Thermodynamic optimization of the Mg–Tb and Mg–Yb systems

The two Mg–RE systems, Mg–Tb and Mg–Yb, have been critically assessed by means of the CALculation of PHAse Diagram (CALPHAD) technique. The solution phases (liquid, body-centered cubic, face-centered cubic and hexagonal close-packed) were modeled with the Redlich–Kister equation. The intermetallic compounds Mg24Tb5 and Mg5Tb were treated as stoichiometric. The compounds, Mg2Tb and Mg3Tb in Mg–Tb system and Mg2Yb in Mg–Yb system, which have a homogeneity range, were modeled using two sublattices Mg2(Mg, Tb), Mg3(Mg, Tb) and (Mg, Yb)2(Mg, Yb), respectively. A three-sublattice model (Mg, Tb)0.5(Mg, Tb)0.5Va3 was applied to describe the intermediate B2 compound (MgTb) in order to cope with the order–disorder transition in the Mg–Tb system. A set of self-consistent thermodynamic parameters of the Mg–Tb and the Mg–Yb system was obtained, respectively.