Energy-state-dependent mechanical and structural heterogeneity in metallic glasses probed by nanoindentation

The plastic deformation behavior of metallic glasses is sensitive to the structural states, i.e., as-cast, aged and rejuvenated states. In the current work, Zr₅₀Cu₄₀Al₁₀ metallic glass with different energy states, which were tuned by high-pressure torsion method, is studied by nanoindentation. With the help of the statistical analysis of the first pop-in event, the cooperative shear model is used to describe the size of shear transformation zones (STZs), revealing the variations of STZs influenced by different energy states. The strain rate sensitivity of the metallic glass with various energy states obtained from creep experiments demonstrated that the mechanical softening effect induced by HPT enhances the plasticity of metallic glasses. Within the framework of molecular dynamics simulations, the structural evolution of metallic glasses at the atomic scale under different loading rates is analyzed. The results provide an atomic-scale explanation for the significant enhancement of plastic deformation in metallic glasses with higher energy state and under high strain rate. The research indicates that severe plastic deformation can nucleate STZs with lower energy barriers during the severe plastic deformation in metallic glasses with high-energy states. It also indicated that severe plastic deformation can introduce multiple shear bands during this process. Furthermore, the intersection of shear bands and the presence of multiple shear bands enhance energy dissipation, potentially improving the plasticity of metallic glasses. The underlying physics of the energy state and strain rate independence of plastic deformation is discussed, providing insights into the nucleation and propagation of STZs and shear bands in metallic glasses.