Microstructural characterization of the η-Ni₃(Ti, Al) phase in a long-term-aged Ni-based superalloy

Microstructural features of a geometrically closed packed hexagonal η-Ni3(Ti, Al) phase precipitated in a long-term-aged Ni-based superalloy are characterised using various kinds of transmission electron microscope (TEM) techniques. The defect features in the η grain interior are clearly revealed at the atomic scale. A nano-sized γ′ phase is found at the defect regions of the micro-sized η phase. The interior γ′ phase keeps a good orientation relationship (OR) of (1 1 1)γ′//(0 0 0 1)η and [1 0]γ′//[2 0]η with the η-Ni3(Ti, Al) phase. However, there is no definitive OR between η-Ni3(Ti, Al) grains and the surrounding matrix. By means of first-principle calculations, we reveal that a symmetrical interfacial feature between γ′ and η corresponds to a lower energy state.