Search

Understanding and controlling the development of deformation twins is paramount for engineering strong and stable hexagonal close-packed (HCP) Mg alloys. Actual twins are often irregular in boundary morphology and twin crystallography, deviating from the classical picture commonly used in theory and simulation. In this work, the elastic strains and stresses around irregular twins are examined both experimentally and computationally to gain insight into how twins develop and the microstructural features that influence their development. A nanoprecession electron diffraction (N-PED) technique is used to measure the elastic strains within and around a $$\left\{ {10\overline{1}2} \right\}$$ 10 1 ¯ 2 tensile twin in AZ31B Mg alloy with nm scale resolution. A full-field elasto-viscoplastic fast Fourier transform (EVP-FFT) crystal plasticity model of the same sub-grain and irregular twin structure is employed to understand and interpret the measured elastic strain fields. The calculations predict spatially resolved elastic strain fields in good agreement with the measurement, as well as all the stress components and the dislocation density fields generated by the twin, which are not easily obtainable from the experiment. The model calculations find that neighboring twins, several twin thicknesses apart, have little influence on the twin-tip micromechanical fields. Furthermore, this work reveals that irregularity in the twin-tip shape has a negligible effect on the development of the elastic strains around and inside the twin. Importantly, the major contributor to these micromechanical fields is the alignment of the twinning shear direction with the twin boundary.

In this perspective, we: (a) characterize the multiscale nature of the mechanical (plasticity, fracture, twinning, and phase transformations) and thermal behavior, as well as the mass transport behavior in a variety of materials with microstructure complexities; (b) examine the applicability of several representative experimental/computational techniques/approaches in identifying the mechanisms underlying the interface-dictated mechanical, thermal, and mass transport; (c) highlight the need for the development of multiscale methods that can address atomistic and continuum descriptions of materials within one framework, together with our preliminary attempts in this regard. This perspective, together with the relevant papers collected in this focus issue, will inspire researchers to further develop advanced theories, algorithms, and software implementation for bottom-up predictive simulation of the deformation, thermal, and diffusion behavior of advanced materials. Such methods can support the design and development of materials with desired combinations of properties, for example high strength/ductility/toughness, low/high thermal/ionic conductivity, corrosion-/irradiation-resistance.

Fe-Al compounds possess a combination of high strength and corrosion resistance at high temperatures. However, increasing Al content to make them lighter results in embrittlement. Here, we investigate the high-temperature behavior of a novel, lightweight, ultra-fine-layered FeAl/FeAl2 material. We report a transition from unstable to stable plasticity at 450 °C. Below 450 °C, deformation is dominated by localized shear deformation within the soft FeAl layers, while above 450 °C, it proceeds by co-deformation between FeAl and the brittle FeAl2 layers. We show that co-deformation is associated with the temperature at which the interface converts from sliding to sourcing dislocations for FeAl2.

The microstructural origins of highly localized elastic strain concentrations in polycrystalline microstructures under monotonic loading are studied using grain-scale, in situ digital image correlation and crystal plasticity finite element method. It is shown that the locations of exceptionally high elastic strain concentrations in the microstructure depend on particular crystallographic and morphological orientations of grains and less so on crystalline details of their local neighborhood. Based on these results, we discuss how topological and crystallographic features of annealing twin boundaries can increase the likelihood of slip band initiation throughout the microstructure of polycrystalline Ni-base superalloys.

By reducing the layer thickness, h, in metal multilayers from micrometer- to nanometer-levels, scale-dependent transitions in behavior are observed. The role of interface properties in these transitions has been extensively investigated. However, reducing h to the nanoscale regime also imposes a physical geometric confinement on defect-related processes occurring within the layers, an effect that is less explored. We present illustrative examples of the influence of nanoconfinement on the strength, structure, and radiation response of multilayers. These examples show that nanoconfinement and interface structure and topology act cooperatively in the nanoscale limit, affecting behaviors that cannot be explained based on either factor alone.

