Phase equilibria in Mg-rich corner of Mg–Ca–RE (RE=Gd, Nd) systems at 400 °C

Abstract

The phase equilibria in Mg-rich corner of Mg–Ca–Gd and Mg–Ca–Nd ternary systems at 400 °C were determined through the equilibrated alloy method by using XRD, SEM, EPMA and DSC. Partial isothermal sections in Mg-rich corner of Mg–Ca–Gd and Mg–Ca–Nd ternary systems at 400 °C were constructed from 13 alloys. A three-phase region of α–Mg, Mg41RE5 and Mg2Ca was determined in both ternary systems. It is formed by a similar ternary eutectic reaction L→α-Mg+Mg2Ca+Mg41RE5 at 499.6 °C and 505.6 °C, respectively. It is found that the maximum solubility of Ca in Mg5Gd is 3.68% (molar fraction) and 3% of Gd can be dissolved in Mg2Ca in the Mg–Ca–Gd system at 400 °C. While in the Mg–Ca–Nd system the maximum solubility of Ca in Mg41Nd5 is 3.57% and 1.24% of Nd can be dissolved in Mg2Ca at 400 °C. Other three-phase equilibria existing in Mg-rich corner of Mg–Ca–Gd system are α-Mg+Mg5Gd+T and Mg5Gd+Mg2Ca+T and the three-phase equilibrium in Mg-rich corner of Mg–Ca–Nd system is Mg3Nd+Mg2Ca+ Mg41Nd5.