Deformation Of Metallic Glass Research Articles

Structural characteristics in deformation mechanism transformation in nanoscale metallic glasses

Deformation of metallic glasses is closely related to their microstructures which depend on the composition, processing method, and the size of the materials. This subtle structure-property relation is fairly complex and remains to be explored. Here, we scrutinize the microstructural evolution in relation to the mechanical properties in metallic glass nanowires with the same composition and size but subtle microstructural differences by controlling the preparing process using molecular dynamics simulations. The results suggest that a structural threshold exists for the transformation of deformation mechanisms in metallic glasses: when the structural feature exceeds the threshold, the deformation changes from homogeneous flow to shear localized deformation.

On the effect of hydrostatic stress on plastic deformation in metallic glasses

Previous experimental observations have demonstrated that large stress gradient results in high plasticity, while the underlying mechanism and constitutive laws are rarely reported. In this work, a micro-mechanics based model was proposed to account for the contribution made by hydrostatic stress. This micro-mechanics model was then incorporated into the free volume model describing the viscous softening behavior and a generalized constitutive law was established. Using a user material subroutine (UMAT) and von-Mises criterion, the new constitutive equations were implemented into a finite element code to simulate the material response and free volume evolution procedure under tensile and compressive loading when hydrostatic stress was taken into account. Furthermore, comparison was made between current simulation and previous theoretical or experimental results, good agreement was successfully obtained. Therefore, it is concluded that the current constitutive law is a good candidate for describing the deformation behavior of metallic glass under complex stress states.

The influence of deformation on the medium-range order of a Zr-based bulk metallic glass characterized by variable resolution fluctuation electron microscopy

The medium-range order (MRO) of both as-cast and deformed states of a Zr-based bulk metallic glass (Zr52.5Cu17.9Ni14.6Al10Ti5 (at.-%) - Vitreloy 105) were analyzed using variable resolution fluctuation electron microscopy (VR-FEM). After deformation significant structural changes in the MRO were observed inside the shear bands and the adjacent matrix; the MRO was altered in terms of types, size and volume fractions. The changes in the matrix MRO confirm the existence of a shear affected zone around the shear band in our deformed sample. Since the MRO correlation length was larger inside the shear bands compared to the surrounding matrix, it is concluded that either frictional heat dissipation or shear induced nucleation and growth also plays a role in the alteration of MRO. The current work illustrates the significance of MRO in deformation of metallic glasses and suggests that the plasticity of monolithic BMGs can be improved by tailoring topological order based on MRO types, sizes and volume fractions.

Elucidating how correlated operation of shear transformation zones leads to shear localization and fracture in metallic glasses: Tensile tests on Cu[sbnd]Zr based metallic-glass microwires, molecular dynamics simulations, and modelling

The plastic deformation of metallic glasses (MG) is well-known to occur via shear transformation zones (STZs) on the scale of atomic clusters, yet fracture of MG takes place via shear bands of the micron scale. So far, understanding on how the operation of STZs leads to shear localization and fracture remains limited. In this work, tensile tests on Cu/Zr-based MG micro-wires show that both the first-yield and fracture stress exhibit the Weibull distribution, and fractography reveals that shear localization in the form of intense shear bands leads to shear fracture. Molecular dynamics (MD) simulations show that shear bands form via the correlated emergence and operation of discrete STZs close to one another. To describe how the stochastic yet correlated occurrence of the discrete STZs leads to shear localization, a model is constructed to relate the probability of the successive operation of discrete STZs, to their nucleation density. The model predicts that, if nucleation density of the STZs grows along the strain path, as prior shear events triggers the emergence of new STZs, then successive occurrence of discrete shear events speeds up rapidly to an asymptotic state which is exactly the condition of shear localization and shear banding. Furthermore, the MD results suggest an exponential growth law for the occurrence of the STZs along the strain path, which also gives predictions in good agreement with the experimental Weibull distributions of the first-yield stress of Cu/Zr-based MGs.

High-throughput simulations for insight into grain boundary structure-property relationships and other complex microstructural phenomena

High-throughput simulations can be a powerful tool in the discovery of new materials and behaviors. As part of a special issue on Rising Stars in Computational Materials Science, this article uses the work of the author to show how high-throughput simulations have had an impact in grain boundary structure-property relationships and other complex microstructural phenomena. The work demonstrates how new tools designed to analyze large datasets produced by high-throughput simulations are enabling comprehensive grain boundary structure-property relationships to be obtained. High-throughput simulations are also used to demonstrate the impact descriptive and inferential statistics has had in extracting key aspects of deformation in metallic glasses. Finally, several different examples are used to show a balanced approach between simulations designed to survey and simulations designed for detailed analysis. Together, the two approaches provide a comprehensive picture of the variety of behaviors that exist, while ensuring that the physics underlying the behaviors are thoroughly understood.

