Growth and Microstructure of Alpha-Al2O3 on Beta-NiAl

Growth of alpha -Al2O3 films and scales on beta -NiAl (Ni--32 wt.-%Al) was investigated in oxygen at 1 atm over the temp. range 1273-1573 deg K. An initial submicrocrystalline oxide film recrystallized during its growth to a textured alpha -Al2O3 film containing polycrystalline ridges. Inert marker measurements demonstrated that the initial film grew by inward diffusion of O. Fully developed alpha -Al2O3 scales contained dissolved Ni (0.5 wt.-% at 1473 deg K). Metallographic evidence indicated that these scales grew by countercurrent Al and O boundary diffusion and Al lattice diffusion. Cavities, which were generated at the alloy /Al2O3 interface, did not influence scale growth since Al was transported at a sufficient sustaining rate by evaporation from the alloy substrate. The square of the average oxide grain size in a growing scale increased proportional to time. The nonparabolic growth kinetics of the alpha -Al2O3 scales were interpreted by a short-circuit diffusion model.31 refs.--AA