Study on Ni-Ti alloys around equiatomic composition by the first-principles phase field method

Ni-Ti alloys around equiatomic composition, i.e., those with 45-55 at.% Ni, at 1200 K are studied by using the first-principles phase field (FPPF) method, which does not rely on any thermodynamic empirical parameter. There is an important tolerant aspect of the method, which does not account for the crystal structure at all but accounts for the value of the corresponding local free energy only. Here we show that this tolerance can be used to handle the correct NiTi2 phase in a tetrahedron approximation in cluster expansion theory. With the correct NiTi2 local free energy, the resulting microstructures are in good agreement with the experimental ones. Moreover, since the energy of the most stable structure is generally very close to the less stable structures, even if one assigned an incorrect crystal structure, for example, NiTi3 instead of the correct NiTi2, the method can still reproduce the experimental microstructures, although the shape of the resulting precipitates slightly changes from angular to round. We explicitly demonstrate this characteristic by comparing the results. These features may become a great advantage in predicting unknown alloys by the FPPF method. We also explicitly clarify that the inclusion of interstitial atoms into calculation causes almost no effect on the final microstructures in the present case.