The role of diffusion in determining the controlling creep mechanisms in Al-Zn solid-solutions: Part II

Abstract

The theoretically appropriate diffusion coefficients for creep in binary solid solutions are verified in Part I. Furthermore it is observed that the observed composition dependence of the steady-state creep rate for Al-Zn alloys may be described by these diffusion coefficients. Part II further examines the creep behavior of the Al-Zn system. Stress exponent and activation energy measurements indicate that a transition from climb-controlled to glide-controlled creep occurs as the Zn concentration increases. It is found that the observed creep behavior can be explained by a combination of climb and glide controlled creep processes. Glide creep is dependent on the chemical diffusivity which approaches zero at the miscibility gap composition and temperature. This causes a minimum in the creep rate at the miscibility gap composition. By using a sequential summation of a semiempirical climb equation and the Cottrell-Jaswon glide equation, the absolute magnitude of the observed creep rate can be rationalized for all compositions. The observed behavior of the stress exponent and activation energy is also predicted.