Intrinsic shear transformations in metallic glasses

TL;DR

This study uses realistic simulations to show that shear transformations in metallic glasses originate from structural inhomogeneities without identifiable defects, involving only a small fraction of atoms and challenging previous models.

Contribution

It provides the first realistic simulation-based characterization of shear transformations in metallic glasses, showing they do not stem from pre-existing structural defects.

Findings

Only about 2% of atoms participate in shear transformations.

Each shear event involves approximately 10 atoms.

Realistic cooling rates correct previous overestimations of liquid-like regions.

Abstract

Plastic flow in amorphous solids is known to be carried by localized shear transformations (STs) which have been proposed to preferentially initiate from some defect units in the structure, akin to dislocations and point defects in crystalline solids. Despite the central role of STs in the mechanical deformation of metallic glasses (MGs), our knowledge of their characteristics has so far been limited to hypothetical models, based on computer simulations using unreleastically high cooling rates. Using combined molecular dynamics (MD) and Monte Carlo (MC) simulations, here we have succeeded in solidifying a metallic liquid at an effective cooling rate as slow as 500 K/s to approach that typical in experiments for producing bulk MGs. Exploiting this realistic MG model, we find that STs do not arise from signature structural defects that can be recognized a priori. Upon yielding, only about 2% of the total atoms participate in STs, each event involving as few as ~10 atoms. These findings rectify the unrealistically high content of liquid-like regions retained in MD-produced glass structures, which has rendered the MG model artificially ductile and under-predicted the sample-wide shear modulus by at least ~20% (with respect to that of experimental BMGs). Our finding sheds light on the scope of intrinsic structural inhomogeneity as well as the indeterministic aspect of the ST emergence under mechanical loading.