ON THE FORMATION OF INTERFACIAL AND INTERNAL VOIDS IN ALPHA-AL2O3 SCALES

Microstructural observations were used as the basis for a discussion o f the formation and growth of voids in alumina scales. Reactive-elemen t additions to alloys and alloy desulfurization appear to inhibit the growth of interfacial voids, thus improving scale adhesion. This pheno menon is analyzed in terms of surface energies. In addition, a model i s proposed for the formation of large internal voids in alpha-Al2O3 sc ales, These voids appear to be too large to form as a result of vacanc y coalescence and are more frequently observed in scales not doped wit h a reactive element. The model is based on a growth mechanism where i nward and outward growing ridges at scale grain boundaries eventually seal off and form internal voids.