Critical issues in η-phase nucleation and MC carbide decomposition in superalloys: Site-dependent precipitation and the role of MC carbides with different sizes

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 M 23 C 6 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.