The Initiation of Shear Band Formation in Deformed Metallic Glasses from Soft Localized Domains

TL;DR

This study demonstrates that shear band formation in metallic glasses originates from strain-induced soft regions identified via atomic displacement metrics, revealing a critical softness threshold that triggers instability akin to turbulence transition.

Contribution

It introduces a novel atomic-scale softness metric based on the Debye-Waller factor to predict shear band initiation in metallic glasses, linking local softness to deformation instability.

Findings

Shear bands form through localized soft regions identified by atomic displacement.

Critical softness distribution triggers shear band formation at a specific strain.

Material thickness influences shear band emergence, with ultra-thin films showing no bands.

Abstract

It has long been thought that shear band (SB) formation in amorphous solids initiates from relatively 'soft' regions in the material in which large-scale non-affine deformations become localized. The test of this hypothesis requires an effective means of identifying 'soft' regions and their evolution as the material is deformed to varying degrees, where the metric of 'softness' must also account for the effect of temperature on local material stiffness. We show that the mean square atomic displacement on a caging timescale <u2>, the 'Debye-Waller factor', provides a useful method for estimating the shear modulus of the entire material and, by extension, the material stiffness at an atomic scale. Based on this 'softness' metrology, we observe that SB formation indeed occurs through the strain-induced formation of localized soft regions in our deformed metallic glass free-standing films. Unexpectedly, the critical strain condition of SB formation occurs when the softness (<u2>) distribution within the emerging soft regions approaches that of the interfacial region in its undeformed state, initiating an instability with similarities to the transition to turbulence. Correspondingly, no SBs arise when the material is so thin that the entire material can be approximately described as being 'interfacial' in nature. We also quantify relaxation in the glass and the nature and origin of highly non-Gaussian particle displacements in the dynamically heterogeneous SB regions at times longer than the caging time.