The η phase is a widely observed high-temperature degradation phase in nickel-based superalloys and is generally considered detrimental to mechanical properties. To date, research on the η phase has largely focused on the transformation from γ′ to η, while key issues such as the nucleation kinetics of η remain insufficiently explored. A major challenge in this field lies in the rapid, statistically meaningful collection of η phase information over large areas (macroscopic scale) while simultaneously covering multiple length scales down to the nanometer regime. In addition, within a complex precipitation–transformation phase assembly, it remains difficult to clearly distinguish and characterize different phases spanning from the micrometer to nanometer scale. In the current study, by exploiting the multi-detector capabilities of advanced scanning electron microscopy, the aforementioned challenges are overcome, enabling a systematic investigation of the effects of precipitation locations, the presence or absence of MC carbides, and MC carbide size on the precipitation kinetics of the η phase. In addition, we identify a pronounced influence of MC size on the final decomposition products, particularly on the formation or suppression of M23C6 carbides. These findings deepen our understanding of the precipitation and growth behavior of the η phase and its strong dependence on the local microstructural environment, and guide more effective control of η phase formation and evolution, for example through regulating the distribution and size of MC carbides and the early-stage abnormal growth of oversized γ′ precipitates at grain boundaries.