In this work, the deformation response, texture evolution, and twinning development of a magnesium (Mg) alloy, Mg-1.3Zn-0.4Ca-0.4Mn, for biocompatible applications are investigated. Further, the alloy’s formability, by examining the instantaneous r-ratio and strain-rate sensitivity (SRS) as a function of strain and loading direction, is investigated. It is found that after rolling and peak aging, the alloy has a rolled texture of moderate intensity with the basal planes contained in the rolling plane and with a bimodal, fine-grained microstructure. The alloy shows both high room-temperature tensile strength (300 MPa) and ductility (25 pct) in the rolling direction (RD) and, remarkably, r-ratios saturating close to unity in all three in-plane testing directions. It is also found that the SRS is relatively high and uniform, with averages ranging from 0.015 to 0.025, depending on the in-plane testing directions. These are outstanding properties compared to pure Mg and most of its biocompatible alloys. Typical of Mg alloys, this alloy has a propensity for twinning by multiple twin modes, which leads to rapid texture evolution, anisotropy, and tension-compression (T-C) asymmetry in yield stress, with compression having the weaker response. These mechanical characteristics along with their microstructural origins are presented and discussed in this article.

Deformation twinning is a prevalent plastic deformation mode in hexagonal close-packed (HCP) materials, such as magnesium, titanium, and zirconium, and their alloys. Experimental observations indicate that these twins occur heterogeneously across the polycrystalline microstructure during deformation. Morphological and crystallographic distribution of twins in a deformed microstructure, or the so-called twinning microstructure, significantly controls material deformation behavior, ductility, formability, and failure response. Understanding the development of the twinning microstructure at the grain scale can benefit design efforts to optimize microstructures of HCP materials for specific high-performance structural applications. This article reviews recent research efforts that aim to relate the polycrystalline microstructure with the development of its twinning microstructure through knowledge of local stress fields, specifically local stresses produced by twins and at twin/grain–boundary intersections on the formation and thickening of twins, twin transmission across grain boundaries, twin–twin junction formation, and secondary twinning.

In this work, strength and microstructural evolution of superalloy Inconel 718 (IN718) are characterized as a function of the initial microstructure created via direct metal laser melting (DMLM) additive manufacturing (AM) technology along with subsequent hot isostatic pressing (HIP) and heat treatments as well as wrought processing. Stress-strain curves are measured in tension and compression from room temperature to 550 °C and crystallographic texture is characterized using neutron diffraction. Furthermore, a recently developed crystal plasticity model incorporating the effects of precipitates is extended to interpret the temperature dependent deformation behavior of the alloy. The model accounts for solid solution, precipitate shearing, and grain size and shape contributions to initial slip resistance, which evolves with a dislocation density-based hardening law considering latent hardening, while non-Schmid effects are taken into account in the activation stress. Part of the experimental data is used for calibration of the model, while the rest is used for experimental validation of the model. It is shown that the model is capable of modeling the data with accuracy. Based on the comparison of the data and model predictions, it is inferred that the grain structure and texture give rise to plastic anisotropy of the alloy, while its tension-compression asymmetry results from non-Schmid effects and latent hardening.

In this work, we use polycrystal modeling to study the interactions between slip and twinning during load reversals of commercially pure titanium. The constitutive response incorporates anisotropic elasticity, crystal plasticity, a dislocation density based hardening law for prismatic slip, basal slip, and pyramidal type I 〈c + a〉 slip, and micromechanical model for twin reorientation on two types: { 101 2 ¯ } extension twinning and { 11 2 ¯ 2 } contraction twinning. The key feature of the model is the inclusion of slip-system level backstress development due to dislocation density accumulation. To demonstrate, the model is used to simulate the stress-strain response and texture evolution in a series of compression-tension and tension-compression tests carried out to different strain levels and applied in two different load directions to a strongly textured CP-Ti plate. Material parameters associated with the slip strengths for the three slip modes are reported. The model identifies the few systems within the pyramidal 〈c + a〉 slip mode as developing the most backstress among the three slip modes. It also indicates that the backstresses that develop in the forward loading path promote pyramidal slip in the reversal loading path. We also find that reverse loading changes negligibly the relative slip mode contributions from monotonic loading but it strongly affects the twinning-detwinning behavior. This work highlights the ability of polycrystal modeling to account for the co-dependent nature of multiple crystallographic slip and twinning modes, the hysteresis in plastic response during the forward-reversal cycle, and the two sources of hardening engendered by history-dependent dislocation density accumulation.

The main attraction of metallic nanolayered composites (MNCs) lies not only with their five-to ten-fold increases in strength over that of their constituents, but also in the tunability of their superior strength with nanolayer thickness. While the size scaling in strength prevails in many MNC material systems, the size scaling cannot be accurately predicted with crystal plasticity framework. Here, we present a crystal plasticity based computational method that considers plasticity to occur in grain boundary-controlled discrete slip events and apply it to predict the deformation response and underlying mechanisms in Cu/Nb MNCs. Predicted tensile stress-strain responses are shown to achieve agreement with measurements for four distinct nanolayer thicknesses, without introducing adjustable parameters. The model predicts the Hall-Petch size scaling of strength on layer thickness and the rising plastic anisotropy as the layer thickness reduces. Analysis of the results indicates that the origin of the layer size effect on strength results from the limits layer thickness places on the lengths of dislocations sources lying in the grain boundaries.

