Oxidation behaviour at 1100°C in air of 25wt-% Cr-containing cobalt-based alloys containing high quantities of hafnium carbides

Hafnium, usually added to improve the high temperature oxidation resistance of alloys, allowed obtaining HfC carbides which are very efficient for the creep-resistance. For that Hf must be added in particularly high quantities which may possibly influence the oxidation behaviour. Three HfC-strengthened cast cobalt alloys were studied all along thermogravimetry tests at 1100 degrees C. They were compared to similar but Hf-free ternary alloys. The mass variation were plotted according to {mxdm/dt=f(-m)} to specify all kinetic oxidation constants, and versus temperature to study the oxidation beginning during heating and the scale spallation during cooling. The presence of many HfC carbides obviously influences the high temperature oxidation: mass gain occurring sooner during heating, faster isothermal mass gains but better behaviour in oxide scale spallation during cooling. This deterioration of oxidation behaviour must be corrected to hope benefiting from the high creep-resistance brought by this new type of strengthening. Le hafnium, habituellement ajoute pour ameliorer la resistance a l'oxydation a haute temperature des alliages, a permis d'obtenir des carbures HfC qui sont tres efficaces pour la resistance au fluage. Pour cela, on doit ajouter le Hf en quantites particulierement elevees, ce qui peut influencer le comportement a l'oxydation. On a etudie trois alliages de cobalt coules renforces par des carbures HfC, au moyen d'essais de thermogravimetrie a 1100 degrees C. On les a compares a des alliages ternaires similaires, mais sans Hf. On a trace une courbe de la variation de la masse d'apres {mxdm/dt=f(-m)} pour specifier toutes les constantes de la cinetique d'oxydation, en fonction de la temperature, afin d'etudier le debut d'oxydation lors du chauffage et l'ecaillage de l'oxyde lors du refroidissement. La presence de nombreux carbures HfC influence evidemment l'oxydation a haute temperature : gain de masse se produisant plus tot lors du chauffage, gains de masse isothermes plus rapides mais meilleure resistance a l'ecaillage d'oxyde lors du refroidissement. On se doit de corriger cette deterioration du comportement a l'oxydation si l'on espere beneficier de la haute resistance au fluage, amenee par ce nouveau type de renforcement.