Stress dependence of the creep rate at constant dislocation structure

Abstract

Stress reduction tests were performed during steady state compressive creep of pure aluminum at 523 K and of Al-5at.%Mg at 573 K in order to determine the dependence of the creep rate ϵdotr at constant (steady state) dislocation structure on (reduced) stress σr. Forward flow with ϵdotr > 0 was found in the whole range σr ⩾ 0, i.e. even after (nearly) full unloading, implying the existence of internal stresses in both materials. For aluminum the relation between log ϵdotr and σr consists of two branches with different stress sensitivities, indicating that the dominant mechanisms of dislocation motion are different for small and for large stress reductions. An interpretation is given in terms of the composite model, which assumes that the thermally activated glide in the (soft) subgrain interior and the recovery processes at (hard) subgrain boundaries are coupled by internal stresses. In the case of AlMg, on the contrary, the shape of the log ϵdotrvs. σr curves indicates that glide and recovery are not separable.