文摘
We report on the microstructural evolution of a polycrystalline Ni-based superalloy (Alloy 617B) for power plant applications at a service temperature of 700 ¡ãC. The formation of secondary M23C6-carbides close to grain boundaries (GBs) and around primary Ti(C,N) particles is observed upon annealing at 700 ¡ãC, where ¦Ã?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 ¦Ã/M23C6 and ¦Ã?M23C6 interfaces as well as at grain boundaries are detected, while no B enrichment is found at ¦Ã/¦Ã?interfaces. It is suggested that segregation of B in conjunction with ¦Ã?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.