Morphology of Al(sub 2)O(sub 3) scales: indicators of phase, growth mechanisms, and grain boundary segregation

摘要

Advanced analysis over the past decade has resulted in increased understanding of the formation, growth, and adhesion of alumina scales. It is proposed that this can now be inferred from SEM images of the AL(sub 2)O(sub 3) scale morphology. The presence of bladelike grains is an indicator of metastable alumina phases and an outward growth mechanism. Large surface ridges indicate that the scale has transformed to (alpha)-Al(sub 2)O(sub 3). A columnar grain structure is associated with an inward growth mechanism, while an equiaxed grain structure typically forms when both Al and O diffusion is occurring. Grain boundary segregation typically results in a fine grain size at the scale-gas interface. Examples are given from Ni- and Fe-base alumina formers ((beta)-NiAl, FeCrAl, iron aluminides) to illustrate these correlations. 5 figs, 35 refs, 1 tab.