Microstructural evolution of a Ni-based superalloy (617B) at 700 °C studied by electron microscopy and atom probe tomography

We report on the microstructural evolution of a polycrystalline Ni-based superalloy (Alloy 617B) for power plant applications at a service temperature of 700 °C. The formation of secondary M₂₃C₆-carbides close to grain boundaries (GBs) and around primary Ti(C,N) particles is observed upon annealing at 700 °C, where γ′ is found to nucleate heterogeneously at M₂₃C₆ carbides. Using atom probe tomography, elemental partitioning to the phases and composition profiles across phase and grain boundaries are determined. Enrichments of B at γ/M₂₃C₆ and γ′/M₂₃C₆ interfaces as well as at grain boundaries are detected, while no B enrichment is found at γ/γ′ interfaces. It is suggested that segregation of B in conjunction with γ′ formation stabilizes a network of secondary M₂₃C₆ precipitates near GBs and thus increases the creep rupture life of Alloy 617B. Calculations of the equilibrium phase compositions by Thermo-Calc confirm the chemical compositions measured by atom probe tomography.