiber texture. These orientations have very different elastic constants, both in and normal to the film plane, leading to significant stress inhomogeneities. Strain transfer between the two orientations leads to complex stress/strain states, hindering interpretations of plastic deformation behaviors such as strain hardening and anelasticity. In this work, we present models to calculate the three-dimensional plastic strains from the three-dimensional elastic strains, and the dislocation densities from the three-dimensional plastic strains, within each texture component of a passivated mixed (111)/(100) fiber texture film during biaxial loading. These models are applied to elastic strain data obtained using X-ray diffraction during thermal cycling of thin Cu films. A measure of the strain transfer is obtained and the plastic behavior, both strain hardening and anelastic recovery, is shown to be very different in the two texture components. [Copyright &y& Elsevier]

Abstract: We develop a first-principles model of thermally activated cross-slip in magnesium in the presence of a random solute distribution. Electronic structure methods provide data for the interaction of solutes with prismatic dislocation cores and basal dislocation cores. Direct calculations of interaction energies are possible for solutes – K, Na and Sc – that lower the Mg prismatic stacking fault energy to improve formability. To connect to thermally activated cross-slip, we build a statistical model for the distribution of activation energies for double kink nucleation, barriers for kink migration and roughness of the energy landscape to be overcome by an athermal stress. These distributions are calculated numerically for a range of concentrations, as well as alternate approximate analytic expressions for the dilute limit. The analytic distributions provide a simplified model for the maximum cross-slip softening for a solute as a function of temperature. The direct interaction calculations predict lowered forming temperatures for Mg–0.7at.%Sc, Mg–0.4at.%K and Mg–0.6at.%Na of approximately 250°C. [Copyright &y& Elsevier]

Abstract: The subsurface microhardness mapping technique of Chaudhri was utilized to determine the shape, size and distribution of plastic strain underneath conical indenters of varying semi-apex angles, α (55°, 65° and 75°). Results show that the elastic–plastic boundary under the indenters is elliptical in nature, contradicting the expanding cavity model, and the ellipticity increases with α. The maximum plastic strain immediately under the indenter was found to decrease with increasing α. Complementary finite-element analysis was conducted to examine the ability of simulations to capture the experimental observations. A comparison of computational and experimental results indicates that the plastic strain distributions as well as the maximum strains immediately beneath the indenter do not match, suggesting that simulation of sharp indentation requires further detailed studies for complete comprehension. Representative strains, εr , evaluated as the volume-average strains within the elastic–plastic boundary, decrease with increasing α and are in agreement with those estimated by using the dimensional analysis. [Copyright &y& Elsevier]

Abstract: We analyze the macroscopic deformation of a polycrystalline solid due to local deformation events in the core of grain boundaries. The central result is an equation that decomposes the effective macroscopic strain into contributions from three deformation modes, namely: (i) the elastic strain in the bulk of the crystallites; (ii) the results of dislocation glide and climb processes; and (iii) the deformation events in the grain boundary core. The latter process is represented by jumps in the displacement vector field that can be decomposed into tangential (“slip”) and normal (“stretch”) components. The relevant measure for the grain-boundary-mediated deformation is not the displacement jump vector but a grain-boundary discontinuity tensor that depends on the displacement jump and on the orientation of the grain boundary normal. Accommodation processes at triple junctions do not contribute significantly to the macroscopic strain. By means of example, the theory is applied to the effective elastic response of nanocrystalline materials with an excess slip compliance at grain boundaries. The predictions, specifically on the size dependence of the Poisson ratio, agree with recent experiments on nanocrystalline Pd. The value of the slip compliance for grain boundaries in Pd is obtained as 18pmGPa−1. [Copyright &y& Elsevier]

Abstract: Plastic deformation of nanocrystalline Mg is studied using molecular dynamics simulation. In a -textured structure, slip and twinning behaviors are observed during tensile loading. Various twinning and slip process are identified, with basal slip and tensile twinning being dominant. For grain sizes larger than ∼30nm, basal slip occurs at a lower strain than twinning; for smaller grain sizes, twinning takes place at a lower strain than slip. For small grain sizes, the system generates partial dislocations; extended or full type dislocations are generated at high stress and large grain sizes. As the external stress increases, pyramidal 〈 c + a 〉 dislocations are also frequently generated, leading to a reduction in twinning activity. Whereas, under low stresses only the tensile twin is created at the grain boundaries, under high stress compressive twins are created at grain boundaries and in the interior of grains. [ABSTRACT FROM AUTHOR]

