Microscopic phase-field method simulation for the in situ transformation of L1₀ phase and L1₂ phase structure

As a Ni₃Al precipitation strengthening alloy, Ni-Cr-Al alloy is one of typical structural materials applied in high temperatures. Much work has been done concerning its structure transformation from fcc to L1₂ during the process of ordering and phase separation. However, the study so far we know on the phase transformation in very early precipitation stage, especially on the phase transformation of unstable pre-precipitation phase, was not yet sufficient. Microscopic phase-filed method is used for describing the temporal and spatial evolution of atomic site in lattice. This method is employed to study the pre-precipitation during the structure transformation from fcc to L1₂ in Ni₈₀Al₁₃Cr₇ alloy in this paper. The relationship between the occupation probability changes in Al and Cr atoms on (100) and (200) planes respectively and structure evolution of Ni₃(Al, Cr) will be inveatigated. The simulation results demonstrate that at in very early aging stage, the composition order parameters of Al and Cr atoms keep unchange on both (100) and (200) planes. However their long range order parameters are equal in these two planes and gradually increase with aging time until the first in situ transformation by congruent ordering at that time the L10 pre-precipitation phase with low long range order is formed. By prolonging aging time, their long range order parameters continue increase once they attain to the respective certain values, the composition order parameters and the long range order parameters of Al and Cr atoms on (100) plane become increasing rapidly but those on (200) plane decreasing quickly, The second in situ transformation occurs and the L1₀ is gradually transformed into L1₂.