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To determine the influence of Re and Ru on the phase morphologies and microstructure in Ni-based superalloys, a series of model alloys derived from the 4th generation TMS-138 superalloy have been investigated with systemically changing Re and Ru additions. All alloys are subjected to solid solution and aging treatments to achieve the typical γ/γ′ two phases microstructure. Phase transition temperatures are measured by using differential scanning calorimetry. The Re is found to increase the γ′-solvus temperature with a slope of roughly 5.4 ℃ per wt% Re in Ru-free alloys and 2.3 ℃ per wt% Re in 2Ru alloys, while the Ru exhibits much weaker effect. Microstructure and composition of the two phases are examined to understand the Re and Ru effects on γ′ size, shape, volume fraction, the elemental partition behavior, and the γ/γ′ lattice misfit. The results show that the addition of Re affects size and shape of γ′ precipitates, and also reduces γ/γ′ lattice misfit significantly. Meanwhile, Re increases partition coefficients of Cr and Ru. In comparison, the addition of Ru seems to have little effect on morphology of γ′ precipitates and the γ/γ′ lattice misfit, but Ru appears to alter partitioning behaviors of other elements. Because Re and Ru are two key elements for high generation Ni-based single crystal (SX) superalloys, this work might provide guidelines for designing next generation of SX superalloys. [Display omitted] • Re effectively increases the γ′ solvus temperature of superalloys, while Ru has slight effect on the γ′ solvus temperature. • Re addition decreases the size of γ′ phase and γ/γ′ lattice misfit, facilitating cubic precipitation of γ′ phase. • Ru addition only has little effect on the γ′ morphology and γ/γ′ lattice misfit of superalloys. • Partitioning behaviors of other alloying elements depend on the Re and Ru additions, especially Ru. [ABSTRACT FROM AUTHOR]

Oxidation induced phase transformation from γ′ to γ phase can severely degrade creep properties of superalloys and has been studied by in-situ environmental transmission electron microscopy, energy dispersive spectrometer and electron energy loss spectroscopy in detail. The in-situ observation showed that, during oxidization, the ordered γ′ phase (L1 2) gradually transformed to disordered γ phase (face-centered cubic (fcc)) with the appearance of oxidation product consisted of γ-Al 2 O 3 crystallites with a strong crystallographic texture. This directly proves that γ′-γ phase transformation are induced by aluminum oxidation with aluminum depleted in γ′ phase. Furthermore, the oxygen was demonstrated to diffuse preferentially through γ′-γ phase interface which is different from previously thought (γ matrix). [ABSTRACT FROM AUTHOR]