Short-range structural origins of serration events in metallic glasses

Plastic deformation of metallic glasses under compression proceeds through sudden stress drops and accumulations known as serration or avalanche events. Experimental studies have suggested that this occurs by stick-slip behaviour of weak spots, while the atomistic structural origins underlying this phenomenon have remained unclear. In this study, we investigate deformation mechanisms underpinning serrated flow in Cu-Zr metallic glasses using molecular dynamics simulations. We show that this behaviour is essentially dictated by the short-range structural order as defined by the relative population of full icosahedra and geometrically unfavourable motifs (GUMs). The slip of atoms belonging to GUMs causes stress drops whose magnitude is directly correlated to the number of atoms involved in the slip. Continued loading leads to the breakdown of full icosahedra into GUMs, causing severe serration behaviour during the later stages of deformation. More severe and frequent stress drops are observed at higher temperatures and lower strain rates, indicating the thermally activated nature of this phenomenon. The compositions with lower Cu content show enhanced serrated flow, suggesting that ductility can be compositionally tuned. The statistical analysis of stress drops exhibits good agreement with previous experimental findings. Insights gained from this study will assist in designing new metallic glasses with superior ductility.

Strategies to tailor serrated flows in metallic glasses

Abstract

On the origin of softening in the plastic deformation of metallic glasses

In the plastic deformation of amorphous materials, dilatation occurs universally with softening. However, the critical question that whether softening is caused by dilatation or not remains an ansatz and requires experimental examination. In this work, by examining the deformation behaviors of 3 metallic glasses (MGs) with distinct Johari-Goldstein (JG) relaxation characteristics before and after crystallization with nanoindentation, it is found that the origin of softening is probably not dilatation but the collective mode in the activation dynamics of shear transformation zones (STZs). Since the deformation in the creep stage of nanoindentation of the MGs originates from the “delayed plasticity” from the loading stage and depends on the softened deforming state of the MGs, the creep depth vs. time curves in the creep stage of nanoindentation of the MGs are examined and display similar loading rate dependent softening phenomena for both the glassy and crystallized MGs. The stretched exponent and relaxation time characterizing the deformation dynamics in the creep stage of nanoindentation are derived by fitting the creep depth vs. time curves with the Kohlrausch-Williams-Watts (KWW) equation. The stretched exponents indicate similar collective modes in the activation dynamics of the elementary deformation units (STZs in glassy MGs and dislocations in crystallized MGs). The relaxation times manifest similar softening effects in the deformation accommodation processes of both the glassy and crystallized MGs, except for a glassy La MG of which the relaxation time is extraordinary for its pronounced JG relaxation. The extraordinary relaxation time of the glassy La MG precludes the possibility of shear dilatation being the main cause for the softening of the glassy La MGs indicated by the loading rate dependent creep depth vs. time curves. The stretched exponent and the relaxation time indicate the homological softening dynamics in the plastic deformation of the glassy and crystallized MGs. For the distinct elementary deformation units, i.e., STZs in glassy MGs and dislocations in crystallized MGs, noting the absence of dilatation in dislocation movements, the softening phenomena in the plastic deformation of both the glassy and crystallized MGs is suggested to originate from the collective mode in the activation dynamics of the elementary deformation units, rather than from local properties of elementary deformation units, such as dilatation in STZs.

Shear-band affected zone revealed by magnetic domains in a ferromagnetic metallic glass

Plastic deformation of metallic glasses (MGs) has long been considered to be confined to nanoscale shear bands, but recently an affected zone around the shear band was found. Yet, due to technical limitations, the shear-band affected zone (SBAZ), which is critical for understanding shear banding and design of ductile MGs, has yet to be precisely identified. Here, by using magnetic domains as a probe with sufficiently high sensitivity and spatial resolution, we unveil the structure of SBAZs in detail. We demonstrate that shear banding is accompanied by a micrometer-scale SBAZ with a gradient in the strain field, and multiple shear bands interact through the superimposition of SBAZs. There also exists an ultra-long-range gradual elastic stress field extending hundreds of micrometers away from the shear band. Our findings provide a comprehensive picture on shear banding and are important for elucidating the micro-mechanisms of plastic deformation in glasses.

Voronoi volume recovery during plastic deformation in deep-notched metallic glasses

Many previous simulations of the plastic deformation in metallic glasses showed that shear banding is the dominant mode accompanied by Voronoi volume continuous generation and softening. In the present work, we report a contrary phenomenon where Voronoi volume recovery is dominant during plastic deformation in CuZr metallic glasses under tension. By creating a notched geometry, the deformation mode of notched samples transits from shear banding to necking due to the introduction of triaxial stress state. With the suppressing of shear banding, a notch strengthening and structure ordering phenomenon together with the recovery of Cu-centered full-icosahedra fraction are also observed. The present work enriches our understanding on the mechanical behavior and deformation mechanism of metallic glasses.

