Slip Avalanches Research Articles

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.

Shear Strain-Induced Two-Dimensional Slip Avalanches in Rhombohedral MoS2.

Slip avalanches are ubiquitous phenomena occurring in three-dimensional materials under shear strain, and their study contributes immensely to our understanding of plastic deformation, fragmentation, and earthquakes. So far, little is known about the role of shear strain in two-dimensional (2D) materials. Here we show some evidence of 2D slip avalanches in exfoliated rhombohedral MoS2, triggered by shear strain near the threshold level. Utilizing interfacial polarization in 3R-MoS2, we directly probe the stacking order in multilayer flakes and discover a wide variety of polarization domains with sizes following a power-law distribution. These findings suggest that slip avalanches can occur during the exfoliation of 2D materials, and the stacking orders can be changed via shear strain. Our observation has far-reaching implications for the development of new materials and technologies, where precise control over the atomic structure of these materials is essential for optimizing their properties as well as for our understanding of fundamental physical phenomena.

Spatial clustering of microscopic dynamics governs the slip avalanche of sheared granular materials

Establishing quantifiable links between individual-particle dynamics and macroscopic response of granular materials has been a longstanding challenge, with implications in material science, geology and industry. Despite sustained efforts in uncovering generic features in both macroscopic flow and microscopic dynamics, further advance on the subject matter demands quantitative correlations to be established. We propose a 3D convolution neural network (CNN) to quantify the link between microscopic dynamics and macroscopic stress fluctuations, including both stress recharge (stick regime) and stress drop (slip regime). Through the model interpretation, microscopic dynamics is found to demonstrate distinctive spatial patterns in the stick and slip regimes, which root in the result of free volume-induced structural rearrangements and contact network dynamics, respectively. We conclude that the spatial clustering of microscopic dynamics governs the occurrence of slip avalanches and acts as the “fingerprint” of macroscopic stress fluctuation. The data-driven framework developed in this paper can be readily extended to other amorphous solids for building cross-scale relations, paving a new way to understand the complex behavior of amorphous solids.

Micro- and macroscopic aspects of the intermittent behaviors of granular materials related by graph neural network

Slowly sheared granular materials deform via intermittent slip avalanches, characterized by repetitive cycles of elastic loading interrupted by sudden stress drops. Although many mesoscopic models have been proposed to describe the relations between generic features in the macroscopic flow and microscopic plasticity, quantitative relations between microscopic plasticity and macroscopic stress drop remain elusive. We simulate the simple shearing of a three-dimensional dense granular assembly using the discrete element method. Microscopic plasticity is more spatially concentrated and tends to generate larger active clusters with the increase of the magnitude of stress drop. The initialization, development, and aggregation of microscopic plasticity constitute the microscopic origin of macroscopic slip avalanches. We then utilize the graph convolution neural network to establish a quantitative relation between microscopic plasticity and macroscopic stress drop. Further analysis on the number of convolution layers reveals that the spatial extent of microscopic plasticity determines the magnitude of macroscopic stress drop.

Machine learning bridges microslips and slip avalanches of sheared granular gouges

Understanding the origin of stress drop of fault gouges may offer deeper insights into many geophysical processes such as earthquakes. Microslips of sheared granular gouges were found to be precursors of large slip events, but the documented relation between microslips and macroscopic stress drops remains largely qualitative. This study aims to quantitatively connect microslips to macroscopic stress fluctuations, including both stress recharges and stress drops. We examine the stick-slip behavior of a slowly sheared granular system using discrete element method simulations. The microslips are found to demonstrate significantly different statistical and spatial characteristics between the stick and slip stages. We further investigate the correlation between the macroscopic stress fluctuations and the features extracted from microslips based on a machine learning (ML) approach. The data-driven model that incorporates the information of the spatial distribution of microslips can robustly predict the magnitude of stress fluctuation. A further feature importance analysis confirms that the spatial patterns of microslips manifest key information governing the macroscopic stress fluctuations. The generalization of ML across granular gouges with different characteristics indicates the proposed model can be applicable to a broad range of granular materials. Our findings in this study may shed lights on the mechanisms governing earthquake nucleation, microslips, friction fluctuations, and their connection during the stick-slip dynamics of earthquake cycles.

