The Effect of Yttria Dispersions on the Growth Mechanisms and Morphology of Chromia and Alumina Scales

Abstract

The oxidation behaviour of a chromia and an alumina forming ODS alloy has been compared with that of conventional wrought model alloys with similar compositions. Scale structure and morphology, as well as scale growth mechanisms, were studied using metallography, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis and an ¹⁸O tracer technique. The main effects of the yttria dispersions on the properties of the chromia and alumina scales were found to be improved adherence, decreased growth rates and enhanced selective oxidation. The results indicate that the main reason for these changes in scale properties is a change in transport mechanism in the scales. The oxides on the conventional alloys grow by both metal and oxygen transport, the addition of oxide dispersions suppresses the outward scale growth. It is shown that the change in properties of the oxide scales on the ODS alloys can be explained by assuming that the reduced outward growth is the dominating effect caused by the addition of yttria dispersions. The results indicate that the effectiveness of the dispersions depends on the amount and the distribution. Because of their faster scale growth rates, chromia forming alloys seem to require larger amounts of yttria than alumina formers in order to obtain the same beneficial effect.