Theoretical description of phase equilibrium in the nickel-titanium system

Solid-solid and solid-liquid phase equilibrium in the Ni-Ti system is studied within the framework of the cluster variation method (CVM). The energy parameters entering the free-energy description of each phase are determined by tight-binding energy calculations using the cluster Bethe lattice method (CBLM). For simplicity, the pair interactions are restricted to first-nearest neighbours in FCC-based structures and to first- and second-nearest neighbours in BCC-based structures. The configurational entropy for disordered solutions and for ordered compounds are obtained using the tetrahedron approximation of the CVM. The three compounds NiTi, Ni₃Ti and NiTi₂ compounds are treated as stoichiometric compounds. The calculated diagram agrees reasonably well with that determined experimentally.