M. Léglise and P. Berthod
University of Lorraine, Lorraine, France
Keywords: cobalt superalloys, tantalum carbides, niobium carbides, high temperature creep, agingCast alloys containing 25wt.%Cr, 0.5wt.%C and 7.5 wt.%Ta (reference alloy) or 3.9 wt.% Nb were cast, then aged (100hours, 1200°C) or not. The as-cast and aged microstructures were observed using a SEM. The liquidus and solidus temperatures were measured using DTA, in both metallurgical states.Parallelepipeds of as-cast and aged alloys were tested in creep resistance at 1100°C, 1150°C and 1200°C under 20 MPa, in 3-points flexural mode. The solidus temperature of the alloy containing TaC is greater than the NbC-containing alloy one, while the melting range is wider for the alloy containing NbC than for the one containing TaC. In the two cases, the two temperatures are more or less lowered by the heat-treatment. But they remain much higher than 1200°C. The creep tests show that, for all temperatures, the creep resistance of the TaC-containing alloy is better than the one of the NbC-containing one. In addition, preliminary aging induces a mechanical weakening for the two alloys. First data concerning the oxidation behavior of the two alloys in their two states were obtained by simple exposure at high temperatures followed by post-mortem characterization of the oxides formed and of the subsurface modifications.