Analysis on the anelasticity of a superplastic Zn–22% Al eutectoid

The kinetics of anelastic deformation of a superplastic Zn–22%Al was examined to provide the systematic information on the anelastic behavior of fine grained superplastic materials showing the extraordinary high relaxation strength compared to large grained materials. For the present analysis, the strain transient data of a superplastic Zn–22%Al after unloading, which was reported in the literature, were reappraised by taking the instantaneous remaining anelastic strain as a driving force causing anelastic deformation. The results showed that anelastic behavior of the alloy could be described by the empirical description similar to that of forward superplastic deformation. In addition, anelastic strain rate was found to be insensitive to the forward applied stress prior to unloading. The anelastic behavior of the alloy was manifested by the presence of three distinct regions in the double logarithmic plot of anelastic strain rate against remaining anelastic strain according to the values of the dependency of anelastic strain rate on remaining anelastic strain, r, the grain size sensitivity, s, and activation energy, Q. Region I (long time after unloading) was characterized by r≈1, s≈5 and Q≈76 kJ mol−1. In region II (intermediate time after unloading), the values of s and Q were similar with those of region I, but r was about 3. The results of the analysis indicated that region III (short time after unloading) could be attributed to the athermal process. Activation energy close to that for forward superplastic deformation and the high value of grain size sensitivity in region I and II are indicative of the important role of grain boundary diffusion on anelastic deformation of fine grained superplastic materials. In addition, with the aid of the grain boundary back tension model for anelastic deformation, a relationship between remaining anelastic strain and the back stress resulting from grain boundary back tension was derived.