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

P.A.J Bagot,O.B.W Silk,J.O Douglas,S Pedrazzini,D.J Crudden,T.L Martin,M.C Hardy,M.P Moody,R.C Reed

doi:10.1016/j.actamat.2016.11.053

P.A.J Bagot, O.B.W Silk + Show 7 more

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https://doi.org/10.1016/j.actamat.2016.11.053

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Journal: Acta Materialia	Publication Date: Dec 10, 2016
Citations: 121	License type: cc-by

Affiliation: University of Oxford, Rolls-Royce (Germany)

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.

Highlights

Where do the atoms in an alloy reside? This question is of fundamental importance to the field of materials science and the discipline of physical metallurgy from which it emerged
We examine in detail element partioning behaviour and inter-relationships in the g0 phases of this alloy, and assess how specific elements segregate to grain boundaries
Fig. 1c) isolates these at high magnification, along with a series of Energy Dispersive X-ray Spectroscopy (EDX) maps identifying them as a Mo-Cr rich boride and a smaller Ta-Hf rich carbide

Summary

Introduction

Where do the atoms in an alloy reside? This question is of fundamental importance to the field of materials science and the discipline of physical metallurgy from which it emerged. For structural alloys e which are usually based upon one of Al, Fe, Ni or Ti e truly unequivocal answers are not yet available. These alloys are usually multicomponent in nature with many alloying elements added. Strengthening phases are nearly always present, and these can be ordered, so that a site preference arises. That segregation and/or grain boundary precipitation can arise. To what extent can a modern high-resolution characterisation techniques answer this critical question?

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Conclusion