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Microstructure and hardness of several chromium-containing Co-based alloys reinforced by MC (TaC, NbC or HfC) carbides in as-cast or heat-treated conditions

Patrice Berthod

The cast Co-based superalloys are usually reinforced by carbides, notably of the MC type. If TaC are often used in this field, it is not the case of other MC carbides, e.g. NbC and HfC. The purpose of this work is to better know the fractions and shapes of these carbides when obtained by solidification and/or heat treatment in a cobalt-based alloy, with comparison with a similar alloy containingTaCcarbides. Three alloys,Co-8Ni-30Cr-0.4C-3.3Nb, or 6.6 Ta/Hf, were cast and submitted to three different heat treatments. The morphologies, surface fractions and distribution of the primary and secondary carbideswere examined and quantified.Macro- andmicro-indentationswere performed for all alloys in the as–cast and treated conditions. The results were analyzed versus the carbides fractions and distributions between grain boundaries andmatrix. PrimaryTaC, NbCand HfC are all script-like eutectic carbides. The highest volume fractions are obtained for the HfC carbides while Nb andHf, almost non-existent in the as-castmatrix, do not allowa real secondary precipitation of carbides.