Precipitation and growth behavior of γ′ phase in Ni3Al-based superalloy under thermal exposure

Precipitation and growth behavior of γ′ (Ni3Al) phase are crucial to evaluate the service stability of superalloys. In this work, precipitation and growth mechanisms of γ′ phase in different areas of a newly designed Ni3Al-based superalloy were investigated under short-term thermal exposure conditions. Results showed that twinning γ′ phase and foliated γ′ phase precipitated in eutectic area and interface area, respectively. γ′ phase went through from sphere to precursor plate and plate twinning eventually in eutectic area, which was related to preferred orientation of γ′ phase and local segregation of Al element. Dislocation and sidestep interface between matrix and the precipitation were the main reasons for the formation of twinning γ′ phase. Meanwhile, the morphology of γ′ phases in interface area went through a process from sphere to rod-like, and foliated γ′ phases formed eventually which grew toward eutectic areas with thermal exposure time increasing. Vacancies, distortions caused by different thermal expansion coefficients, lattice mismatch between dual-phase area and eutectic area contributed to the formation and growth of spherical phase. Moreover, reduction in interface energy promoted the transformation of spherical phase to rod-like and foliated γ′ phases. In addition, concentration gradient of Al element and adequate holding time were responsible for the directional growth of foliated γ′ phase.