| Severe plastic deformation (SPD), which is often used to obtain ultrafine grains to improve mechanical properties of materials, has drawn much attention in recent years. Equal channel angular extrusion (ECAE), being one of the severe plastic deformation methods that can produce the bulk materials with ultrafine grain size, has received much more attention. The advantages of this processing technique can be not only to refine grain size but also obviously to improve plasticity of materials, which is especially important for magnesium alloys with the worse plastic deformation ability. Thus, the superplastic deformation behavior and mechanism for AZ91, AZ81 and AZ61 magnesium alloys subjected to equal channel angular extrusion has been investigated in order to provided the reliable basis for development of high ductility magnesium alloys and their application.Through tensile tests performed under both different temperatures and strain rates, the influence of temperature and strain rate on the superplastic deformation behaviors of the ECAEed AZ91, AZ81 and AZ61 were studied. The strain rate sensitivity and active energy of plastic flow were determined. Moreover, the suitable extrusion route, optimum temperature and strain rate in which the ECAEed AZ91, AZ81 and AZ61 could exhibit the best superplasticity were obtained. In addition, the surface morphologies of three ECAEed magnesium alloys after the superplastic deformation were observed using scanning electron microscope (SEM). And the corresponding mechanisms of superplastic deformation were discussed.The experimental results reveal that the elongation to failure at 300℃and an strain rate of 2×10-4s-1 for the ECAEed AZ91 alloy subjected to 2 passes of ECAE by the route BC is 410%. It means that the ECAEed AZ91 alloy exhibit the better superplasticity. The elongation-to-failure of the AZ81 alloy ECAEed for 2 passes by the route A is 731% at 250℃and an strain rate of 1×10-3s-1. It implies that the ECAEed AZ81 alloy can show the excellent low temperature superplasticity (LTS). The elongation-to-failure of the ECAEed AZ61 processed for 2 passes by route BC can reach to 272% at 200℃and an strain rate of 1×10-3s-1, that is, the ECAEed AZ61 magnesium alloy can attain the low temperature superplasticity. It is suggested that for three magnesium alloys subjected to equal channel angular extrusion, the superplastic deformation mechanism mainly is grain-boundary slipping controlled by grain-boundary diffusion, where the cavity plays an important role in the grain-boundary sliding as an accommodated mechanism.
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