Time-Dependent Oxidation and Scale Evolution of a Wrought Co/Ni-based Superalloy

Abstract

Understanding how protective oxide scales evolve over time is necessary for improving the long term resistance of superalloys. This work investigates the time-dependent oxidation behavior of an ingot-processable Co/Ni-based superalloy oxidized in air at $800~^\circ\mathrm{C}$ for $20$, $100$, and $1000~\mathrm{h}$ . Mass-gain and white-light interferometry measurements quantified oxidation kinetics, surface roughness, and spallation, while high-resolution STEM-EDX characterized oxide morphology and nanoscale elemental partitioning. Atom probe tomography captured the key transition regions between the chromia and alumina scales, and X-ray diffraction was used to identify a gradual transition from NiO and (Ni,Co)-spinel phases to a compact, dual phase chromia and alumina-rich scale. The oxidation rate evolved from near-linear to parabolic behavior with time, consistent with diffusion-controlled growth once a continuous Cr$_2$O$_3$/$\alpha$-Al$_2$O$_3$ scale formed. These observations help link kinetics, structure and chemistry, showing how an originally porous spinel layer transforms into a dense, adherent chromia + alumina scale that provides long-term protection in wrought Co/Ni-based superalloys.