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

摘要

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 Mg_2Si in the alloys changes from large dendritic (>100 nm) to fine polygon (<20 μm). Meanwhile, the eutectic Mg_2Si changes gradually from Chinese script shape to a fine fibre morphology with increasing Ca content. The grain size of the primary Mg_2Si 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 Mg_2Si 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 Mg_2Si, and also decrease the amount of the primary Mg_2Si. 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 °C. Optimal mechanical properties can be achieved with a Ca addition of 0.4%.