Ternary compounds and solid-state phase equilibria in Mg-rich side of Mg-Zn-Ca system at 300 °C

Abstract

The phase equilibria and compositions in Mg-rich side at 300 °C were investigated in Mg-Zn-Ca ternary system through the equilibrated alloy method by using scanning electron microscopy, electron probe microanalysis, X-ray diffraction and transmission electron microscopy. The results show that two ternary compounds T 1 and T 2 can be in equilibrium with the Mg-based solid solution in Mg-Zn-Ca system. T 1 phase is a linear compound with the composition region (molar fraction) of 15% Ca, 20.5%-48.9% Zn and balanced Mg at 300 °C. Its hexagonal structure parameters decrease with increasing Zn content, i.e. a=0.992-0.945 nm, c=1.034-1.003 nm. T 2 phase has hexagonal structure with the composition region of 26.4%-28.4% Mg, 63.2%-65.5% Zn and 7.1%-8.4% Ca. At 300 °C, the solubility of Zn in the Mg-based solid solution increases for the addition of Ca, the maximum solubility of Zn is 3.7%. Three-phase fields consisting of α-Mg+Mg 2Ca+ T 1, α-Mg+ T 1+ T 2, α-Mg+ T 2+MgZn and MgZn+ T 2+Mg 2Zn 3 exist in the Mg-Zn-Ca system at 300 °C.