Bi-metal interfaces act as sinks for radiation defects and hence play an important role in the design of radiation-resistant materials. Here, we report that under neon ion bombardment, the hetero-twin Cu/Ag interface serves as a vacancy pump, transferring vacancies from Cu to Ag, a mechanism first observed under helium radiation (Acta Materialia 160 (2018) 211–223). By comparing helium and neon ion irradiated samples, we reveal that helium likes to aggregate with vacancies and slow down the recombination rate of radiation-induced interstitials and vacancies, which leads to significant swelling, radiation-enhanced diffusion and a stronger vacancy pump effect.

The impact of twin transmission between neighboring grains as a contributor to overall twin activity is considered via a neighborhood viscoplastic self-consistent (NVPSC) model. The NVPSC model is an extension of a stochastic model for twin nucleation developed by Niezgoda et al. and a dislocation density based hardening law model developed by Knezevic et al. Beyond the baseline combined framework, the new model tracks sets of neighboring grains and allows twin transmission between them under certain conditions. The influence of grain boundary (GB) character is included in the stochastic models of twin nucleation and transmission. The starting texture from a rolled magnesium alloy AZ31B sheet was obtained using electron backscatter diffraction (EBSD) for initial input into the NVPSC. The sample was further deformed by uniaxial compression to encourage twin formation and the corresponding texture information was collected using EBSD. The accuracy of simulated twin activity was determined by comparing it with the twin activity seen in the deformed sample. The total number of predicted twins and the number of transmission twins is found to agree favorably with those observed via the EBSD scans. This validation demonstrates the significance of incorporating twin transmission as a twin formation mode in predictive models for this material.

An important fundamental question regarding deformation twinning is whether it is possible for twins to nucleate at boundaries or interfaces when specific stress fields are present. A corollary that follows from this question is: if this is indeed possible, what determines the proper stress field and how does it occur at the nanoscale? Here, we demonstrate the application of an in-situ nanoindentation approach to confine and dynamically capture the stages in the formation of a deformation twin at an internal twin boundary in single crystal Mg. We observe the formation of contraction twin embryos at the pre-existing extension twin boundary, and the subsequent propagation of the twin embryos into the crystal. We reveal an intermediate step, involving the coalescence of tiny embryos into a larger embryo before the nucleus emanates into the crystal. De-twinnning of the twin embryos is captured during unloading and shown to leave a remnant nanosized twin ( dislocations and boundary structure (incoherent vs. coherent) in embryo formation, as suggested by the TEM and modeling analyses, are discussed.

In this work, we investigate the tensile strength and ductility of bulk Cu/Nb nanolaminates after exposure to high temperatures. We show that the interface transforms from flat to wavy at a transition temperature of 700 °C, and tensile strength is linearly proportional to H-1/2 (H = layer thickness). Moreover, the wavy interfaces give rise to a higher slope of the Hall-Petch law. This result can be attributed to greater resistance to slip transmission across wavy interfaces compared to planar interfaces. After 1000 °C for 1 h, the material still exhibits a high strength of 468 MPa and enhanced elongation.

This paper studies the evolution in strength of alloy AA6022-T4 sheets that have been pre-deformed by a continuous-bending-under-tension (CBT) process. Significant improvements in strength are observed only after a few CBT cycles. Less appreciable improvements in strength are observed with more CBT cycles and with every cycle the ductility reduces. These observations are rationalized by characterizing microstructural evolution using transmission electron microscopy and electron backscattered diffraction. It is found that evolution of texture and grain shape during CBT slightly differ from those in simple tension (ST). Also, the precipitates do not change their shape during CBT or ST. It is, therefore, concluded that these microstructural features have only a secondary effect on the strength behavior of the alloy. Consistent with earlier observations in the literature, we find that dislocation structures form within grains during monotonic ST and that they are disorganized and not as well defined. In contrast, cellular substructures are observed to form very early during CBT processing, even after the first cycle and to evolve from loose tangles of dislocations to well-defined walls during subsequent cycles. These dislocation patterns are found responsible for the observed behavior of the alloy. Therefore, the strength of the material is determined not only by the achieved effective strain level but also by achieved microstructure.

