Study On Mechanical Anisotropy Of AZ31 Magnesium Alloy At Low Temperatures

AZ31 magnesium alloy is a high performance wrought alloy widely used around the world, careful examination of the low-temperature mechanical behaviors of the alloy would contribute to expand its range of engineering application. In this paper, the mechanical anisotropies of AZ31 magnesium alloy rolled sheet, extruded bar and annealing bar was studied by using low temperature uniaxial tension, and uniaxial compression tests as well as optical microscopy, scanning microscopy(SEM) and X-ray diffraction(XRD) analysis techniques, within the temperature range from room temperature(RT) to liquid nitrogen(77K) temperature.Tensile test results of rolled sheet showed that within the temperature range of RT to 77 K, the AZ31 magnesium alloy sheet exhibits obvious plastic deformation anisotropy behavior. Along the RD and TD, the ultimate tensile strength and yield strength of AZ31 magnesium alloy increases with decreasing tension temperature, and the elongation decreases. The ultimate tensile strength and yield strength along RD increase from 282 MPa and 170 MPa at RT, respectively, to 399 MPa and 264 MPa at 77 K, while the elongation from 16% to 5%. Meanwhile, the ultimate tensile strength and yield strength along TD direction increase from 276 MPa and 163 MPa at room temperature, respectively, to 346 MPa and 240 Mpa at 77 K, and elongation is 19% at RT while is 3% at liquid nitrogen temperature. At the same tensile temperature, the strength of AZ31 magnesium alloy sheet along the RD direction is higher than the TD direction, and the elongation along TD direction is higher than the RD direction. SEM observation results show that with decreasing tensile temperature, tensile fracture mode of AZ31 magnesium alloy sheet along the RD and TD varied from dimple-based plastic fracture at RT to ductile brittle mixed fracture at 77 K. Optical observation on deformed microstructure of AZ31 magnesium alloy sheet along the RD and TD directions showed that the tensile deformation twins near the fracture surface is reduced with decreasing temperature, while the grain size did not change significantly.The compression test results of extruded bars along different orientations showed that within the temperature range from RT to 77 K, the AZ31 magnesium alloy bar displays significant presence of plastic deformation and compression anisotropy. In parallel to the extrusion and perpendicular to the extrusion direction, the compressive strength and yield strength of AZ31 magnesium alloy bar also increase with decreasing temperature, while little change in compression fracture strain. The compressive strength and yield strength in the extrusion direction increase from 418 MPa and 100 MPa at room temperature, respectively, to 538 MPa and 124 MPa at 77 K. While the compressive strength and yield strength along the normal direction increase from 321 MPa and 89 MPa at room temperature to 476 MPa and 125 MPa at 77 K. At the same compression temperature, the strength of AZ31 magnesium alloy bar along the extrusion direction is higher than the normal direction, while the compressive fracture strain is lower than normal direction. Optical observations on deformation microstructure of the AZ31 magnesium alloy bar along both directions show that with decreasing compression temperature down, the amount of mechanical twins found in deformed AZ31 magnesium alloy bar decreases, and the number of broken grains increases.The uniaxial tension and uniaxial compression test results of the annealed AZ31 magnesium extruded bars along the extrusion direction show that compared to the original extruded bar, the annealed AZ31 magnesium alloy exhibits a higher compressive strength and yield strength, which increases significantly, reaching 607 MPa and 151 Mpa, respectively, at 77 K. Within the temperature range of 77 K to RT, the annealed AZ31 magnesium alloy bar display a remarkable tension and compression yield asymmetry, with the tensile and compressive yield ratio is 1.6 at room temperature, while is more than 2 at 77 K.