The deformation structure of the superplastic Zn–Al alloy

Fine grained (d≅1 μm) and coarse grained (d>20 μm) samples of the Zn-1.1 wt.% Al alloy were strained in a broad range of temperatures and strain rates corresponding both to the optimum superplastic region II and to the non-superplastic region I. The evolution of the microstructure during straining was studied and compared with the assumptions of the co-operative grain boundary sliding model. No pronounced localization of grain boundary sliding was found during straining at optimum superplastic strain rates. A tendency toward sliding of groups of grains was observed during straining at low strain rates. The stress dependence of the size of these grain groups might be a possible reason for the origin of the non-superplastic region I at low strain rates.