A phenomenological description of creep transients based on anelasticity

Abstract

A phenomenological model of creep anelasticity is developed. It is applied to previously reported data for polycrystalline aluminum and copper, and single crystals of aluminum. The results demonstrate that an excellent description of the magnitude and kinetics of backflow can be obtained using two Voigt solids in series, one with a linear dashpot and the other incorporating a power law dashpot with an exponent of 2. This phenomenological model is combined with constant substructure forward creep data to demonstrate that stress reduction creep transients can be represented by a superposition of anelasticity and forward flow. While operationally successful, it is argued that this superposition is valid only if the two mechanisms operate independently. Neither the data nor the results of the model permit the mechanisms of backflow to be identified completely. However, several indirect observations indicate that the processes of backflow are directly related to the creep substructure. Further, the power 2 stress dependence suggests that the initial backflow processes are controlled by dislocation glide on noncompact planes. The linear behavior observed at longer times is then probably associated with relaxation of subgrain walls. Finally, it is demonstrated that forward flow and reverse flow under constant structure conditions cannot be represented by the same kinetic laws. This finding indicates that the two mechanisms are different, and supports the contention that creep transients can be described as a superposition of forward and reverse flow.