On the effect of Re addition on microstructural evolution of a CoNi-based superalloy
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 hours, reduces the γ/γ' lattice misfit by {\approx} 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) reveals Re partitions to the γ phase (K_Re=0.34) and also results in the partitioning reversal of Mo to the γ phase (K_Mo=0.90) from the γ' precipitate. An inhomogeneous distribution of Gibbsian interfacial excess for the solute Re (Γ_Re, 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 hours) suggests that the coarsening of γ' precipitates occurs solely by an 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 extensive alignment of γ' along <100> directions as a sign of microstructural instability. The γ' coarsening rate exponent (K_r) and γ/γ' interfacial energy are estimated to be 1.41 x 10-27 m3/s and 8.4 mJ/m2, which are comparable and lower than Co-Al-W based superalloys.