Role of Impurities in Superplastic Flow and Cavitation

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9500316 Mohamed This is a fundamental investigation of superplastic flow and cavitation in fine-grained materials. The main objective is to assess if the characteristics of superplastic flow at low stresses are a consequence of impurity atom segregation at boundaries and to address several issues regarding the substructure details and origin of superplastic deformation and cavitation at intermediate stresses. The experimental approach involves: (a) an in-depth study of impurity type and level on the cavitation processes; (b) a characterization of impurity distribution at boundaries before and after deformation; (c) an investigation of microstructural details associated with superplastic deformation (boundary sliding and migration, grain shape, grain rotation, ligaments, and lattice dislocations) as a function of impurity level and type, strain, and deformation, mode (tension and double shear); and (d) an examination of the effects on creep behavior and microstructure of introducing oxide dispersoids into a superplastic alloy using the new process of Reactive Atomization and Deposition. Three different model materials, Zn-22%Al, Pb-62%Sn, and Al-33%Cu, are used with controlled amounts of impurities. %%% This project will not only enhance the understanding of the role of impurity atoms in the deformation and fracture of superplastic alloys, but also provide new insights into the origin of deformation processes controlling superplastic flow. ***