The high-temperature stability of the primary tantalum carbides is a problem of importance for chromium-rich cast alloys, based on cobalt or nickel. The focus of this study was nickel-based alloys, as these alloys are particularly sensitive to a lack of TaC carbides in the ascast state and by dissolution due to high-temperature exposure. In this work, a possible way for promoting the formation of many TaC carbides by changing from the usual 30wt.% chromium content was investigated. Five alloys with Cr content varying from 10 to 50 wt.% were prepared and then subjected to microstructure characterization and to oxidation tests. In contrast with what was expected, decreasing the Cr content in comparison to the Ni-30Cr-0.4-6Ta reference alloy did not succeed in obtaining more TaC carbides, but instead had the opposite effect. Concerning the high-temperature oxidation behavior at 1127 and 1237°C, loss of resistance was observed only for a Cr content at the lower level of 10 wt.%. It was noticed that a subscale CrTaO 4 developed during oxidation and seemed to promote oxide spallation during cooling.
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.
Tantalum carbides (TaC) are classical strengthening particles, like chromium carbides, in equiaxed cast Cr-rich cobalt-based alloys. When they are of a eutectic nature and in quantity high enough, they may be of great interest for applications at elevated temperature, due to their favorable script-like morphology and their high stability at high temperature. Unfortunately they cannot be so easily obtained in Cr-rich nickel-based alloys of similar composition. This can be resolved by adding cobalt in substitution to nickel but such chemical modification may change the high temperature oxidation behavior in the wrong direction. In this work several alloys, all containing 25%Cr, 0.4%C and 6% Ta (wt.%), were elaborated and studied at a high temperature: 1127°C. It appears that the 27wt.% Co content is high enough to allow obtaining primary carbides which are essentially TaC. The alloys rich enough in Co containing only tantalum carbides are clearly disadvantaged in term of oxidation resistance at high temperature by the high Co content and by the absence of interdendritic chromium carbides known to act as useful Cr reservoirs. Among the six studied alloys these are the three ones for which Co/Ni >1 that demonstrated starts of catastrophic oxidation before 24 hours of exposure to air.
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