Microscopic phase-field simulation of atomic site occupation in ordering process of NiAl₉Fe₆ alloy

The process of γ(fcc)→γ(fcc)+γ′(L1₂) phase transformation was simulated by using microscopic phase-field method for the low supersaturation NiAl₉Fe₆ alloy. It is found that in the γ′ phase, the ordering degree of Al atoms is obviously higher than that of Fe atoms, and the ordering of Al atoms precedes their clustering, while the case of Fe atoms is opposite. The α site is mainly occupied by Ni atoms, while the β site is occupied in common by Al, Fe and Ni atoms. At order-disorder interphase boundary, the ordering degree of Al atoms is higher than that of Fe atoms, and at the β site, the Fe atomic site occupation probabilities vary from high to low during ordering; the Al atomic site occupation probabilities are similar to those of Fe atoms, but their values are much higher than those of Fe atoms; Ni atoms are opposite to both of them. Meanwhile, during the ordering transformation, γ′ phase is always a complex Ni₃(AlFeNi) single-phase, and it is precipitated by the non-classical nucleation and growth style. Finally, in the alloy system, the volume of γ′ ordered phase is less than that of γ phase, and the volume ratio of order to disorder is about 77%.