Microstructural and diffusional aspects of the growth of alumina scales on β-NiAl

The transport properties of alumina scales formed on undoped and Y-doped β-NiAl have been characterized at 1100°C by diffusion measurements, either by isotopic exchange (16O2–18O2) or from a thin film of chromium, in order to simulate the cationic inward diffusion. The penetration profiles have been established by secondary ion mass spectrometry (SIMS). To determine anionic and cationic diffusion coefficients, the microstructure, determined by transmission electron microscopy (TEM), and the surface roughness of the scale have been considered. The diffusion coefficients determined on scales are compared to results on massive alumina extrapolated to 1100°C. The oxidation constant is then calculated, using the theory of Wagner, and compared either with the values experimentally obtained by thermogravimetry, or with the oxidation constant calculated from electrochemical measurements. The value calculated from the electrochemical results is in good agreement with the experimental value, while the calculation from diffusion data leads to an oxidation constant smaller than the experimental value. The yttrium effect on transport properties is small.