Experimental Study on the Effect of Cooling Rate on the Secondary Phase in As-Cast Mg-Gd-Y-Zr Alloy

The effect of cooling rate on the composition, morphology, size, and volume fraction of the secondary phase in as-cast Mg-Gd-Y-Zr alloy is investigated. In the study, a casting containing five steps with thickness of 10-50mm is produced, in which cooling rate ranging from 2.6 to 11.0Ks(-1) is created. The secondary phase is characterized using optical microscope (OM), scanning electron microscope (SEM), and electron probe micro-analyzer (EPMA). The volume fraction of the secondary phase is determined using OM and quantitative metallographic analysis, and Vickers hardness test is conducted to verify the analysis results. The effect of the cooling rate on the volume fraction of the secondary phase is discussed in detail. The result shows that with the increase of the cooling rate, the size of the secondary phase decreases. The effect of the cooling rate on the volume fraction of the secondary phase is complicated somewhat. A comprehensive analysis on the experimental data shows that a critical cooling rate may exist, over which the volume fraction of the secondary phase decreases with the increase of the cooling rate, however under which the volume fraction increases with the increase of the cooling rate.