Precipitation of η phase in a nickel based superalloy during aging

The microstructure evolution of a nickel based superalloy aged at 800 °C for 0–10000 h was studied by scanning electron microscopy, transmission electron microscopy, and atom probe tomography. A high number density of γ′-Ni 3 (Al, Ti, Nb) phase homogeneously distributed at matrix was observed at the early stage of aging. Tiny plate-like η-Ni 3 (Ti, Al, Nb) phase precipitated at the grain boundaries after aging for more than 40 h. Both the γ′ phase and η phase have coherent orientation relationship with the nearby matrix. Stacking faults were observed occasionally within the γ′ phase and most of η phase. The composition evolution of γ′ phase and η phase have been analyzed. Based on the experimental results, the precipitation and growth behaviors of η phase are discussed. Stacking faults and nano-twin are proposed to facilitate the phase transformation of γ′ phase to η phase. The results are expected to contribute to the design of nickel based superalloy with optimized composition and heat treatment process with improved mechanical properties.