Search for the maximal threshold Co content for the oxidation behaviour point of view for 25wt.%Cr-containing TaC-strengthened cast superalloys designed for use at 1200°C on long time

The base of chromia-forming cast superalloys is the most often constituted of nickel and/or cobalt. Both elements may be simultaneously present, for instance Co for promoting good mechanical properties and Ni for stabilizing the austenitic matrix and favouring good ductility and machinability. In service, superalloys must also resist hot oxidation, property favoured by Ni but disadvantaged by Co. In (Ta,C)-rich cast superalloys exposed at high temperature, the intergranular strength may be enhanced by the very efficient TaC carbides while the higher the Co content the stronger the matrix creep resistance. In this work, six alloys designed to be chromia-forming and TaC-strengthened were exposed to laboratory air at 1200°C for 170 hours, to examine how the deterioration by oxidation and the evolution with time of the strengthening carbides in the bulk may be influenced by the Co/Ni ratio in the case of this important family of superalloys. The characterization of the formed oxide scales and of the morphologic evolution of the carbides allowed identifying a {0.6Co-0.4Ni}-proportion for the base elements. Such combination obviously allows keeping a chromia-forming behaviour and a good adherence of the protective oxide scale on the alloy, as well as a good resistance of the TaC against fragmentation.