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MG alloys have many attractive properties, including low density and high strength. Currently, the application of Mg alloys has been limited by the poor workability and low ductility at room temperature because of its hexagonal closest packed (hcp) crystal lattice structure that has limited operating slip systems.[1] In contrast, it has been reported that Mg alloys with fine grains can exhibit superplasticity during hightemperature deformation, which is desirable for forming complex components such as those encountered in automobile applications.[2–4] Friction stir processing (FSP) techniques, originally developed from friction stir welding (FSW) for metallic alloys that are hard to weld using fusion welding, are effective in improving forming or bending properties of engineering materials substantially through grain refinement[5–8] and texture modification.[9] For example, Sato et al.[5] proposed multipass FSP for large-scale grain refinement in die-cast AZ91D Mg sheet, and a final grain size of 2.7 lm was achieved leading to a six-fold increase in the elongation. Yu et al.[9] used the FSP technique to modify the texture of Mg plate to increase the ductility at room temperature drastically. During FSP, the grain refinement and texture development are governed by dynamic ZHENZHEN YU, Post Doctor, WEI ZHANG, Staff Member, and ZHILI FENG, Group Leader, are with the Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. HAHN CHOO, Associate Professor, is with the Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996. Contact e-mail: [email protected] Manuscript submitted July 19, 2010. Article published online September 7, 2011 724—VOLUME 43A, FEBRUARY 2012

recrystallization and the active deformation modes corresponding to various deformation conditions (such as strain rate and temperature history experienced by the material). Chang et al.[2] reported that during FSP with a tool rotation speed ranging from 180 to 1800 rpm and travel speed of 1.5 mm/s, the strain rate range was estimated to be approximately 1 to 100 seconds1 and the peak temperatures in stir zone (measured with embedded thermocouples) was observed to be approximately 523 K to 723 K (250 °C to 450 °C) in Mg alloy AZ31. Using in situ neutron diffraction, Woo et al.[10] reported a maximum temperature of 635 K (362 °C) near the top surface of Al alloy 6061-T6 plate during FSW. Frigaard et al.[11] used thermocouples and measured a peak temperature of approximately 823 K (550 °C) during FSW of Al alloy 6082-T6 and estimated the strain rate to be approximately 1 to 20 seconds1. In addition to these experiment-based studies, computational models have been developed for improved understanding of heat and material flow during FSW and FSP of Al alloys[11–20] and steels.[21,22] There has been few publications on modeling material flow and heat transfer during FSP of Mg alloys, although a detailed thermomechanical history is essential for the understanding of active deformation mechanisms and tex