Stochastic thermal feedback in switching measurements of a superconducting nanobridge caused by overheated electrons and phonons

We study correlated switchings of a superconducting nanobridge probed with a train of current pulses. For pulses with a low repetition rate each pulse transits the superconducting bridge to the normal state with probability $P$ independent of the outcomes in the preceding pulses. We show that with the reduction of the time interval between pulses long-range correlation between pulses occurs: stochastic switching in a single pulse raises the temperature of the bridge and affects the outcome of the probing for the next pulses. As a result, an artificial intricate stochastic process with an adjustable strength of correlation is produced. We identify the regime where apparent switching probability exhibits the thermal hysteresis with discontinuity at a critical current amplitude of the probing pulse. This engineered stochastic process can be viewed as an artificial phase transition and provides an interesting framework for studying correlated systems. The process resembles the familiar transition from the superconducting to normal state in the current-bias nanowire, proceeding through a phase slip avalanche. Due to its extreme sensitivity to the control parameter, i.e., electric current, temperature, or magnetic field, it offers the opportunity for ultrasensitive detection.

Strain rate-dependent avalanches in bulk metallic glasses

According to a simple mean-field model, two distinct types of slip avalanches during serrated plastic flows in bulk metallic glasses are extracted to distinguish the slipping modes of shear bands under various strain rates. Small avalanches propagate progressively, while large avalanches follow a simultaneous propagation. A weakening parameter and the critical size are defined for characterizing the two completely disparate shearing modes. The larger weakening and lower critical size mean more activated STZs, providing us a new method to explore more plasticity in bulk metallic glasses.

Similarity of internal and external friction: Soft matter frictional instabilities obey mean field dissipation through slip avalanches

Similarity of internal and external friction: Soft matter frictional instabilities obey mean field dissipation through slip avalanches

Applied-force oscillations in avalanche dynamics.

Until now most studies of discrete plasticity have focused on systems that are assumed to be driven by a monotonically increasing force; in many real systems, however, the driving force includes damped oscillations or oscillations induced by the propagation of discrete events or "slip avalanches." In both cases, these oscillations may obscure the true dynamics. Here we effectively consider both cases by investigating the effects of damped oscillations in the external driving force on avalanche dynamics. We compare model simulations of slip avalanches under mean-field dynamics with observations in slip-avalanche experiments on slowly compressed micrometer-sized Au specimens using open-loop force control. The studies show very good agreement between simulations and experiments. We find that an oscillatory external driving force changes the average avalanche shapes only for avalanches with durations close to the period of oscillation of the external force. This effect on the avalanche shapes can be addressed in experiments by choosing suitable specimen dimensions so that the mechanical resonance does not interact with the avalanche dynamics. These results are important for the interpretation of avalanche experiments with built-in oscillators, and for the prediction and analysis of avalanche dynamics in systems with resonant vibrations.

Stochastic Crystal Plasticity Models with Internal Variables: Application to Slip Channel Formation in Irradiated Metals

Stochastic crystal plasticity models are originally introduced to study slip avalanche phenomena that are ubiquitous features of the microscale plasticity of crystalline solids. Herein, a method is proposed to couple such models to the evolution of internal variables to account for microstructural hardening and softening phenomena. Specifically, strain hardening is described in terms of a Kocks–Mecking‐type dislocation density and the structural softening of irradiated metals in terms of the density of irradiation‐induced point defect agglomerates, which are cut and eliminated by moving dislocations. The interplay of both effects results in the formation of defect‐free slip channels. Critical conditions for slip channel formation are formulated, the statistical morphology of the ensuing slip channel patterns is investigated and compared with surface observations. Finally, the magnitude and nature of stress concentrations that emerge if slip channels interact with platelet‐like hard inclusions are discussed.

