Composition and Temperature Stability of η and δ Phases for Future Nickel-Base Superalloys for Turbine Disks Application

To provide precipitation hardening of nickel-base superalloys, γ′ and γ″ phases can be partially replaced by phases like eta (η) and delta (δ), which may be stable up to temperatures higher than 800 °C. Nevertheless, there is still a lack of information about these phases in terms of crystal structure, composition, thermodynamic stability, and precipitate morphology. Therefore, the present study focuses on the composition and temperature stability of η and δ phases in alloys with high Nb and Ta contents. Various experimental nickel-base alloys were designed using literature and thermodynamic calculations. After solution and aging treatments, they were characterized using SEM to determine precipitate morphology and distribution, and TEM-EDS to determine crystal structure and composition of precipitated phases. Combined in situ HE-XRD experiments followed by metallographic analysis were performed to determine the solvus temperature of η phase in several alloys. Results on the nature of the precipitated phases show that thermodynamic calculations (TCNI7 database) and composition criteria are not always consistent with experimental data. The investigation also reveals that Ta is more favorable to form η phase than Nb and that other elements than Nb, Ta, Al, and Ti have an effect on η and δ phases, such as Cr, Co, and Fe. Hence, the composition criteria for the formation of η and δ phases in alloys with high Ta content are discussed.