Influences of the Co content and of the level of high temperature on the microstructure and oxidation of cast {Ni, Co}-based Cr-rich TaC-containing cast alloys

A series of six alloys derived from a Ni-25Cr-0.4C-6Ta (wt.%) base one was developed by substituting nickel by cobalt. They were synthesized by casting and exposed to oxidative environment at two high temperatures. Their bulk microstructures were studied in as-cast condition and in two high temperature aged states. Their surfaces after oxidation during aging were characterized. The cobalt enrichment succeeded in avoiding chromium carbides formation and in stabilizing the TaC carbides at high temperature. As the high temperature morphologic stability of TaC was not perfect, it was much better than the one of the chromium carbides, but can be improved by the total removal of nickel. Unfortunately, at the same time, the oxidation behavior, initially good, shows increased rate of the oxides formation. The room temperature hardness was also significantly increased by the substitution of Ni by Co, and decreased after aging when carbides became rounder or fragmented.