Lindemann-like rule between average thermal expansion coefficient and glass transition temperature for metallic glasses

The correlations between the average linear thermal expansion coefficient < α > and glass transition temperature Tg and between GVm (G is shear modulus and Vm is molar volume) and Tg for a variety of metallic glasses (MGs) were investigated. A Lindemann-like equation, <α > ×Tg = 0.75 × 10−2, is observed, indicating that the glass transition of MGs follows Lindemann-like rule. The relationship provides a new perspective to understand the mechanism of glass transition and a simple method to predict Tg for a given MG. Additionally, Tg is found to vary linearly with GVm, implying that an inherent relevance between glass transition and plastic deformation of MGs.

Plastic Deformation Mechanism of Ductile Fe50Ni30P13C7 Metallic Glass

Shear band (SB) multiplication is considered as an essential characteristic of the plastic deformation in metallic glasses (MGs). In this work, the evolutionary characteristics of SBs and serrated behavior in ductile Fe50Ni30P13C7 MGs were studied by the finite element method simulation. The study demonstrated that the stress field would redistribute and become inhomogeneous during SB sliding, where the stress perpendicular to the original SB gradually accumulates until reaching the magnitude of yielding strength and triggering new SBs. The results of simulation are in good agreement with the SBs intersection morphologies observed in SEM images and the serration flows on the stress–strain curves. Furthermore, several factors affecting stress field distribution in MGs, such as contact friction, aspect ratio, and boundary confinement, were also analyzed and discussed. The overall results indicate that the ductility of Fe-based MGs could be achieved without any inhomogeneous structure, and scientifically explicate the dependence of SB multiplication on both material properties and loading condition, which can provide us with a deeper understanding on plastic deformation mechanism of MGs.

Nanometer-scale gradient atomic packing structure surrounding soft spots in metallic glasses

The hidden order of atomic packing in amorphous structures and how this may provide the origin of plastic events have long been a goal in the understanding of plastic deformation in metallic glasses. To pursue this issue, we employ here molecular dynamic simulations to create three-dimensional models for a few metallic glasses where, based on the geometrical frustration of the coordination polyhedra, we classify the atoms in the amorphous structure into six distinct species, where “gradient atomic packing structure” exists. The local structure in the amorphous state can display a gradual transition from loose stacking to dense stacking of atoms, followed by a gradient evolution of atomic performance. As such, the amorphous alloy specifically comprises three discernible regions: solid-like, transition, and liquid-like regions, each one possessing different types of atoms. We also demonstrate that the liquid-like atoms correlate most strongly with fertile sites for shear transformation, the transition atoms take second place, whereas the solid-like atoms contribute the least because of their lowest correlation level with the liquid-like atoms. Unlike the “geometrically unfavored motifs” model which fails to consider the role of medium-range order, our model gives a definite structure for the so-called “soft spots”, that is, a combination of liquid-like atoms and their neighbors, in favor of quantifying and comparing their number between different metallic glasses, which can provide a rational explanation for the unique mechanical behavior of metallic glasses.

Accelerating coupled finite element-kinetic Monte Carlo models: 200 $$\times $$ × speedup of shear transformation zone dynamics simulations

Shear transformation zone dynamics models of metallic glass deformation access experimentally-relevant time scales by using the kinetic Monte Carlo method to simulate small, fast, often discrete events, while the finite element method calculates macroscopic shape change and continuum-level interactions within samples. The most time-consuming portion of these models is the finite element method calculation on each step. However, in cases where the finite element mesh geometry and element elastic properties do not change from step to step, the finite element stiffness matrix (and its Cholesky factors) from previous steps can be reused. This strategy improves the asymptotic complexity of these models and in practice accelerates their execution by nearly 200 $$\times $$ . This enables simulation of larger samples in more reasonable time. A set of three-dimensional shear transformation zone dynamics simulations, with larger length scales than any currently in the literature, illustrates the utility of this approach.

Predicting Shear Transformation Events in Metallic Glasses.

Shear transformation is the elementary process for plastic deformation of metallic glasses, the prediction of the occurrence of the shear transformation events is therefore of vital importance to understand the mechanical behavior of metallic glasses. In this Letter, from the view of the potential energy landscape, we find that the protocol-dependent behavior of shear transformation is governed by the stress gradient along its minimum energy path and we propose a framework as well as an atomistic approach to predict the triggering strains, locations, and structural transformations of the shear transformation events under different shear protocols in metallic glasses. Verification with a model Cu_{64}Zr_{36} metallic glass reveals that the prediction agrees well with athermal quasistatic shear simulations. The proposed framework is believed to provide an important tool for developing a quantitative understanding of the deformation processes that control mechanical behavior of metallic glasses.

