Uncovering two-step shear banding initiation in metallic glasses via complex network theory-based spatial structural parameters

Abstract

The structural origin of shear banding in metallic glasses remains elusive due to its inherent multi-scale complexity and nonlinearity. Although significant efforts have been made to characterize medium-range-order structures (MROs) formed by interconnected icosahedra, the spatial features of these structures and the atomic-level origin of shear banding are still ambiguous. In this study, two spatial parameters: “icosahedral packing tightness” and “structural integrity threshold”, are proposed based on complex network theory. These two parameters integrate local connections within neighboring icosahedra and their spatial distribution, quantitatively describing the structural evolution under interactions dominated by shear, dilatation, and rotation. A two-step structural softening mechanism for shear banding is revealed: first, the loose boundaries of icosahedral clusters are peeled by small-scale atomic behaviors, leaving densely packed cores that resist softening. Under increasing load, the dilatation expands until the general icosahedral arrangement within the stiff clusters becomes relaxed. The accumulated energy is subsequently released through the further enhancement of rotational softening of the whole rigid structure in a narrow band, leading to shear band formation. Our approach provides a comprehensive framework for characterizing structural features from short range to medium-to-long range in metallic glasses, offering new insights into the detailed origin of shear banding.