Gd–Mg–Sm Thermodynamic Description
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 Mg5Gd and Mg5Sm, Mg3Gd and Mg3Sm, Mg2Gd and Mg2Sm in the Gd–Mg–Sm system were described according to the formulae Mg5(Gd,Sm), Mg3(Gd,Sm), and Mg2(Gd,Sm), respectively. The compound Mg41Sm5 with a homogeneity range was treated as a line compound Mg41(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.