MICROSTRUCTURAL AND DIFFUSIONAL ASPECTS OF THE GROWTH OF ALUMINA SCALES ON BETA-NIAL

The transport properties of alumina scales formed on undoped and Y-dop ed beta-NiAl have been characterized at 1100 degrees C by diffusion me asurements, either by isotopic exchange (O-16(2)-O-18(2)) or from a th in film of chromium, in order to simulate the cationic inward diffusio n. The penetration profiles have been established by secondary ion mas s spectrometry (SIMS). To determine anionic and cationic diffusion coe fficients, the microstructure, determined by transmission electron mic roscopy (TEM), and the surface roughness of the scale have been consid ered. The diffusion coefficients determined on scales are compared to results on massive alumina extrapolated to 1100 degrees C. The oxidati on constant is then calculated, using the theory of Wagaer, and compar ed either with the values experimentally obtained by thermogravimetry, or with the oxidation constant calculated from electrochemical measur ements. The value calculated from the electrochemical results is in go od agreement with the experimental value, while the calculation from d iffusion data leads to an oxidation constant smaller than the experime ntal value. The yttrium effect on transport properties is small. (C) 1 997 Elsevier Science S.A.