Local Structural Signatures of Shear Bands in Metallic Glasses via Electron Nanodiffraction

Abstract

Structural changes in a glass due to deformation are subtle and difficult to quantify using conventional imaging and diffraction techniques. Additionally, transmission electron microscopy (TEM) sample preparation using energetic ions often causes structural modifications that are challenging to detect in disordered materials. By preparing inverted cross-sectional transmission electron microscopy lamellae of shear bands formed during bending, and employing cryogenic ion polishing to minimize preparation artefacts, we preserve the intrinsic atomic structure. Using sensitive, new parameters derived from electron nano-diffraction, we directly probe the local nano-scale structure in the plastic zone beneath surface shear steps in metallic glasses. Mapping of local centrosymmetry and strain reveals nanoscale, stripe-like regions oriented at 45 degree to the applied strain where strain has localized. These regions exhibit a high density of local atomic structures that have transformed to configurations with reduced centrosymmetry and increased magnitudes of shear and normal strain. Our results demonstrate that plastic deformation in metallic glasses arises from coordinated nanoscale structural transformations, providing direct experimental insight into a long-standing problem.