Effects of extrusion ratio and subsequent heat treatment on the tension-compression yield asymmetry of Mg-4.58Zn-2.6Gd-0.18Zr alloys

Effects of extrusion ratio and subsequent solution + aging treatment on the tension-compression yield asymmetry (TCYA) of extruded Mg-4.58Zn-2.6Gd-0.18Zr alloys were investigated. When the extrusion ratio increased from 10:1 to 15:1 and 30:1, the TCYA, which was determined as the ratio between compressive yield strength to tensile yield strength, was gradually improved from 0.79 to 0.84 and 0.89. The texture intensity decreased and deviation angle of the basal plane from extrusion direction increased with increasing the extrusion ratio, and resulted in the improvement of the TCYA. However, the TCYA of the extruded alloy with extrusion ratio of 30:1 was aggravated after solution treatment at 480 °C for 2 h, and it was attributed to the grain coarsening but not caused by the texture evolution. The TCYA was further deteriorated after the following aging treatment at 180 °C for 32 h. The interaction model of the peak-aged MgZn₂ phases with prismatic <a> slip and {101(_)2} extension twinning were established, and the ability of the Orowan model to predict the strengthening effect produced by MgZn₂ precipitates in the peak-aged alloy was evaluated quantitatively, as a result, the rod-like MgZn₂ precipitates lead to the reduction of yield asymmetry, which was in accordance with the experimental results.