Memory effect in enthalpy relaxation of two metal-alloy glasses

The change in enthalpy on structural relaxation of two bulk metal-alloy glasses, Mg65Cu25Tb10 and Zr46.75Ti8.25Cu7.5Ni10Be27.5 has been investigated by differential scanning calorimetry (DSC) using an anneal-and-scan procedure. Two aspects of this relaxation have been investigated: (i) a monotonic decrease in the enthalpy with time, (ii) an increase in enthalpy with time and then a decrease, or the so-called 'memory effect'. The non-exponential, non-linear model with a structure-independent parameter P fits the C, scans, but yields a lower P value than an equally good fit to the isothermal enthalpy decrease without non-linearity. The memory effect is more pronounced in Zr46.75Ti8.25Cu7.5Ni10Be27.5 glass for which # is smaller. Despite the chemical and topological short-range order in their structures and no rotational diffusion, these glasses mimic the molecular, polymeric and network structure glasses. It is suggested that a reliable analysis of the memory effect data requires including a contribution from localized motions and avoiding the use of a time-temperature superposition. Even if this is done, molecular interpretation of the effect would depend upon whether a non-exponential relaxation is seen as a multiplicity of relaxation modes each with its own time and hence T-f, or another process that yields a characteristic overall time and T-f. (C) 2007 Elsevier B.V. All rights reserved.