Quasi phase transition model of shear bands in metallic glasses

A quasi phase transition model of shear bands in metallic glasses (MGs) is presented from the thermodynamic viewpoint. Energy changes during shear banding in a sample-machine system are analyzed following fundamental energy theorems. Three characteristic parameters, i.e. the critical initiation energy ΔG_c, the shear band stability index k₀, and the critical shear band length l_c, are derived to elucidate the initiation and propagation of shear bands. The criteria for good plasticity in MGs with predominant thermodynamic arrest of shear bands are proposed as low ΔG_c, large k₀, and small l_c. The model, combined with experimental results, is used to analyze some controversial phenomena of deformation behavior in MGs, such as the size effect, the effect of testing machine stiffness and the relationship between elastic modulus and plasticity. This study has important implications for a fundamental understanding of shear banding as well as deformation mechanisms in MGs and provides a theoretical basis for improving the ductility of MGs.