Influences of load direction and grain size on tension-compression yield asymmetry of extruded AZ31 magnesium alloy

Mechanical tests were conducted to investigate the influences of load direction and grain size on tension-compression yield asymmetry of an extruded Mg-3Al-1Zn alloy. A criterion for the activation of deformation modes was proposed to analyze the effect of load direction on twinning activity. When the load angle was 45o, the twin area fractions after tension and compression resembled each other, and the corresponding ratio between compression yield stress and tension yield stress equaled 1.02. As the load direction was parallel or perpendicular to the extrusion axis, the yield stress and twinning activity in tension differed obviously from those in compression, showing marked tension-compression yield asymmetry. Although grain-coarsening promotes twinning in tension along extrusion axis, it cannot reduce the yield asymmetry. Further, the contributions of twinning to strain during yield deformation were evaluated based on the quantitative statistic of twin area fraction.