Dependence on the Chromium Content of the High-Temperature Oxidation Behavior of Ta-Rich Nickel-Based Cast Alloys

Abstract

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.

Oxide scale spallation behaviour of cast chromia-forming TaC-strengthened superalloys

“…This difference may be a result of the easiness of diffusion of tantalum towards the oxidation front. First, tantalum possibly diffuses less easily through a nickel-based matrix than through a cobalt-based matrix, as previously observed in earlier works concerning similar alloys [23,24]. The resulting kinetic competition of outward diffusion of tantalum atoms and the inward diffusion of oxygen allows the oxidation reaction close to the chromia/alloy interface (nickel-based alloys) or deeper and more widely distributed in the carbide-free zone (cobalt-based alloys).…”

“…The isothermal oxidation behaviours in air of alloys based on cobalt and nickel simultaneously and containing 25 wt-%Cr in addition to these 6 wt-%Ta and 0.4 wt-%C were recently studied at high temperatures. These previous tests were performed for short durations at temperatures close to 1150°C and 1250°C [22,23], to look for the best {Co,Ni} combinations allowing both good resistance of carbides against morphologic evolution and good oxidation resistance. Despite these high test temperatures the alloys appeared to be well oxidation resistant, but problems of spallation of the protective external oxide scales during cooling were pointed out, especially for the alloys richer in nickel than in cobalt.…”