Plastic Deformation Of Metallic Glasses Research Articles

Research progress on the shear band of metallic glasses

Metallic glasses have received considerable attention in the fields of materials science and condensed matter physics. The disordered structure of metallic glasses gives rise to unique mechanical properties, such as high strength, large elasticity and a distinct deformation mechanism in contrast to conventional crystalline alloys. The plastic deformation of metallic glasses at temperatures well below the glass transition is generally carried out through the formation of highly localized shear bands. Therefore, a thorough understanding of shear banding behavior is vital for improving the mechanical properties of metallic glasses and tailoring the properties of post-deformed metallic glasses. This review article aims to provide an up-to-date overview of the atomic mechanisms underlying shear band nucleation, propagation and interaction. Additionally, the dynamics of shear bands and their correlation with plasticity are also presented. This article also focuses on the progress in the fundamental structure and properties of mature shear bands, such as density fluctuation, medium-range order, shear-band affected zone, and diffusivity. Furthermore, the fracture mechanism based on the development of cavities is elaborated. Finally, we identify a number of key unsolved issues on this subject.

Influence of structural heterogeneity on shear bands in fracture-affected zones of metallic glasses

The catastrophic fracture of metallic glasses (MGs) caused by shear banding limits its engineering application. Structural heterogeneity of MGs strongly affects the shear banding process during plastic deformation. Here, we study the microstructure of MG fibers of different Poisson's ratios or structural heterogeneities after stretching them to fracture using synchrotron X-ray nano-computed tomography with the combination of high-resolution transmission electron microscopy. It is found that interlaminated high- and low-density layers induced by shear banding appear in the fracture-affected zone of the MG fibers. Meanwhile, the fraction of crystal-like order regions in the fracture-affected zone is larger than that of the unaffected matrix. With the increase of Poisson's ratio, the number of low-density shear bands (SBs) increases, and the thickness of a single high-density layer around SBs decreases in fracture-affected zone. The size of fracture-affected zone increases from ∼310 nm for La60Ni15Al25 MG to ∼920 nm for Pd40Cu30Ni10P20 MG. The results help to understand the influence of structural heterogeneity on the plastic deformation of MGs.

Effect of stress triaxiality on plastic deformation of metallic glasses

Aiming at fabrication of cellular metallic glasses with high mechanical properties, double edge notched samples were designed by orthogonal design method. Compressive tests on those samples were carried out to investigate mechanical responses induced by different notch settings. It was found that the notch tip distance and stress triaxiality are two key factors controlling yield strength and plasticity of notched samples. Besides, free volume theory factoring in hydrostatic stress was incorporated into finite element model to simulate shear band (SB) deformation process for different samples. Combined with experiments, simulations as well as microstructure observations, it was concluded that the notch tip distance effect could be ascribed by transition process between sparsely-intersected and densely-intersected SBs, and the later is the key to plasticity enhancement. Additionally, large stress triaxiality could improve the global plasticity by introducing densely-intersected SBs so as to hinder sample from fracturing along one major SB. The current work could offer useful data in providing direct evidence on fabrication of cellular materials as well as enriching shear band formation mechanism of monolithic metallic glasses.

Evaluating the predictive power of machine learning model for shear transformation in metallic glasses using metrics for an imbalanced dataset

Plastic deformation of metallic glasses, which show no long-range structural order, proceeds by shear transformation of a local group of atoms referred to as the shear transformation zone (STZ). Unlike crystalline solids, it is difficult to identify STZs and predict the onset of plasticity from a random atomic configuration under a given loading. Recently, significant efforts have been made to predict the shear transformation with initial atomic properties using machine learning. However, despite the class imbalance, where the atoms participating in shear transformation is much rarer compared to the others, few studies have explored the issue of the proper predictive metric choice, with most studies considering widely used metrics such as Recall or AUC in the machine learning community. Therefore, here we train a graph neural network that predicts the initially activated STZ and evaluate its predictive power using various metrics considered to be proper for handling imbalanced datasets. We find that the AUC value is significantly overestimated due to the class imbalance and too many atoms are misclassified as initial STZ, so other metrics such as the precision, f1, MCC, and AP indicate very low predictive power close to zero. Additionally, we reveal that the predictive performance changes significantly over the threshold value of non-affine displacement, above which an atom is classified as the initially activated STZ, due to the change in the degree of class imbalance. Our study implies that it is crucial to use an identical threshold for this type of classification (i.e., the class ratio) for a fair assessment of ML models adapted in different studies and to holistically evaluate the predictive performance based on various metrics.

