Cascade at local yield strain for silica and metallic glass

TL;DR

This study reveals that silica and metallic glasses exhibit early, cascade-like plastic events at strains much lower than the yield point, with their likelihood influenced by material ductility and softness, shedding light on brittle-ductile transition mechanisms.

Contribution

It uncovers the existence of cascade plastic events at very low strains in glasses and links their occurrence to material ductility and softness, providing new insights into the brittle-ductile transition.

Findings

First plastic events occur at strains two orders of magnitude below yield strain.

Cascade events are not system-spanning but resemble avalanches.

More ductile glasses are more prone to early plastic events.

Abstract

We report observations of unusal \emph{first} plastic events in silica and metallic glasses in the shear startup regime at applied strain two orders of magnitude smaller than yield strain. The (non-Affine) particle displacement field during these events have complex real space structure with multiple disconnected cores of high displacement appearing at the \emph{same} applied strain under athermal quasistatic simple shear deformation, and identified by a ``cell based cluster analysis'' method. By monitoring the stress relaxation during the first plastic event by Langevin dynamics simulation, we directly show the cascade nature of these events. Thus these first plastic events are reminiscent of avalanches in the post-yielding steady state, but unlike the steady state avalanches, we show that these events are not system spanning. To understand the nature of these events, we tune three factors that are known to affect brittleness of a glass. These are (i) sample preparation history, (ii) inter-particle interactions and (iii) rigidity of the background matrix applying a ``soft matrix'' probe recently developed by some of us. In each case we show that such first plastic events are more probable in more ductile glasses. Our observations are consistent with the picture that more ductile materials are softer, implying that understanding the role of softness may be a promising route to develop microscopic quantifiers of brittleness and thus clarifying the physical origin of brittle-to-ductile transition.