Activation Of Shear Transformation Zones Research Articles

Atomistically informed mesoscale modelling of deformation behavior of bulk metallic glasses

Both atomistic and mesoscale simulation techniques have been extensively employed to gain fundamental understanding of the structures, deformation mechanisms, and structure-property relationships in bulk metallic glasses (BMGs), each with its unique strengths and limitations. Nevertheless, there is a limited degree of synergistic integration between the two approaches. In this study, we extract key properties of shear transformation zones (STZs) directly from the atomistic simulations, including their size, number of shear modes, eigenstrain, and most importantly, the activation energy barrier spectrum as a function of cooling history and strain rate. We then incorporate these STZ properties into a heterogeneously randomized STZ dynamic model implemented in a kinetic Monte Carlo algorithm to study parametrically the deformation microstructure, shear band formation and stress-strain behavior of BMGs. Two important characteristics of STZ activation that dictate the strength and ductility of a glass are identified. One is the average of the activation energy barrier spectrum (approximated by a Gaussian distribution), determined by the glass composition and processing history such as the cooling rate. The other is the amount of shift of the Gaussian distribution towards smaller activation energy barrier values during deformation, which is determined by the initial structural states and strain rate during deformation and exhibits a saturation value. These findings have allowed us to gain important fundamental insights into the correlation between the degree of shear-induced softening and the general deformation behavior of BMGs, leading to a better understanding of the correlation between the processing history/loading condition and the mechanical properties.

Micromechanical response of an electrodeposited NiP metallic glass by pillar compression under extreme conditions

In this work, the mechanical properties of pulse electrodeposited amorphous nickel phosphorous (NiP), with ∼16 wt% P, was investigated for an improved understanding of the deformation behavior of metallic glasses at extreme conditions, particularly at the microscale. Upon decreasing the temperature to sub-ambient, both the yield strength and the shear band density increase. While the change of yield strength with temperature can be well explained by a cooperative shear model, the increase in the number of shear bands can be related to an increase in potential sites for activation of shear transformation zones due to the enhanced structural heterogeneity at lower temperatures. This, however, hinders the percolation of deformation units and leads to a decrease in the shear band propagation speed, from ∼50 nm/s at 295 K to ∼20 nm/s at 195 K. By testing micropillars across seven orders of magnitude in strain rate, a two-stage strain rate sensitivity is observed, from nearly negligible at the quasi-static range (m1 = 0.001 ± 0.002 for 10−3 s−1 to 10−1 s−1) to highly positive at dynamic high strain rate range (m2 = 0.02 ± 0.004 for 1 s−1 to 1000 s−1). This transition implies a change in the yield-controlling process, from the diffusion-assisted relaxation at quasi-static conditions to the rate of energy barrier crossing of shear transformation zones at high strain rates. These findings highlight several distinctive features of microscale metallic glasses, including the reduced shear band velocity at sub-ambient temperature and the two-stage strain rate sensitivity.

Investigating the influence of medium range order defects on shear instability in Cu64Zr36 metallic glass

The occurrence of shear instability in metallic glasses is profoundly influenced by the spatial perturbation of material properties. Despite the considerable research on the origin of shear transformation zones (STZ), it remains a challenge to understand the behavior from a structural perspective. In this study, extensive molecular simulations (MD) and synchrotron diffraction were employed to characterize the structural features of medium-range order (MRO) defects in Cu64Zr36 metallic glass. These defects manifest as line-like interruptions in the connections between full icosahedral clusters. And it was also observed that the size and content of these defects are closely related to the history of the glass formation process. Moreover, we explored the contribution of these MRO defects, which are comparable in size to the STZ, in the genesis of shear instability. We uncovered that a large strain gradient exists in these regions under external loads, providing a convenient pathway for STZ activation. Notably, the generation of MRO defects are also going on as the STZ evolves. These insightful findings provide valuable insights and offer a promising avenue for the design and development of advanced metallic glass materials with enhanced structural integrity.

