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NTRODUCTION

A prerequisite for achieving structural superplasticity is a fine grain size, typically less than 15 m. Various processing techniques such as thermomechanical processing (TMP),[1,2] equal-channel angular pressing,[3] torsional deformation under pressure,[4] and friction stir processing[5,6] have been developed to produce fine-grained aluminum alloys. Among these, friction stir processing, developed based on the principles of friction stir welding (FSW),[7–11] is particularly attractive because it provides a very simple and effective approach for obtaining fine-grained regions for specialized applications. Grain sizes ranging from 0.8 to 12 m have been reported by various investigators in the friction stir–welded and FSP aluminum alloys.[5,6,8–11] More importantly, friction stir processing can result in the generation of high-strain-rate superplasticity (HSRS) in commercial aluminum alloys. A recent study[12] has indicated that FSP 7075Al with a fine grain size of 3.8 m exhibited HSRS. A superplastic ductility of 1250 pct was obtained at 480 °C and high strain rates of 3
103 to 3
102s1. The optimum strain rate for the FSP 7075Al was more than one order of magnitude higher than the previous best TMP effort on 7075Al.[2] This shows that friction stir processing is a very effective processing approach to produce fine-grained materials amenable to HSRS. Furthermore, the FSP aluminum alloy exhibited a much lower cavity level and higher critical strain for cavitation compared to the TMP alloy.[13] However, fine grain size is a necessary, but not always sufficient, condition to obtain superplasticity. If the finegrained microstructure is not stable at high temperature, superplastic elongation will be significantly reduced. A recent Z.Y. MA, Professor, formerly with the Department of Materials Science and Engineering, University of Missouri, is with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. R.S. MISHRA, Associate Professor, is with the Department of Materials Science and Engineering, University of Missouri, Rolla, MO 65409. Contact e-mail: [email protected] M.W. MAHONEY is with Rockwell Scientific, Thousand Oaks, CA 91360. R. GRIMES is with the Department of Materials, Imperial College of Science, Technology and Medicine, London, SW7 2BP, United Kingdom. Manuscript submitted January 19, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A

study[14] revealed that fine-grained 7475Al prepared by friction stir processing did not exhibit superplastic elongation due to abnormal grain growth at high temperatures. Similarly, abnormal grain growth was also observed at high temperatures in the FSP 7050 and 2519 aluminum alloys.[15] Fine Al3Zr dispersoids can inhibit grain growth in fine-grained aluminum alloys at high temperature. Supral aluminum alloys (Al-Cu-Zr) have exhibited good superplasticity at a high strain rate of 102s1.[16,17] Recently, Grimes and coworkers[18,19] developed an Al-Mg-Zr alloy for enhanced superplastic properties. They showed that very goo