Understanding the stretched exponential decay of strained metallic glasses by a free-volume model

Physical ageing of a strained metallic glass has caused its nonexponential decay to shift from compressed to stretched, yet the reason is unknown. Compared to structural relaxation taking place at temperatures close to the glass-transition temperature, where most ageing data were made, straining a metallic glass at room temperature causes rejuvenation. We discovered that the varying stretching-factor values are due to the changing rate of free volume, as calculated from a theoretical model using shear stress and temperature as inputs. Furthermore, independent of relaxation, equilibrium, or rejuvenation, the rate magnitude rather than the rate sign is what matters for stretching factor. The correlation of the stretching factor and free-volume changing rate is confirmed by a tensile creep test that promotes viscoplastic flow inside the metallic glass. Stretched exponential decay is interpreted by a modified diffusive-trapping model that allows a new band to develop through straining.