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 degrees 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 degrees C, where gamma' 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 gamma/M23C6 and gamma'/M23C6 interfaces as well as at grain boundaries are detected, while no B enrichment is found at gamma/gamma' interfaces. It is suggested that segregation of B in conjunction with gamma' 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. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.