Internal Degrees of Freedom and Anelasticity in Glasses

Abstract

An extension of the AHV-Phillips model is proposed which describes the anelastic and timedependent behaviour of glasses over a range of temperatures. The model assumes the existence of g;roups of atoms with two or more metastable locations in the matrix. Transitions between these locations are assumed to be thermally activated, and the parameters of the potential seen by such groups are assumed to be continuously distributed. The active groups are a realization of internal degrees of freedom in the sense of irreversible thermodynamics. The thermodynamic description is particularly convenient when strain couplings are involved, since Maxwell relations determine the internal contribution ,to the stress in terms of the strain dependence of the internal potentials. Several consequences of the model, such as time-dependent heat capacities, time-varying stress-strain relations etc., are investigated and found to be consistent with existing experiments. In particular, Wiedmann's elastic after-effect measurements of 1886 can be fitted and lead to values of 0·6-0·9 eV for the width of the distribution of energy barriers. However, the predicted ultrasonic attenuation from active groups conflicts with experiment, and reasons for this are discussed.