Distinct avalanche dynamics detected in metallic glasses with high energy state revealing the crack-like shear banding mechanism

When a sufficiently high stress is applied to a metallic glass, causing plastic deformation, the material undergoes structural reconfiguration through dissipative slip avalanche events that release local stresses. By utilizing isothermal annealing and cold rolling techniques to tune the energy levels of metallic glasses, it has been observed that structural rejuvenation is accompanied by structural relaxation, as evidenced by distinct changes in avalanche dynamics. We present detailed statistics of the avalanche dynamics during shear band formation in energy-tuned metallic glasses, ranging from structurally relaxed to rejuvenated states. By analyzing shear band characteristics and examining scaling exponents, avalanche durations, and stress relaxation rates, we can establish a connection between the local activation of shear transformation zones and the formation of macroscopic shear bands. The statistics of avalanche duration indicate that an increase in soft zones within metallic glasses can alleviate stress release and stabilize plastic flow, as evidenced by the characteristics of shear bands. We attribute the significant transition of serrated flow, observed at different energy levels (i.e., as-cast, relaxed, and rejuvenated states) to the variations in nucleation and multiplication of shear bands that originate from local weak spots. Analysis of the distinct avalanche dynamics suggests that in lower energy level metallic glasses, the nucleation and propagation of shear bands exhibit localized crack-like behavior, while in higher energy level metallic glasses, they display diffused crack-like characteristics. Indeed, our results strongly support that the decreased avalanches observed in the high energy level metallic glasses originate from the nucleation of numerous small shear bands, which directly compete with the propagation of the main local shear band. These findings deepen our fundamental understanding of the relationship between the microscopic mechanism of slip avalanche dynamics and shear banding, providing a pathway to control the plasticity of metallic glasses.

Composition-dependent fracture energy in metallic glasses

The interplay between metallic glasses (MGs) mechanical properties, fracture energy $(G)$, and glass-forming ability (GFA) and their dependence on alloy composition remain poorly understood. Here, we perform molecular dynamics simulations to investigate the intrinsic composition dependence of $G$ in ${\mathrm{Cu}}_{x}{\mathrm{Zr}}_{100\ensuremath{-}x}\mathrm{MGs}$ $(x=20,30,40,50,64)$. The results indicate that the value of $G$ increases with Cu content. In addition, it is revealed that MGs with higher $G$ values display higher Poisson's ratio (\ensuremath{\nu}) and GFA, suggesting a close correlation between fracture toughness, mechanical properties, and GFA. This correlation between G, \ensuremath{\nu}, and GFA can be understood based on the fragility ($m$) of supercooled liquids, which is directly related to the structural heterogeneity in MGs. Larger $m$ values are related to dynamic slowdown and supercooled liquid stabilization, which enhance GFA and the formation of pronounced structural heterogeneity, comprised of loosely packed regions that favor \ensuremath{\beta} relaxation and the activation of shear transformation zones. Those concurrently promote the expansion of the plastic zone at the crack tip, enhancing the observed value of $G$. These simulation results shed light on the intrinsic relationship between fracture toughness, mechanical properties, and alloy composition in MGs.

The effect of pressure-promoted thermal rejuvenation on the fracture energy of metallic glasses

Despite that the pressure-promoted thermal rejuvenation is a promising remedy for the plasticity enhancement of metallic glasses (MGs), there has been so far little studies of the effect of rejuvenation on the fracture toughness of MGs. Herein, molecular dynamic simulations are performed to study the influence of annealing pressure (Pa) on the fracture energy (G) of rejuvenated MGs. Our results manifest that the G increases at first and then decreases with Pa, showing a two-stage behavior. The increase of G is attributed to the improved plasticity but marginal change of strength, while the decrease of G is caused by the significant reduction of strength. Another key finding is that the improved plasticity arises from the more homogeneous activation of shear transformation zones due to its lower activation energy in the high-energy rejuvenated state. The present study establishes the influence of Pa on G based on the internal structure analyses.

Molecular dynamics study of fracture and plastic deformation of Cu/Cu64Zr36 crystalline/amorphous composites with a pre-existing void

In this paper, the fracture behavior and damage mechanism of Cu/Cu64Zr36 crystalline/amorphous composites with a pre-existing void are investigated by molecular dynamics (MD) simulation. The results show that the incorporated Cu crystalline phase improves the global plasticity by dislocation slip, effectively avoids catastrophic localization and hinders shear band propagation by controlling the volume ratio of the crystalline/amorphous composite. As the volume fraction of crystalline phase increases, the dislocation density increases, while the degree of strain localization decreases which is associated with homogeneous activation of shear transformation zones (STZs). Numerous dislocation movements and homogeneous STZ activation become the main reason for enhancing the global plasticity of the composites. In addition, the existence of voids changes the distribution of shear strain: the peak shear strain is distributed around the void, where it is most likely to activate dislocation movement and STZ percolation as well as embryonic shear band formation.

