The Effect of Ti Replacement with Nb on the Long-Term Oxidation Behavior at High Temperature of Ni-Base Superalloys for Turbine Disc Applications

This work focuses on the effect of partial and total replacement of Ti with Nb on the long-term oxidationOxidation at 800 °C up to ~ 2000 h of Ni-based superalloysNi- based superalloys for turbine discTurbine disc applications. The replacement of Ti with Nb leads to a lower mass gain of the specimens, in both pseudo and isothermal oxidationOxidation conditions. The alloys presented a relatively smooth oxide scale surface; however, the roughness of the Nb-free/high-Ti alloy is twice of that of the Ti-free/high-Nb alloy. From XRD data, the Nb-free/high-Ti alloy presented Cr2O3, TiO2, Al2O3, (Ni,Co)Cr2O4, and γ + γ′ (matrix) phases, while the Ti-free/high-Nb alloy presented Cr2O3, Al2O3, (Ni,Co)Cr2O4, γ + γ′ (matrix), and δ-Ni3Nb phases. The chromia layer thickness of the Ti-free/high-Nb alloy ranged from 1.0 to 1.5 µm, the thinnest among all the alloys for any given oxidationOxidation time and condition, with no mass change after approximately 280 h exposure. The exceptional oxidationOxidation behavior is attributed to the absence of Ti in this specific alloy composition. The semi-continuous alumina formation underneath the chromia scale as well as the formation of δ-Ni3Nb phase filling up the spaces between the alumina intrusions is probably the reason of delayed outward Cr diffusionDiffusion.