A COMBINED APPROACH - ISOTOPIC EXPOSURE SIMS ANALYSIS SEM TO STUDY THE EARLY STAGES OF OXIDATION OF BETA-NIAL AT 1473 K

The early oxidation stages of unmodified and yttrium-implanted beta-NiAl have been studied at 1473 K using sequential isotopic exposure (so-called two-stage oxidation) combined with low and high resolution SIMS analysis and SEM characterization. The scales growing on unmodified material comprised typically a cracked morphology with oxide ridges growing outward from the cracks. Some ridges were formed at the reaction temperature during the first oxidation stage and continued growing during the second stage. This resulted in a network of ridges on the outer surface of the scale. Two regions : non-cracked areas and cracked round patches were observed in the scale on yttrium-implanted beta-NiAl. No ridges but thin lace-like oxide was formed in cracks and round particles extruded from some crack centres. No network was formed by the lace-like oxide. Inward oxygen transport contributed significantly to the overall matter transport in the patches, while its contribution in the remaining scale was much less pronounced. The very thin outermost layer of most patches was enriched in Cr, which was present at a level of ca. 90 ppm only in the starting material. This finding is discussed in terms of the effect of Cr on the phase transformation of unstable aluminas into the stable alpha-Al2O3. The results showed that the scales were inhomogeneous regarding both composition and microstructure. Therefore, only analytical methods having respectable sensitivity and resolution might be used to give reliable information regarding the growth mechanisms of scales. This applies to SIMS as well as to the electron microscopy methods.