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Friction stir processing was applied to hot-rolled Mg-Zn-Y-Zr alloy to produce a fine-grained structure 4.5 mu m in size with fine, uniformly distributed Mg(3)Zn(3)Y(2) particles and predominant high-angle grain boundaries (HAGBs) of 91%. A maximum superplasticity of 1110% was achieved at a high strain rate of 1 x 10(-2) s(-1) and 450 degrees C. The superior superplasticity at this high strain rate is attributed to the excellent thermal stability of the fine-grained structure and the high percentage of HAGBs. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.