An Atom Probe Tomography study of site preference and partitioning in a nickel-based superalloy

Abstract

Atom Probe Tomography (APT) has been utilised for an in-depth examination of the commercial polycrystalline Ni-based superalloy RR1000, assessing compositions of the primary, secondary and tertiary γ′ phases. Clear differences in the phase chemistries are noted, particularly for the tertiary γ′ to which much of the alloy strength is attributed. Trace amounts of Hf are found to segregate strongly to the primary and secondary γ′ phases, but also exhibit an extended diffusion profile across the γ-γ′ interface up to 80 nm wide. Ti, Al and Mo demonstrate similar, yet not as pronounced diffusion profiles, indicating assumed phase chemistries may not be representative of those regions adjacent to the γ-γ′ interface. Within γ′, unique element site-occupancy preferences for this alloy were identified. Finally, the grain boundary chemistry across a γ-γ interface and that of an intragranular boride were analysed, identifying the latter as a mixed M5B3 boride rich in Mo and Cr. These demonstrate further the depth of information on Ni-alloys accessible by APT, while the overall implications of results in comparison with other in-service/model alloys are also discussed.