Abstract: The evolution of dislocation boundaries, comprising a distinct equiaxed cell block structure formed within the cube-oriented grains during tensile straining of an aluminium alloy AA5005, has been investigated in detail using transmission electron microscopy. The observed dislocation substructure displayed a hierarchical character resulting from a gradual development of larger-angle planar dense dislocation walls (DDWs), bounding roughly equiaxed cell blocks, on a background of slightly misoriented tangled cell boundaries. These walls displayed a complex, non-crystallographic character largely governed by the macroscopically imposed deformation geometry. A high degree of freedom in dislocation rearrangement, allowed by the present experimental conditions, resulted in a “random” arrangement of DDWs, in which misorientation vectors were largely scattered and misorientations across consecutive walls displayed a tendency to become more cumulative with increasing strain. The present observations appear to support the dislocation/disclination concept, describing the evolution of DDWs as nucleation, growth and immobilization of partial disclination dipoles. [ABSTRACT FROM AUTHOR]

Abstract: The functional form of the equation describing the superposition of contributions to the flow stress due to various strengthening mechanisms is analyzed. Considering that the superposition can be written as a sum to which each mechanism contributes through function f, that the dependence of the critical resolved shear stress on the density of obstacles to dislocation motion is given by function g, and that f and g have the same functional form for all strengthening mechanisms considered, it is shown that these two function are necessarily power functions, and their exponents are related. Furthermore, requiring that the superposition law is valid both at finite temperatures and at 0K leads to an equivalent expression for the strain rate sensitivity and imposes restrictions on the way in which contributions of various mechanisms to the flow stress are evaluated at finite temperatures. [ABSTRACT FROM AUTHOR]

Abstract: Micro-structural evolution and grain refinement in ANSI 304 stainless steel subjected to multiple laser shock processing (LSP) impacts were investigated by means of cross-sectional optical microscopy and transmission electron microscopy observations. The plastic strain-induced grain refinement mechanism of the face-centered cubic (fcc) materials with very low stacking fault energy was identified. The micro-structure was obviously refined due to the ultra-high plastic strain induced by multiple LSP impacts. The minimum grain size in the top surface was about 50–200nm. Multidirectional mechanical twin matrix (MT)–MT intersections led to grain subdivision at the top surface during multiple LSP impacts. Furthermore, a novel structure with submicron triangular blocks was found at the top surface subjected to three LSP impacts. The grain refinement process along the depth direction after multiple LSP impacts can be described as follows: (i) formation of planar dislocation arrays (PDAs) and stacking faults along multiple directions due to the pile up of dislocation lines; (ii) formation of submicron triangular blocks (or irregularly shaped blocks) by the intersection of MT–MT (or MT–PDA or PDA–PDA) along multiple directions; (iii) transformation of MTs into subgrain boundaries; (iv) evolution by continuous dynamic recrystallization of subgrain boundaries to refined grain boundaries. The experimental results and analyses indicate that a high strain with an ultra-high strain rate play a crucial role in the grain refinement process of fcc materials subjected to multiple LSP impacts. [ABSTRACT FROM AUTHOR]

Abstract: Aspects of the ductile vs. brittle response of nanocrystalline and ultrafine-grained (UFG) materials are theoretically examined. The focus of this study is on the combined effects of grain boundaries (GBs) and blunting of cracks on the fracture toughness of nanocrystalline and UFG materials in a typical situation where crack blunting and growth processes are controlled by dislocation emission from crack tips. Within our description, lattice dislocations emitted from cracks are stopped at GBs, resulting in blunting of cracks. Both crack blunting and the stress fields of the arrested dislocations hamper further dislocation emission from cracks in nanocrystalline and UFG materials. As a result, crack blunting is suppressed, while crack growth is enhanced. The combined effects of GBs and blunting of cracks on the dislocation emission, further crack blunting and growth processes depend on grain size and material parameters. The dependence of the maximum number of dislocations, emitted by a crack, and the critical stress intensity factor on grain size (ranging from 10 to 300nm) in Al and α-Fe is calculated. It is demonstrated that a decrease in grain size from 300 to 10nm in Al and α-Fe decreases their critical stress intensity factors by a factor of 2–3 and thereby dramatically reduces the toughness/ductility of these materials. [ABSTRACT FROM AUTHOR]

