DISTRIBUTION AND TRANSPORT OF YTTRIUM IN ALUMINA SCALES ON IRON-BASE ODS ALLOYS

The oxidation behaviour of yttria-containing ODS alloys of the type MA 956 and PM 2000 was investigated at 1100 and 1200-degrees-C. Main emp hasis was placed on studying the growth mechanism of the protective al umina scales and the distribution and transport of yttrium in these sc ales. For this purpose alumina scales formed during long time air expo sure as well as during two-stage oxidation using O-18-tracer were anal ysed by SIMS and TEM. It was found, that the yttrium, which is initial ly present in the alloy in form of a thermodynamically stable yttria d ispersion, is incorporated in the alumina scale where it forms precipi tates on the grain boundaries and diffuses to the oxide surface. This behaviour of the yttrium is believed to be responsible for the modific ation of the growth phenomena of the oxide scales compared to those of yttria-free alloys. Large yttria containing dispersions, which are so metimes present in the bulk alloy due to improper mechanical alloying, are incorporated in the alumina scale and have a negative effect on l ong time scale integrity.