Design of low mass density γ/γ′ CoNi-based superalloys with promising high-temperature mechanical properties

Abstract

We report new compositions of CoNi-based superalloys that have low mass densities (7.84–8.20 g/cc), negative γ / γ ′ lattice misfit, and high γ ′ solvus temperature up to 1094 °C with promising high-temperature mechanical properties. The γ / γ ′ in the base Co-30Ni-10Al-5Cr alloy was stabilized with Nb and further developed by varying Ti and Cr. The γ′ precipitates in these alloys are richer in Ni than the γ matrix phase, unlike for γ ′ in other CoNi-based superalloys. The atomic-scale compositional analysis indicates that the γ ′ has a stoichiometry of ( Ni , Co ) 3 ( Al , Nb , Ti , Cr ) . In these alloys, the γ ′ microstructure is stable at 900 °C up to 1000 h. In an alloy with a γ ′ solvus of 1089 °C, the coarsening kinetics of γ′ at 900 °C follows the Lifshitz-Slyozov-Wagner model with a rate constant of 6.6 ± 0.7 n m 3 / s , comparable with high-density Co-Al-W-based superalloys. The alloys also show an anomalous increase in strength, with temperature having a peak of 0.2% proof stress (PS) and specific PS (SPS) of ∼710 MPa and ∼ 91 MPa g – 1 cc , respectively, at 770 °C. The low density and high strength make this class of alloys a potential and cost-effective alternative to the other Co-based superalloys and comparable with Ni-based superalloys.