Abstract
The oxidation behaviour of yttria-containing ODS alloys of the type MA 956 and PM 2000 was investigated at 1100 and 1200°C. Main emphasis was placed on studying the growth mechanism of the protective alumina scales and the distribution and transport of yttrium in these scales. For this purpose alumina scales formed during long time air exposure as well as during two-stage oxidation using 18O-tracer were analysed by SIMS and TEM. It was found, that the yttrium, which is initially present in the alloy in form of a thermodynamically stable yttria dispersion, is incorporated in the alumina scale where it forms precipitates on the grain boundaries and diffuses to the oxide surface. This behaviour of the yttrium is believed to be responsible for the modification of the growth phenomena of the oxide scales compared to those of yttria-free alloys. Large yttria containing dispersions, which are sometimes present in the bulk alloy due to improper mechanical alloying, are incorporated in the alumina scale and have a negative effect on long time scale integrity.
Published Version
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