Thermodynamic description on the miscibility gap of the Mg-based solid solution in the Mg–Zn, Mg–Nd and Mg–Zn–Nd systems

With the consideration of the existence of the metastable miscibility gaps, the thermodynamic parameters of the Mg-based solid solution have been assessed for the Mg–Zn and Mg–Nd systems. The new semi-empirical equation proposed by Kaptay was used to describe the interaction parameters between components to prevent the appearance of the artificial miscibility gap at high temperature. The obtained thermodynamic descriptions can reproduce the metastable miscibility gaps and the spinodal decomposition curves in the Mg–Zn and Mg–Nd systems. The driving force calculation can explain well the precipitation of the second hcp_A3 phase (Guinier–Preston zones) in the primary Mg-based hcp_A3 matrix, as well as the decomposition sequence following a quench from a single-phase supersaturated solid solution to a two-phase regime. The miscibility gap of the Mg-based phase in the Mg–Zn–Nd ternary system was predicted on the basis of the descriptions of the Mg–Zn and Mg–Nd binary systems without consideration of the three-component interactions.