Shear Avalanches Research Articles

In situ coherent x-ray scattering reveals polycrystalline structure and discrete annealing events in strongly coupled nanocrystal superlattices

Solution-phase bottom up self-assembly of nanocrystals into superstructures such as ordered superlattices is an attractive strategy to generate functional materials of increasing complexity, including very recent advances that incorporate strong interparticle electronic coupling. While the self-assembly kinetics in these systems have been elucidated and related to the product characteristics, the weak interparticle bonding interactions suggest the superstructures formed could continue to order within the solution long after the primary nucleation and growth have occurred, even though the mechanism of annealing remains to be elucidated. Here, we use a combination of Bragg coherent diffractive imaging and x-ray photon correlation spectroscopy to create real-space maps of supercrystalline order along with a real-time view of the strain fluctuations in aging strongly coupled nanocrystal superlattices while they remain suspended and immobilized in solution. By combining the results, we deduce that the self-assembled superstructures are polycrystalline, initially comprising multiple nucleation sites, and that shear avalanches at grain boundaries continue to increase crystallinity long after growth has substantially slowed. This multimodal approach should be generalizable to characterize a breadth of materials in their native chemical environments, thus extending the reach of high-resolution coherent x-ray characterization to the benefit of a much wider range of physical systems. Published by the American Physical Society 2024

Cooperative Shear in Bulk Metallic Glass Composites Containing Metastable β-Ti Dendrites.

We report a novel plastic deformation mechanism of bulk metallic glass composites (BMGCs) containing metastable β-Ti dendrites. Plastic deformation of the BMGCs beyond the ultimate tensile strength is mediated by cooperative shear events, which comprise a shear band in the glassy matrix and a continuous ω-Ti band with a thickness of ∼10 nm in the β-Ti dendrite. The cooperative shear leads to serrated shear avalanches. The formation of narrow ω-Ti bands is caused by high local strain rates during the cooperative shear. The cooperative shear mechanism enriches the deformation mechanisms of BMGCs and also deepens the understanding of ω-Ti formation.

Study on stochastic nature of plasticity of Cu/Zr metallic glass micropillars

Abstract Micro-sized metallic glass pillars with a diameter ranging 2.5∼7.5 μm were subjected to compression experiments using a nanoindentation tester equipped with a flat punch, to characterize their deformation behaviors. The strain-stress response and scattering mechanical properties were studied in statistics. The intermittent and trigger-aftershock shear avalanches exhibited a stochastic nature of plasticity of metallic glass specimens under microscale. The size distribution of strain bursts versus the stress where they occurred indicated the flow dynamics as a self-organized behavior with collective motion of multiple shear bands, and the maximum burst size increased with applied stress following in a power-law manner independent of burst order and pillar size. According to the experimental observation, universal features were proposed to characterize the current jerky deformation, meanwhile a Monte Carlo simulation method was developed to predict the stochastic plasticity of metallic glass under microscale.

Serration and shear avalanches in a ZrCu based bulk metallic glass composite in different loading methods

In the current research, serrated flow is investigated under tensile and compressive loading in a ZrCu-based bulk metallic glass composite (BMGC) that is well known for its plastic deformability, which is higher than that of metallic glasses. Statistical analysis on serrations shows a complex, scale free process, in which shear bands are highly correlated. The distribution of the elastic-energy density stored in each serration event follows a power-law relationship, showing a randomly generated serrated event under both tension and compression tests. The plastic deformation in the temporal space is explored by a time-series analysis, which is consistent with the trajectory convergent evolution in critical dynamic behavior even in the low strain rate regime in both tests. The results demonstrate that the secondary phase in the BMGC can stabilize the shear band extension and facilitate the critical behavior in the low strain rate regime. This study provides a strong evidence of serrated flow phenomenon in BMGC under tension test, and offers a deep understanding of the correlation between serrations and shear banding in temporal space.

