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.

This article reviews recent basic research on two categories of metal-based nanolaminates: those composed of metal/metal constituents and those composed of metal/ceramic constituents. We focus primarily on studies that aim to understand—via experiments, modeling, or both—the biphase interface structure and its role in changing the mechanisms that govern strength and deformability at a fundamental level. We anticipate that, by providing a broad perspective on the latest advances in nanolaminates, this review will aid design of new metallic materials with unprecedented combinations of mechanical and physical properties.

Two-phase nanolaminates are known for their high strength, yet they suffer from loss of ductility. Here, we show that broadening heterophase interfaces into "3D interfaces" as thick as the individual layers breaks this strength-ductility trade-off. In this work, we use micropillar compression and transmission electron microscopy to examine the processes underlying this breakthrough mechanical performance. The analysis shows that the 3D interfaces stifle flow instability via shear band formation through their interaction with dislocation pileups. To explain this observation, we use phase field dislocation dynamics (PFDD) simulations to study the interaction between a pileup and a 3D interface. Results show that when dislocation pileups fall below a characteristic size relative to the 3D interface thickness, transmission across interfaces becomes significantly frustrated. Our work demonstrates that 3D interfaces attenuate pileup-induced stress concentrations, preventing shear localization and offering an alternative way to enhanced mechanical performance.

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.

Tungsten displays high strength in extreme temperature and radiation environments and is considered a promising plasma facing material for fusion nuclear reactors. Unlike other metals, it experiences substantial irradiation hardening, which limits service life and presents safety concerns. The origin of ultrahigh-irradiation hardening in tungsten cannot be well-explained by conventional strengthening theories. Here, we demonstrate that irradiation leads to near 3-fold increases in strength, while the usual defects that are generated only contribute less than one-third of the hardening. An analysis of the distribution of tagged atom-helium ions reveals that more than 87% of vacancies and helium atoms are unaccounted for. A large fraction of helium-vacancy complexes are frozen in the lattice due to high vacancy migration energies. Through a combination of in situ nanomechanical tests and atomistic calculations, we provide evidence that irradiation hardening mainly originates from high densities of atomic-scale hidden point-defect complexes.

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 strength and plasticity of metallic composites is reviewed in terms of (1) hierarchical morphologies in metallic nanocomposites, such as Cu–Mo, Zr–Nb, Al–Si, and (2) heterogeneous interface behavior and compositionally graded interfaces in metallic laminates, such as Cu/bronze, Ti/Al, Fe/Cu, Cu/Nb. Hierarchical architectures of multiple phases with varying sizes and morphologies are particularly effective in enhancing yield strength, due to the strong glide dislocation interactions with their nanoscale features; plastic deformability, due to increased strain hardening from microstructure hierarchy. Enhanced toughening is attributed to impedance of crack growth via deflection along interfaces that have relatively low shear strength and bridging via relatively softer microscale composite domains. In laminates, the constraint from an interface affected zone arising from geometrically necessary dislocations can lead to enhanced strain hardening and plasticity.

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.

In this work, we calculate the local slip resistances (LSRs) in equal-molar MoNbTi multi-principal element alloy via molecular static simulations. We consider dislocations of either screw or edge character gliding on four types of slip planes, {110}, {112}, {123}, and {134}, in either forward or backward sense of the 111 slip direction. As references, we also compute the Peierls stresses of the same dislocations in two natural reference metals, Mo and Nb, and a synthetic one, the mean-field, A -atom potential-based MoNbTi. Further, we compare the LSRs with the corresponding ideal shear strengths that do not account for the lattice distortions induced by dislocation cores. We show that for neither dislocation character is the LSR on the {110} plane the lowest in MoNbTi, in contrast to Mo and Nb. For edge dislocations, slip on the {134} plane is the easiest, but for the screw dislocations, it is the hardest. For screw dislocations, the {112} glide plane is the most favored, while for edge dislocations, it is the least favored. We also find that the screw-to-edge ratio in the slip resistance is reduced by one order of magnitude in MoNbTi compared to that of any pure reference metal for the same type of slip plane. These results suggest that, in contrast to pure body-centered cubic (BCC) metals, BCC MPEAs could deform by a multiplicity of slip modes due to the lower screw-to-edge ratios and the lower LSRs for edge dislocations on the three higher order planes.

