Microstructure and tensile properties of AZ91 alloy with individual and combined Calcium and Strontium additions

The effect of individual and combined additions of Ca and Sr additions on the microstructure and tensile properties of AZ91 Mg alloy has been studied. The optical microscope (OM) and field emission scanning electron microscopy (FE-SEM) were used to characterise the microstructure, and an X-ray diffractometer was used to analyse the phases in the cast samples. The evolution of Al₄Sr and Al₂Ca phases was found with the suppression of the β-Mg₁₇Al₁₂ after Ca and Sr additions. Individual Sr addition resulted in the formation of needle-shaped Al₄Sr phases. Individual Ca and combined Ca+Sr additions, on the other hand, resulted in a skeletal-like structure of secondary phases. After both individual and combined Ca and Sr additions, the grain refinement of AZ91 has been observed. The microhardness values and tensile properties of the samples are measured using a Vicker's microhardness tester and universal tensile testing machine (UTM). The microhardness was found to improve after the Ca+Sr additions. The optimum combination of tensile properties was obtained at 1.9 wt.% Ca+Sr addition. The strength of AZ91 alloy decreased when Ca+Sr additions exceeded 1.9 wt%. (author)