The pre-precipitation processes of γ＇ phase in Ni-Al-Ti alloy with different chemical compositions are investigated by microscopic phase-field dynamics model. The simulation results show that the early precipitated phase with L10 structure is formed before the formation of γ＇ phase. With the increase of Al content, the composition of precipitated phase is transformed from the co-existence of γ＇ phase and material phase into single y phase. Furthermore, excessive Al content can stimulate the production of γ＇ phase, and the conversion time from L10 phase to L12 phase will lag behind other samples with lower Al con-tent. By analyzing the occupation probability of Ti, it can be found that the solid solution strengthening effect of Ti is reduced with the increase of Al component.
The early precipitation process of Ni（75）Al（14）Mo（11） alloy was simulated by microscopic phase-field model at different temperatures.The microstructure of the alloy,the precipitation time of Llo structure and occupation probability of the three kinds of atoms were investigated.It is indicated that the non-stoichiometric Ll0（Ⅰ/Ⅱ） phases are found in the precipitation process.With the temperature increasing,the appearance time of Ll0 is brought forward.The Ll0（Ⅱ） structure always precipitates earlier than the Ll0（Ⅰ） structure.Compared with lower temperature,higher temperature brings the formation time of Ll0 phase forward and makes Ll0 phase have a higher order degree.But lower temperature shortens the process time of the Ll0 phase to the Ll2 phase.Al and Mo atoms tend to occupy γ site,Ni atom tends to occupy a and β sites.At the same temperature,Al atom has stronger occupation ability than Mo atom in the same site.Ni,Al and Mo collectively form the composited Ll2 structure.
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.
