A thermodynamic description of the Gd–Mg–Sm system

Abstract

The Mg–Sm, Gd–Sm and Gd–Mg–Sm systems were thermodynamically optimized using the CALPHAD technique. The solution phases, liquid, bcc, hcp and rhombohedral, were described by the substitutional solution model. The isostructural compounds, MgGd in the Gd–Mg system and MgSm in the Mg–Sm system with a B2 structure was assumed to form a continuous range of solid solutions in the Gd–Mg–Sm system. The order–disorder transition between the bcc solution with an A2 structure and compound Mg(Gd, Sm) with a B2 structure in the system has been taken into account and thermodynamically modeled. The other isostructural compounds Mg 5Gd and Mg 5Sm, Mg 3Gd and Mg 3Sm, Mg 2Gd and Mg 2Sm in the Gd–Mg–Sm system were described according to the formulae Mg 5(Gd,Sm), Mg 3(Gd,Sm), and Mg 2(Gd,Sm), respectively. The compound Mg 41Sm 5 with a homogeneity range was treated as a line compound Mg 41(Gd,Sm) 5 in the Gd–Mg–Sm system. Based on the experimental data in the Mg-rich corner of the Gd–Mg–Sm system, a set of thermodynamic parameters describing the Gibbs energies of individual phases of the Gd–Mg–Sm system as functions of composition and temperature was obtained. In addition, the complete ternary phase diagram of the Gd–Mg–Sm system were predicted.