J. Jedlinski et al., A COMBINED APPROACH - ISOTOPIC EXPOSURE SIMS ANALYSIS SEM TO STUDY THE EARLY STAGES OF OXIDATION OF BETA-NIAL AT 1473 K, Werkstoffe und Korrosion, 46(5), 1995, pp. 297-305
The early oxidation stages of unmodified and yttrium-implanted beta-Ni
Al 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 m
aterial comprised typically a cracked morphology with oxide ridges gro
wing outward from the cracks. Some ridges were formed at the reaction
temperature during the first oxidation stage and continued growing dur
ing the second stage. This resulted in a network of ridges on the oute
r surface of the scale. Two regions : non-cracked areas and cracked ro
und patches were observed in the scale on yttrium-implanted beta-NiAl.
No ridges but thin lace-like oxide was formed in cracks and round par
ticles 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 leve
l of ca. 90 ppm only in the starting material. This finding is discuss
ed in terms of the effect of Cr on the phase transformation of unstabl
e aluminas into the stable alpha-Al2O3. The results showed that the sc
ales 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 g
rowth mechanisms of scales. This applies to SIMS as well as to the ele
ctron microscopy methods.