Thermodynamic description of the Mg–Nd–Zn ternary system

Abstract

A thermodynamic description of the Mg–Nd–Zn system was developed by means of the CALPHAD (CALculation of PHAse Diagrams) method. The constituent binary systems Mg–Nd and Nd–Zn were re-optimized based on the experimental phase equilibria and thermodynamic properties available in the literature. Combining with the thermodynamic parameters of the Mg–Zn system cited from the reference, the Mg–Nd–Zn ternary system was evaluated. The Gibbs energies of the solution phases (liquid, BCC_A2, DHCP, HCP_A3 and HCP_Zn) were described by the subregular solution model with the Redlich–Kister polynomial function, and those of the stoichiometric compounds, Nd 2Zn 17, NdZn 11_H, NdZn 11_L, Nd 3Zn 22, Nd 13Zn 58, Nd 3Zn 11, NdZn 3, NdZn 2 and Mg 2Nd, were described by the sublattice model. The compounds Mg 3Nd and Mg 41Nd 5 in the Mg–Nd–Zn system were treated as the formulae (Mg, Zn) 3(Mg, Nd) and (Mg, Nd, Zn) 41(Mg, Nd) 5. The order–disorder transition between BCC_B2 and BCC_A2 phases was treated using a two-sublattice model (Mg, Nd, Zn) 0.5(Mg, Nd, Zn) 0.5. Based on experimental data, four stable ternary compounds τ 1(Mg 7Nd 1Zn 12), τ 2(Mg 7Nd 2Zn 11), τ 3(Mg 6Nd 1Zn 3) and τ 4(Mg 6Nd 3Zn 11) were taken into consideration in this system. A set of self-consistent thermodynamic parameters of the Mg–Nd–Zn system was obtained. Projection of the liquidus surface, selected vertical and isothermal sections were calculated using the proposed thermodynamic description. Comprehensive comparisons between the calculated and measured phase diagrams show that almost all the accurate experimental information is satisfactorily accounted for by the present thermodynamic description.