Deformation Of Metallic Glass Research Articles

Integrating dynamic relaxation with inelastic deformation in metallic glasses: Theoretical insights and experimental validation

Metallic glasses (MGs), a metastable material far from equilibrium, exhibit intricate dynamic relaxation behaviors. The challenge lies in developing a model that accurately describes the dynamics and deformation mechanisms of MG. This paper introduces a model integrating dynamic relaxation with deformation behavior. Validation through dynamic mechanical analysis, stress relaxation, creep, and strain recovery tests confirm the existence of four deformation modes: elasticity, anelasticity from β relaxation, anelasticity from α relaxation at low temperatures, and viscoplasticity from α relaxation at high temperatures. The model captures all of these deformation modes. The dynamical mechanical spectrum and stress relaxation spectrum unveil dynamic features during glass to liquid transition, and a simple and effective experimental method was developed for identifying ultra-low-frequency dynamic relaxation. This work provides new perspectives on the study of relaxation dynamics in glassy states and establishes important connections between dynamic relaxation behavior and deformation mechanisms. These findings lay a theoretical and experimental foundation for the broad application of MGs.

Rheological behavior during thermoplastic deformation of metallic glasses: A molecular dynamic simulation

Nanoimprint is a low-cost and effective method to manufacture nanostructures for metallic glasses with a precision replication of the mould shape and control of size. However, the atomic structure of the materials changes during the processing, which affects the practical application and the performance of the nanostructures. This crucial issue has not been addressed as thoroughly as it deserves. Here, we investigate the rheological behaviors of metallic glasses under the deformation with different temperatures and strain rates via molecular dynamic simulation. We find an abnormal rheological mode change in the non-Newtonian region, which is attributed to the evolution of short- and medium-range ordered structures. Mechanical deformation leads to the destruction and regeneration of atomic short-range ordered structures at all strain rate ranges. Their total amount and distribution remain at a similar level when the strain rate is low. When the strain rate exceeds a critical value, the deformation accelerates the relaxation by shortening the β-relaxation, resulting in the decrease of the total amount of short-range ordered structure and reorganization in the medium-range ordered structure. Furthermore, the results show a significant inheritability from the sample during deformation to the cooled-down sample, demonstrating the influence of deformation history on the properties of materials manufactured with the nanoimprint. The deformation during the nanoimprinting could reduce the ultimate strength and increase the plasticity of the materials, which provides a potential method to precisely turn the properties of metallic glasses by controlling the rheological process in the nanoimprint possessing.

Extracting and analyzing the governing model for plastic deformation of metallic glasses

This paper first detects hidden system from plastic deformation of metallic glasses by sparse identification. The extracted model simulates four types of stress-time curves and displays the prediction of serrated events. This interpretation effectively explains various experimental phenomena of repeated yielding. Further, in terms of parametric sensitivity analysis to the model, two parameters are taken as bifurcation parameter, and the analysis of codimension-one and codimension-two bifurcation are carried out to excavate the causes of dynamic transformation, including saddle–node bifurcation, Hopf bifurcation, Bogdanov–Takens bifurcation and cusp bifurcation. Different bifurcation points correspond different types of stress-time curves. The homologous phase diagrams including periodic orbit, unstable orbit and chaotic behavior are presented to show the dynamics diversity of the model. In addition to dynamic analysis, statistical analysis for plasticity values is also applied to excavate the crossover between periodic and chaotic plastic dynamic transitions. Our results provide a novel perspective on the deformation of metallic glasses from the viewpoint of dynamic model and are also important for evaluating the plastic deformation properties of metallic glasses in practical applications.

Deformation-Induced Crystal Growth or Redissolution, and Crystal-Induced Strengthening or Ductilization in Metallic Glasses Containing Nanocrystals.

It is generally known that the incorporation of crystals in the glass matrix can enhance the ductility of metallic glasses (MGs), at the expense of reduced strength, and that the deformation of MGs, particularly during shear banding, can induce crystal formation/growth. Here, we show that these known trends for the interplay between crystals and deformation of MGs may hold true or become inverted depending on the size of the crystals relative to the shear bands. We performed molecular dynamics simulations of tensile tests on nanocrystal-bearing MGs. When the crystals are relatively small, they bolster the strength rather than the ductility of MGs, and the crystals within a shear band undergo redissolution as the shear band propagates. In contrast, larger crystals tend to enhance ductility at the cost of strength, and the crystal volume fraction increases during deformation. These insights offer a more comprehensive understanding of the intricate relationship between deformation and crystals/crystallization in MGs, useful for fine-tuning the structure and mechanical properties of both MGs and MG-crystal composites.

