# Atomistic origin of stress overshoots and serrations in a CuZr metallic glass

## TL;DR

This study uses molecular dynamics to reveal that stress overshoots in CuZr metallic glass are due to structural symmetry relaxation in shear bands, not free volume changes, with implications for understanding fatigue and deformation.

## Contribution

It identifies atomistic mechanisms behind stress overshoots and serrations, emphasizing shear band symmetry relaxation over free volume effects in metallic glasses.

## Key findings

- Atomic volume decreases during tensile relaxation in shear bands.
- Structural symmetry increases during relaxation, correlating with stress overshoots.
- Stress serrations are linked to shear event avalanches, not shear band relaxation.

## Abstract

In this work we use molecular dynamics simulations to study the stress overshoots of metallic glass Cu$_{50}$Zr$_{50}$ in three scenarios (unloading-reloading, slide-stop-slide, and stress serrations) that are associated with shear band relaxation. We found that, after the elastic recovery effect is factored out, atomic volume in the shear band barely changes during compressive relaxation but decreases during tensile relaxation, while local fivefold symmetry increases consistently for both cases. We propose that the atomistic mechanism for the related stress overshoots is due to the relaxation of structural symmetry, instead of free volume, in the shear band. Upon unloading, a propagating shear band continues for some time before arrested, which results in a stress undershoot and could contribute to material fatigue under cyclic elastic loads. We did not directly observe stress serrations via molecular dynamics simulations due to the very high simulated strain rates. While athermal quasistatic simulations produce serrated flow stress, we note that such serrations result from global avalanches of shear events rather than the relaxation of the shear band.