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

TL;DR

This study investigates the time-dependent oxidation behavior of a Co/Ni-based superalloy at 800°C, revealing how oxide scale evolution enhances long-term corrosion resistance through detailed structural and chemical analysis.

Contribution

It provides new insights into the transformation of oxide scales over time, linking oxidation kinetics with microstructural and compositional changes in the superalloy.

Findings

Oxidation rate shifts from near-linear to parabolic over time.

Porous spinel transforms into a dense chromia and alumina scale.

A continuous Cr2O3/α-Al2O3 scale forms, improving protection.

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.