The formation of η-Ni3Ti phase microstructure in a cast nickel-based superalloy with high Ti/Al ratio

By over-aging at 900 °C and 1000 °C, a high volume-fraction of η-Ni3Ti phase was prepared in a cast nickel-based superalloy with high Ti/Al ratio. EBSD analysis shows that the η phase obeys a SN-type orientation relationship with γ matrix: <11 2‾ 0>η//<1 1‾ 0>γ, {0001}η//{111}γ. Four kinds of η phase variants with a 70.5° misorientation to each other are clarified. From the η/γ interface structure as detected by HRTEM, the thin plate-like morphology of η phase is attributed to the preferential growth of the incoherent {11 2‾ 0}η//{1 1‾ 0}γ interface. The formation of η phase at the grain boundary (GB) was by a discontinuous precipitation reaction, during which the original GB migrates from the η-nucleating side into the adjacent grain following the elongation of η phase to form a cellular structure. The detailed GB migration mechanism is investigated by EBSD and rationalized from the aspect of interface energy. For the formation of η phase in grain interior, three relative reactions are clarified: (i) MC+γ→M23C6+OA-γ′, OA-γ′→η, (ii) γ→M23C6+OA-γ′, OA-γ′→η, and (iii) γ′→OA-γ′, OA-γ′→η, where OA-γ′ is the newly precipitated γ′ phase during over-aging which shares a similar elemental distribution with η. The intermediate phase OA-γ′ is formed due to the comparably low nucleation barrier from γ while its subsequent transformation to η is excited by the higher thermodynamic stability of η as revealed by the calculation results by using the VASP software. At last, as conformed by STEM-HAADF, the structure transformation of OA-γ′→η is aided by the introduce of superlattice intrinsic stacking faults.