Experimental investigation and thermodynamic calculation of phase relations in the Mg–Nd–Y ternary system

Mg–Nd–Y alloys ingots were cast and annealed at 753 K, and then analyzed with scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron-probe microanalysis (EPMA) to determine phase equilibria in the Mg-rich part of the Mg–Nd–Y system. Existence of the ternary compound β with Mg5Gd-type structure was verified, which is in the equilibrium with hcp_A3 (Mg). Furthermore, thermodynamic description of the Mg–Nd–Y system was carried out on the basis of the present experimental results. The thermodynamic parameters of the boundary Mg–Nd and Nd–Y binary systems were directly cited from literatures, while most of the parameters for the Mg–Y binary system were taken from the latest literature with an exception of a minor revision for the bcc_B2 phase in this paper. The ordered bcc_B2 and disordered bcc_A2 phases were described with a sublattice model (Mg,Nd,Y)0.5(Mg,Nd,Y)0.5 and their Gibbs energies were expressed with same function. Most of the binary intermetallic phases, except for Mg3Nd, were assumed to have no ternary solubility. The Mg3Nd and ternary compound β were treated as semistoichiometric compounds with mutual substitution between Nd and Y. Reasonable agreement of the phase equilibria in the Mg–Nd–Y ternary system between thermodynamic extrapolation and experiments was achieved.