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High temperature oxidation of cobalt-based alloys strengthened by hafnium carbides. Part 1: Thermogravimetry results

Patrice Berthod, Elodie Conrath

Hafnium is one of the most important elements for favouring the high temperature oxidation resistance of alloys and superalloys when present in low quantity. It may also lead to the development of interdendritic eutectic HfC carbides in cobalt alloyswhen several weight percents are added to their compositions. Such carbides are especially stable at high temperature and then potentially very efficient for the mechanical resistance at high temperature. Three alloys strengthened by eutectic HfC carbides were tested for 50 hours in oxidation at 1200°C in synthetic air with thermogravimetrymeasurements. The mass gains recorded during the heating, the isothermal high temperature stage and during the cooling were all analyzed and the results compared with the ones obtained with ternary Co-Cr-C alloys with the same chromium and carbon contents, in the same operating conditions. It appeared that hafnium strongly influences the oxidation kinetic during heating and during the isothermal stage, as well as concerning the oxide spallation at cooling. The effects of so high hafnium contents are either beneficial or detrimental for the high temperature oxidation behaviour, this depending on the criterion followed.