Simulation of the pre-precipitate in Ni₇₅Al₅Cr₂₀ alloy by microscopic phase-field method

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.