Abstract: Surface mechanical attrition treatment (SMAT), an efficient way to create nanostructured surface/subsurface layers, has been extensively exploited in the last decade. However, the impact velocity of the balls in the treatment has not yet been measured in detail. The motivation of the present paper was to investigate the ball velocity and the effect of the number of balls on the resulting mechanical properties and the associated microstructures. Employing a high-speed camera, the maximum impact velocities of balls were quantified. This velocity is affected by the density and size of the ball. In the present paper an optimum number of balls for SMAT was also identified. With a detailed knowledge of the ball velocity we were able to accurately estimate the strain rate at different depths by analytical modeling and to study the correlation between the resulting microstructures and the strain/strain rate history of the material. [Copyright &y& Elsevier]

Abstract: The dislocation interactions that stop threading dislocations (threads) during relaxation at increasing applied strains in single-crystal thin films are investigated using large-scale three-dimensional dislocation dynamics simulations. Threads were observed to stop via interactions with both threads and misfit dislocations (misfits). Both types of interactions were shown to depend on stress inhomogeneity. Low-stress regions enabled threads to stop in weak thread–misfit interactions even at high average film stresses. Threads were also concentrated in low-stress regions, which facilitated their interaction with other threads. Threads accumulated in thread–thread interactions, and stopped only temporarily in thread–misfit interactions. The mean free path for dislocation motion is shown to be accurately predicted from details of the inhomogeneous stress state arising from the applied strain and the misfit structure. These behaviors are analyzed to present a more complete picture of film strength, strain hardening and relaxation. [Copyright &y& Elsevier]

Abstract: The plastic behavior of four pure face-centered-cubic (fcc) metals (Al, Au, Cu and Ni) was studied at room temperature up to a strain value of ∼8 achieved by severe plastic deformation. It is shown that at high strains the saturation flow stress and the total dislocation density can be scaled with the melting point, and the deformation process can be regarded as steady-state flow leading to a uniform description of the plastic behavior of these fcc metals at all temperatures. The results suggest that the plastic flow at room temperature of fcc metals processed by severe deformation is primarily attributable to diffusion-controlled and thermally activated processes due to the high concentration of deformation-induced vacancies. [Copyright &y& Elsevier]

Abstract: In situ transmission electron microscopy straining experiments with concurrent macroscopic stress–strain measurements were performed to study the effect of microstructural heterogeneity on the deformation behavior of nanocrystalline metal films. In microstructurally heterogeneous gold films (mean grain size d m =70nm) comprising randomly oriented grains, dislocation activity is confined to relatively larger grains, with smaller grains deforming elastically, even at applied strains approaching 1.2%. This extended microplasticity leads to build-up of internal stresses, inducing a large Bauschinger effect during unloading. Microstructurally heterogeneous aluminum films (d m =140nm) also show similar behavior. In contrast, microstructurally homogeneous aluminum films comprising mainly two grain families, both favorably oriented for dislocation glide, show limited microplastic deformation and minimal Bauschinger effect despite having a comparable mean grain size (d m =120nm). A simple model is proposed to describe these observations. Overall, our results emphasize the need to consider both microstructural size and heterogeneity in modeling the mechanical behavior of nanocrystalline metals. [Copyright &y& Elsevier]

Abstract: The influences of surface step state and temperature on the elastic limit for dislocation nucleation from a surface step were analyzed by means of atomic scale simulations in face-centered cubic metals. When varying the step height, two regimes were found: for smaller steps, local effects dominate, whereas for larger steps, the stress concentration prevails. The differences observed for the elastic limit were correlated to relevant properties of the different potentials. Finally, for aluminum, the implication of the activation parameters in the nucleation strain was studied in greater detail. This study is particularly relevant to nanostructures, where plasticity is most often governed by dislocation nucleation rather than dislocation multiplication. [Copyright &y& Elsevier]

