A phenomenological theory of steady state creep based on average internal and effective stresses

Abstract

A phenomenological theory of steady state ceep is formuated which explicitly includes the development of an average internal back stress. The theory is based on the premise that dislocation glide and recovery are separate kinetic processes driven by different components of the applied stress, the effective and internal stress respectively. Phenomenological relations patterned after expressions derived from dislocation theory are presented to describe the stress and temperature dependencies of the elementary processes of strain hardening and recovery as well as dislocation glide. It is argued that the stress and temperature dependence for steady state creep is, in general, a complex function of the separate stress and temperature dependencies of dislocation glide and recovery. It is shown that the applied stress dependence for high temperature creep can be predicted in a straightforward manner from the stress dependence of strain hardening, recovery and glide if the relative magnitudes of the internal and effective stresses are known. Measurements of the internal stress in polycrystalline samples of Al + 5 % Mg at 350°C are presented and it is shown that the prediction of the applied stress dependence for the steady state creep rate compares favorably with the measured dependence. It is also shown that the activation energies for the separate processes of glide and recovery can be obtained, in principle, from the apparent activation energy (based on holding the applied stress constant) and from measurements of the steady state internal stress as a function of temperature at a given applied stress.