O-18/SIMS CHARACTERIZATION OF THE GROWTH-MECHANISM OF DOPED AND UNDOPED ALPHA-AL2O3

Sequential oxidation experiments at 1200-degrees-C and 1500-degrees-C using O-16 and >95% O-18-enriched environments were conducted on undoped and Y- and Zr-doped beta-NiAl and FeCrAl alloys. After oxidation, samples were analyzed by SIMS sputter depth profiling. At 1200-degrees-C, a clear pattern was established where the undoped alpha-Al2O3 was found to grow by the simultaneous transport of both Al and O. Zr-doped alpha-Al2O3 was found to grow mainly by the inward transport of oxygen. The profiles in all cases indicate O diffusion primarily by short-circuit pathways. Results at 1500-degrees-C (only on beta-NiAl) indicated a similar behavior but were less conclusive. Y and Zr were found to segregate to the oxide grain boundaries at 1200-degrees-C and 1500-degrees-C The segregation of these dopants is believed to impede the cation transport in the alpha-Al2O3 scale and thereby change the oxidation mechanism.