On the effect of Re addition on microstructural evolution of a CoNi-based superalloy

Abstract

In this study, the effect of rhenium (Re) addition on microstructural evolution of a new low-density Co-Ni-Al-Mo-Nb based superalloy is presented. Addition of Re significantly influences the γ′ precipitate morphology, the γ/γ′ lattice misfit and the γ/γ′ microstructural stability during long term aging. An addition of 2 at.% Re to a Co-30Ni-10Al-5Mo-2Nb (all in at.%) alloy, aged at 900 °C for 50 h, reduces the γ/γ′ lattice misfit by ∼ 40% (from +0.32% to +0.19%, measured at room temperature) and hence alters the γ′ morphology from cuboidal to round-cornered cuboidal precipitates. The composition profiles across the γ/γ′ interface by atom probe tomography (APT) reveal Re partitions to the γ phase (KRe=0.34) and also results in the partitioning reversal of Mo to the γ phase (KMo=0.90) from the γ′ precipitate. An inhomogeneous distribution of Gibbsian interfacial excess for the solute Re (ΓRe, ranging from 0.8 to 9.6 atom.nm−2) has been observed at the γ/γ′ interface. A coarsening study at 900 °C (up to 1000 h) suggests that the coarsening of γ′ precipitates occurs solely by evaporation–condensation (EC) mechanism. This is contrary to that observed in the Co-30Ni-10Al-5Mo-2Nb alloy as well as in some of the Ni-Al based and high mass density Co-Al-W based superalloys, where γ′ precipitates coarsen by coagulation/coalescence mechanism with an extensive alignment of γ′ along <100> directions as a sign of microstructural instability. The γ′ coarsening rate constant (Kr) and γ/γ′ interfacial energy are estimated to be 1.13 × 10−27 m3/s and 8.4 mJ/m2, which are comparable and lower than Co-Al-W based superalloys.