Surface Shear Transformation Zones in Amorphous Solids

TL;DR

This study investigates the properties of shear transformation zones (STZs) at free surfaces of amorphous solids under tensile load, revealing how temperature and strain rate influence their behavior and symmetry.

Contribution

It introduces a combined atomistic simulation approach to analyze surface STZs at finite temperature and strain rates, highlighting new surface-specific behaviors and transitions.

Findings

Surface STZs decay exponentially with distance from the surface.

At high strain rates, surface and bulk STZs exhibit similar characteristics.

At experimental strain rates, surface STZs show a loss of tensile-compression symmetry.

Abstract

We perform a systematic study of the characteristics of shear transformation zones (STZs) that nucleate at free surfaces of two-dimensional amorphous solids subject to tensile loading using two different atomistic simulation methods, the standard athermal, quasistatic (AQ) approach and our recently developed self-learning metabasin escape (SLME) method to account for the finite temperature and strain-rate effects. In the AQ, or strain-driven limit, the nonaffine displacement fields of surface STZs decay exponentially away from their centers at similar decay rates as their bulk counterparts, though the direction of maximum nonaffine displacement is tilted away from the tensile axis due to surface effects. Using the SLME method at room temperature and at the high strain rates that are seen in classical molecular dynamics simulations, the characteristics for both bulk and surface STZs are found to be identical to those seen in the AQ simulations. However, using the SLME method at room temperature and experimentally-relevant strain rates, we find a transition in the surface STZ characteristics where a loss in the characteristic angular tensile-compression symmetry is observed. Finally, the thermally-activated surface STZs exhibit a slower decay rate in the nonaffine displacement field than do strain-driven surface STZs, which is characterized by a larger drop in potential energy resulting from STZ nucleation that is enabled by the relative compliance of the surface as compared to the bulk.