Elemental partitioning of platinum group metal containing Ni-base superalloys using electron microprobe analysis and atom probe tomography

The partitioning of platinum group metal (PGM) additions in polycrystalline Ni-base superalloys has been investigated. Alloys with a baseline composition of 15Al-5Cr-1Re-2Ta-0.1Hf (at.%) with systematic variations in Pt, Ir, Ru and W were cast and heat-treated to produce bimodal two-phase γ-γ' microstructures. Electron microprobe analysis was used to measure the composition of coarse γ' particles. Selected alloys were studied using atom probe tomography. Pt and Ru weakly partition to the γ' and γ phases, respectively, whereas Ir partitions equally between both phases. Pt and Ru do not change partitioning behavior appreciably with additional W and Re. Interestingly, Ir lowers γ' volume fraction and reduces partitioning of W, Re, Ru, Al and Pt. Tungsten partitioning coefficients exhibit an unusually strong temperature dependence, with increased partitioning of W to the matrix at lower temperatures. The thermodynamics of these PGM-containing systems are analyzed and the implications for partitioning are discussed.