COMPOSITION, STRUCTURE AND PROTECTIVE PROPERTIES OF ALUMINA SCALES ON IRON-BASED OXIDE DISPERSION STRENGTHENED ALLOYS

The oxidation behaviour of a number of iron/chromium/aluminium based oxide dispersion strengthened (ODS) alloys of the type MA 956 was investigated at temperatures between 900 and 1200-degrees-C. Apart from studies on long-time cyclic oxidation resistance, the main emphasis was placed on the effect of alloy yttria content on scale composition and structure as a function of time and temperature. The detailed scale analyses were made by using Rutherford backscattering analysis in addition to electron-optical methods. It was found that during short-term oxidation the scale growth rate at 900-degrees-C showed a deviation from the Arrhenius-type temperature dependence observed at higher temperatures. This could be attributed to formation of a metastable alumina modification in the initial stages of oxidation at 900-degrees-C. Studies of a series of alloys with variations of alloy yttria content in the range 0.17-0.7% showed an increase in the oxide growth rate with increasing yttria content. The reason for this technologically important effect is discussed against the background of various studies on scale growth mechanisms.