Mechanical Properties and Microstructure of AZ31 Magnesium Alloy during High Temperature Deformation

In this study, uniaxial and biaxial deformations were applied to a magnesium alloy of AZ31 to reproduce the stresses that occur during product processing and to compare the mechanical properties and microstructure of the alloy after being deformed at each load stress. We conducted a high-temperature tensile test on uniaxial and biaxial deformations in the range of initial strain rate of 2.7×10-4 to 2.7×10-1 s-1. The uniaxially deformed alloy achieved a fracture elongation of 230% at an initial strain rate of 2.7×10-4 s-1 and also about 100% at that of 2.7×10-1 s-1. On the other hands, the biaxially deformed alloy indicated higher values of yield stress and tensile strength, 45 MPa and 62 MPa at maximum, respectively, than the uniaxially deformed one, but a fracture elongation of only about 30%. The cross section of the uniaxially deformed alloy was significantly reduced due to the distortion of its sheet thickness and width, while the biaxial deformation was restricted due to the application of a vertical and bi-directional principal stress. For this reason, the biaxial deformed alloy received a higher average stress than the uniaxially deformed one, so it ruptured without huge elongation. Although the biaxially deformed alloy suffered less deformation under rupture than the uniaxially deformed alloy, microvoids with a length of about 1 μm that could not be observed after the uniaxial deformation were found in grain boundaries where a fibrous structure could be seen. The formation of this fibrous structure caused local stress and nonuniform strain during deformation, so the grains became corrugated and smaller at a strain rate of 2.7×10-4 s-1 and fine grain of about 2 μm were formed among coarse grains.