Calorimetric Investigation of Structural Relaxation of Bulk Metallic Glasses

Enthalpy and entropy changes on structural relaxation of Mg65Cu25Tb10 glass have been studied after keeping its samples for varying periods of time t(a) at several temperatures T(a)s, and after keeping for fixed t(a) at various T(a)s. At a fixed T-a, the decrease in the enthalpy and entropy occurred with time according to a non-exponential kinetics. When the sample was kept for the same t(a), but at different T(a)s, the decrease in the enthalpy and entropy showed a peak at a temperature when the sample reached an equilibrium state for that t(a). The rate of heat release from the DSC heating scan was analyzed in terms of the model for non-exponential, non-linear enthalpy relaxation. A single set of parameters that fitted the data for un-annealed glass did not fit the data for the highly annealed glass. This is expected in view of the approximations made in the model and the contribution from the Johari-Goldstein (JG) relaxation. It is shown that the distribution of relaxation times leads to memory effect for a glass sample of complex thermal history. This has been investigated by measurement of the enthalpy change on structural relaxation of two bulk metallic glasses, Mg65Cu25Tb10 and Zr46.75Ti8.25Cu7.5Ni10Be27.5 by DSC using two-step annealing temperature procedure. The memory effect was observed as an increase in enthalpy with time and then a decrease.