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Metallic glasses with secondary crystalline phase have been reported to either enhance or degrade the global plasticity. Here, we employed molecular dynamics simulation to investigate a generic glass-forming system, in which crystal-glass composites can be obtained via in situ crystallization. Ex situ composite samples were also constructed by embedding perfect crystals into monolithic glasses in order to delineate the precise influences of the composite microstructure. Our simulations demonstrate both toughening and embrittlement effects due to nanocrystallization. The double-edged-sword role of nanocrystals is mainly controlled by elasticity mismatch between the crystalline phase and the glass matrix. The effects of volume fraction and the size of nanocrystals can be understood in the perspective of cumulative probability of shear bands formation and propagation being interfered by the presence of nanocrystals. [ABSTRACT FROM AUTHOR]

Under sharp contact loading, glass deforms elastically and then plastically via densification as well as shear flow. Stress build-up during loading, residual stress build-up after unloading, and possible ultimate cracking all depend sensitively on the competition and interplay between densification and shear flow. The crack resistance was shown in experimental studies to generally improve with the increasing contribution of densification in glasses where shear deformation plays a dominant role under indentation. In this work, the role of densification in deformation behaviors of model metallic glasses under sharp contact loading was studied by 3-D nanoindentation tests using indenters with different sharpness in classical molecular dynamics simulations. Starting from a model metallic glass that favors shear deformation, a Lennard-Jones potential was modified to describe model metallic glasses with different abilities of instantaneous densification under compression and permanent densification after decompression. Our studies show that model metallic glass with a higher densification ability under indentation has less stress build-up and less localized shear deformation during loading, as well as smaller residual stress build-up after unloading. However, our study indicates that both instantaneous and permanent densification need to be tuned for designing damage resistant glasses. [ABSTRACT FROM AUTHOR]

We report a heuristic approach to measure the fracture energy of model metallic glasses using molecular dynamics simulation. Specifically, we adopted the Rivlin-Thomas method, simplified by Suo et al., which is applicable even with the presence of plastic flow. We further modified the testing condition with semi-rigid holders in our molecular simulations, to avoid unintended fracture near the holders. This method was first applied in measuring the fracture energy of a brittle model glass, which agrees well with direct K I C and J I C measurements (both measurements are independent of the crack size). Furthermore, the fracture energy values of a family of model metallic glasses, ranging from brittle to ductile (BTD), were measured. The Poisson's ratio-fracture energy (v-G, or v-G/2γ, normalized by the surface energy) relation obtained here exhibits a BTD transition at a critical Poisson's ratio of 0.31–0.32, consistent with experimental results. [ABSTRACT FROM AUTHOR]

Using molecular dynamics simulations, model Zr50Cu40Al10 metallic glass (MG) nanopillars were subjected to simulated irradiation processes followed by uniaxial compression tests. As the intensity or dosage of irradiation increases, the plastic deformation mode of the MG nanopillars transits from localized shear banding to homogeneous shear flow. The suppression of shear banding in MG nanopillars is due to irradiation-induced structural disordering. Furthermore, a correlation is found between the average potential energy of MG nanopillars and their deformation modes, common to both irradiation processing and thermal processing. Our results imply that the homogeneous shear flow observed in experimental MG nanopillars carved by focused ion beam may be due to irradiation damage instead of size effect. [ABSTRACT FROM AUTHOR]

For the mechanical behaviors of the BMGs, Rezvan I et al. i report the deformation-mode-sensitive behavior under dynamic loading. This topical collection is to honor Professor Peter K. Liaw for his significant contributions to materials science and engineering and to The Minerals, Metals & Materials Society (TMS). Besides HEAs, Wang I et al. i report the formation and mechanical behavior of a body-centered-cubic medium-entropy alloys (MEAs). [Extracted from the article]

We report compression-compression fatigue simulations on model metallic-glass nanowires up to a 4% maximum compressive strain and 100 cycles using the molecular dynamics method. The distribution and irreversibility of the fatigue-induced deformations are characterized by the local shear strain and rise of the system temperature, respectively. Furthermore, the system potential energy, structural signatures in terms of icosahedral centers, and radial distribution functions are obtained to identify structural transformations during cyclic loading. No noticeable changes on the aforementioned isotropic structural signatures have been found during fatigue tests. Based on a binary view of the constituting local structures (stable clusters and floppy clusters) in metallic glasses, we attribute the irreversible deformation to inter-conversion of those floppy clusters, which leads to a constant composition of stable and floppy clusters. Finally, uniaxial compression tests were carried out on the cyclically loaded samples. The mechanical behaviors of fatigue-tested samples are rather similar to the original sample. Upon cyclic loading, unlike macroscopic samples, a metallic-glass nanowire is free of permanent structural damage, and therefore does not exhibit deteriorated mechanical behaviors. The excellent fatigue-resistance behavior of a metallic-glass nanowire might be related to its defect-free structure (no stress concentrators). It appears that defects may play a central role in fatigue damages in experimental metallic-glass systems. [ABSTRACT FROM AUTHOR]

