Phase field model was employed to study the variations of interatomic potentials of Ni 3 Al (L1 2 phase) and Ni 3 V (DO 22 phase) as a function of temperature and concentration. The long-range order (LRO) parameter related interatomic potentials equations formulated by Khachaturyan were utilized to establish the inversion equations for L1 2 and DO 22 phases, with which interatomic potentials could be calculated. The interatomic potentials of Ni-Al and Ni-V exhibited approximately linear increases and decreases, individually, with enhanced Al concentration. Substituting the inverted interatomic potentials into the microscopic phase field equations led to three cases of precipitation sequence: the DO 22 phase preceded L1 2 phase precipitating at the interatomic potentials of Ni-V > Ni-Al; the vice cases; and two phases precipitated simultaneously at interatomic potentials of Ni-V and Ni-Al were equal.
DONG WeiPing WANG YongXin YANG Kun CHEN Zheng LU YanLi
Based on the microscopic phase-field model, the structure and migration characteristic of ordered domain interfaces formed between DO22 and L12 phase are investigated, and the atomistic mechanism of phase transformation from L12 （Ni3Al） to DO22 （Ni3V） in Ni75AlxV25-x alloys are explored, using the simulated microstructure evolution pictures and the occupation probability evolution of alloy elements at the interface. The results show that five kinds of heterointerfaces are formed between DO22 and L12 phase and four of them can migrate during the phase transformation from L12 to DO22 except the interface （002）D//（001）L. The structure of interface （100）D//（200）L and interface （100）D//（200）L·^1/2[001] remain the same before and after migration, while the interface （002）D//（002）L is formed after the migration of interface （002）D//（002）L·^1/2[100] and vice versa. These two kinds of interface appear alternatively. The jump and substitute of atoms selects the optimization way to induce the migration of interface during the phase transformation, and the number of atoms needing to jump during the migration is the least among all of the possible atom jump modes.
Based on the microscopic phase-field model, ordered domain interfaces formed between D022 （Ni3V） phases along [001] direction in Ni75AlxV25-x alloys were simulated, and the effects of atomic structure on the migration characteristic and solute segregation of interfaces were studied. It is found that the migration ability is related to the atomic structure of interfaces, and three kinds of interfaces can migrate except the interface （001）//（002） which has the characteristic of L12 （Ni3Al） structure. V atoms jump to the nearest neighbor site and substitute for Ni, and vice versa. Because of the site selectivity behaviors of jumping atoms, the number of jumping atoms during the migration is the least and the jumping distance of atoms is the shortest among all possible modes, and the atomic structures of interfaces are unchanged before and after the migration. The preferences and degree of segregation or depletion of alloy elements are also related to the atomic structure of interface.
Kinetics of order-disorder transition at antiphase domain boundary （APDB） formed between L12 （Ni3A1） phases is investigated using microscopic phase-field model. The results demonstrate that whether order-disorder transition happens or not depends on the atomic structure of the APDB. Accompanied with the enrichment of V and depletion of Ni and A1, the ordered APDB with phase-shift vector of a/2[100] transforms into a thin disordered phase layer. Whereas at the APDB with phase shift vector of a/2[110], which remains ordered with temporal evolution, Ni and A1 enrich and V depletes. Composition evolution of APDB with order-disorder transition favors the nucleation of DO22 phase, and the formation of disordered phase layer accelerates the growth of DO22 phase. The disordered phase caused by order-disordered transition of the APDB can be considered as the transient phase along the precipitation path of DO22 phase.
By utilizing phase field method combined with analysis on free energy and interatomic potentials, pre-precipitation phase formation and transformation process of Ni0.75Al0.05Fe0.2 alloy in early precipitation stage during the ageing process under 1 000 K were studied. And free energy, microstructures, compositions and volume fractions of pre-precipitation phase and equilibrium phase were analyzed. The simulation results indicate that nonstoichiometric Llo pre-precipitation phase formed first, and then would gradually transform into L12 equilibrium phase. It is discovered that the phase transformation process was closely related to free energy and interatomic potentials. Additionally, it is revealed that free energy of Llo pre-precipitation phase was higher and interatomic potential was smaller than that of L12 equilibrium phase. Therefore, it is concluded that Llo phase was unstable, and phase transformation would occur to L12 which was more stable.
The precipitating kinetics of Ni75Al17Zn8 alloy was studied at both 873K and 973K by microscopic phase-field model.The calculation results show that the order-disorder transformation experiences the matrix→lowly-ordered L10 phase→L12 phase at 973 K.And the nucleation of L12 particles belongs to the spinodal decomposition mechanism.As temperature increases,orderings of Al and Zn atoms are resisted,but coarsening of L12 particles is promoted.The value of coarsening kinetic exponents approaches to 1/2.In addition,the discussions about Ni-Al anti-site defect and Zn substitutions for Ni site and Al site exhibit that the higher the temperature,the more distinctive the Ni-Al anti-site defect,but the less the Zn substitution.
Correlation between site occupation evolution of alloying elements in L12 phase and growth of DO22 phase in Ni75Al7.5V17.5 was studied using microscopic phase field model. The results demonstrate that the growing process of DO22 phase can be divided into two stages. At the early stage, composition in the centre part of L12 phase almost remains unchanged, and the nucleation and growth of DO22 phase is controlled by the decrease of interface between L12 phases. At the late stage, part of V for growth of DO22 phase is supplied from the centre part of L12 phase and mainly comes from Al sublattice, the excess Ni spared from the decreasing L12 phase migrates into the centre part of L12 phase and occupies the Ni sublattices exclusively, while the excess Al mainly occupies the Al sublattice. At the late stage, the growth of DO22 phase is controlled by the evolution of antisite atoms and ternary additions in the centre part of L12 phase.
