Early Stages of Precipitation in γ' Phase of a Ni–Al–Ti Model Alloy: Phase-Field and First-Principles Study

The early stages of precipitation process of the γ' phase of a Ni–Al–Ti alloy are investigated by microscopic phase-field and first-principles calculations. The simulated results indicate that a pre-precipitate with L1 0 structure appears before the L1 2 ordered phase, and then this metastable phase gradually transforms to L1 2 ordered phase; finally, the precipitated phase is composed of γ' ordered phase and γ matrix phase. The occupation probabilities of Al, Ni, and Ti atoms also illustrate the formation of the L1 0 phase and its situ conversion to L1 2 ordered phase constituted by a complicated compound Ni 3 (AlTi). Through the analyses of order parameter and occupation probability, the precipitation mechanism of γ' phase is drawn as a combination of congruent ordering and destabilization decomposition. Meanwhile, we also find that the growth and coarsening of the γ' phase occur via mixed mechanisms of Ostwald ripening and coalescence coarsening of neighboring precipitates. Moreover, the first-principles method is applied to calculate the thermodynamic parameters and validate further the appearance of the metastable phase and the site preference of Ti atom, which offers an explanation for atomic occupancy characteristics in the precipitate.