Multicomponent γ’-strengthened Co-based superalloys with increased solvus temperatures and reduced mass densities

Several Co-Al-(W)-based γ-(FCC)/γ’-(L1₂) alloys are investigated to combine recent results indicating significant increases in the γ′-solvus temperature with additions of Ni, Ta, and Ti, and reduced mass density with the substitution of Mo and Nb for W. A maximum solvus temperature of 1167 ± 6 °C is achieved for an alloy with the composition Co-30Ni-7Al-4Ti-7W-1Ta (mole fraction × 100); while the composition Co-30Ni-7Al-4Ti-3Mo-2W-1Nb-1Ta (L19) exhibits a promising combination of high γ′ volume fraction and solvus temperature, low mass density, and excellent two-phase γ-γ′microstructural stability. Atom probe tomographic measurement of L19 aged for 4 h at 900 °C indicates that Ni, Al, Ti, W, Nb, and Ta partition preferentially to the γ′-precipitates while Co partitions strongly to the γ-matrix. Molybdenum segregates at the γ/γ′ interface, resulting in a reduction in the interfacial free energy of 1.63 ± 0.85 mJ m⁻². Decreasing the mole fraction of Ni from 30% to 10% decreases the partitioning of Al and Ti to the γ′-phase and increases partitioning of Co, Mo, W, Nb, and Ta to the γ′-phase. From an analysis of coarsening kinetics (Ostwald ripening) at 900 °C in Co-xNi-7Al-4Ti-3Mo-2W-1Nb-1Ta (x = 10 and 30) interfacial free energies of 35.0 ± 18.6 mJ m⁻² and 29.2 ± 15.5 mJ m⁻² are calculated for mole fractions of Ni of 10% and 30%, respectively. This decrease in interfacial free energy with increasing Ni-concentration is attributed partially to both Mo-segregation at the γ-γ′ interface and a decrease in the lattice parameter misfit between the γ′-precipitate and γ-matrix, and concomitantly the misfit strain energy.