Tensile deformation of metallic glass and shape memory alloy nanolaminates

Mechanical deformation of Cu50Zr50 metallic glasses (MGs) with CuZr B2 layers was explored using tensile tests under iso-strain and iso-stress conditions using molecular dynamics simulations. Atomic structure identification was conducted through machine learning techniques, enabling a distinct examination of the deformation exhibited by each phase. Our results revealed early-stage plasticity driven primarily by shear transformation zones at the amorphous/crystalline interface, rather than shear band formation and propagation observed in the monolithic MG, leading to nanolaminates with enhanced strength. Furthermore, under iso-strain deformation, B2 layers underwent martensitic transformation promoted by the nucleation of voids. However, void growth quickly supersedes these effects under both iso-strain and iso-stress conditions, culminating in sample fracture. In summary, this work advances our understanding of amorphous/crystalline nanolaminates, providing valuable insights into their mechanical complexities.

Creep deformation in metallic glasses: A global approach with strain as an indicator within transition state theory

Within the framework of transition state theory, the isothermal creep behavior of metallic glasses is elucidated through a unique global approach, where the topological state is exclusively linked to measured strain. Our methodology allows the computation of the average activation volume and activation energy of deformation units as a function of strain. Experimental data from four representative metallic glasses (La30Ce30Ni10Al10Co20, La65Ni15Al25, La56.16Ce14.04Ni19.8Al10, and Cu46Zr46Al8) reveal two distinct characteristics. Below the glass transition temperature, the mechanical response is primarily influenced by secondary relaxation processes and excess configuration entropy, with activation volume increasing with strain. Upon reaching the glass transition temperature, the activation volume becomes notably larger and strain-independent. Additionally, the activation energy exhibits an increase with strain, and deformation units of varying sizes are progressively activated, from smaller to larger units. The decoupling and competition among relaxation events are correlated with the increase in the activation volume of deformation units. These findings provide valuable insights into the dynamic behavior of metallic glasses and their mechanical response across different states.

Anomalous temperature dependence of elastic limit in metallic glasses

Understanding the atomistic mechanisms of inelastic deformation in metallic glasses (MGs) remains challenging due to their amorphous structure, where local carriers of plasticity cannot be easily defined. Using molecular dynamics (MD) simulations, we analyzed the onset of inelastic deformation in CuZr MGs, specifically the temperature dependence of the elastic limit, in terms of localized shear transformation (ST) events. We find that although the ST events initiate at lower strain with increasing temperature, the elastic limit increases with temperature in certain temperature ranges. We explain this anomalous behavior through the framework of an energy-strain landscape (ESL) constructed from high-throughput strain-dependent energy barrier calculations for the ST events identified in the MD simulations. The ESL reveals that the anomalous behavior is caused by the transition of ST events from irreversible to reversible with increasing temperature. An analytical formulation is developed to predict this transition and the temperature dependence of the elastic limit.

Relevance of structural defects to the mechanism of mechanical deformation in metallic glasses

It is known that deformation in disordered materials such as metallic glasses and supercooled liquids occurs via the cooperative rearrangement of atoms or constituent particles at dynamical heterogeneities, commonly regarded as point-like defects. We show via molecular-dynamics simulations that there is no apparent relationship between atomic rearrangements and the local atomic environment as measured by the atomic-level stresses, kinetic and potential energies, and the per-atom Voronoi volume. In addition, there is only a weak correlation between atomic rearrangements and the largest and smallest eigenvalues of the dynamical matrix. Our results confirm the transient nature of dynamical heterogeneities and suggest that the notion of defects may be less relevant than that of a propensity for rearrangement.

Plasticity in cyclic indentation of a Cu-Zr-based bulk metallic glass after tensile loading: An experimental and molecular dynamics simulation study

In this study, we combined experiments and molecular dynamics simulations to investigate the deformation of metallic glasses subjected to previous tensile deformation. The experimental investigations were conducted on a Zr67Cu10.6Ni9.8Ti8.8Be3.8 (wt%) sample and the molecular dynamics simulations were performed on a Cu64.5Zr35.5 sample. Initially, plasticity was induced in the specimens by tensile deformation through the formation of shear bands, and we studied the further evolution of plasticity using cyclic indentation. We found that the macroscopic properties measured in experiments, such as hardness and hysteresis width, qualitatively agree with simulations. We use the molecular dynamics simulations to analyze the microscopic properties resulting from cyclic indentation and determine the polyhedral packing structures (‘motifs’) after tensile deformation in regions outside and around the shear bands. We then followed their evolution after each indentation cycle. Our findings indicate that areas outside the shear bands experience higher shear strain during cyclic indentation and that all motifs increased in fraction but evolved differently inside and outside the shear bands. The first indentation leads to the strongest change in microstructure, affecting both the material within and outside of shear bands. Repeated indentation mostly acts on the shear bands. These changes lead to a stabilization of the material in the sense that motifs with high coordination number and hence bond formation are established.