Pathways for ductility Alloys containing multiple elements can be very strong but often suffer from poor ductility. F. Wang et al. found that different mechanisms accommodated plasticity in a molybdenum-niobium-titanium multiprincipal element alloy (see the Perspective by Cairney). Instead of so-called “screw” dislocations, deformation is accommodated by multiple pathways that include “edge” dislocations and activation of crystallographic slip planes. These results offer a design paradigm for developing new high-strength alloys. Science , this issue p. 95 ; see also p. 37

As the numbers of medium- to high-entropy alloys being studied and impressive structural properties they exhibit increase rapidly, questions regarding the role played by their complex chemical fluctuations rise concomitantly. Here, using a combination of large-scale molecular dynamics (MD), a hybrid MD and Monte-Carlo simulation method, and crystal defect analysis, we investigate the role lattice distortion (LD) and chemical short-range order (CSRO) play in the nucleation and evolution of dislocations and nanotwins with straining in single crystal and nanocrystalline CoCrNi, a medium entropy alloy (MEA). LD and CSRO effects are elucidated by comparisons with responses from a hypothetical pure A-atom alloy, which bears the same bulk properties of the nominal MEA but no LD and no CSRO. The analysis reveals that yield strengths are determined by the strain to nucleate Shockley partial dislocations, and LD lowers this strain, while higher degrees of CSRO increase it. We show that while these partials prefer to nucleate in the CoCr clusters, regardless of their size, they find it increasingly difficult to propagate away from these sites as the level of CSRO increases. After yield, nanotwin nucleation occurs via reactions of mobile Shockley partials and is promoted in MEAs, due to the enhanced glide resistance resulting from LD and CSRO.

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.

s Manipulation of the atomic-scale structure of two-dimensional (2D) interfaces have been shown to provide nanocomposites with enhanced strength, deformability, and radiation damage resistance. In comparison with 2D interfaces, here we investigate the mechanical response of nanocomposites containing three-dimensional (3D) Cu/Nb interfaces consisting of a chemical/structural gradient separating pure Cu and Nb layers, through which the lattice mismatch between face-centered cubic Cu and body-centered cubic Nb is accommodated over a distance of several nanometers. It is demonstrated that 3D interfaces increase the yield and flow strength by 50% and 22%, respectively, over composites containing 2D interfaces at similar layer thicknesses. After 14% compressive strain, the onset of shear banding results in co-deformation of both Cu and Nb phases within and outside of the shear band. We conclude with a discussion of the role of interface structure in shear band formation and growth in 3D Cu/Nb.

In the present study, we combine comprehensive site-specific microstructural and mechanical characterization studies to evaluate the toughening effect induced by applying a surface SPEX milling (SSM) approach to commercially-pure Mg. Our results show that the high frequency, multi-directional deformation strain induced by SSM generated gradients in the grain size and orientation with depth from the surface. More importantly, it also resulted in a gradient in the density of twin meshes, which are defined as two or more intersecting arrays of twins, along the sample thickness direction. Tensile tests at room temperature indicate that after SSM, the samples have higher ultimate tensile strengths and two-fold increases in ductility as compared to those of the untreated samples. The effect of the initial texture and SSM parameters on the microstructural evolution and texture randomization were analyzed by electron backscattered diffraction (EBSD). Furthermore, in order to correlate the variation in microstructure with the site-specific mechanical response, TEM and in-situ micropillar compression testing in SEM were performed at various distances from the SSM-treated surface. The results show that twin meshes may be responsible for the activation of more slip systems and higher strain hardening, which result in higher uniform plastic strain.

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.

Voids, helium bubbles and stacking fault tetrahedra (SFTs) are common irradiation-induced defects in face-centered cubic (FCC) metals and their alloys that have detrimental effects on their deformation behavior and lifetime. The formation mechanisms of voids and SFTs have been investigated in single crystals but the potential augmentation of these mechanisms by a heterophase interface has not been well studied. Here, using transmission electron microscopy (TEM), we report on the stability of both SFTs and voids at interfaces in an irradiated Cu/Ag nanolayered composite. With atomistic simulations, we show that the heterophase interface can promote the transformation of voids (

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.