Metallic glasses (MGs) are arguably one of the most exciting metallic systems in the past 30 years, attracting significant research effort and undergoing rapid development. Parallel to research on crystalline metals, the sample size has been exploited as property-tuning parameters for MGs. It has been shown that nanometre-sized MG samples exhibit higher-than-bulk elastic limit, tensile strength, and non-zero tensile ductility. While some of the size-related observations have been generally accepted, others have been poorly understood, even hotly debated. Of particular interests is the importance of sample preparation in experiments and model generation in simulations. Here, we will review how the sample size affects mechanical properties including the elastic, plastic, fracture properties, and fatigue endurance, as well as various proposed size-dependent mechanisms and relevant length scales. [ABSTRACT FROM AUTHOR]

In molecular dynamics simulations, the local stress state in the shear band is examined in six different model metallic glasses and one amorphous Si system (also has been perceived as a metallic glass analog) under different loading conditions. For all but the FeP and the amorphous Si systems, the running shear band (RSB) exhibits a liquid-like hydrostatic plus shear stress state. Our results suggest that the liquid feature of a RSB is not due to temperature rise or plastic confinement but due to the disorder driven by flow, which can be offset by strong directionality in bonding, phase segregation, or aging. The knowledge of the liquid-like stress state can be conveniently utilized in experiments to infer the local stress state of the RSB from the global tensile stress for metallic glasses. [ABSTRACT FROM PUBLISHER]

Tensile fracture of metallic glasses due to shear-band-to-crack transition was studied by molecular dynamics. Such transition is difficult to characterize experimentally as it occurs within nanoseconds in the buried nanometer-thin shear band. We show that the sample fractures via shear band cavitation under high initial tensile stress, or with long sample length. The critical thermomechanical conditions for shear band cavitation, strongly influenced by shear, were identified. An analytical model was developed, leading to a length scale and a time scale for shear-band-to-crack transition. [ABSTRACT FROM AUTHOR]

Abstract: Simulated nanoindentation tests on a three-dimensional model of a binary metallic glass-forming alloy reveal how the stress field and material structure interact to control deformation beneath the indenter. Initially, the stress field follows the Hertzian solution with fluctuations due to heterogeneities. Homogeneous or localized plastic deformation arises, depending on the processing history of the material. In the case of localized deformation, the first shear band initiates sub-surface, relaxing the local shear stress. The subsequent shear band morphology is observed to be rate-dependent. Deformation induced changes in material structure are characterized in terms of short-range ordering and free volume, with the former providing significantly greater signal-to-noise. [Copyright &y& Elsevier]

Abstract: Nanoindentation was simulated in a two dimensional model metallic glass thin film using molecular dynamics. Strain localization was observed in simulations where the system was sufficiently structurally relaxed prior to deformation. Indentation simulations utilized an atomic indenter that adhered to the surface or a frictionless indenter. Boundary conditions were varied to constrain the film or allow the film to relax in-plane to examine the effect on shear band formation. The most constrained system, i.e. that with the atomic indenter and the constrained boundaries exhibited the highest hardness. [Copyright &y& Elsevier]

A simulation study demonstrates that localization can arise as the result of the breakdown of stable quasicrystal-like atomic configurations. Samples produced at elevated quench rates and via more energetic processes contain a lower fraction of such configurations and exhibit significantly less pronounced localization and shorter spacing between bands. In the samples produced by the lowest quench rates, localization is accompanied by the amorphization of material with initially quasicrystal-like medium range order. This result is of particular significance in light of recent experimental evidence of local quasicrystal order in the most stable of the bulk metallic glasses. [ABSTRACT FROM AUTHOR]

Understanding the dynamics of shear band propagation in metallic glasses remains elusive due to the limited temporal and spatial scales accessible in experiments. In micron-scale molecular dynamics simulations on two model metallic glasses, we studied the propagation of a dominant shear band under uniaxial tension with a macroscopic strain of 3-5%. For both materials, the shear band can be intersonic with a propagation speed exceeding their respective shear wave speeds. The propagation exhibits intrinsic instability that manifests itself as microbranching and considerable fluctuations in velocity. The shear strain singularity ahead of propagating shear band tip scales as 1/ r (r is the distance away from the tip), independent of the macroscopic tensile strain. In addition, we studied the intersection of two shear bands under uniaxial tension, during which path deflection, speed slowing-down, and temperature rise at the junction region were observed. The dynamics of propagating shear band shown here indicate that shear band in metallic glasses can be viewed as shear crack under the framework of weakly nonlinear fracture mechanics theory. [Display omitted] [ABSTRACT FROM AUTHOR]

