Correlation between microstructure evolution and cavity stringer formationin the superplastic Zn-22 wt% Al alloy

In the present investigation, microstructure evolution during the superplas tic deformation of Zn-22 wt% Al was examined to provide insight into the or igin of cavity stringers that form parallel to the tensile axis in the allo y. Substructural data show that groups of fine alpha (Al-rich) and beta (Zn -rich) phases are encompassed by former alpha boundaries (F alpha Bs) which consist of fine elongated alpha grains and which divide the microstructure into equiaxed domains (F alpha B domains). By examining the correspondence between the behaviour of these boundaries and the development of cavity st ringers, it is suggested that the alignment of cavities along the tensile a xis can be explained in terms of two processes: firstly the preferential nu cleation of cavities at F alpha Bs during the initial stages of deformation , and secondly the tendency of F alpha Bs and their associated cavities to change their orientation and align along the tensile axis during superplast ic deformation. Evidence in support of this explanation is provided by pres ent experimental results on firstly the correlation between the overall num ber of aligned F alpha Bs and the total length of cavity stringers, secondl y the effect of impurities on both the average size of F alpha Bs domains a nd the total length of cavity stringers, and thirdly the direct corresponde nce between the morphology of F alpha Bs and that of cavity stringers in no tched specimens which, upon deformation, exhibit a distinctive local strain gradient.