Decreasing activation energy of fast relaxation processes in a metallic glass during aging

Many of the macroscopic properties of a glass are determined by the degree of structural relaxation. When the nonequilibrium system ages toward a thermodynamically more favorable state, the accompanying densification leads to an increase of the activation energies found for the alpha and especially the beta relaxation processes. In this work we experimentally quantify the low-energy mechanical relaxation spectrum of a metallic glass at cryogenic temperatures, and show that these relaxation processes intriguingly show the opposite trend. The energy scale as well as the relaxation strength decrease during the aging process below the glass transition temperature with a surprisingly strong dependence on the annealing time. The experimental results are analyzed in the framework of established models and the temporal behavior of the typical energy V-0 is assessed. We compare the derived values to the values of the thermal energy available at the estimated fictive temperature of the given state and find that the absolute values as well as their temporal behavior show a high degree of correlation for the studied metallic glass. The decreasing characteristic energy values found in the present experiment directly depict the evolution of the structure toward a hypothetical lowest entropy state before the glass becomes structurally indistinguishable from a crystalline material.