Strain-dependent Transition of the Relaxation Dynamics in Metallic Glasses

Non-exponential relaxation is pervasive in glassy systems and intimately related to unique thermodynamic features, such as glass transition and aging; however, the underlying mechanisms remain unclear. The time scale of non-exponential relaxation goes beyond the time limit (nanosecond) of classic molecular dynamics simulation. Thus, the advanced time scaling atomistic approach is necessary to interpret the relaxation mechanisms at the experimental timescale. Here, we adopted autonomous basin climbing (ABC) to evaluate the long-time stress relaxation. At the same time, based on the energy minimization principle, we carried out simulations at continuum levels on the long-time stress relaxation kinetics of Cu-Zr metallic glass over timescales greater than 100 s. Combined with atomistic and continuum models, we demonstrate that a strain-dependent transition from compressed to stretched exponentials would happen, consistent with recent experimental observations on metallic glasses. Further examination of the spatial and temporal correlations of stress and plastic strain reveals two predominant driving forces: the thermal energy gradient governs in the compressed regime and leads to a release of the local internal stress; in the stretched regime, the strain energy gradient rules and causes long-range structural rearrangements. The discovery of the competition between two driving forces advances our understanding of the nature of aging dynamics in disordered solids.