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 metall urgy (PM) routes. The threshold stress is found to increase with incre asing grain size in the IM alloy. The true value of the parameter p(t) 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 st rain rates. A significantly higher threshold stress in the PM alloy ma y result from the prevailing small angle character of grain boundaries . (C) 1997 Elsevier Science S.A.