In situ mechanical crystallization of amorphous alloys

Metallic glasses or amorphous solids are metastable and therefore they transform to the equilibrium stable phases on thermal annealing. In recent years it has been shown that deformation of metallic glasses also results in their crystallization, referred to as mechanical crystallization. Two different types of mechanical crystallization processes have been identified. One is the ex situ mechanical crystallization, a two-stage process, where a metallic glass, produced by any of the non-equilibrium processes, crystallizes on subjecting it to mechanical deformation. The other is in situ mechanical crystallization, a single-stage process, where a metallic glass or an amorphous solid produced by mechanical alloying crystallizes on continued milling for a longer period. This route seems to offer many advantages. The present article reviews the current literature – briefly on ex situ mechanical crystallization, and in detail on in situ mechanical crystallization – and presents the prospects of this interesting route of producing alloys containing either amorphous or crystalline, or different combinations of these two types.

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.

Topologically close-packed structure characteristics of the plastic deformation regions of amorphous Cu64.5Zr35.5

Understanding the structure of the plastic deformation region is important for improving the mechanical property of amorphous alloys. What kind of structure is conducive to the formation of deformation zone is however still unclear yet. The uniaxial compression of amorphous Cu64.5Zr35.5 has been conducted by a large-scale molecular dynamics simulation. The structure characteristics of the system, especially the deformation regions are systematically investigated based on the newly developed topologically close-packed cluster (TCP) method and the shear transformation zone theory. Results indicate that the deformation process is divided into three stages that can be reasonably explicated by the population and heredity of the 30 top largest standard clusters (LaSCs). It is also found that the response of topologically close-packed (TCP) atoms to strain is closely related with the bond number connected to icosahedra atoms, and the densely packed TCP atoms are more prone to non-affine displacement than the loosely packed ones, which act as the carrier of accommodating plastic deformation. In addition, the plastic deformation units are composed of various types of LaSCs, and the region with local translational symmetry is observed. These findings greatly improve the understanding of the relationship of microstructure and deformation of 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

A hierarchically correlated flow defect model for metallic glass: Universal understanding of stress relaxation and creep

Due to the structurally disordered arrangement of atoms and deviation from thermodynamic equilibrium, the physical and mechanical properties of metallic glasses can vary with time, temperature and magnitude of strain or stress. The current work provides a theoretical framework based on the hierarchically correlated atomic theory, which allows a quantitative description of the non-elastic deformation in metallic glasses. The defect concentration is adopted as an order parameter, which can evolve with temperature and non-elastic strain owing to correlated atomic movements. Through our hierarchically correlated atomic theory, we derive the characteristic times for local shear events in metallic glasses that entail activation, growth and/or annihilation of flow defects, which however are not accounted for in the previous mean field theories. Finally, we demonstrate that the current theoretical framework can be validated by the stress relaxation and creep experiments on typical La-based metallic glasses, which in turn provides quantitative insights into the non-elastic deformation mechanisms in metallic glasses.

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.

Cycle deformation enabled controllable mechanical polarity of bulk metallic glasses

Tuning anisotropy in bulk metallic glasses, ideally isotropic, is of practical interest in optimizing properties and of fundamental interest in understanding the amorphous structure and its instability. By employing the quasi-elastic asymmetric mechanical cycling method, we effectively induce the polarized plastic response of a model bulk metallic glass, without damaging the sample or introducing significant annealing or rejuvenation effects. Moreover, the polarized anelastic limit can be well controlled by regulating the amplitude of mechanical cycling. Through the atomic-level analysis of nonaffine displacement, it is found that the asymmetric cycling can only exhaust the plastic atomic rearrangements, and the survived anelastic rearrangements govern the anelastic limit of the training direction. While usual structural indicators are not sensitive probes for the induced anisotropy, the changes in local yield stress distributions capture the polarization well. This polarized plastic response of metallic glasses is originated from the plastic-event-exhaustion induced asymmetry of local potential energy surface, rather than the frozen-in anelastic strain. Our study is of fundamental importance, which furthers our understanding of the mechanical deformation of metallic glasses and sheds some light on the prospects for improved properties through induced anisotropy.