Metallic glasses (MGs) usually suffer brittle fracture under uniaxial tension due to size-dependent shear band cavitation. Here we developed a concurrent multi-scale simulation method to describe the uniaxial tension of a binary Lennard-Jones (BLJ) model glass up to 88 microns in length. No cavitation or brittle fracture was found even for the longest BLJ sample. As the length increases, the shear band temperature increases, then saturates, while the elastic unloading from shear-off diminishes. We conjecture that the shear band of a BLJ sample, even with a macroscopic length, cannot reach the herein estimated critical cavitation conditions. Thus, BLJ samples appear to be free of size-induced tensile brittleness. Based on the shear band evolution and the critical cavitation conditions, we propose three classes of MGs in terms of tensile ductility: brittle-MGs (brittle for all lengths), normal-MGs (ductile for short samples, brittle for long samples), cohesive-MGs (ductile for all lengths). Our simulation results illustrate limitations of existing molecular models, and suggest that certain experimental metallic glasses may be free of size-induced tensile brittleness. [ABSTRACT FROM AUTHOR]

Abstract: The yielding of metallic glasses (MGs) is generally considered to be dependent on pressure or normal stress, which is usually described by the Mohr–Coulomb yield criterion. Experimentally, the orientation of the shear band angle or fracture angles in MG is roughly 40° to 42° in uniaxial compression and 50° to 59° in uniaxial tension, both with respect to the loading direction. However, the amounts of deviation from the maximum shear stress direction (45°) under uniaxial tension and compression are quite different, which seems to be inconsistent with the Mohr–Coulomb yield criterion. Here we carried out molecular dynamics simulations to study the incipient plasticity of model Dzugutov glasses subjected to various uniaxial and biaxial stress states. The yield points are defined as the sharp rise of the population of atoms participating in non-affine deformations, in addition to the traditional offset method. The yield surfaces thus created agree well with the Mohr–Coulomb yield criterion. Furthermore, the orientations of the embryonic shear bands (instead of the fully-grown shear bands, or fracture planes) are measured, which also follow predictions based on the Mohr–Coulomb criterion. Our simulation results call for orientational measurement of incipient shear bands in experiments to critically examine the Mohr–Coulomb yield criterion in MG systems. [Copyright &y& Elsevier]

Simulated nanoindentation tests on a model metallic glass reveal that the crack initiates inside a shear band via cavitation. The load-displacement curve was shown to be insensitive to the crack initiation but sensitive to subsequent crack propagation. The critical conditions for crack initiation were identified at both the macroscopic and microscopic levels. At the macroscopic level, the indenter geometry affects the overall critical load for crack initiation. Interestingly, the indentation volume at crack initiation appears to be a constant for different indenter geometries, based on which an analytical formula of the critical load as a function of the indenter geometry was derived. At the microscopic level, cavitation occurs once the normal stress perpendicular to the shear band exceeds a temperature-dependent critical cavitation stress. This critical cavitation stress was shown to reduce significantly upon shear deformation. [ABSTRACT FROM AUTHOR]

Low-cycle fatigue fracture of metallic glass nanowires was investigated using molecular dynamics simulations. The nanowires exhibit work hardening or softening, depending on the applied load. The structural origin of the hardening/softening response was identified as the decrease/increase of the tetrahedral clusters, as a result of the non-hardsphere nature of the glass model. The fatigue fracture is caused by shear banding initiated from the surface. The plastic-strain-controlled fatigue tests show that the fatigue life follows the Coffin–Manson relation. Such power-law form originates from plastic-strain-dependent microscopic damage accumulation. Lastly, the effect of a notch on low-cycle fatigue of nanowires in terms of failure mode and fatigue life was also discussed. [ABSTRACT FROM AUTHOR]

Metallic glasses, like many other glasses and ceramics, generally have zero tensile ductility. Distinct from common wisdom which seeks to toughen brittle glasses with ductile phases, we show that composites made from two brittle glasses can be ductile, strong and even harden under tension as a result of microstructure and stiffness contrast. Using molecular dynamics simulations, we designed and tested composites consisting of alternating wavy nanofilaments of two brittle glasses with different stiffness. The composites exhibit failure strain over 40% and strain hardening modulus over 2 GPa. The tensile ductility is determined by both the flexibility of nanofilaments and the propensity of crack deflection, with which a ductile composite design map can be constructed. The strain hardening is due to filament alignment analogous to linear polymer. The brittle-brittle composite design strategy proposed here is not material-specific and should be applicable to, for instance, ultra-hard materials as constituents in achieving new damage-resistance composites. [Display omitted] [ABSTRACT FROM AUTHOR]