The dependence of the threshold stress for superplastic flow on structure

The model of a threshold stress for superplastic flow is tested in Al-Zn-Mg-Cu based alloys prepared both using ingot (IM) and powder metallurgy (PM) routes. The threshold stress is found to increase with increasing grain size in the IM alloy. The true value of the parameter pt characterizing the grain size dependence of the strain rate is close to two at low and medium strain rates and decreases to zero at high strain rates. A significantly higher threshold stress in the PM alloy may result from the prevailing small angle character of grain boundaries.