SUPERPLASTIC FLOW AND CAVITATION IN ZN-22 PCT AL DOPED WITH CU

The sigmoidal relationship between stress and steady-state strain rate that has been reported for micrograin superplastic alloys is characte rized by the presence of three regions: region I at low stresses, regi on II (the superplastic region) at intermediate stresses, and region I II at high stresses. Recent results on the superplastic Zn-22 pct Al e utectoid have shown that the characteristics of region I are influence d by the impurity level of the alloy, and that neither region I nor si gnificant cavitation is observed when such a level is reduced to about 6 ppm. These observations are in agreement with the suggestion that t he origin of region I is related to strong impurity segregation at bou ndaries. The present investigation was conducted to study the effect o f Cu. as a selected impurity, on superplastic deformation and cavitati on in Zn-22 pct Al. The results show that Zn-22 pct Al-0.13 pct Cu exh ibits two primary characteristics: region I is absent and cavitation i s not extensive. These characteristics, which are essentially similar to those reported previously for high-purity Zn-22 pct Al but are diff erent from those documented for a grade of the alloy containing a comp arable atomic concentration of Fe, suggest that Cu has little or no te ndency to segregate at boundaries. Indirect evidence in support of thi s suggestion is inferred from studying the effect of impurities on for mer alpha boundaries that form in the microstructure of Zn-22 pct Al a s a result of solution treatment above the eutectoid temperature. Alth ough further studies are needed to provide direct evidence for the abs ence of Cu segregation at boundaries, the present results clearly indi cate that superplastic flow and cavitation at low stresses are control led not only by the impurity level, but also by its type.