Effects of content and distribution of Zn and Gd on formation ability of I phase and W phase in Mg‒Zn‒Gd‒Zr alloy

Abstract

The solid solution and compounds in Mg‒Zn‒Gd‒Zr magnesium alloy semicontinuous casting ingots were effectively separated by phase separation technique，the solid solution quantity of Zn and Gd as well as the mass ratio of Mg3GdZn6 (I phase) and Mg3Gd2Zn (W phase) compounds were determined, and the mass fraction of I phase and W phase were calculated. The results show that increasing the initial Zn/Gd ratio in as-cast alloys increases the ratio of I and W phases, increasing the content of Zn and Gd under a certain Zn/Gd ratio increases the amount of I and W phases simultaneously. I phase and W phase form in the residual melt of primary α-Mg crystallization, the changes of the distribution of Zn and Gd in the alloy will affect the composition of the residual melt, thus affecting the forming ability of I phase and W phase. After homogenization, I phase disappears, W phase increases and total amount of ternary phase decreases. The Gd element in the newly formed W phase comes from the decomposed I phase and the supersaturate solid solution, while the Zn decomposed from I phase and more than that required for the newly formed W phase diffused to the solid solution.