The Role of Impurities in Superplastic Deformation and Cavitation

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9810422 Mohamed This research focuses on the substructural details of superplastic flow in metal alloys with special focus on the superplastic region (region II). This includes a systematic examination of the effects of selected types of impurity atoms on superplastic flow and cavitation. Such an examination addresses synergistic interactions among various impurities on cavitation and ductility. An in-depth study of the effects of impurity type and level on the cavitation process is carried out, looking at cavity nucleation and the relationship between deformation and cavity growth. Impurity distribution at boundaries is characterized. Microstructural details associated with superplastic deformation (boundary sliding and migration, grain shape, grain rotation, and ligaments) are determined as a function of impurity level and type, large strains (30%), and deformation mode (tension and double shear). Three different model materials, Zn-22% Al, Pb-62% Sn, and Al-33% Cu, are examined. For these alloys, high-purity grades and grades doped with impurities are tested. In addition, selected commercial superplastic alloys are included to examine the orientation of cavity stringers as a function of strain rate in light of a new mechanistic model proposed for the stringers. Also examined are the effects of major impurities, such as Cu and Mg, on cavitation and ductility behavior in two near-nano scale A1 alloys, which exhibit high strain rate superplasticity. An innovative technique employs flat double-shear specimens to simplify the test procedure and avoid problems associated with torsional and tensile loading. %%% The results of this program should have significant impact in the science and technology of metal alloy superplasticity. It will advance understanding of the processing of commercial superplastic alloys and could lead to improvements in the performance of these materials. ***