On the formation of interfacial and internal voids in α-Al₂O₃ scales

Microstructural observations were used as the basis for a discussion of the formation and growth of voids in alumina scales. Reactive-element additions to alloys and alloy desulfurization appear to inhibit the growth of interfacial voids, thus improving scale adhesion. This phenomenon is analyzed in terms of surface energies. In addition, a model is proposed for the formation of large internal voids in α-Al₂O₃ scales. These voids appear to be too large to form as a result of vacancy coalescence and are more frequently observed in scales not doped with a reactive element. The model is based on a growth mechanism where inward and outward growing ridges at scale grain boundaries eventually seal off and form internal voids.