Serrated flow during inhomogeneous bending deformation of bulk metallic glasses: From self-organized critical to chaotic dynamics

In this research, the dynamic behavior of serrated flows observed in the flexural stress-deflection curves during three-point bending (3 PB) tests on Zr-, Ti- and La-based bulk metallic glasses (BMGs) is statistically analyzed. Furthermore, the effect of strain rate on the serrations is studied. Based on the ductility of alloy, two distinct types of dynamics are detected. For more ductile alloys (Zr- and Ti-based BMGs) the stress drop magnitude of serrations obeys a power-law distribution of shear avalanches, which is an indicator of the self-organized critical (SOC) state in dynamics. In this case, the size of avalanches has no characteristic scale. In contrast, the size of serrations corresponding to the less ductile alloy (La-based BMG) exhibits a characteristic length scale and implies that the underlying dynamics is chaotic. Results indicate that the plasticity in less ductile alloy can be related to the stick-slip process of a few shear bands, but in the case of more ductile alloys, it can be attributed to the simultaneous nucleation and interaction of a large number of shear bands. Above a critical strain rate, the serrations on flexural stress-deflection curves do not appear and a transition occurs from the serrated to non-serrated plastic flow. The comparison of apparent activation energy for the onset of serrated flow and the STZ activation energy indicates that these two processes are interrelated. Using these values, it is possible to determine the dynamic behavior of serrated flows in BMGs.

Utilization of high entropy alloy characteristics in Er-Gd-Y-Al-Co high entropy bulk metallic glass

In the present study, we carefully evaluate the role of high entropy alloy (HEA) characteristics on glass-forming ability (GFA) and mechanical responses in a newly developed Er-Gd-Y-Al-Co high entropy bulk metallic glass (HE-BMG) by comparison with the Er-Al-Co and Er-Y-Al-Co bulk metallic glasses (BMGs). The addition of multiple rare-earth elements (REs) to Er-Al-Co BMG results in both higher fragility according to Adam-Gibbs theory and slower crystallization kinetics according to confusion principle. Based on these mutual competitions, the Er18Gd18Y20Al24Co20 HE-BMG can be successfully fabricated up to 5 mm in diameter for glass formation. In particular, the HE-BMG exhibits unexpected structural bias and more fragile manner via preferential covalent bonding of Co-Al and various RE-Al clusters, which leads to larger deviation from the typical BMGs' behavior in the relation between fragility and elastic properties. Furthermore, the HE-BMG effectively retards crystallization kinetics, which was confirmed by exceptionally sluggish nanocrystallization of multi-precipitates from metastable liquid and longer post-recalescence plateau duration in cooling curve of solidification from stable liquid. Indeed, the increased structural instability due to HEA characteristics (preferential short-range order and sluggish crystallization) in HE-BMGs reduces cut-off size of self-organized shear avalanches and increases the number of chaotic shear avalanches, which can distribute the applied strain more homogeneously, resulting in enhanced intrinsic ductility. Consequently, these results provide a guideline on how to design HE-BMGs with promising properties by utilizing HEA characteristics, and thus can give us a strategy for bridging BMG and HEA.

Modulation of plastic flow in metallic glasses via nanoscale networks of chemical heterogeneities

We systematically investigate the microstructures of metallic glasses with nanoscale networks of chemical heterogeneities introduced by the presence of a metastable miscibility gap, and their effects on modulating plastic flow of the alloys. Microstructural analysis of as-quenched alloys and the associated thermodynamic assessment in Cu-Zr-Al-Y metallic glass-forming system suggest that the existence of a metastable miscibility gap can induce not only phase-separated microstructures with sharp phase interfaces but also compositional fluctuations without a clear interface ranging from atomic scale to a few-nanometer scale in the fully amorphous alloys. The statistical analysis of shear avalanches in such compositionally heterogeneous metallic glasses reveals that chemical heterogeneities extending over a few nanometers promote a relatively large population of shear deformation units jammed before the nucleation of mature shear bands. This leads to the multiple nucleation of shear bands and sluggish deformation behavior along them. However, phase interfaces formed by phase separation inside the miscibility gap promote rapid propagation of shear bands at low flow stress, while compositional fluctuations creating non-sharp interfaces emerging at the outside of miscibility gap have relatively high resistance against shear band propagation. We hence suggest that the optimization of nanoscale compositional fluctuations in metallic glasses in terms of topology, percolation and magnitude can be an effective route for improving the materials' damage tolerance upon plastic flow.