In our previous study, we observed a lack of $\left\{ {10\bar 12} \right\}$ twinning in a deformed Mg–Y alloy, which contributed to the observed yield “symmetry.” However, the effects of texture and grain size on polycrystalline deformation made it difficult to fully understand why twinning was not active. Therefore, we report herein in-depth study by in situ transmission electron microscopy, i.e., in situ TEM. The in situ deformation of nano-sized Mg–Y pillars revealed that prismatic slip was favored over twinning, namely, the critical stress required to activate prismatic slip was lower than that for twinning. This finding diametrically differs from that reported in other nano/micro-pillar deformation studies, where twinning is always the dominant deformation mechanism. By measuring the critical stresses for basal, prismatic, and pyramidal slip systems, this in situ TEM study also sheds light on the effects of the alloying element Y on reducing the intrinsic plastic anisotropy in the Mg matrix.

In this work, we study the effect of crystallographic orientation and applied triaxiality on the growth of intragranular voids. Two 3D full-field micromechanics methods are used, the dilatational visco-plastic fast-Fourier transform (DVP-FFT) and the crystal plasticity Finite Elements (CP-FE), both of which incorporate a combination of crystalline plasticity and dilatational plasticity. We demonstrate with several select cases that predictions of void growth from both formulations agree qualitatively. With the more computationally efficient DVP-FFT, additional effects of polycrystalline microstructure and the influence of nearest neighborhood are investigated. Crystals bearing a single intracrystalline void are studied in three types of 3D microstructural environments: isolated single crystals, individual equal-sized grains within a regular polycrystal, and individual variable sized grains within a polycrystal with grains and voids randomly located. We show that loading type plays a significant role. In strain-rate controlled conditions, voids in the hardest [111]-crystals grow the fastest in time, whereas in stress-controlled conditions, voids in the softest [100]-crystal grow the fastest in time. The analysis reveals that on average void growth is slower for the same starting orientation in the polycrystal than in the single crystal. We find that at the highest triaxiality tested that the correlation between crystal orientation and void growth rate in the polycrystal strengthens, drawing closer to that seen in the isolated single crystals. These results and model can help guide the microstructural design of polycrystalline materials with high strength and damage-tolerance in high-rate deformation.

Recent molecular dynamics simulations revealed that 〈c + a〉 dislocations in Mg were prone to dissociation on the basal plane, thus becoming sessile. Basal dissociation of 〈c + a〉 dislocations is significant because it is a major factor in the limited ductility and high work-hardening in Mg. We report an in situ transmission electron microscopy study of the deformation process using an H-bar-shaped thin foil of Mg single crystal designed to facilitate 〈c + a〉 slip, observe 〈c + a〉 dislocation activity, and establish the validity of the largely immobile 〈c + a〉 dislocations caused by the predicted basal dissociation. In addition, through detailed observations on the fine movement of some 〈c + a〉 dislocations, it was revealed that limited bowing out movement for some non-basal portions of 〈c + a〉 dislocations was possible; under certain circumstances, i.e., through attraction and reaction between two 〈c + a〉 dislocations on the same pyramidal plane, at least portions of the sessile configuration were observed to be reversed into a glissile one.

Quantification of mesoscale microstructures of polycrystalline materials is important for a range of practical tasks of materials design and development. The current protocols of quantifying grain size and morphology often rely on microstructure metrics (e.g., mean grain diameter) that overlook important details of the mesostructure. In this work, we present a quantification framework based on directionally resolved chord length distribution and principal component analysis as a means of extracting additional information from 2-D microstructural maps. Towards this end, we first present in detail a method for calculating chord length distribution based on boundary segments available in modern digital datasets (e.g., from microscopy post-processing) and their low-rank representations by principal component analysis. The utility of the proposed framework for capturing grain size, morphology, and their anisotropy for efficient visualization, representation, and specification of polycrystalline microstructures is then demonstrated in case studies on datasets from synthetic generation, experiments (on Ni-base superalloys), and simulations (on steel during recrystallization).

Predictions of the mechanical response of polycrystalline metals and underlying microstructure evolution and deformation mechanisms are critically important for the manufacturing and design of metallic components, especially those made of new advanced metals that aim to outperform those in use today. In this review article, recent advancements in modeling deformation processing-microstructure evolution and in microstructure–property relationships of polycrystalline metals are covered. While some notable examples will use standard crystal plasticity models, such as self-consistent and Taylor-type models, the emphasis is placed on more advanced full-field models such as crystal plasticity finite elements and Green’s function-based models. These models allow for nonhomogeneity in the mechanical fields leading to greater insight and predictive capability at the mesoscale. Despite the strides made, it still remains a mesoscale modeling challenge to incorporate in the same model the role of influential microstructural features and the dynamics of underlying mechanisms. The article ends with recommendations for improvements in computational speed.

