Abstract:Inelastic deformation of metallic glasses occurs via slip events with avalanche dynamics similar to those of earthquakes. For the first time in these materials, measurements have been obtained with sufficiently high temporal resolution to extract both the exponents and the scaling functions that describe the nature, statistics and dynamics of the slips according to a simple mean-field model. These slips originate from localized deformation in shear bands. The mean-field model describes the slip process as an avalanche of rearrangements of atoms in shear transformation zones (STZs). Small slips show the predicted power-law scaling and correspond to limited propagation of a shear front, while large slips are associated with uniform shear on unconstrained shear bands. The agreement between the model and data across multiple independent measures of slip statistics and dynamics provides compelling evidence for slip avalanches of STZs as the elementary mechanism of inhomogeneous deformation in metallic glasses. 2 One Sentence Summary:We show that bulk metallic glasses deform via slip avalanches of "weak spots", by demonstrating agreement of new high temporal resolution measurements of the slip-statistics and dynamics with the predictions of a simple mean field model for plastic deformation. Main Text:We show here that slowly sheared metallic glasses deform plastically via slip avalanches of weak spots. The weak spots are shear transformation zones (STZs), which are collective rearrangements of 10-100 atoms [1].During high temperature deformation of metallic glasses (close to the glass transition), STZs operate independently and the material flows homogeneously, in agreement with STZ theory predictions over several orders of magnitude of stress and strain rate [1,2]. At lower temperatures metallic glasses deform inhomogenously via intermittent slips on narrow shear bands [3]. At low strain rates, these slip events are manifested as sudden stress drops, called serrated flow. Analytical [4,5] and computational investigations [6,7,8] suggest STZ operation, but experimental support has been challenging because slip events are both fast (with millisecond durations) and highly localized (with thicknesses <1 µm) [3]. Here we report experimental results on the stress drop dynamics and statistics, finding excellent agreement with analytic model predictions for the slip avalanche statistics of weak spots or STZs.Many other materials-including crystals and densely packed granular solids-exhibit sudden slips during inelastic deformation. Although the mechanisms of deformation differ, the statistics and dynamics of the slip events are described by the same simple mean-field model of plastic deformation [9,10]. The model assumes that weak spots slip and then restick whenever the local shear stress exceeds a local slip threshold. Weak spots in crystals are dislocations, while in a metallic glass they are STZs. Through elastic interactions a slipping weak spot can trigger others to slip creating a slip avalanche. In crystal...

The melting transition of two-dimensional systems is a fundamental problem in condensed matter and statistical physics that has advanced significantly through the application of computational resources and algorithms. Two-dimensional systems present the opportunity for novel phases and phase transition scenarios not observed in 3D systems, but these phases depend sensitively on the system and, thus, predicting how any given 2D system will behave remains a challenge. Here, we report a comprehensive simulation study of the phase behavior near the melting transition of all hard regular polygons with 3 ≤ n ≤ 14 vertices using massively parallel Monte Carlo simulations of up to 1 × 10 6 particles. By investigating this family of shapes, we show that the melting transition depends upon both particle shape and symmetry considerations, which together can predict which of three different melting scenarios will occur for a given n. We show that systems of polygons with as few as seven edges behave like hard disks; they melt continuously from a solid to a hexatic fluid and then undergo a first-order transition from the hexatic phase to the isotropic fluid phase. We show that this behavior, which holds for all 7 ≤ n ≤ 14, arises from weak entropic forces among the particles. Strong directional entropic forces align polygons with fewer than seven edges and impose local order in the fluid. These forces can enhance or suppress the discontinuous character of the transition depending on whether the local order in the fluid is compatible with the local order in the solid. As a result, systems of triangles, squares, and hexagons exhibit a Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) predicted continuous transition between isotropic fluid and triatic, tetratic, and hexatic phases, respectively, and a continuous transition from the appropriate x-atic to the solid. In particular, we find that systems of hexagons display continuous two-step KTHNY melting. In contrast, due to symmetry incompatibility between the ordered fluid and solid, systems of pentagons and plane-filling fourfold pentilles display a one-step first-order melting of the solid to the isotropic fluid with no intermediate phase.

Ingots of the bulk metallic glass (BMG), Zr64.13Cu15.75Ni10.12Al10 in atomic percent (at. %), are compressed at slow strain rates. The deformation behavior is characterized by discrete, jerky stress-drop bursts (serrations). Here we present a quantitative theory for the serration behavior of BMGs, which is a critical issue for the understanding of the deformation characteristics of BMGs. The mean-field interaction model predicts the scaling behavior of the distribution, D(S), of avalanche sizes, S, in the experiments. D(S) follows a power law multiplied by an exponentially-decaying scaling function. The size of the largest observed avalanche depends on experimental tuning-parameters, such as either imposed strain rate or stress. Similar to crystalline materials, the plasticity of BMGs reflects tuned criticality showing remarkable quantitative agreement with the slip statistics of slowly-compressed nanocrystals. The results imply that material-evaluation methods based on slip statistics apply to both crystalline and BMG materials.

High-entropy alloys (HEAs) are new alloys that contain five or more elements in roughly-equal proportion. We present new experiments and theory on the deformation behavior of HEAs under slow stretching (straining), and observe differences, compared to conventional alloys with fewer elements. For a specific range of temperatures and strain-rates, HEAs deform in a jerky way, with sudden slips that make it difficult to precisely control the deformation. An analytic model explains these slips as avalanches of slipping weak spots and predicts the observed slip statistics, stress-strain curves, and their dependence on temperature, strain-rate, and material composition. The ratio of the weak spots’ healing rate to the strain-rate is the main tuning parameter, reminiscent of the Portevin-LeChatellier effect and time-temperature superposition in polymers. Our model predictions agree with the experimental results. The proposed widely-applicable deformation mechanism is useful for deformation control and alloy design.

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