First-principles calculations and thermodynamic modeling of the Ni-Mo system

The phase equilibria and thermodynamic properties of the Ni-Mo system were analyzed by combining a first-principles approach and calculation of phase diagram (CALPHAD) technique. The first-principles calculation results indicate that Ni₂Mo and Ni₈Mo are stable in addition to Ni₃Mo and Ni₄Mo, and δ-NiMo is not stable at 0 K, both in contradiction to the existing phase relationships in the Ni-Mo system. The enthalpies of the mixing of the bcc and fcc solid solution phases were also predicted by first-principles calculations using the special quasirandom structures. In the present work, the non-stoichiometric δ-NiMo and Ni₃Mo phases were modeled using three- and two-sublattice models, respectively. The Ni₂Mo, Ni₄ Ni₈Mo phases were treated as stoichiometric compounds. Based on the first-principles data from the present work and experimental data in the literature, the Gibbs energy functions of individual phases were evaluated, and a new Ni-Mo phase diagram was presented.