Cavity distribution pattern in a superplastically deformed aluminum alloy
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I,
INTRODUCTION
F O R M I N G o f superplastic materials by gas pressure is a commercially viable process. However, cavitation after superplastic deformation has been observed in many commercial alloys and, in particular, in aluminum alloys. tl-51 Since cavitation can drastically degrade service properties o f superplastically deformed materials, much effort has been directed toward the understanding o f the formation and growth mechanisms o f cavities under superplastic deformation conditions, t6-~J While some progresses have been achieved in the study o f cavity growth mechanisms, t6'7'SJ the nucleation process o f cavities is still not entirely clear, r9,1°,1~] With regard to cavitation characteristics, the changes o f cavity volume fraction and size distribution during superplastic deformation have been extensively studied, ~1-51 whereas insufficient attention has been paid to the investigation o f the cavity morphology. Furthermore, cavitation during biaxial deformation, which is very closely related to commercial manufacturing processes, has not been adequately studied compared to superplastic cavitation under uniaxial tensile deformation. Therefore, the aim o f this research was to investigate the evolution o f cavity morphology in a high-strength aluminum alloy during biaxial superplastic deformation and to understand the mechanism o f cavity nucleation and growth. II.
EXPERIMENTAL P R O C E D U R E
AA7475 sheets o f 2-ram thickness, supplied by Swiss Aluminum Ltd., were used in this study. The sheets have a fine grain structure, which was produced by a thermomechanical processing method. Due to the nonhomogeneity o f the rolling deformation in the thermomechanical processing, the grains are slightly coarser at the center o f the sheet than those near the sheet surface. The average grain size o f the alloy was approximately 1 5 / z m in the rolling plane and 9/zm in the M.G. ZELIN, Postdoctoral Research Fellow, formerly Associate Professor with the Department of Materials Science, Ufa Aviation Institute, H.S. YANG, Postdoctoral Research Fellow, and A.K. MUKHERJEE, Professor, are with the Department of Mechanical, Aeronautical and Materials Engineering, University of California-Davis, D a v i s , C A 95616. R.Z. VALIEV, Professor, is with Ufa Aviation Institute, Ufa 450025, Russia. Manuscript submitted March 23, 1992. METALLURGICAL TRANSACTIONS A
thickness direction. The composition (wt pct) o f the alloy is 5.40 pct Zn, 2.27 pct Mg, 1.50 pct Cu, 0.20 pct Cr, 0.06 pct Fe, 0.04 pct Si, 0.01 pct Mn, and the balance AI. At superplastic deformation temperatures (--790 K), the material contains a solid solution o f Zn, Mg, and Cu elements, constituent particles formed by impurities, and Cr-containing dispersoids which pin grain boundaries and minimize grain growth at high temperature. The pressure-forming equipment used for forming domes and top hats has been described in a previous article, t12] Sheets o f 127-mm diameter were used. The forming tests were carried out at 790 -+ 2 K and a strain rate o f --1
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