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 β fits the Cp scans, but yields a lower β 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 β 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 Tf, or another process that yields a characteristic overall time and Tf.