In this work, the deformation mechanisms underlying the room temperature deformation of the pseudomorphic body centered cubic (BCC) Mg phase in Mg/Nb nanolayered composites are studied. Nanolayered composites comprised of 50% volume fraction of Mg and Nb were synthesized using physical vapor deposition with the individual layer thicknesses h of 5, 6.7, and 50 nm. At the lower layer thicknesses of h = 5 and 6.7 nm, Mg has undergone a phase transition from HCP to BCC such that it formed a coherent interface with the adjoining Nb phase. Micropillar compression testing normal and parallel to the interface plane shows that the BCC Mg nanolayered composite is much stronger and can sustain higher strains to failure than the HCP Mg nanolayered composite. A crystal plasticity model incorporating confined layer slip is presented and applied to link the observed anisotropy and hardening in the deformation response to the underlying slip mechanisms.

Mechanical behavior of alloys is influenced by segregation of solute atoms, which affects deformation mechanisms, such as slip and twinning. In this study, we report on an atomic-scale investigation into room temperature, deformation-induced solute segregation in a Mg-Y alloy. High concentrations of Y were observed at the dislocation cores. In addition, we found that { 10 1 ¯ 2 } twins were bounded by coherent twin boundaries and basal-prismatic facets, which contained periodic segregation of Y-rich columns and nano-sized Y-rich clusters, respectively. The observed segregation arrangement was energetically attributed to the fact that it minimizes the overall lattice distortion and is kinetically assisted by the dynamic interaction between solute atoms and crystallographic defects and the slip-twin interaction during plastic deformation. Moreover, segregated Y atoms exert a pinning effect and lead to anisotropy on the mobility of twin boundaries. This finding offers a potentially new alloy design path to control the mechanical response of Mg alloys.

We perform molecular dynamics simulations to investigate shock-induced amorphization in CoCrNi, a medium entropy alloy (MEA) and its mean-field variant without lattice distortion. We show that a critical velocity exists above which amorphization occurs. At a low shock velocity of 800 m/s, dislocation slip and twins dominate and amorphization does not happen, but as the shock velocity increases, the deformation mechanism transitions from slip and twinning to solid-state amorphization. Under ultra-high shock velocities, extensive amorphization occurs, following the precursor of shock wave, eliminating anisotropy in spall strength. Compared to the mean-field model, lattice distortion in the MEA causes substantially more amorphization, resulting in a lower spall strength, since voids nucleate and grow preferentially in the amorphous regions.

The design and development of structural materials, which can survive under the extreme conditions of operation, are critical to next generation aerospace and energy technologies. Selectively designed multi-principal element alloys (MPEAs), which are solid solution phases with three or more principal elements on simple underlying lattices, are expected to fulfill such requirements. The combination of refractory metals with elements known for enhancing oxidation resistance, high temperature strength, and thermal stability makes them ideal candidates for high temperature applications. Due to their unique microstructures and chemical compositions, MPEAs may exhibit excellent mechanical properties, such as high strength at elevated temperatures and improved hardness. Improving the mechanical properties of MPEAs requires knowledge of their plastic deformation mechanisms, at the core of which is dislocation slip, which is intimately connected to the local slip resistances (LSRs). In this work, atomistic calculations are conducted to obtain LSRs of edge and screw dislocations on three slip planes – {110}, {112}, and {123} – in four refractory MPEAs, CrMoNbTa, CrNbTaW, MoNbTaV, and MoNbTaW, that attain high melting temperatures and have been previously studied for their oxidation resistance. Many candidate MPEAs have been identified as promising for their oxidation resistance; however, it is also important that they are at the same time strong. The goal of this work is to determine the LSR and the role that lattice distortion has. We find that the two MPEAs containing Cr bear an increased lattice distortion and achieve the highest LSR values and lowest anisotropy in LSR. It is also shown that the MPEAs possess much lower slip resistance anisotropy than pure metals.

The role of microstructure and strain rate on the development of geometrically necessary dislocation (GND) density in polycrystalline copper subjected to compression is assessed via crystal plasticity modelling and electron microscopy. Micropolar crystal plasticity finite element (MP-CPFE) simulations show that GND density is strongly dependent on crystal orientation, with the highest values in grains with a direction parallel to the compression axis. Experimental analysis shows that this relationship breaks down and demonstrates that orientation is only one of many microstructural features that contributes to dislocation density evolution. Texture development as a function of strain rate is also considered, and it is found that the commonly observed compression texture is delocalized from that pole at high strain rate. Furthermore, quantitative analysis of the role of grain boundaries in GND density evolution highlights their role as strong dislocation sources.

