Bulk Metallic Glasses Deform via Slip Avalanches

Abstract

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...

Avalanche statistics from data with low time resolution

“…More importantly, we introduce improved analysis methods that circumvent these problems. We then apply these analysis methods to downsampled high-resolution experimental data on compressed bulk metallic glasses and find good agreement with results previously obtained at high resolution [13]. The results of this study resolve an apparent discrepancy between the results from experiments at different time resolutions reported in the literature.…”

“…The often-observed power-law distribution of avalanche sizes is believed to reflect proximity to a nonequilibrium critical point. A simple mean-field model [18] that exhibits tuned critical behavior in the depinning universality class agrees with experimental data on slip avalanches during the slow deformation of crystalline materials [10], amorphous materials [13], and granular materials [7,19], as well as many other systems with yielding behavior [7].…”

“…The large avalanches are less frequent but almost regularly recurring and do not show a power-law size distribution. The large shear bands are clearly related to the propagation of fully formed, system-spanning shear bands [13,23]. We focus here on the small events.…”

“…There are two types of avalanche events observed for BMGs. We have previously referred to these events as small and large avalanches [13,23]. The small avalanches show a power-law size distribution and self-similar dynamics and are likely related to incipient shear bands.…”

“…Avalanches are observed in a variety of dynamical systems, including magnetic materials [1,2], charge density waves [3,4], vortices in superconductors [5], earthquakes [6,7], crystal plasticity [7][8][9][10][11], and amorphous plasticity [7,8,[12][13][14][15][16][17]. For these last two cases, deformation occurs through small jumps caused by slipping weak spots in the material.…”