Seismic-like size dynamics of slip avalanches in bulk metallic glasses

The size dynamics of slip avalanches is instructive for revealing plastic deformation mechanisms of bulk metallic glasses (BMGs). Analogies of the productivity law of and the Båth’s law of seismic activity are used in the analysis of avalanches in BMGs. Two distinct types of avalanches are observed. Avalanche statistics consisting of the complementary cumulative size distribution and the relationship between avalanche durations and avalanche sizes conform with the mean-field theory. The aftershock number increases as a power-law with the mainshock size for small avalanches, which accords with the productivity law of seismic activity and can be deduced from the mean-field theory. Different statistical pictures of the Båth’s law of seismic activity are respectively identified in small and large avalanches. These results imply that room-temperature avalanches in BMGs agree with the mean-field theory and have seismic-like size dynamics.

Temperature rises during strain-rate dependent avalanches in bulk metallic glasses

Localized heating during slip avalanches is relevant to the understanding of the transition from shear banding to cracking in bulk metallic glasses (BMGs). Here we focus on the released elastic energies (incorporating the influence of machine stiffness) and avalanche durations for regularly appearing large avalanches to study the maximum temperature increases of a Zr-based BMG compressed at three strain rates of 3 × 10−5, 3 × 10−4, and 3 × 10−3 s−1. The maximum released elastic energies during slip avalanches decrease with the increase of strain rates. The durations of the avalanches are on the scale of tens of milliseconds for each strain rate. As strain rate increases, the maximum temperature rises due to shear-band slips are decreased from 2.6, 2.3 to 1.9 K. Moderate heating during the large slip avalanches for three strain rate implies that thermal softening has only a small effect on the shear-banding instability.

Beyond classical thermodynamics: Dislocation-mediated plasticity

It is suggested a thermodynamic framework to describe plastic deformation in FCC metals at low strain rates and temperatures when the major mechanism of deformation is dislocation motion. The theory involves new thermodynamic parameters: dislocation polarization tensor, stress resistance tensor, entropy and temperature of microstructure. They are linked by constitutive equations. Entropy and temperature of microstructure are associated with the rate of slip avalanches and average energy drop in slip avalanches, respectively. The theory is illustrated by comparison with experimental data.

Crackling to periodic dynamics in granular media.

We study the local and global dynamics of sheared granular materials in a stick-slip experiment, using a slider and a spring. The system crackles, with intermittent slip avalanches, or exhibits irregular or periodic dynamics, depending on the shear rate and loading stiffness. The global force on the slider during shearing captures the transitions from the crackling to the periodic regime. We deduce a dynamic phase diagram as a function of the shear rate and the loading stiffness and associated scaling laws. Using photoelastic particles, we also capture the grain-scale stress evolution, and investigate the microscopic behavior in the different regimes.

Dislocation avalanche mechanism in slowly compressed high entropy alloy nanopillars

Crystals deform by the intermittent multiplication and slip avalanches of dislocations. While dislocation multiplication is well-understood, how the avalanches form, however, is not clear, and the lack of insight in general has contributed to “a mass of details and controversy” about crystal plasticity. Here, we follow the development of dislocation avalanches in the compressed nanopillars of a high entropy alloy, Al0.1CoCrFeNi, using direct electron imaging and precise mechanical measurements. Results show that the avalanche starts with dislocation accumulations and the formation of dislocation bands. Dislocation pileups form in front of the dislocation bands, whose giveaway trigs the avalanche, like the opening of a floodgate. The size of dislocation avalanches ranges from few to 102 nm in the nanopillars, with the power-law distribution similar to earthquakes. Thus, our study identifies the dislocation interaction mechanism for large crystal slips, and provides critical insights into the deformation of high entropy alloys.

