FORMATION OF CAVITY STRINGERS DURING SUPERPLASTIC DEFORMATION

Zn-22 wt%Al was utilized as a model material to investigate cavity str inger formation during superplastic deformation. The experimental resu lts show that residual phase boundaries, which are referred to as form er cc boundaries (F alpha Bs), form as a consequence of solution treat ment above the eutectoid temperature. A comparison between the behavio r of FaBs and the characteristics of cavity stringers indicates a clos e correspondence between these two substructural features. On the basi s of such a correspondence, a mechanism that explains the formation of cavity stringers in Zn-22 wt%Al is proposed. In addition, a general m echanism for cavity stringer formation is introduced by considering th e characteristics of F alpha Bs and available information on superplas tic flow and cavitation. This mechanism is based on the concept that c avity alignment in the form of stringers is essentially assisted by su perplastic flow. The mechanics and pre dictions of the mechanism are c onsistent with available experimental evidence. (C) 199S Acta Metallur gica Inc.