The deformation mechanism of the extruded cylinder of AZ31 magunesium alloy depending on a condition of temperature and loading path was studied by a multi-loading test under various temperature conditions and an observation of its microscopic structure after the plastic deformation. From the experimental results, AZ31 magnesium alloy is plastically deformed at room temperature by a twin deformation mechanism. AZ31 magnesium alloy is hard to be plastically deformed at room temperature because the critical shear stress of the twin deformation is much larger than that of a trans-granular slip. Thus, due to the twin deformation mechanism, the yield surface of AZ31 magnesium alloy at room temperature does not coincide with the surface estimated by von Mises criterion, and then the yield stresses of a compressive test and torsion test are lower than the yield stress of a tensile test. From the shape of the yield surface and the existence of the twin in microscopic structure, the deformation mechanism at 423K is similar to the mechanism at room temperature. The superplastic deformation driven by a grain boundary sliding and the grain refinement occurs at 673K. Then, concerning the yield surface, the yield stress of a compressive test is equal to that of a tensile test, and the shear yield stress shows 80% of the tensile yield stress. Such as a shape of the yield surface appears at the temperature of 473K, 498K and 523K and the grain refinement is observed in the specimen deformed at 523K. Therefore, at these temperature conditions, the influence of the grain boundary sliding mechanism appears on the plastic deformation. Because the grain boundary sliding mechanism and the grain refinement leading to the superplastic deformation reveal at a temperature more than 473K, it is supposed that the plastic workability of AZ31 magnesium alloy can be facilitated at a comparatively lower temperature.
