Superplastic deformation mechanism of an ultrafine-grained aluminum alloy produced by friction stir processing

An ultrafine-grained (UFG) Al-4Mg-1Zr alloy with a grain size of similar to 0.7 mu m with predominantly high-angle boundaries of 97% was produced by friction stir processing (FSP). The UFG Al-4Mg-1Zr retained submicrometer grains even after static annealing at 425 degrees C, and exhibited excellent superplasticity at 175-425 degrees C. High strain rate and low-temperature superplasticity of >1200% were observed at 1 x 10(-2)-1 x 10(-1) s(-1) and 300-350 degrees C. Even at 425 degrees C, a superplasticity of 1400% was achieved at 1 s(-1). A linear relationship between log (epsilon) over dot(opti) and T was observed (where (epsilon) over dot(opti) is the optimum strain rate, and T is the temperature). The analyses on the superplastic data revealed the presence of threshold stress, a stress exponent of 2, an inverse grain size dependence of 2, and an activation enemy of 142 kJ mol(-1). This indicated that the dominant deformation mechanism was grain boundary sliding, which was controlled by lattice diffusion. Based on this notion, a constitutive equation has been developed. A new superplastic deformation mechanism map for FSP aluminum alloys is proposed. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All