The relationship between microstructure, dislocation motion and mechanical response of metallic multilayered nanomaterials is investigated. Several competing theories for the dependence of hardness on layer thickness, namely Confined Layer Slip (CLS) and Hall-Petch (H-P) theories are discussed. Analysis of homophase and heterophase experimental data suggests that Hall-Petch with modified coefficients provides a good fit down to layer thicknesses of about 5 nm, below which experimental data starts to deviate. We suggest that at this layer thickness, dislocations accumulate in the interface, and assuming there is a constant dislocation density in each interface, the strength varies as h −1/2 .

Twinning-induced plasticity (TWIP) steels can exhibit high ultimate strength and ductility, but low yield strength and thermal stability. Here we approach this problem by introducing a heterogeneous microstructure comprised of soft, hard and thermally stable regions in a model composition of Fe-22Mn-0.6 C TWIP steel. This target microstructure is achieved via a three-step processing route: cold rolling to introduce nanotwin bundles, an aging treatment to transform highly defective regions to thermally stable pearlite nano-lamellae, and an annealing step for relatively large, ductile grains. We show that this microstructure generates a good balance between high yield strength, good ductility, high ultimate tensile strength, and good thermal stability. The main deformation mechanism of this unique heterogeneous structure is deformation twinning. The high thermal stability can be attributed to the transformation of the shear bands, introduced by cold rolling, into pearlite during the aging process, and into the composite of nanograined austenite and nanograined cementite formed during the subsequent isothermal annealing.

Due to the geometry of the hexagonal close-packed (HCP) lattice, there are two types of pyramidal slip modes: { 10 1 ¯ 1 } 〈 11 2 ¯ 3 ¯ 〉 or type I and { 1 ¯ 1 ¯ 22 } 〈 11 2 ¯ 3 〉 or type II in HCP crystalline materials. Here we use crystal plasticity to examine the importance of crystallographic slip by pyramidal type I and type II on texture evolution. The study is applied to an Mg-4%Li alloy. An elastic-plastic polycrystal model is employed to elucidate the reorientation tendencies of these two slip modes in rolling of a textured polycrystal. Comparisons with experimental texture measurements indicate that both pyramidal I and II type slip were active during rolling deformation, with pyramidal I being the dominant mode. A single-slip-mode analysis is used to identify the orientations that prefer pyramidal I vs. II type slip when acting alone in a crystal. The analysis applies not only to Mg-4%Li, but identifies the key texture components in HCP crystals that would help distinguish the activity of pyramidal I from pyramidal II slip in rolling deformation.

In this work, we study the deformation behavior of a direct chill cast WE43 Mg alloy. This material initially has equiaxed grains approximately 40 µm in diameter and a random texture. The room temperature, quasi-static response exhibits little plastic anisotropy when evaluated parallel to and normal to the solidification direction and no initial yield tension-compression asymmetry. The deformation at room temperature is accompanied by significant basal texture development and formation of three types of deformation twins: { 10 1 2 } 〈 1 011 〉 , { 10 1 1 } 〈 10 12 〉 , and { 11 2 1 } 〈 1 1 26 〉 as well as double twins { 10 1 1 } 〈 10 12 〉 - { 10 1 2 } 〈 1 011 〉 , although each in small amounts 1.0 true strain) without fracturing.

This paper reports a strong effect of multi-step forging (MIF) followed by elevated temperature isothermal rolling (IR) on the yield stress in ZK60 Mg alloy. After the MIF stage, the yield stress is slightly higher in the rolling direction (RD) than in the transverse direction (TD). After IR at 300 °C, the anisotropy remains small. In contrast, after IR at a lower temperature 200 °C, a significant difference in yield stress between the RD and TD directions is observed, found to increase with rolling strain, and even reverse from being higher to being much lower in the RD than in the TD with rolling reduction at 200 °C. To help determine the possible causes for the anisotropy and its evolution with straining, we use a multi-scale elasto-plastic self-consistent polycrystal model that accounts for dislocation density (Taylor hardening), precipitate hardening, texture, and grain size. The model is extended here to also include the effects of type I and type II residual stresses. With a combination of modeling and electron microscopy, we find that texture, grain size distribution, residual stresses, Taylor hardening, and Orowan hardening only have moderate effects on the plastic anisotropy and cannot fully explain the observations. We rationalize that the primary origin of the yield anisotropy is the evolution of the secondary precipitates, which become distributed in the plane of the rolled sheet during rolling. They become more aligned in the RD plane during IR, causing strengthening in tensile yield stress in the TD over that in the RD.