On the deformation and failure mechanisms of hydrogen alloyed metallic glasses

While microalloying of metallic glasses (MGs) using hydrogen emerged a few years ago, the underlying mechanism of how hydrogen microalloying influences mechanical behaviors is currently still under debate. Herein, we perform atomistic simulations to investigate the effect of hydrogen microalloying on the mechanical profiles of MGs. The mitigation of strain localization accompanied with plenty of shear transformation zones (STZs) is exhibited in H-alloyed MGs and becomes more significant with the increase of H content. The addition of hydrogen induces a loose atomic structure with fertile liquid-like regions and abundant free volume, which triggers more STZ events and the softening behavior. Furthermore, hydrogen doping lowers the STZ activation energy barrier, which is verified by a theoretical model based on the concept of energetic criterion. Our work provides some new insight on how to tune the local packing of MGs and further alter the deformation behavior.

Interface dominated deformation transition from inhomogeneous to apparent homogeneous mode in amorphous/amorphous nanolaminates

• A length-scale-dependent mechanical properties were observed in the A/ANLs. • The hardness of A/ANLs was closely related with their deformation mode. • The deformation mode of A/ANLs was determined by the interface. The amorphous/amorphous nanolaminates (A/ANLs) have aroused great attentions owing to their tunable structure and enhanced mechanical properties. However, the plastic deformation mechanism of A/ANLs have yet been clarified. Here, we systematically examined the mechanical properties and deformation behavior of series of NiNb/ZrCuNiAl A/ANLs via nanoindentaion test. It was found that both the amount and morphology of amorphous/amorphous interface (A/AIs) played crucial roles in the plastic deformation of A/ANLs. Less and straighter A/AIs facilitated multiple shear banding deformation, of which the hardness increased with decreasing layer thickness, as the A/AIs hindered the propagation of shear bands (SBs). Whilst, more and wavier A/AIs promoted homogeneous deformation, of which the hardness stayed at a much lower value and was relatively irrelevant with the layer thickness, for the promoted activation of shear transformation zones by A/AIs. Our results provide guidance for modifying the mechanical properties of amorphous alloys with interface engineering design.

Dynamic mechanical relaxation and thermal creep of high-entropy La30Ce30Ni10Al20Co10 bulk metallic glass

Dynamic mechanical relaxation is a fundamental tool to understand the mechanical and physical properties of viscoelastic materials like glasses. Mechanical spectroscopy shows that the high-entropy bulk metallic glass (La30Ce30Ni10Al20Co10) exhibits a distinct β-relaxation feature. In the present research, dynamic mechanical analysis and thermal creep were performed using this bulk metallic glass material at a temperature domain around the β relaxation. The components of total strain, including ideal elastic strain, anelastic strain, and viscous-plastic strain, were analyzed based on the model of shear transformation zones (STZs). The stochastic activation of STZ contributes to the anelastic strain. When the temperature or external stress is high enough or the timescale is long enough, the interaction between STZs induces viscous-plastic strain. When all the spectrum of STZs is activated, the quasi-steady-state creep is achieved.

Shear Transformation Zone (STZ) plasticity analysis of constrained shear

The response in constrained shear of materials characterized by discrete Shear Transformation Zone (STZ) plasticity is investigated. A plane strain strip of finite thickness is analyzed within a small deformation framework and with the STZs modeled as Eshelby inclusions in an isotropic elastic solid. The strip is unbounded and consists of periodic unit cells in the shear direction. Within each unit cell, potential STZ sites are randomly distributed with a specified density and with randomly distributed critical values of shear strain energy density for STZ activation. For both a narrow and a broad distribution of values of critical shear strain energy density, the maximum shear stress is essentially independent of cell geometry but the post-maximum stress–strain response can vary with the geometry and size of the unit cell. The distribution of critical values of STZ shear strain energy density plays a key role in determining the post-maximum stress–strain response, with the post stress maximum shear stress dropping sharply for a narrow distribution of values of critical shear strain energy density and more gradually for a broader distribution of values of critical shear strain energy density. The deformation mode involves bands of enhanced shear strain that are nearly uniformly spaced for a narrow distribution of values of critical shear strain energy density, while for a broad distribution of values of critical shear strain energy density, the band spacing is more irregular. Within the bands the Mises effective stress magnitude can exceed the theoretical strength estimate E∕10.

Molecular Dynamics Study of the Nanoindentation Behavior of Cu64Zr36/Cu Amorphous/Crystalline Nanolaminate Composites.