Evolution of local densities during shear banding in Zr-based metallic glass micropillars

Shear bands (SBs) play an important role in plastic deformation of metallic glasses (MGs), but the evolving mechanism of SBs is still elusive. We study the structural formation and evolution of SBs inside MGs using synchrotron X-ray nano-computed tomography with the combination of high-resolution transmission electron microscopy. It is found that discrete anisotropic low-density regions, as manifestation of the activations of accumulated shear transformation zones, are created in the elastic deformation regime, which form SBs of density fluctuations under large plastic deformation. With further increasing plastic strain, the thickness of SBs increases from ∼ 105 nm to ∼ 180 nm, and their local densities decrease. Before the formation of cracks, the low-density regions in SBs gradually evolve into nanovoids with the matrix around densified. Compared to the unaffected matrix, densified regions with more crystal-like order (CLO) structure appear around thick SBs of severe bond breaking, and the densification is more apparent near crack surface with CLO structure further increasing. The results are significant for understanding the structural evolution during shear banding and plastic deformation mechanism for glassy materials.

Extracting governing system for the plastic deformation of metallic glasses using machine learning

Extracting governing system for the plastic deformation of metallic glasses using machine learning

In Situ Generated Shear Bands in Metallic Glass Investigated by Atomic Force and Analytical Transmission Electron Microscopy

Plastic deformation of metallic glasses performed at temperatures well below the glass transition proceeds via the formation of shear bands. In this contribution, we investigated shear bands originating from in situ tensile tests of Al88Y7Fe5 melt-spun ribbons performed under a transmission electron microscope. The observed contrasts of the shear bands were found to be related to a thickness reduction rather than to density changes. This result should alert the community of the possibility of thickness changes occurring during in situ shear band formation that may affect interpretation of shear band properties such as the local density. The observation of a spearhead-like shear front suggests a propagation front mechanism for shear band initiation here.

A Comparison of in- and ex-situ Shear Bands in Metallic Glass by Transmission Electron Microsopy

Plastic deformation of metallic glasses performed at temperatures well below the glass transition proceeds via the formation of shear bands. In this contribution shear bands originating from in situ tensile tests of Al88Y7Fe5 melt-spun ribbons conducted in a transmission electron microscope are compared with ones which had formed ex situ during cold rolling. The observed contrasts for shear bands generated in situ at the edges of thin electron-transparent foils are shown to be related to a meniscus-like foil thickness reduction. In comparison with ex situ samples, alternating contrast changes due to volume changes are missing and the shear band width is a factor of 15 larger. The meniscus-like shear band profile is accounted for by the thin foil allowing volume increase due to shear deformation to annihilate via the free surfaces of the thin foil and the sheared zones to widen.

Unveiling the Local Atomic Arrangements in the Shear Band Regions of Metallic Glass.

The prospective applications of metallic glasses are limited by their lack of ductility, attributed to shear banding inducing catastrophic failure. A concise depiction of the local atomic arrangement (local atomic packing and chemical short-range order), induced by shear banding, is quintessential to understand the deformation mechanism, however still not clear. An explicit view of the complex interplay of local atomic structure and chemical environment is presented by mapping the atomic arrangements in shear bands (SBs) and in their vicinity in a deformed Vitreloy 105 metallic glass, using the scanning electron diffraction pair distribution function and atom probe tomography. The results experimentally prove that plastic deformation causes a reduction of geometrically favored polyhedral motifs. Localized motifs variations and antisymmetric (bond and chemical) segregation extend for several hundred nanometers from the SB, forming the shear band affected zones. Moreover, the variations within the SB are found both perpendicular and parallel to the SB plane, also observable in the oxidation activity. The knowledge of the structural-chemical changes provides a deeper understanding of the plastic deformation of metallic glasses especially for their functional applications and future improvements.

From elastic excitations to macroscopic plasticity in metallic glasses

Shear transformation zone (STZ) remains the fundamental unit to explain plastic flow in metallic glasses (MGs). Although STZs have been known and characterized for decades now the morphological and dynamical characteristics of STZs are not fully understood yet. Here, simulating athermal quasistatic shearing processes, the atomic-level mechanisms underlying elastic and plastic deformation in MGs are disclosed. Given the highly heterogeneous nature of glassy materials and the related rugged energy landscape the activation of STZs is observed from an early level of elastic deformation until the final shearing stage. The STZs identified within the elastic and plastic ranges have similar fingerprints but different magnitude and dynamics. Long-range overlapping STZ fields constrain atomic rearrangements and limit quantifying structure-dynamics correlations that explains the difficulty in establishing an ideal structure-property relationships for MGs.

