Superplastic flow and cavitation in Zn-22 pct Al doped with Cu

Abstract

The sigmoidal relationship between stress and steady-state strain rate that has been reported for micrograin superplastic alloys is characterized by the presence of three regions: region I at low stresses, region II (the superplastic region) at intermediate stresses, and region III at high stresses. Recent results on the superplastic Zn-22 pct Al eutectoid have shown that the characteristics of region I are influenced by the impurity level of the alloy, and that neither region I nor significant cavitation is observed when such a level is reduced to about 6 ppm. These observations are in agreement with the suggestion that the origin of region I is related to strong impurity segregation at boundaries. The present investigation was conducted to study the effect of Cu, as a selected impurity, on superplastic deformation and cavitation in Zn-22 pct Al. The results show that Zn-22 pct Al-0.13 pct Cu exhibits two primary characteristics: region I is absent and cavitation is not extensive. These characteristics, which are essentially similar to those reported previously for high-purity Zn-22 pct Al but are different from those documented for a grade of the alloy containing a comparable atomic concentration of Fe, suggest that Cu has little or no tendency to segregate at boundaries. Indirect evidence in support of this suggestion is inferred from studying the effect of impurities on former α boundaries that form in the microstructure of Zn-22 pct Al as a result of solution treatment above the eutectoid temperature. Although further studies are needed to provide direct evidence for the absence of Cu segregation at boundaries, the present results clearly indicate that superplastic flow and cavitation at low stresses are controlled not only by the impurity level, but also by its type.