采用微观相场法模拟了Ni₇₅Al₅Cr₂₀合金的时效过程, 研究了Cr和Al原子在无序fcc固溶体(001)和(002)面上序参数变化与相结构演化之间的关系. 结果表明, 合金时效序列为: 无序基体→L1₀(M=1)预析出相→DO₂₂相. 在时效的最初阶段, 原子只在面内跃迁, 发生面内原子有序化, 形成L1₀(M=1)结构; 此后, 原子在面间跃迁, 原子在(001)面上进一步有序化, 在(002)面上无序化, L1₀(M=1)结构向DO₂₂结构转变. 随温度降低, L1₀(M=1)预析出相孕育期和存在期缩短, 从而导致DO₂₂相的孕育期缩短; L1₀(M=1)预析出相在低温下容易形成.

DO₂₂ phase with long period ordered structure is the main strengthening phase in Ni-Al-Cr alloy applied in high temperatures, as it hinders the slipping of dislocations during the deformation. Structure transformation from fcc to DO₂₂ during the ordering process and phase separation have been studied extensively, but there are only a few of reports on the phase transformation in early precipitation stage, especially on the phase transformation of metastable pre-precipitate. In this paper, the microscopic phase--field method was employed to study the pre-precipitate during the transformation from fcc to DO₂₂ in Ni₇₅Al₅Cr₂₀ alloy. The early ordering process of Ni₇₅Al₅Cr₂₀ alloy are simulated at four different temperatures, and the relationship between the order parameter evolutions of Cr and Al atoms on (001) and (002) planes of disordered fcc solid solution and structure evolution will be investigated. The simulation results demonstrate that the parent phase transformed into L1₀(M=1) pre-precipitate with long period ordered structure before DO₂₂  phase formed. At the early aging stage, the atomic ordering is proceeded on each plane as the atoms migrate on planes only, and the L1₀(M=1) pre-precipitate formed in the way of congruent ordering. Then, the further ordering of atoms on (001) plane was induced by the migration of atoms between different planes, and the pre-precipitate with L1₀(M=1) structure is gradually transformed into DO₂₂ phase. The incubation period and existent time of L1₀(M=1) phase are prolonged with the temperature increasing, as a result, the incubation period of DO₂₂ phase is longer when the aging temperature is higher, and the formation of L1₀(M=1) pre--precipitate is easier when the temperature is lower.