Effects of heat treatment on microstructure and tensile deformation of Mg AZ80 alloy at room temperature

The plasticity of coarse and grain-refined Mg AZ80 alloys in the as-cast, gamma-dissolved and homogenized states was investigated by specialized tensile testing at room temperature. Results indicate that microstructural parameters such as the activation volume and mean free path are important descriptors for these materials and capture the nature of the solute and second phase effect on strength and ductility. The as-cast alloys contain a microstructure consisting of alpha-Mg matrix, and divorced eutectic alpha-Mg/gamma-Mg17Al12 phase with non-uniform Al solute content in the alpha-Mg. Dissolution of the majority of gamma-phase occurs after annealing 5 h at 420 degrees C, and an almost uniform solid solution is obtained after 20 h at 420 degrees C. The yield strength is dependent upon the volume fraction of gamma-phase and grain size. All alloys yield initially by basal slip and they exhibit different work hardening behaviour. The as-cast alloys show the fastest initial hardening and earliest saturation, and ultimately the lowest ductility. In contrast the solutionized alloys show a lower initial work hardening rate that is sustained, and enhanced ductility. The flow stress dependence of the strain rate sensitivity indicates that dynamical recovery processes associated with the dislocation-dislocation interactions, which develop in the as-cast alloys after small amount of deformation, lead to strain localizations and early failure. Results reveal that reducing the grain size and dissolving the gamma-phase will enhance the ductility of AZ80 at room temperature. (C) 2008 Elsevier B.V. All rights reserved.