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

Abstract

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 Mg 24Tb 5 and Mg 5Tb were treated as stoichiometric. The compounds, Mg 2Tb and Mg 3Tb in Mg–Tb system and Mg 2Yb in Mg–Yb system, which have a homogeneity range, were modeled using two sublattices Mg 2(Mg, Tb), Mg 3(Mg, Tb) and (Mg, Yb) 2(Mg, Yb), respectively. A three-sublattice model (Mg, Tb) 0.5(Mg, Tb) 0.5Va 3 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.