In this work, new non-orthogonal computational grids are implemented into a phase field model called Phase Field Dislocation Dynamics (PFDD). We demonstrate that the new non-orthogonal grid can accommodate multiple slip planes in either the face centered cubic (FCC) or body centered cubic (BCC) crystallographic systems. We show that they avoid numerical errors induced when modeling glide on inclined slip planes in an orthogonal grid. The Gibbs effect that arises in the orthogonal or rotated orthogonal grids is substantially diminished when a non-orthogonal grid is employed. A few test cases demonstrate the effectiveness of using non-orthogonal grids in solving systems with multiple non-planar slip systems.

In this work, we investigate the relationship between an intense slip band (ISB) and the zone of large lattice rotations that forms ahead of the tip of the ISB. We develop a crystal plasticity finite element model of a discrete ISB lying within an oligocrystalline assembly and calculate the local crystalline stress and lattice rotation fields generated by the ISB. The calculations demonstrate that, first, a region of severe lattice rotations, commonly referred to as a microvolume, does not form without the ISB, and second, large amounts of accumulated slip in the ISB are required to enlarge the microvolume to sizes and rotation magnitudes observed experimentally. Ahead of the ISB tip, the quintessential plastic zone always forms, but the atypical microvolume forms when non-concentrated and spatially diffuse slip is activated by the ISB-induced stress field. This result suggests that the detrimental ISB/microvolume pair will likely appear in pairs of crystals in which transmission of the slip from the ISB is severely blocked by the grain boundary, a hypothesis that we verify with a few target cases.

In this review article, we present and discuss recent mesoscale modeling studies of slip transmission of dislocations through biphase interfaces. Specific focus is given to fcc/fcc material systems. We first briefly review experimental, atomistic, and continuum-scale work that has helped to shape our understanding of these systems. Then several mesoscale methods are discussed, including Peierls–Nabarro models, discrete dislocation dynamics models, and phase field-based techniques. Recent extensions to the mesoscale mechanics technique called phase field dislocation dynamics are reviewed in detail. Results are compiled and discussed in terms of the proposed guidelines that relate composite properties to the critical stress required for a slip transmission event.

In this work, we employ the recently developed framework for the explicit modeling of discrete twin lamellae within a three-dimensional (3D) crystal plasticity finite element (CPFE) model to examine the effects of dislocation densities in the twin domain on twin thickening. Simulations are carried out for 1 ¯ 012 " open=" 10 1 ¯ 1 extension twins in a magnesium AZ31 alloy. The model for the twin lamellae accounts for the crystallographic twin-matrix orientation relationship and characteristic twin shear transformation strain. The calculations for the mechanical fields as a result of twinning consider that one of three types of twin-dislocation density interactions have occurred. One case assumes that the expanding twin retains in its domain the same dislocation density as the parent. The second one considers that twin expansion has lowered the dislocation density as the twin thickens, and the last one, the Basinski effect, assumes that when twin sweeps the region, the dislocation density incorporated in the twin domain is amplified. In the modeling approach, the twin is thickened according to a criterion that maintains the stress state in the vicinity of the grain at a pre-defined characteristic twin resistance. The calculations show that most of the averaged properties, such as the rate of dislocation storage in the entire twin grain, the twin growth rate, the stress field in the twinned grain and neighboring grains, and the slip activity in the parent matrix are not significantly altered by dislocation storage in the twin. The results indicate that, however, the slip activity in the twinned domain is affected. In particular, in the increased dislocation density case, the rate of dislocation density in the twin domain increases at low strains when the twin is first growing from 2% to 5% volume fraction. This initial boost in the dislocation density storage rate causes the newly expanded dislocation twin to contain more stored dislocations than the other cases for all strain levels. Another interesting difference concerns the preference for one or two twins for the same total twin volume fraction; for the increased dislocation twin or twin that retains the dislocation density as it grows, formation of two twins is favored. For a twin that removes dislocation density, only one twin is preferred. The results imply that in the case with reduced dislocation density leads to lower stored dislocations and dislocation storage rates, and lower pyramidal slip activity.

