Determination of Oxygen Diffusion Along Nickel/Zirconia Phase Boundaries by Internal Oxidation

Internal oxidation tests with nickel alloys that contained up to 8 at.% zirconium were carried out. All alloys were two-phase consisting of γ-Ni and the intermetallic phase Ni₅Zr. Their behavior under low oxygen partial pressures in the range of 800–1,000?°C could not be described by the Wagnerian analysis. Oxygen diffusivity along the interface nickel/monoclinic zirconia plays an important role for the rate of internal oxidation. The early stages of internal oxidation show the in situ mode where diffusion of the less noble element zirconium cannot diffuse in the matrix and is oxidized instantly. Later in the process the mode shifts from in situ towards the diffusive mode as zirconium has the possibility to diffuse. This change could also be observed as the size of the oxide particles varied with ongoing oxidation. A method for the determination of the oxygen diffusivity in nickel/monoclinic zirconia phase boundaries is presented.