Extraordinary creep relaxation time in a La-based metallic glass

The structural relaxation times of metallic glasses (MGs) are generally believed to decrease with increasing flow rate, also known as shear thinning. In this work, an extraordinary loading rate-independent creep relaxation time, in contrast to the decreasing Maxwell relaxation time with increasing loading rate, is observed for a La-based MG with pronounced Johari–Goldstein relaxation under nanoindentation. The Johari–Goldstein relaxation in the flow of MGs is suggested to be a form of local Newtonian flow and may be not crucial in shear thinning, whereas the main α relaxation is proposed to predominate in the shear thinning of MGs, based on the disparity between the creep relaxation time and the Maxwell relaxation time. These results improve the current understandings on the structural relaxation and its role in the shear thinning and plastic deformation of MGs.

Quantifying the microstructural inhomogeneity of Zr46Cu46Al8 metallic glasses based on change ratio of polyhedral volume

The microstructural inhomogeneity of Zr46Cu46Al8 metallic glasses is investigated quantitatively by molecular dynamics simulation. A parameter, the change ratio of polyhedral volume, is used to characterize the microstructural inhomogeneity in metallic glasses. Based on identifying the polyhedral volume expansion and contraction around each atom, the microstructural heterogeneity of metallic glasses can be revealed and quantified. The relation between the redistribution of the free volume and production or annihilation of defects has been established. By molecular dynamics simulations of dynamic mechanical tests, the distribution of the change ratio of polyhedral volume is correlated to dynamic heterogeneities in metallic glasses at the atomic level. Our work opens a new technologically avenue towards the characterization of microstructural inhomogeneity that affects the thermodynamics, dynamics and deformation in metallic glasses.

Design of fracture-resistant silicon structure with molecular dynamics simulation

The inherent brittleness of crystalline silicon is a cause of concern for processing, handling and application of the material. Here a simulation-based design is proposed to reduce the risk of catastrophic brittle fracture of devices and components made of silicon. In this scheme, ultrathin layers of amorphous silicon are introduced within the crystalline silicon to create a multilayered laminated structure. Using molecular dynamics simulations, we demonstrate that in this nanolaminate, a crack growing within the crystalline region gets pinned at the amorphous layers. This phenomenon occurs due to the observed ductility in amorphous silicon, which resembles the shear-band mediated deformation of metallic glasses. Asa consequence, the catastrophic failure is averted by localizing the damage and preventing it from extending across the material.

Investigating shear band interaction in metallic glasses by adjacent nanoindentation

The plastic deformation of metallic glass (MG) at room temperature progresses mainly through shear banding. Although many previous studies have addressed the behaviors of the shear band (SB), there is still some debate regarding the SB processes, particularly its reactivation and interaction. Hence, it is essential to further understand the SB processes in MGs. In this paper, a new method via adjacent nanoindentation was introduced to investigate the SB interaction, which makes it easy to distinguish between the pre-existing and newly generated SBs. Nanoindentation was performed on Zr-based MG under various spatial intervals, and the resultant residual morphologies and serrated flows were comparatively analyzed. With a change in the interval, various SB behaviors, such as straight SBs, SB reactivation, stoppage, intersection, narrowing, and suppression were clearly observed in the interaction region. The evolution of the SBs indicated that the main carrier of the plastic deformation in the interaction region changed between the newly generated SBs and the pre-existing SBs. Corresponding to the strong SB interaction, serrated flows in the load-depth (P-h) curves were also found to be promoted. These findings are expected to enhance our understanding of SB processes, which is meaningful for controlling the SBs and tuning the surface plasticity of MGs.

Shear banding in metallic glasses described by alignments of Eshelby quadrupoles

Plastic deformation of metallic glasses performed well below the glass transition temperature leads to the formation of shear bands as a result of shear localization. It is believed that shear banding originates from individual stress concentrators having quadrupolar symmetry. To elucidate the underlying mechanisms of shear band formation, microstructural investigations were carried out on sheared zones using transmission electron microscopy. Here we show evidence of a characteristic signature present in shear bands manifested in the form of sinusoidal density variations. We present an analytical solution for the observed post-deformation state derived from continuum mechanics using an alignment of quadrupolar stress field perturbations for the plastic events. Since we observe qualitatively similar features for three different types of metallic glasses that span the entire range of characteristic properties of metallic glasses, we conclude that the reported deformation behavior is generic for all metallic glasses, and thus has far-reaching consequences for the deformation behavior of amorphous solids in general.