An elasto-plastic polycrystal plasticity model is developed and applied to an Inconel 718 (IN718) superalloy that was produced by additive manufacturing (AM). The model takes into account the contributions of solid solution, precipitates shearing, and grain size and shape effects into the initial slip resistance. Non-Schmid effects and backstress are also included in the crystal plasticity model for activating slip. The hardening law for the critical resolved shear stress is based on the evolution of dislocation density. Using the same set of material and physical parameters, the model is compared against a suite of compression, tension, and large-strain cyclic mechanical test data applied in different AM build directions. It is demonstrated that the model is capable of predicting the particularities of both monotonic and cyclic deformation to large strains of the alloy, including decreasing hardening rate during monotonic loading, the non-linear unloading upon the load reversal, the Bauschinger effect, the hardening rate change during loading in the reverse direction as well as plastic anisotropy and the concomitant microstructure evolution. It is anticipated that the general model developed here can be applied to other multiphase alloys containing precipitates.

A spatially-resolved crystal plasticity Fast Fourier Transform (FFT)-based model is employed to study the effect of alloying addition on twin thickening and twin transmission in hexagonal close packed (HCP) magnesium. In the simulations, the influence of alloying additions is represented through the differences in the critical resolved shear stress (CRSS) of different slip and twinning modes. The results show that for the same grain orientation, twin type and boundary conditions, anisotropy in the CRSS values have a significant effect on twin thickening and twin transmission. Those with large differences in CRSS favor both twin thickening and twin transmission, and vice versa for those with small differences. However, less difference among the CRSS values enhances the dependence of thickening and transmission on the neighboring grain orientation.

In this work, we describe a finite element (FE) implementation of an elasto-plastic self-consistent (EPSC) polycrystal plasticity model termed FE-EPSC, which is intended for simulations of metal forming. To this end, we present an analytical Jacobian, which is necessary for the implicit coupling and ensuring a fast convergence. Every FE integration point is a material point that can be represented either by a single crystal or a polycrystalline material. The constituent crystal can deform by a combination of anisotropic elasticity, crystallographic slip, and deformation twinning. The model is validated and applied to a suite of tests, including monotonic compression, cyclic forward loading, unloading and reverse loading and non-monotonic four-point bending, and materials, such as different alloy compositions, crystal structures, and initial microstructures. The same FE-EPSC framework is applied for all these cases with the main differences pertaining to intrinsic properties, such as the available slip and twinning deformation modes, and the material parameters for activating and hardening of these modes. Full characterization for these parameters for high-purity α-Ti is presented here for the first time. Through these examples we show that, in addition to being predictive with great accuracy, the key advantage of this model lies in its versatility. It accounts for the development of backstress aided dislocation glide, thermally activated storage of dislocations, elastic anisotropy, crystallographic slip and deformation twinning via multiple modes, and de-twinning as well as multi-level homogenizations.

We study pyramidal I and pyramidal II edge (and mixed) dislocations in Mg using a combination of experiment, dislocation theory, and atomic-scale modeling. With high-resolution transmission electron microscopy (HR-TEM) of a deformed Mg sample, a single 1 6 [ 11 2 ¯ 3 ] partial dislocation on the ( 11 2 ¯ 2 ) plane emanating from a { 10 1 ¯ 2 } twin boundary is observed, suggesting the possibility of a dissociation of a 〈 c + a 〉 dislocation into two 1 2 〈 c + a 〉 partials on the ( 11 2 ¯ 2 ) plane. Using first-principles density functional theory (DFT) calculations, we find that achieving this dissociation requires additional relaxations in the atomic positions normal to the slip direction. With molecular statics (MS) simulations, employing a modified embedded atom method (MEAM) potential, the full pyramidal-II 〈 c + a 〉 edge dislocation is shown under no stress to split into two equal value partials 1 6 [ 11 2 ¯ 3 ] + 1 6 [ 11 2 ¯ 3 ] . When a resolved shear stress is applied, dislocations of edge and mixed character are glissile and the stacking fault in-between them narrows or widens depending on the sense of shear. With further analysis of this model, we show that the HR-TEM observation can be explained if one of the partials is pinned at the twin boundary. Last, with these atomic scale methods, we show for the first time that the full edge pyramidal-I 〈 c + a 〉 dislocation dissociates into two equal value partials of 1 6 [ 20 2 ¯ 3 ] and 1 6 [ 02 2 ¯ 3 ] Burgers vectors consistent with recent experimental observations. In contrast to the extended pyramidal-II dislocation, the extended pyramidal-I dislocations of similar edge or mixed character cannot move under an applied resolved shear since only one of the two partials is glissile.