Effect of Ca on the microstructure and tensile properties of Mg-Zn-Si alloys at ambient and elevated temperature

The microstructures and tensile properties of a Mg-6Zn-4Si alloy with calcium additions at ambient and elevated temperature were investigated by means of X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and standard high temperature tensile testing. The results indicate that the morphology of the primary Mg2Si in the alloys changes from large dendritic (>100 l mu m) to fine polygon (<20 mu m). Meanwhile, the eutectic Mg2Si changes gradually from Chinese script shape to a fine fibre morphology with increasing Ca content. The grain size of the primary Mg2Si decreases initially and then slowly increases with increasing Ca content. Experimental analysis shows that the concentration distribution of Ca atoms on the growth front of the Mg2Si cause a "poisoning effect". The coarse CaMgSi compound is formed due to an excessive Ca addition, which resulted in the over-modification. Therefore, it can be concluded that the appropriate level of Ca addition can both effectively modify and refine the primary Mg2Si, and also decrease the amount of the primary Mg2Si. Tensile testing results show that the addition of Ca improves the ultimate strength, yield strength and elongation of the Mg-6Zn-4Si alloys at both ambient temperature and 150 degrees C. Optimal mechanical properties can be achieved with a Ca addition of 0.4%. (C) 2012 Elsevier B.V. All rights reserved.