Amorphous/crystalline nanolaminate composites have aroused extensive research interest because of their high strength and good plasticity. In this paper, the nanoindentation behavior of Cu64Zr36/Cu amorphous/crystalline nanolaminates (ACNLs) is investigated by molecular dynamics (MD) simulation while giving special attention to the plastic processes occurring at the interface. The load–displacement curves of ACNLs reveal small fluctuations associated with shear transformation zone (STZ) activation in the amorphous layer, whereas larger fluctuations associated with dislocations emission occur in the crystalline layer. During loading, local STZ activation occurs and the number of STZs increases as the indentation depth in the amorphous layer increases. These STZs are mostly located around the indenter, which correlates to the high stresses concentrated around the indenter. When the indenter penetrates the crystalline layer, dislocations emit from the interface of amorphous/crystalline, and their number increases with increasing indentation depth. During unloading, the overall number of STZs and dislocations decreases, while other new STZs and dislocations become activated. These results are discussed in terms of stress distribution, residual stresses, indentation rate and indenter radius.

Effect of the initial temperature on the shock response of Cu50Zr50 bulk metallic glass by molecular dynamics simulation

The effect of initial temperature on the shock response of Cu50Zr50 bulk metallic glass (BMG) is investigated by molecular dynamics simulations using the multiscale shock technique. The shock Hugoniot relationship of Cu50Zr50 BMG shows an obvious temperature dependence. At the Hugoniot elastic limit (HEL), the critical shear stress decreases with increasing initial temperature; this is modeled based on the activation of shear transformation zones. Shock at high temperatures shows a pressure-dependent HEL. The deformation map reveals that there are three deformation regimes under different shock intensities and temperatures: elastic, plastic, and shock-induced melting. The flow stress decreases with increasing initial temperature confirming that high temperatures reduce the shear resistance of the Cu50Zr50 BMG. At high temperatures and pressures, a shock-induced melting regime is identified using a diffusivity-based analysis.

Enhanced ductility in Cu64Zr36 metallic glasses induced by prolonged low-energy ion irradiation: A molecular dynamics study

The irradiation induced variations in microstructure and mechanical properties of the Cu64Zr36 metallic glasses (MGs) are investigated via large-scale molecular dynamics simulations. The irradiation condition is modeled by sequential and prolonged collision cascades. The simulation results show a simultaneous decrease of the density and the fraction of Cu-centered full icosahedron (CCFI) clusters in the irradiated samples. This microstructural evolution induced by the prolonged irradiation is proposed to be responsible for the change of deformation mode of tested samples under uniaxial tensile loading. When increasing the irradiation dose, the ultimate tensile strength of MG samples decreases gradually, while their deformation mechanism switches from localized shear banding to ductile necking and finally to ideally plastic flow, enhancing the MG ductility. Careful atomic-scale examinations on the initiation and development of shear transformation zones (STZs) are carried out for a better understanding of the irradiation-induced ductility enhancement in MGs. Two related effects of the irradiation are unveiled. On the one hand, the irradiation releases the stored strain energy by promoting the STZ activation within the MG matrix. On the other hand, the irradiation damages restrain the propagation of shear band through the innovatively proposed two-unit “STZ-vortex” mechanism. Under the combined effect of these two mechanisms, the low-temperature ductility of irradiated MGs is improved and the brittle-like failure driven by single shear banding is prevented.

Guiding shear bands in bulk metallic glasses using stress fields: A perspective from the activation of flow units

Controlling shear band propagation is the key to obtain ductile metallic glasses. Here, we use a residual stress field to vary the direction of shear band propagation. We ascribe this behavior to the effect of the stress field on the activation of shear transformation zones (STZs) along their characteristic direction and we quantify this contribution to the energy of the process. Because of the progressively adverse orientation of the stress field, the energy stored as shear in the STZ decreases to a level where shear band propagation at alternative angles becomes energetically more favorable.

Intrinsic and extrinsic effects on the brittle-to-ductile transition in metallic glasses

The effects of cooling rate, temperature, and applied strain rate on the tensile deformation behavior of a Cu64Zr36 metallic glass (MG) are investigated using large-scale molecular dynamics simulations. An increase in the quenching rate during sample preparation, as well as an increase of the temperature or the applied strain rate, affects the activation of shear transformation zones (STZs) and, consequently, the shear-banding processes, which ultimately causes a brittle-to-ductile transition in the deformation behavior of MGs. A quantitative interpretation for the observed enhanced ductility in MGs with an increasing quenching rate is obtained by sampling the saddle points on the potential energy surface. High quenching rates lead to lower energy barriers for activation of a local atomic rearrangement (STZ) as compared to those MGs obtained at low quenching rates. Although the glassy structure does not show significant variations with increasing temperature, the kinetic energy of the atoms increases dramatically, which allows the atoms to rearrange easily; therefore, the probability of homogeneous thermal activation of STZs increases. Finally, a large number of STZs can also be activated by deformation at high strain rates when a large amount of elastic energy is stored in the glassy matrix. Consequently, a high density of STZ events and, therefore, a more complex percolation process results in a low probability for strain localization and formation of critical shear bands. Our results provide an atomistic understanding for the strain localization mechanisms in metallic glasses and shed more light on the brittle-to-ductile transition.

