Several Co-Al-(W)-based - alloys are investigated in an attempt to combine recent results indicating significant increases in solvus temperature with additions of Ni, Ta, and Ti, and reduced density with the substitution of Mo and Nb for W. A maximum solvus temperature of 1167±6 °C is achieved in an alloy with the composition (by mole fraction ×100) Co-30Ni-7Al-4Ti-7W-1Ta; while the composition Co-30Ni-7Al-4Ti-3Mo-2W-1Nb-1Ta (L19) exhibits a promising combination of high volume fraction and solvus temperature, low density, and excellent two-phase -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 -precipitates while Co partitions strongly to the -matrix . Mo segregates to the / interface, resulting in a reduction in interfacial energy of -1.63±0.85 mJ·m-2. Decreasing the mole fraction of Ni from 30 % to 10 % results in decreased partitioning of Al and Ti to the γ-phase and increases partitioning of Co, Mo, W, Nb, and Ta. From an analysis of coarsening behavior at 900 °C in Co-xNi-7Al-4Ti-3Mo-2W-1Nb-1Ta (x=10 and 30) interfacial energies of 19.3 mJ·m-2 and 10.0 mJ·m-2 are calculated for mole fractions of Ni of 10 % and 30 %, respectively. This decrease in interfacial energy with increasing Ni-content is potentially due to both Mo-segregation to the interface and a decrease in lattice misfit between the precipitate and matrix, and thus misfit strain energy.
Citation: ACTA Materialia
Pub Type: Journals
Co-based, superalloy, microstructural evolution, alloy development