Yield asymmetry and fracture behavior of Mg–3%Al–1%Zn–(0–1)%Sr alloys extruded at elevated temperatures

Tensile and compression experiments were carried on Mg–3Al–1Zn alloys containing different levels of Sr (0.05, 0.4 and 0.8 wt%) extruded at elevated temperatures of 250, 300, 350 and 400 °C and the results were compared to AZ31 alloy. The addition of 0.05 wt%Sr to the alloy produces the highest strength and the alloy containing 0.4 wt%Sr has the highest elongation during tensile tests. High levels of Sr (0.8 wt%) reduce the yield strength by decreasing the solid solubility of Al in α-Mg and thereby decreasing the solute pining effect of dislocations under both compression and tension loading. The ultimate tensile strength (UTS) is also reduced due to the cracking of Al–Sr precipitates perpendicular to the tension direction. Cracking is also seen in compressed samples; however, the cracks which initiate are parallel to the compression axis, which impedes crack opening and the formation of large voids. As a result, fracture in compression becomes less sensitive to high concentrations of Sr. It is found that increasing the extrusion temperature increases the yield asymmetry and the improvement in yield symmetry is less significant in alloys containing the high levels of Sr.