Enhanced mechanical properties of as-cast AZ91 magnesium alloy by combined RE-Sr addition and hot extrusion

The synergetic effects of strontium (Sr) and rare earth elements (RE) on the modification of the microstructure and enhancement of the mechanical properties of as-cast Mg-9Al-1Zn alloy were studied. Based on the microstructural analysis, phase identification, and tensile tests, it was revealed that the combined RE and Sr addition results in superior mechanical properties compared to those of solo additions of RE or Sr. This was related to both morphological enhancement of the beta-Mg17Al12 intermetallics notably by RE towards the disappearance of the intergranular network and modification of the Al(11)RE(3 )particles by Sr. The enhancement of tensile properties was achieved by increasing the RE/Sr ratio in the Sr-RE containing alloys and the best combination of mechanical properties was observed for the Mg-9Al-1Zn-0.9RE-0.1Sr alloy. Owing to the dissolution of the p phase and solid solution strengthening effect of Al, the tensile properties of the Mg-9Al-1Zn-0.9RE-0.1Sr alloy were enhanced by homogenization treatment. Moreover, the hot extrusion process remarkably augmented the tensile properties by combined effects of grain refinement and fracturing and distributing the intermetallic particles.