Manipulation of thermal and mechanical stability by addition of multiple equiatomic rare-earth elements in Al-TM-RE metallic glasses

In the present study, we demonstrate that the addition of multiple equiatomic rare-earth elements (REs) in the order of electronegativity in Al84Ni10RE6 (RE = La, Ce, Nd, Gd, Y) metallic glasses (MGs) can result in stabilizing supercooled liquid region and destabilizing shear avalanches via controllably induced compositional complexity. In detail, the addition of multiple equiatomic REs leads to a relatively fragile glass, but interrupts easy crystallization behavior in Al84Ni10(LaCeNdGdY)6MG, which causes a larger supercooled liquid region and higher thermoplastic formability. Furthermore, the increased structural instability reduces cut-off size of self-organized shear avalanches and increases the number of chaotic shear avalanches, which can distribute the applied strain more homogeneously, resulting in enhanced intrinsic ductility. These results provide a guideline on how to design promising Al-based MGs with high thermal stability and improved mechanical stability, which is a key step to developing ductile Al-based bulk metallic glasses through thermoplastic forming process.

Serrated plastic flow behavior and microstructure in a Zr-based bulk metallic glass processed by surface mechanical attrition treatment

The serrated plastic flow, microstructure and residual stress of a Zr33 Cu30 Ni5 Al10 bulk metallic glass (BMG) undergone surface mechanical attrition treatment (SMAT) have been investigated by a combination of compression tests with scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and the incremental hole-drilling strain-gage method. It is found that SMAT leads to various microstructural modifications and residual stress distribution in the surface layers of the Zr-based BMG due to the mechanically-induced nanocrystallization and generation of shear bands. As a result, the BMG alloy exhibits a remarkable work-hardening like behavior and significant increase of plastic strain from less than 1 % to 15 %, and its plastic deformation dynamics yields a power-law distribution of shear avalanches. Based upon the analysis of the experimental results, it is indicated that this can be connected to the SMAT-induced microstructural modifications and the resulting residual compressive stress in the Zr-based BMG.

Loading-rate-independent delay of catastrophic avalanches in a bulk metallic glass.

The plastic flow of bulk metallic glasses (BMGs) is characterized by intermittent bursts of avalanches, and this trend results in disastrous failures of BMGs. In the present work, a double-side-notched BMG specimen is designed, which exhibits chaotic plastic flows consisting of several catastrophic avalanches under the applied loading. The disastrous shear avalanches have, then, been delayed by forming a stable plastic-flow stage in the specimens with tailored distances between the bottoms of the notches, where the distribution of a complex stress field is acquired. Differing from the conventional compressive testing results, such a delaying process is independent of loading rate. The statistical analysis shows that in the specimens with delayed catastrophic failures, the plastic flow can evolve to a critical dynamics, making the catastrophic failure more predictable than the ones with chaotic plastic flows. The findings are of significance in understanding the plastic-flow mechanisms in BMGs and controlling the avalanches in relating solids.

Chaotic state to self-organized critical state transition of serrated flow dynamics during brittle-to-ductile transition in metallic glass

We study serrated flow dynamics during brittle-to-ductile transition induced by tuning the sample aspect ratio in a Zr-based metallic glass. The statistical analysis reveals that the serrated flow dynamics transforms from a chaotic state characterized by Gaussian-distribution serrations corresponding to stick-slip motion of randomly generated and uncorrelated single shear band and brittle behavior, into a self-organized critical state featured by intermittent scale-free distribution of shear avalanches corresponding to a collective motion of multiple shear bands and ductile behavior. The correlation found between serrated flow dynamics and plastic deformation might shed light on the plastic deformation dynamic and mechanism in metallic glasses.

Self-organized Criticality Behavior in Bulk Metallic Glasses

Serrated flows are known as repeated yielding of bulk metallic glasses (BMGs) during plastic deformation under different loading conditions, which are associated with the operation of shear banding. According to the statistics of some parameters, the shear avalanches can display a self-organized critical state, suggesting a large ductility of BMGs. The emergence of the self-organized criticality (SOC) behavior in different BMGs is due to the temperature, strain rate, and chemical compositions. The SOC behavior is accompanied with the following phenomena: the interactions occur in the shear bands; the incubation time is longer than the relaxation time; the time interval is lacking of typical time scale; and the spatial or temporal parameters should display a power-law distribution.

