The reversible enthalpy relaxation in glassy metal alloys and polymers

Abstract

Spontaneous changes in the enthalpy and entropy of six isothermally annealed Ni-based glassy metal alloys and one network structure organic polymer have been studied by differential scanning calorimetry. Both time and temperature effects of annealing have been investigated. The two classes of materials show a remarkably similar structural relaxation, thus creating a difficulty in reconciling the earlier conclusions that the so-called reversible relaxation observed for glassy metal alloys is unique, involves both chemical and topological short-range orders and needs to be modelled within the concepts of independent two-level systems. This is resolved by showing that the reversible relaxation is a reflection of the time- and temperature-dependent regain of the equilibrium state at temperatures when the enthalpy of the annealed state is less than that of the equilibrium state. Those local groups of atoms, whose diffusion is fast enough to allow loss of enthalpy and entropy on their approach to an equilibrium configuration state of low energy on annealing, absorb heat to reach their new equilibrium Configuration state of higher energy at a higher temperature on heating at a certain rate. Thus each mode of atomic diffusion in the structure has its own ‘mini glass transition temperature’, when the gain of its configurational entropy conies into evidence on heating at a certain rate. The origin of the reversible relaxation is in the broad distribution of activation energy for diffusion times arising from the temporal and spatial variations in the environments of atoms, similar or dissimilar, in the structure of a glassy metal alloy. The reversible relaxation can be modelled in terms of the non-exponential nonlinear character of structural relaxation in glasses, and its strength is expected to be less for glasses with a narrow distribution of diffusion times and lacking a significant contribution from other relaxations.