A thermodynamic description of the Gd-Mg-Sm system

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.