Shear avalanches in plastic deformation of a metallic glass composite

Changes in intermittent shear avalanches during plastic deformation of a Cu50Zr45Ti5 (atomic percent) alloy in response to variant structures including fully glassy phase and/or nanocrystal/glass binary phase are investigated. Second crystalline phases are introduced into the glassy-phase matrix of a Cu50Zr45Ti5 metallic glass to interfere with the shear-avalanche process, which can release the shear–strain concentration, and then tune the critically-dynamic behavior of the shear avalanche. By combining microstructural observations of the nanocrystals with the statistical analysis of the corresponding deformation behavior, we determine the statistic distribution of shear-avalanche sizes during plastic deformation, and established its dependence on the geometric distribution of nanocrystals. The scaling behavior of the distribution of shear-avalanche sizes follows a power–law relation accompanied by an exponentially-decaying scaling function in the pure metallic glass, and the metallic glass containing the small nanocrystals, which can be described by the mean-field theory. The large shear-avalanche events are dominated by structural tuning-parameters, i.e., the resistance of shear banding, and the size and volume fraction of the second crystalline phase in metallic glasses.

Shear avalanches in metallic glasses under nanoindentation: Deformation units and rate dependent strain burst cut-off

Indented metallic glasses at the nanoscale deform via strain bursts. Conventional continuum descriptions are not appropriate for such highly stochastic, intermittent deformations. In this study, after a statistical analysis of strain bursts in five metallic glasses, the dependence of the cut-off of the strain burst size on deformation units and loading rate is established. For soft metallic glasses with smaller deformation units, cut-off of the strain burst size truncates the scale-free behavior at larger strain burst sizes. For hard metallic glasses, scale-free behavior occurs in a wide range of strain burst sizes.

Correlation between the microstructures and the deformation mechanisms of CuZr-based bulk metallic glass composites

The variation of the transformation-mediated deformation behavior with microstructural changes in CuZr-based bulk metallic glass composites is investigated. With increasing crystalline volume fraction, the deformation mechanism gradually changes from a shear-banding dominated process as evidenced by a chaotic serrated flow behavior, to being governed by a martensitic transformation with a pronounced elastic-plastic stage, resulting in different plastic deformations evolving into a self-organized critical state characterized by the power-law distribution of shear avalanches. This is reflected in the stress-strain curves by a single-to-“double”-to-“triple”-double yielding transition and by different mechanical properties with different serrated flow characteristics, which are interpreted based on the microstructural evolutions and a fundamental energy theorem. Our results can assist in understanding deformation behaviors for high-performance metastable alloys.

Plastic dynamics transition between chaotic and self-organized critical states in a glassy metal via a multifractal intermediate

Changes in intermittent serrated flow behavior during plastic deformation of Zr${}_{64.13}$Cu${}_{15.75}$Ni${}_{10.12}$Al${}_{10}$, a representative glassy metal with characteristic ductility, in response to variant strain rates and temperatures were examined. The influence of strain rates and environmental temperatures on the stress-time sequence of the plastic strain regime was investigated using comprehensive dynamical, statistical, and multifractal analyses. Three distinct spatiotemporal dynamical regimes were explored. Under small strain rates or high temperatures, the time-stress sequence exhibited a chaotic behavior. Conversely, under large strain rates or low temperatures, a transition to the self-organized critical state was observed. In addition to chaotic time series and statistical analysis, multifractal analysis was also applied to study the crossover between these two unique plastic dynamic transitions. This plastic dynamical behavior was elucidated based on the interactions between shear avalanches in the glassy metal.

Plasticity of Ductile Metallic Glasses: A Self-Organized Critical State

We report a close correlation between the dynamic behavior of serrated flow and the plasticity in metallic glasses (MGs) and show that the plastic deformation of ductile MGs can evolve into a self-organized critical state characterized by the power-law distribution of shear avalanches. A stick-slip model considering the interaction of multiple shear bands is presented to reveal complex scale-free intermittent shear-band motions in ductile MGs and quantitatively reproduce the experimental observations. Our studies have implications for understanding the precise plastic deformation mechanism of MGs.

Self-organized intermittent plastic flow in bulk metallic glasses

Under stress, bulk metallic glasses irreversibly deform through shear banding processes that manifest as serrated flow behavior. These serration events exhibit a shock-and-aftershock, earthquake-like behavior. Statistical analysis shows that the shear avalanches can self-organize to a critical state (SOC). In analogy to the smooth macroscopic-scale crystalline plasticity that arises from the spatio-temporal averages of disruptive earthquake-like events at the nanometer scale, shear avalanches in glassy metals are another model system that can be used to study SOC behavior. With our understanding of SOC behavior, we further demonstrate how to enhance the plasticity of glassy (brittle) materials. It is expected that the findings can be extended to other glassy or brittle materials.