Abstract: In situ straining experiments have been carried out at low temperature in pure Fe, in order to study the change of mechanism occurring at around 250K. The local stress necessary to move individual screw dislocations is in good agreement with the macroscopic yield stress at various temperatures. In the lower temperature range, straight screw segments have a jerky motion in {110} planes, at variance from the steady motion observed near room temperature. The distributions of waiting times in locked positions, and jump distances, the temperature variation of the average jump distance, and the stress/temperature variation of the macroscopic activation areas, are inconsistent with the kink-pair mechanism observed above 250K. They have been interpreted in terms of a locking–unlocking mechanism, already proposed in hexagonal-closed-packed metals. Under such conditions, the change of mechanism at 250K can account for the surprisingly low value of the flow stress extrapolated to 0K (much lower than the theoretical Peierls stress). [Copyright &y& Elsevier]

Abstract: In situ straining experiments have been carried out in pure Fe, in order to determine the geometry and the kinetics of dislocation glide at room temperature. Straight screw dislocations glide slowly in {110} elemental slip planes, at a velocity proportional to their length, whereas curved non-screw parts are highly mobile. The exact loop shape can yield the local stress as well as the difference of core energy between pure screw and near-screw orientations. The velocity–stress dependence of screws has been measured at the scale of a single dislocation source, and compared with macroscopic activation areas. The results are discussed in terms of the kink-pair mechanism. [Copyright &y& Elsevier]

Abstract: Magnetic shape-memory alloys tend to deform via magnetic-field-induced and stress-induced twin-boundary motion. The rather low martensite transformation temperature of ternary Ni–Mn–Ga limits the operating temperature for potential applications. By alloying 5at.% cobalt, the martensite transformation temperature and the Curie temperature was increased from 70 and 110°C respectively up to 160°C. In the single crystalline samples two non-modulated structures with tetragonal and orthorhombic lattices were found. The non-modulated orthorhombic structure has similar lattice parameters to the pseudo-orthorhombic 14M Ni–Mn–Ga phase. The single crystal specimen with the non-modulated orthorhombic structure exhibited a cyclic permutation of all three crystallographic axes in response to uniaxial loading. The parallelepiped-shaped sample was compressed repeatedly in all three directions. While maximizing work done by the load during deformation required three different martensite variants to result from deformation in three different directions, only two different martensite variants were found. The analysis of the sample shape revealed two variants mutually related through cyclic permutation of the lattice parameters, which cannot result from a single twinning event. The cyclic permutation is discussed in the light of Crocker’s double twinning mechanism. [Copyright &y& Elsevier]

Abstract: The deformation of nanocrystalline aluminum was studied using molecular dynamics simulation at homologous temperatures up to 0.97. The microstructures and stress–strain response were examined in a polycrystalline and bicrystal configuration. The activation energies for dislocation-based deformation as well as grain boundary sliding and migration were quantified by fitting simulation data to temperature using an Arrhenius relation. The activation energy for the flow stress response suggests that deformation is largely accommodated by sliding and migration of grain boundaries. This is in agreement with simulated microstructures, indicating a negligible degree of dislocation interaction within each grain, and microstructural observations from high strain rate processes are also consistent with this result. A steady-state grain size is maintained in the recrystallized structure following yielding due to boundary migration and grain rotation mechanisms, rather than by diffusion-based dislocation climb. [Copyright &y& Elsevier]

Abstract: Polycrystalline Ni of two purities (99.967% (4N) and 99.5% (2N)) was deformed to an ultra-high strain of εvM =100 (εvM , von Mises strain) by high pressure torsion at room temperature. The 4N and 2N samples at this strain are nanostructured with an average boundary spacing of ∼100nm, a high density of dislocations and a large fraction of high angle boundaries (>15°) of 0.68–0.74, as determined by transmission electron microscopy, and 0.8–0.83, as determined by electron backscattering diffraction. The deformed samples were annealed isochronally for 1h at temperatures from 100 to 600°C, and the evolution of the structural parameters (boundary spacing, average boundary misorientation angle and the fraction of high angle boundaries), crystallographic texture and hardness were determined. Based on microstructural parameters the energy stored in the deformed state was estimated to be 14MPa and 24MPa for 4N Ni and 2N Ni, respectively. The isochronal annealing leads to a drop in hardness in three stages: a relatively small decrease at low temperatures (recovery), followed by a rapid decrease at intermediate temperatures (recrystallization) and a slow decrease at high temperature (grain growth). Both recovery and recrystallization of the 2N Ni are strongly retarded by the presence of impurities reducing the mobility of boundaries. In the recrystallization stage, changes in hardness, microstructure and texture show that the 4N Ni recrystallizes discontinuously, in spite of a large fraction of high angle boundaries in the deformed state. This finding contradicts previous experiments and theory, which suggest that recrystallization is continuous when the fraction of high angle boundaries is high. In the 2N Ni, the observations suggest that some structural coarsening (continuous recrystallization) may take place simultaneously with discontinuous recrystallization. The findings emphasize the importance of alloying, which can delay the process of recovery and recrystallization and thereby enable tailoring of the microstructure and properties through an optimized annealing treatment. [Copyright &y& Elsevier]

