Phase equilibria and thermodynamic properties in the RE-Ni (RE = rare earth metals) binary systems

The RE-Ni (RE = La, Ce, Nd, Tb and Ho) binary systems have been optimized thermodynamically using the CALPHAD (CALculation of PhAse Diagram) method based on experimental phase equilibrium and thermodynamic property data. The solution phases including the liquid phase and terminal solid solution phases were modeled by the substitutional solution model with the excess Gibbs energy being described by the Redlich–Kister polynomials. The binary intermetallic compounds REaNib were treated either as stoichiometric phases or by using the two-sublattice model to consider the homogeneity ranges of the compounds. A set of self-consistent thermodynamic parameters of the RE-Ni (RE = La, Ce, Nd, Tb and Ho) binary systems was obtained, which can be used to reproduce well the experimental data. Furthermore, in combination with the reported assessments of the RE-Ni (RE = Pr, Sm, Gd, Dy, Er and Y) binary systems, the phase equilibria and thermodynamic properties of the RE-Ni (RE = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er and Y) binary systems were examined systematically. A trend was shown for the variation of phase equilibria and thermodynamic properties as a function of the RE atomic number. In general, as the atomic number of the rare earth metals increases, the phase transition/reaction temperatures of the RE-Ni intermetallic compounds become higher and higher, and the enthalpies of mixing of liquid RE-Ni (except for Sm-Ni) alloys as well as the enthalpies of formation of the RE-Ni (except for Y-Ni and Sm-Ni) intermetallic compounds become increasingly negative. This trend demonstrates stronger RE-Ni bonds in binary systems containing heavier rare earth elements.