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

摘要

An ultrafine-grained (UFG) Al-4Mg-1Zr alloy with a grain size of ~0.7 urn with predominantly high-angle boundaries of 97 was produced by friction stir processing (FSP). The UFG Al-4Mg-lZr 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 x 10~(-2)-1 x 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)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.