Cooling curve thermal analysis and microstructure characterization of Mg-5Zn-1Y-xCa (0-1 wt%) alloys

The effect of 0-1 wt% Ca addition on solidification pathway of Mg-5Zn-1Y alloy and its solidification characteristics such as nucleation transformation and intermetallics formation temperatures was investigated via plotting cooling and the corresponding first derivative curves. The results revealed the presence of three peaks in the first derivative curve of the ternary Mg-5Zn-1Y alloy, referring to the formation of α-Mg primary phase and intermetallics Mg3Zn3Y2 (W-phase) and Mg3YZn6 (I-phase). One more peak, corresponding to the formation of intermetallic Ca2Mg6Zn3 phase, appeared in that of the quaternary Mg-5Zn-1Y-xCa alloy when Ca content exceeds 0.1 wt%. The cooling curves showed that increasing Ca content from 0 to 1 wt% results in reducing the liquidus temperature from 659 °C to 636 °C and the average grain size from 2864 μm to 1719 μm in the Mg-5Zn-1Y alloy. The increase in Ca content also decreased the formation temperature of Ca2Mg6Zn3 phase significantly, about 29 °C, while it increased the solidus temperature from 219 °C to 233 °C. The microstructure constitution of the alloys and the formation of intermetallic phases were further confirmed through metallographic techniques including scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction analysis.