Enthalpy of anelasticity and rejuvenation of metallic glasses

Elastostatic loading and cryogenic thermal cycling are known to be capable of increasing the enthalpy of metallic glasses and improving their mechanical properties, effects of particular interest as these treatments operate in the nominally elastic regime. The increases in volume and stored energy induced by these treatments are studied for two prototypical (Pd- and Zr-based) bulk metallic glasses. The energy increase induced by elastostatic loading is associated with unrecovered anelastic strain, and not with viscoplastic strain as previously proposed. This energy increase decays similarly to that induced by thermal cycling, implying a storage mechanism that is similar for these two treatments and distinct from that operating under plastic deformation. For different as-cast and annealed states of metallic glasses, increased energy shows a linear correlation with increased volume. Property changes induced by elastostatic loading and thermal cycling also show a linear correlation, but with a different gradient, indicating greater excess volume for a given energy. It is proposed that the changes induced by treatments within the elastic limit do not constitute ‘rejuvenation’ of a metallic glass, but rather structural excitation localized at shear-transformation zones. The energetics of such excitation are analysed.