Identification of the threshold stress and true activation energy for characterizing the deformation mechanisms during hot working

Abstract

For characterizing the origins of threshold stress and the evolution of deformation mechanisms in hot working region, the hyperbolic-sine constitutive analyses were conducted to determine the threshold stress σ0, the true stress exponent nt and the true activation energy Qt in AA7050 Al alloys. The origins of σ0 could be related to the four bypassing mechanisms of dislocations overcoming an obstacle. The origins of σ0 are the local climb at 603–633K, and it may change from the local climb to the detach stress with increasing strains at 663–693K. The nt and Qt after Young's modulus and threshold stress correction can be associated with the specific deformation mechanism. The deformation mechanisms at peak strains may change from the climb-controlled dislocation creep (lattice diffusion) at 603K to the Zn diffusion controlled solute drag creep which is controlled by (sub)grain boundary diffusion at 693K. The nt and Qt decrease with increasing strains may be attributed to the grain boundary self-diffusion accelerated by the fluxes of vacancies for lower temperatures (603–663K) and the atom diffusion controlled solute drag creep accelerated by the vacancies, (sub)grain boundaries, and the motion of high angle grain boundaries for higher temperatures (663–693K), respectively.