Abstract: In situ mechanical tests were carried out to measure the tensile behavior of single-crystalline face-centered cubic (fcc) gold (Au) and body-centered cubic (bcc) molybdenum (Mo) nano-pillars with diameters between 250 and 1μm, and to compare this with the compression results of these materials at the equivalent sizes. In Au, we observed similar tensile and compressive flow stresses at ∼10% strain although strain-hardening in tension is somewhat more pronounced than it is in compression. In Mo, the amount of strain-hardening in tension is significantly lower than that in compression, leading to a distinct tension–compression asymmetry in the flow stress at ∼5% strain. The dissimilarities between tensile and compressive behavior in both crystals are discussed in terms of sample geometry constraints and dislocation behavior in bcc crystals. [Copyright &y& Elsevier]

Abstract: We have made measurements of the temporal and spatial features of the evolution of strain during the serrated flow of Pd40Ni40P20 bulk metallic glass tested under quasistatic, room temperature, uniaxial compression. Strain and load data were acquired at rates of up to 400kHz using strain gages affixed to all four sides of the specimen and a piezoelectric load cell located near the specimen. Calculation of the displacement rate requires an assumption about the nature of the shear displacement. If one assumes that the entire shear plane displaces simultaneously, the displacement rate is approximately 0.002ms–1. If instead one assumes that the displacement occurs as a localized propagating front, the velocity of the front is approximately 2.8ms−1. In either case, the velocity is orders of magnitude less than the shear wave speed (∼2000ms−1). The significance of these measurements for estimates of heating in shear bands is discussed. [Copyright &y& Elsevier]

Abstract: A custom in situ scanning electron microscopy mechanical testing system has been used to study the compressive deformation behavior of single-slip oriented Rene N5 single-crystal Ni superalloy microcrystals. Two different compression platens were used to explore the effect of lateral stiffness on the resultant mechanical response, which approximated either a high-friction or effectively zero-friction case. The change in the lateral constraint of the test system had a demonstrable effect on many aspects or attributes of plastic flow: the yield stress and strain-hardening behavior, the intermittency of strain bursts, the spatial distribution of slip bands, and the development of internal crystal rotations. Finite-element modeling of the microcompression experiments using an anisotropic crystal plasticity framework provided insight regarding changes in the internal stress field and resultant activity of slip systems. The experimental findings are rationalized based on these simulation results. [Copyright &y& Elsevier]

Abstract: Aluminium–5wt.% magnesium open cell foam produced by replication and tested in tension at 300, 350 or 450°C creeps at rates between 10−3 and 10−8 s−1. The behaviour of the foam matches that of the alloy from which it is made: three-power law creep with the same activation energy as for Al–Mg alloy creeping by viscous dislocation glide in the high stress regime and five-power law creep in the low stress regime. The model of Mueller et al. [Mueller R, Soubielle S, Goodall R, Diologent F, Mortensen A. Scripta Mater 2007;57:33], itself a simplified adaptation of previous variational estimates, predicts well the measured foam creep rates, in terms of both absolute value and dependence on temperature and applied stress. Agreement with the model of Andrews et al. [Andrews EW, Gibson LJ, Ashby MF. Acta Mater 1999;47:2853] is somewhat inferior, but nonetheless satisfactory. A strong dependence of creep rate on relative density is found; this feature is also captured by the variational estimate. [Copyright &y& Elsevier]

Abstract: Pure Cu was deformed at different strain rates and temperatures, i.e. with different Zener–Hollomon parameters (Z) ranging within ln Z =22–66, to investigate the effect of Z on its microstructures and mechanical properties. It was found that deformation twinning occurs when ln Z exceeds 30, and the number of twins increases at higher Z. The average twin/matrix lamellar thickness is independent of Z, being around 50nm. Deformation-induced grain refinement is enhanced at higher Z, and the mean transverse grain size drops from 320 to 66nm when ln Z increases from 22 to 66. The grain refinement is dominated by dislocation activities in low-Z processes, while deformation twinning plays a dominant role in high-Z deformation. An obvious increment in yield strength from 390 to 610MPa was found in deformed Cu with increasing Z, owing to the significant grain refinement as well as the strengthening from nanoscale deformation twins. [Copyright &y& Elsevier]

