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 termaging. 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 ~ 40% (from +0.32% to +0.19%, measured at room temperature) and hence alters the g' 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 g phase ( "# = 0.34) and also results in the partitioning reversal of Mo to the g phase ( *+ = 0.90) from the γ' precipitate. An inhomogeneous distribution of Gibbsian interfacial excess for the solute Re (Γ "# , 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 ( / ) and γ/γ' interfacial energy are estimated to be 1.41 ´ 10 -27 m 3 /s and 8.4 mJ/m 2 , which are comparable and lower than Co-Al-W based superalloys.Keywords: Rhenium (Re); Cobalt-based superalloys; Lattice misfit; Atom probe tomography (APT); Coarsening kinetics. all transition metals in fcc-Co [56]. Due to the higher vacancy formation energy in Co, the diffusivity and activation energy barrier of Re is greater than Ni [56,57]. However, Kolb et al. [54] and Volz et al. [55] showed Re addition to a single crystalline Co-Ni-Al-W based alloy leads to a marginal or no improvement in creep properties compared to the role of Re in the 2 nd and 3 rd generation Ni-Al based superalloys. This was attributed to the low partitioning of Re to the γ matrix phase. Hence, it was concluded that Re is not crucial for Co-Al-W based superalloys.The addition of Re could have an important role for the long term microstructural stability of Co-Al based superalloys. However, the role of Re in the coarsening behavior of cobalt based alloys, in particular that of the low-density Co-Al-Mo-Nb/Ta based superalloys, has not been explored rigorously. Some studies on the kinetics and associated models have been reported for Re added Ni-Al based superalloys [27,28,58]. They show that the coarsening of precipitates follows Lifshitz-Slyozov-Wagner (LSW) based Ostwald ripening mechanism with slight variation in temporal exponents.The present work reports a detailed study on the effect of Re addition on a low density W-free g/g¢ Co-Al based superalloys. Microstructural evolut...