Shear band relaxation in a deformed bulk metallic glass

Relaxation of shear bands in a Pd₄₀Ni₄₀P₂₀ bulk metallic glass was investigated by radiotracer diffusion allowing to determine for the first time the effective activation enthalpy of diffusion along shear bands in a deformed glass. The shear bands relax during annealing below the glass transition temperature and the diffusion enhancement reveals unexpectedly a non-monotonous, cross-over behavior. The development of shear bands and the subsequent relaxation of stresses after the shear had been switched off are characterized on microscopic to mesoscopic length scales by molecular dynamics simulation subjecting a model glass to a constant strain rate. Mean-squared displacements as well as strain maps indicate that the heterogeneity, as manifested by shear bands in the systems under shear, persist after the shear was switched off. We observe a continued relaxation of residual stresses that remain localized in regions where the shear band has been present before, although the system is – different from the macroscopic experiment – homogeneous with respect to the local density. These results indicate that even on a local scale one may expect strong dynamic heterogeneity in deformed glassy solids due to shear banding that correlates with the existence of short-circuit type diffusion on a macroscale. The results thus suggest that plastically deformed metallic glasses present poly-amorphous systems that necessitate descriptions that are analogous to multiphase materials including the presence of heterophase interfaces.