Abstract: An analytical model is presented in order to derive a general expression for the flow stress in polycrystalline films which encompasses and correlates dimensional constraints and strengthening effects. The model is based on the Thompson approach, which is extended to take into account both different grain aspect ratios and distinct strengthening contributions. It allows an accurate prediction of the growth textures in polycrystalline CdTe thick films when grain growth is driven by strain energy minimization. The model also matches the experimental data concerning the grain size and film thickness dependences of the yield stress in polycrystalline Cu thin films either deposited on a substrate or freestanding. Interestingly, the yield stress is found to be fitted by a modified Hall–Petch relation resulting in a d − n dependence in which the exponent n varies between ½ and 1 as a function of the grain size for a given thickness. [Copyright &y& Elsevier]

[Display omitted] Shear-coupled grain boundary (GB) migration is an effective plastic mechanism in absence of dislocation activity, ie. more favorably in nanocrystalline metals. For a given GB, several stress induced migration mechanisms, referred as coupling modes participate to the decrease of the elastic energy produced by the shear. They operate through the nucleation and motion of interfacial defects known as disconnections, carrying elementary shear strain characterized by their Burgers vector. However, so far, the coupling modes have been studied only under a simple shear, a situation much less complex than expected in a strained polycrystal, where multiple components of the stress tensor are present. Here we propose a more systematic investigation of the coupling modes when a composite shear is applied. This promotes the activation of new coupling modes. Using Molecular Dynamics simulations, we evidence these multiple coupling modes and the operation of their associate disconnections. Moreover, we also show that, even at low temperature, GB migration may occur by the successive occurrence of two modes: the relaxed shear appears then as an effective parameter, resulting from the combination of two operating elementary mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: A NiAl–Mo eutectic was directionally solidified to produce composites with well-aligned single-crystal Mo-alloy fibers embedded in a NiAl matrix. They were pre-strained by compressing along the fiber axis and then the matrix was etched away to expose free-standing micropillars having different sizes (360–1400nm) and different amounts of pre-strain (0–11%). Compression testing of the pillars revealed a variety of behaviors. At one extreme were the as-grown pillars (0% pre-strain) which behaved like dislocation-free materials, with yield stresses approaching the theoretical strength, independent of pillar size. At the other extreme were pillars pre-strained 11% which behaved like the bulk, with reproducible stress–strain curves, relatively low yield strengths, stable work-hardening and no size dependence. At intermediate pre-strains (4–8%), the stress–strain curves were stochastic and exhibited considerable scatter in strength. This scatter decreased with increasing pre-strain and pillar size, suggesting a transition from discrete to collective dislocation behavior. [Copyright &y& Elsevier]

Abstract: The lattice strains in AL6XN stainless steel specimens subjected to in situ biaxial stress states have been measured by neutron diffraction. The biaxial stress states were generated in tubular specimens using a specially designed loading apparatus that is capable of applying axial loads to specimens under internal pressure. Lattice strains in the axial and hoop directions were measured for different levels of stress biaxiality in numerous loaded and partially unloaded states. The results revealed the role of the biaxial stress state in the differences in average lattice strains between various crystallographic fibers. These trends are examined in light of the orientational dependencies of the lattice strains on the stress biaxiality under an assumption of uniform stress. Possible additional factors contributing to the observed trends are discussed. [Copyright &y& Elsevier]

Abstract: Micron-sized aluminum pillars, fabricated by focused-ion beam milling, were subjected to compression in a nanoindenter using a flat-ended tip to examine their deformation behavior. The deformation was jerky and the statistical distributions of the sizes of the bursts, their occurrence frequency, as well as the stresses at which they occurred were analyzed. The burst size was found to increase with stress in an approximately exponential manner. Post-mortem transmission electron microscopy investigation of the dislocation structures revealed that the dislocation density of the micro-pillars did not grow significantly after severe deformation. Based on the experimental observations, a Monte Carlo model was developed to describe the stochastic nature of deformation of these micro-pillars. [Copyright &y& Elsevier]