Superplastic Deformation of AZ61 Magnesium Alloy having Fine Grains

摘要

Extruded Mg-6mass percent Al-l mass percent Zn (AZ61) alloy was grain-refined utilizing Equal Channel Angular Extrusion (ECAE) processing. Initially, the extruded bar of the alloy was ECAE-processed 2-times at 473 K. Subsequently, it was processed 4-times at 448 K. As a result, the grains are refined to less than 1 mum and a large amount of fine Mg_(17)Al_(12) compound precipitates. Subsequently, the superplastic properties of the ECAE-processed specimens were investigated. Large fracture elongations of over 300 percent are obtained at 423 K and 448 K, which is below T_m/2 (T_m: melting point of the alloy), at strain rates above 1 x 10~(-4) s~(-1) and 1 x 10~(-3) s~(-1), respectively. That is. low temperature superplasticity occurs. Furthermore, at a high strain rate of 1 x 10~(-2) s~(-1), superplasticity occurs with the elongation of 242 percent and 398 percent at relatively low temperatures of 473 K and 523 K, respectively. In the extraordinarily elongated specimens, significant grain boundary sliding is observed with strain rate sensitivity of 0.3 approx 0.4. The activation energy for superplastic deformation is about 91 kJ/mol, which is close to that for grain boundary diffusion of pure magnesium. It is concluded that the superplastic deformation mechanism of the investigated alloy would be grain boundary sliding accommodated by dislocation slip controlled by grain boundary diffusion.