Correlation between former alpha boundary growth kinetics and superplasticflow in Zn-22 pct Al

Former alpha boundaries (F alpha Bs) are residual grain boundaries that dev elop in Zn-22 pet Al during a heat treatment, which is normally applied to produce the fine structure necessary for micrograin superplasticity. They r epresent domains consisting:of fine elongated alpha grains, which encompass groups of fine alpha (Al-rich) and beta (Zn-rich) phases (the superplastic microstructure). The results of a detailed investigation conducted on F al pha B growth kinetics in five grades of Zn-22 pet Al with various impurity contents reveal a direct correspondence between the level of impurities in the alloy and the characteristics associated with F alpha Bs (average size of F alpha Bs and the value of the F alpha B growth exponent). This corresp ondence, which, according to available evidence, is the result of impurity segregation at F alpha Bs, lends strong support to the interpretation of su perplastic behavior at low stresses in terms of phenomena arising from boun dary segregation. It is suggested that information on F alpha B growth kine tics in Zn-22 pet Al can be utilized to predict the low-stress Superplastic characteristics of the alloy, such as the existence of region I or the occ urrence of extensive cavitation.