Superplastic deformation in the low stress region

Abstract

Superplastic alloys generally exhibit a three-stage-sigmoidal variation of stress (f) with strain rate (s), the stages being named region I, II and III according to the increasing order of stress or strain rate. In the recent years, two different types of papers have been published on the plastic deformation of Zn-22\% Al eutectoid in region I differing in the strain-rate sensitivity m ($= d\ell{n} f/d\ell{n} \dot{s}$). In this paper, the data of the two groups have been analyzed by applying Kim and Ree``s theory of superplastic deformation. (1) We obtained the parametric values of $X_{gj}/\alpha_{gj}$ and $\beta_{gj}$ (g: grain boundary, j = 1,2 indicating flow units) appearing in Kim and Ree``s theory [Eq. (2a)]. (2) It was found that the value of $X_{g2}/\alpha_{g2}$ is small for the group data with small m, i.e.,$\alpha_{g2}$ which is proportional to the size of flow unit g2, is large whereas $\alpha_{g2}$ is small for the groups data with large m, i.e., the size of the flow unit g2 is small. In other words, the two types of behavior occur by the size difference in the flow units. (3) From the $\beta_{gj}$ value, which is proportional to the relaxation time of flow unit gj, the $\triangle{H}_{gj}^\neq$ for the flow process was calculated, and found that $\triangle{H}_{g2}^\neq$ is large for the group data with small m whereas it is small for the group data with large m. (4) The flow-unit growth was studied, but it was concluded that this effect is not so important for differentiating the two groups. (5) The difference in $\alpha_{g2}$ and in the growth rate of flow units is caused by minute impurities, crystal faults, etc., introduced in the sample preparation.