Influence of Co Content on the High-Temperature Oxidation Performance of (Ni,Co)–25Cr–0.4C–6Ta Cast Alloys

International audienceTantalum carbides (TaC) are classical strengthening particles, like chromium carbides, in equi-axed 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

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

International audienceThe 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 as-cast 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

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

International audienceA 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

Influence of the Base Element on the Thermal Properties of Non- Ferrous Chromium-Rich TaC-Containing Alloys Elaborated by Conventional Casting: Part 1: Thermodynamic Approach, Melting Ranges, as-Cast Microstructures and Thermal Expansion

After preliminary thermodynamic calculations for verifying their refractoriness, six (Ni,Co)-based alloys were synthesized by casting. They contain chromium, carbon and tantalum to achieve interesting chemical and mechanical high temperature properties. Their microstructures in the as-cast state were observed by electron microscopy (SEM) to discover the carbides characteristics. Differential thermal analysis (DTA) was carried out for all of them to assess their melting points notably, for having better knowledge about the level of high temperature at which they can be used potentially. Thereafter thermomechanical analyses (TMA) were run to explore their behaviour in thermal expansion. As shown by the thermodynamic calculations all the alloys are theoretically possible to be shaped by conventional foundry due to liquidus temperatures all below 1400°C. According to these same results, the solidus temperatures of all alloys would stay over 1250°C, this suggesting that all alloys would be able to be used under moderate mechanical stresses at temperatures as high as 1200°C. As suggested by calculations, the as-cast microstructures are all dendritic and the interdendritic spaces are occupied by carbides. According to calculations again, the Ni-richest alloys contain chromium carbides, but tantalum carbides are also present, a presence which was not expected. In contrast the Co-richest versions contain only TaC carbides. The DTA experiments show that the solidus and liquidus temperatures both increase by going from the Ni-richest alloys to the Co-richest ones. The TMA experiments demonstrate that the thermal expansions and thermal contractions are rather continuous, without any irregularities, and the average thermal expansion coefficients, all close to 20 × 10-6K-1, do not systematically depend on the respective proportions of nickel and cobalt. This first part of the whole work will be followed by two other parts dealing with the effect of these Ni and Co proportions on the high temperature oxidation phenomena, for temperature variations and for isothermal conditions respectively