On the threshold stress for superplasticity in Mg-Al-Zn alloys

High threshold stresses for superplasticity are often reported in both Mg-Al-Zn alloys and reinforced magnesium alloys. However, the threshold stresses in Mg-Al-Zn and reinforced alloys differ in their temperature and grain size dependence. It is shown in this work that the origin of the apparent threshold stresses in the Mg-Al-Zn alloys arise from an inadequate assumption of an n = 2 power law in the transition zone between grain boundary sliding and slip creep or to a grain growth effect on the flow stress at low strain rates.