Effect of Stress Triaxiality on Plastic Deformation of Metallic Glasses

Effect of Stress Triaxiality on Plastic Deformation of Metallic Glasses

Deformation-enhanced hierarchical multiscale structure heterogeneity in a Pd-Si bulk metallic glass

The multiscale structures in a Pd82Si18 binary bulk metallic glass before and after deformation were studied using electron microscopies, high-energy synchrotron X-ray diffraction, and small-angle scattering techniques. The experimental results revealed an enhancement of hierarchical structure heterogeneities on multiple length scales after deformation. Hierarchical multiple shear bands of high number density were observed after bending, introducing complex but periodically distributed residual strain. Pair distribution function analysis revealed that the connectivity of the short-range clusters on the medium-range scale determines the packing density difference between the tension side and the compression side in the sample after bending. In-situ synchrotron X-ray diffraction study also revealed a transformation of connection modes among short-range clusters under uniaxial tension and compression, which is consistent with those of triaxial tension/compression parts upon bending in Pd82Si18 glassy alloys. The nanoscale heterogeneities for metallic glasses after deformation observed by small-angle scattering and transmission electron microscopy may be attributed to the nanoscale amorphous phase separation and interacting multiple shear bands enhanced by plastic deformation. Our findings suggested that the enhancement of hierarchical heterogeneous structure on multiple length scales may explain the excellent plasticity of Pd-Si glassy alloys, deepening the understanding of structure-property relation during plastic deformation in metallic glasses.

Defects activation in CoFe-based metallic glasses during creep deformation

Maxwell-Voigt model with two Kelvin units and one Maxwell unit was utilized to analyze the microalloying effect of Cu on the creep behavior of CoFe-based metallic glasses at different loading rates. The defect activation during creep deformation was detected by the relaxation time spectrum based on this model. The defect, with respect to a short relaxation time in relaxation spectra, intends to be activated at a quasi-static loading mode in the alloy with 0.5 at.% Cu addition. With further increasing loading rates, more defects with a large size were provoked activated at both hard and soft regions in the Cu-containing sample. A softening with the reduction of elastic modulus and hardness about 10 % and 15 %, respectively, was also observed in the Cu-doped sample. It is consistent with the pronounced viscoplastic deformation of this alloy along with the decrease of viscosity. Our work provides a microscopic insight into structural evolution during creep deformation in a Cu-doped metallic glass, which might help for understanding the plastic deformation of metallic glasses upon a minor addition.

Notch Effect on Plastic Deformation of Metallic Glass: A Numerical Study by Revised Free-Volume Theory

By means of a revised free-volume theory, the notch effect on metallic glass was systematically investigated by the numerical method. Simulations on specimens without notches demonstrated that the parameters being determined in this work could reasonably describe the strength asymmetry of tension and compression. Moreover, four samples with different notches were used to numerically investigate the notch effect on global strength and plasticity. A better agreement could also be achieved between current simulations with existing experimental results, compared with another free-volume model. Combined with the free-volume distribution during deformation process, it was proven that the intersection of two major shear bands is the cause for the strength and plasticity enhancement found in sample with two symmetric notches. Besides, strength asymmetry between tension and compression was also found for notched samples. Compressive strengths are accordingly higher than tensile ones. Moreover, with the augment of the aspect ratio, the plasticity for specimens with two symmetric notches was found to increase firstly and then decrease afterwards.

Irradiation-induced homogeneous plasticity in amorphous/amorphous nanolaminates

Achieving homogeneous plastic deformation in metallic glasses is a long-standing goal yet to be solved in materials science. Here we investigate the effect of ion irradiation on the plastic deformation behavior of ZrCu/ZrCuNiAlSi amorphous/amorphous nanolaminates (A/ANLs) via nanoindentation testing. The experimental results indicate a dramatic change in deformation mode from multiple shear banding events to homogeneous compressive deformation before and after ion irradiation on the A/ANLs in the areas underneath the indenter. Ion irradiation-induced changes of both fraction and distribution of free volume inside each constituent layer and interfacial state in the A/ANLs may be responsible for the unusual homogeneous deformation behavior. Our results suggest that the mechanical property of A/ANLs could be modified by tuning both the inner and interfacial structure via ion irradiation.

Strain perceptibility of elements on the diffusion in Zr-based amorphous alloys

With the discovery of bulk metallic glasses (BMGs), there has been considerable interest in understanding their mechanical behavior. In spite of these previous observations on the relation between plastic deformation of metallic glasses and their diffusion behavior, a detailed understanding on the diffusion of BMGs is still unexplored. We evaluated the contribution of deformation-induced structural transformations (elastic, anelastic, viscoplastic or viscoelastic responsive and plastic strain) on the diffusion of Zr-based bulk metallic glasses in as-cast, elastostatically stressed and plastically deformed states. Experimental investigations of the diffusion process and the elemental distributions in the glassy alloy were performed following plastic deformation by multiple cold rolling and elastostatic cyclic compression, respectively. We compared the vacancy model and the transition state model to verify the diffusion mechanism in the deformed bulk metallic glass. The diffusion of tracer atoms, i.e., Fe, in the bulk metallic glass is affected by viscoelastic responsive strain governing the transition-state model. In contrast, the diffusion of constituent atoms, i.e., Ti, Zr, in the bulk metallic glass is dominantly affected by plastic strain governing the vacancy model. The results reveal that the diffusion behavior of bulk glassy alloys can be changed by variation of the constituent elements and applying different strain modes upon 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.

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.

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