Aftershocks in slowly compressed bulk metallic glasses: Experiments and theory.

We observe two distinct interevent time patterns in the slip avalanches of compressed bulk metallic glasses (BMGs). Small slip avalanches cluster together in time, but large slip avalanches recur roughly periodically. We compare the timing patterns of BMG slip avalanches with timing patterns of earthquakes and with the predictions of a mean-field model. The time clustering of small avalanches is similar to the known time clustering of earthquake foreshocks and aftershocks.

Vanishing of room-temperature slip avalanches in a face-centered-cubic high-entropy alloy by ultrafine grain formation

High-pressure torsion (HPT) was applied to Al0.3CrFeCoNi high-entropy alloy to study the effect of severe grain-refinement on the deformation behavior. After 10 rotations of HPT, the grain size was reduced to tens of nanometers. Nano-twins and stacking faults were found in the nano-grained samples. This nano-grain formation led to an obvious increase in micro-Vickers hardness, and vanishing of slip avalanches during nanoindentation which was evidenced by the absence of pop-ins in the loading segment. These phenomena can be attributed to the affluent grain boundaries that acted as extra mediators for plastic deformation and obstacles for the propagation of lattice dislocations.

Entropy and temperature of microstructure in crystal plasticity

It has been argued for quite a while that thermodynamics of solids must include two additional thermodynamic parameters, entropy of microstructure and temperature of microstructure. Entropy of microstructure was associated with the phase volume in microstructure phase space. However, the details of this picture for particular microstructures remained vague. This paper suggests an interpretation of microstructure entropy in crystal plasticity when deformation is due to motion of dislocations. In phase space the special states, p−states, are introduced which are local minima of energy for fixed values of dislocation polarization tensor p. Microstructure entropy is logarithm of the number of p−states. It is argued that negative derivative of microstructure entropy describes the rate of slip avalanches. A proposition is made on what should be an ergodic hypothesis for dislocation dynamics. This proposition yields an interpretation of microstructure temperature as an average energy dissipation in slip avalanches. The constitutive equations for internal resistance stresses are given in terms of microstructure entropy.

Nanomechanics of slip avalanches in amorphous plasticity

Discrete stress relaxations (slip avalanches) in a model metallic glass under uniaxial compression are studied using a metadynamics algorithm for molecular simulation at experimental strain rates. The onset of yielding is observed at the first major stress drop, accompanied, upon analysis, by the formation of a single localized shear band region spanning the entire system. During the elastic response prior to yielding, low concentrations of shear transformation deformation events appear intermittently and spatially uncorrelated. During serrated flow following yielding, small stress drops occur interspersed between large drops. The simulation results point to a threshold value of stress dissipation as a characteristic feature separating major and minor avalanches consistent with mean-field modeling analysis and mechanical testing experiments. We further interpret this behavior to be a consequence of a nonlinear interplay of two prevailing mechanisms of amorphous plasticity, thermally activated atomic diffusion and stress-induced shear transformations, originally proposed by Spaepen and Argon, respectively. Probing the atomistic processes at widely separate strain rates gives insight to different modes of shear band formation: percolation of shear transformations versus crack-like propagation. Additionally a focus on crossover avalanche size has implications for nanomechanical modeling of spatially and temporally heterogeneous dynamics.

Effect of annealing on nanoindentation slips in a bulk metallic glass

We measure and analyze the statistics of nanoindentation pop-ins in a bulk metallic glass that has been annealed after being subjected to high pressure torsion. We argue that these pop-ins are avalanches of slip events. Larger slip avalanches are observed after annealing at higher temperatures. The slip avalanche size distribution shows scaling behavior and suggests the existence of a critical annealing temperature. Results are shown to be consistent with the predictions of a simple mean field theory. This implies that the statistics of slip avalanches in nanoindentation can be used to extract information about the structure, history, density, and free volume of the material. The methods employed here are expected to be applicable to a wide range of materials.