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 ∼0.7 μm 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 °C, and exhibited excellent superplasticity at 175–425 °C. High strain rate and low-temperature superplasticity of >1200% were observed at 1 × 10−2–1 × 10−1 s−1 and 300–350 °C. Even at 425 °C, a superplasticity of 1400% was achieved at 1 s−1. A linear relationship between log ε˙opti and T   was observed (where ε˙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 energy 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.