Long-Time Oxidation of Cr2AlC Between 700 and 1,000 °C in Air

Cr2AlC compounds were synthesized via a powder metallurgical route and their long-term oxidation behavior studied. Oxidation at temperatures between 700 and 1,000 A degrees C for up to 360 days in air resulted in formation of a thin, adherent Al2O3 surface layer and a narrow Cr7C3 sublayer, accompanied by evaporation of carbon from the Cr2AlC. Preferential oxidation of Al on the surface suppressed oxidation of the less mobile Cr in Cr2AlC. In the Al2O3 layer, (0.7-8.3) at.% Cr was incorporated. In the Cr7C3 sublayer, Al was either absent or incorporated. When Cr2AlC oxidized at 850 and 1,000 A degrees C for 30-360 days, metastable theta-Al2O3 blades formed on the alpha-Al2O3 layer. However, such blades were scarcely visible when the oxidation was carried out above 1,100 A degrees C, because of the fast theta A -> A I +/--transition. Moreover, the theta-Al2O3 were not noticeable during oxidation at 700 A degrees C for 30-360 days, due to a small extent of oxidation.