The deformation structure of the superplastic Zn-Al alloy

Fine grained (d congruent to 1 mu m) and coarse grained (d > 20 mu m) sampl es of the Zn-1.1 wt.% Al alloy were strained in a broad range of temperatur es and strain rates corresponding both to the optimum superplastic region I I and to the non-superplastic region I. The evolution of the microstructure during straining was studied and compared with the assumptions of the co-o perative grain boundary sliding model. No pronounced localization of grain boundary sliding was found juring straining at optimum superplastic strain rates. A tendency toward sliding of groups of grains vc as observed juring straining at low strain rates. The stress dependence of the size of these g rain groups might be a possible reason for the origin of the non-superplast ic region I at low strain rates. (C) 1999 Elsevier Science S.A. All rights reserved.