Thermodynamic evaluations of the iron–lutetium and iron–thulium systems

• Both systems were thermodynamically reassessed to correct the invariant reactions previously calculated by Konar and in disagreement with the experiments.
• The Gibbs free energies of solution phases were described by the substitutional solution model.
• A coherent set of thermodynamic parameters was obtained for both systems.
• An agreement between the calculated results and experimental data was obtained for both systems.
• This work is the start point for the study of The Fe–Lu–M (M=Ti, V) ternary systems.

The iron–lutetium and iron–thulium binary systems needed to be reassessed after the previous thermodynamic evaluation by Konar (2012) [1] because significant discrepancies were observed with the experimental data. Furthermore new thermodynamic data were published in the meantime. In the present work, the modelings were carried out with the help of the CALPHAD (CALculation of PHAse Diagram) method. The seven intermediate phases Fe23Lu6, Fe3Lu, Fe2Lu, Fe17Tm2, Fe23Tm6, Fe3Tm and Fe2Tm in these two binary systems have been treated as stoichiometric compounds while the Fe17Lu2 substoichiometric intermetallic compound in Lu, in the Fe–Lu binary system which has a homogeneity range, was treated by a two-sublattice model with convenient substitution in each sublattice (Sundman et al., 1985 [2]). A solution model has been used for the description of the liquid phase and the (Fe), (Lu) and (Tm) solid solutions. The excess term of the Gibbs energy of the solution phases was assessed with the Redlich–Kister (Redlich and Kister, 1948 [3]) polynomial equation. The calculations based on the thermodynamic modeling are in good agreement with the phase diagram data and experimental thermodynamic values available in the literature.