Intermediate temperature ductility under uniaxial compression in a bulk metallic glass

Quasi-static compression tests were conducted on a Zr-based bulk metallic glass in a temperature ranges from 77 K to 370 K. It is observed that the compressive plasticity presents a maximum value at 273 K, which is about 0.4Tg of the BMG. This is different from usually reported better compressive plasticity under lower ambient temperature. The free volume assisted activation of shear transformation zones and the thermal conduction between shear layer and the rest of the specimen are validated to be suitable to rationalize these observations.

Aspect ratio-dependent nanoindentation behavior of Cu64Zr36 metallic glass nanopillars investigated by molecular dynamics simulations

In this paper, we study nanoindentation in Cu64Zr36 metallic glass (MG) nanopillars with different aspect ratios by molecular dynamics simulations. The activation of shear transformation zones (STZs) and the deformation behavior of MG pillars are discussed during nanoindentation loading and unloading processes. Buckling and serrated flow are the two types of deformation behaviors observed during nanoindentation. For large aspect ratio pillars, a sudden stress drop in the load–displacement curve is found that relates to the buckling process, while smaller aspect ratio pillars exhibit large stress fluctuations. The serrated flow is associated with STZ activation. STZs are locally activated, and their number gradually increases with increasing indentation depth during loading, whereas their number decreases during unloading. For pillars with a large aspect ratio, no new STZs are activated and their number decreases rapidly once the indenter has left the sample because of the buckling deformation. In contrast, new STZs are activated for pillars with smaller aspect ratio during the unloading process. Analysis of STZ activation and shear localization reveals an inhomogeneous deformation process and an increase in the degree of structural heterogeneity as the aspect ratio of the pillars increases for both loading and unloading stages. The present work provides an insight into the atomic-scale plastic deformation behavior of MG nanopillars during nanoindentation loading and unloading processes.

Room temperature activated slow β relaxation and large compressive plasticity in a LaCe-based bulk metallic glass

Activation of the β relaxation of metallic glasses is usually related to the activation of the structure units which correlates with the activation of shear transformation zones. In the present work, a pronounced slow β relaxation behavior was observed around ambient temperature in (La25Ce75)65Al11Co24 metallic glass. Based on the compression tests at 223 K and 300 K, the results demonstrated that compression achieves large compressive plasticity with true plastic strain over 160% at room temperature, while the compressive plastic strain less than 1% at 223 K. From the perspective of structural heterogeneity, the (La25Ce75)65Al11Co24 metallic glass provides a model alloy to probe the mechanisms of deformation at room temperature induced shear transition behaviors.

PREDICTION OF SHEAR TRANSFORMATION ZONES IN METALLIC GLASSES BASED ON LAPLACIAN OF ATOMIC VOLUME

Shear transformation zone (STZ), as a basic characteristic unit of plastic events in metallic glasses (MGs), has been widely accepted by researchers, but the source of its origin and activation mechanism are still controversial. Deformation behaviours of Cu$_{64}$Zr$_{36}$ MGs under simple shear loadings are investigated using molecular simulation method in this paper. The results indicate that the activation locations of STZ are related to the initial configuration of MGs. Though the field of atomic volume and its gradient are a direct representation of the local atomic structural heterogeneity of MGs, they lack an obvious correlation to the regions of STZ activation. A new local structural parameter $\xi $ is proposed in this paper based on the initial configuration of MG to predict the potential regions of STZ. $\xi $ is the product of two factors: the Laplacian of atomic volume field (AVF) and the absolute difference between components of the gradient of AVF. Vectors of the AVF gradient present a distribution pattern of pointing inside if the Laplacian of AVF is negatively large, representing the localized soft regions in MGs. The absolute difference of AVF gradient components is used to select different patterns of the AVF gradient distribution. Furthermore, the relationship among structural parameter $\xi $, nonaffine displacement and shear localization is established, revealing that only certain patterns of AVF gradient distribution would lead to nonaffine displacements field strengthening shear localization, which is more likely to result in activation of STZs. The correlation analysis shows that the averaged spatial correlation index of $\xi $ and STZ is larger than 78%, so $\xi $ can be used as an effective parameter for predicting the activation regions of STZs in MGs. Moreover, the ideology of using Laplacian of local AVF in predicting potential STZ regions in MGs would bridge the analysis between atomic simulations of MGs, the mechanism of STZ activations and the traditional mechanical theory.