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In this work, HfC–and TaC–reinforced cast versions of the equimolar CoNiFeMnCr alloy (Cantor) were tested in oxidation at 1000°C in laboratory air. Chromium seemingly shared, with manganese, its role as a selectively oxidized element. The principal oxide to form externally was an M2O3 one in which Mn and Cr were present in various proportions depending on the proximity with the alloy’s surface. The HfC carbides close to the oxidation front were oxidized in situ in HfO2, with the release of carbon diffusing inwards after that and inducing the solid-state precipitation of blocky or acicular chromium carbides. The behavior of the TaC carbides present close to the oxidation front was different since they dissolved, and Ta diffused towards the oxidation front to form CrTaO4 oxides. Even after 50 hours at 1000°C, the subsurfaces were more or less deeply impoverished in Cr and Mn, with minimal contents meager, especially for Mn. The evaluation of the Cr and Mn quantities and the exploitation of results in terms of equivalent oxide thickness and kinetic oxidation constant demonstrates that the resistance of the alloys against oxidation is feeble and must be significantly improved to allow benefiting of the superior high-temperature mechanical resistances of such alloys in practical application. For that, some ways are proposed.

In this study, two new alloys, resulting of the addition of carbon and hafnium to a well–known high entropy alloy (HEA) – the equimolar CoNiFeMnCr one – to promote the formation of HfC carbides, were produced by conventional casting under inert atmosphere and characterized. The as–cast microstructures of the obtained HEA/HfC alloys were studied by X–ray diffraction, electron microscopy, energy dispersion spectrometry, and Vickers indentation. These HEA/HfC alloys are double–phased, with an austenitic matrix and interdendritic eutectic script–like HfC carbides. In terms of chemical composition and of crystallographic network, the matrix of the alloys is identical to the CoNiFeMnCr alloy. In that way, the two alloys can be considered as composite materials resulting of a HEA matrix strengthened by hard HfC particles.

Unlike gamma/gamma prime nickel-based single crystals, some carbide-strengthened polycrystalline superalloys can be efficiently helped by their reinforcing particles to resist creep deformation at ...

In this work two new alloys were obtained by extrapolation from a wellknown high entropy alloy, the equimolar CoNiFeMnCr one. This was doneby the addition of carbon and of tantalum, Ta being one of the strongestMC-former elements. They were produced by conventional casting underinert atmosphere. The obtained microstructures were characterized by X-raydiffraction, metallography, electron microscopy, and energy dispersionspectrometry. Their hardness was also measured by hardness indentation.In parallel, the original CoNiFeMnCr alloy was also synthesized andcharacterized for comparison. The reference HEA alloy is single-phasedwith an austenitic structure, while the two {Ta, C}-added alloys aredouble-phased, with an austenitic matrix and interdendritic script-likeTaC carbides. The matrixes of these HEA/TaC alloy are equivalent toan equimolar CoNiFeMnCr alloy to which 2 wt.% Ta is present in solidsolution. The presence of the TaC carbides caused a significant increase inhardness which suggests that the HEA/TaC alloys may be mechanicallystronger than the HEA reference alloy at high temperature.

International audience; Polycrystalline alloys based on cobalt and elaborated by classical foundry are known since the middle of the last century. They are currently still used, notably for geometrically complex components working at high temperature. Beyond the oldest ones reinforced by chromium carbides, new principles of carbides-strengthened cobalt-based alloys have recently appeared. MC-type refractory mono-carbides allow maintaining the melting start temperature at a high level when they are present as single carbide phase. Their high temperature stabilities and script-like morphologies also favor high mechanical properties at elevated temperatures. Optimized MC-carbides fractions can be obtained with carbon contents, close to 0.4 wt.%C, for achieving significant strengthening without threat for the low and high temperature toughness. Controlling the carbon content is thus of prior importance. Unfortunately the most common metallographic apparatus used for measuring the chemical composition of alloys-the Energy Dispersion Spectrometers (EDS) attached to Scanning Electrons Microscopes (SEM)-are not able to analyze, with sufficient accuracy, carbon with so low contents. A simple indirect method using both EDS and SEM is proposed here to get some information about the carbon content in several MC-reinforced Co-based superalloys.

Six {Ni,Co}-based alloys containing TaC were oxidised at 1250°C. The mass variations files were exploited to follow the progress of the spallation of the oxide scales during cooling. The oxides for...

In this work, six refractory nickel and cobalt-based cast alloys, rich in chromium and designed to be strengthened by tantalum carbides, were cast in controlled atmosphere. Samples were especially prepared to investigate their surface behavior in synthetic air at 1250 °C for three days. Their oxidized states were first characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) on surface, prior to following metallography processing. Second, oxides and subsurfaces were examined by SEM and energy dispersion spectrometry (EDS) on cross sections. Results show that the surface and subsurface deterioration by oxidation clearly depend on the relative quantities in Ni and Co in the chemical composition of the alloys. All alloys well-resisted oxidation due to their chromium contents, but beginnings of catastrophic oxidation were noticed for the alloys containing much more cobalt than nickel. Tantalum, present with particularly high contents, played an important role in the oxidation phenomena and for the microstructure stability at elevated temperature. It notably influenced the adherence of the external scale and led to highly stable heat-resistant TaC carbides, the fraction of which is governed by the base element.

Three cast cobalt-based alloys containing or not chromium carbides and tantalum carbides were exposed at 900°C to 100% water vapour for 46 or 100 h. The oxidised samples were characterised by XRD, ...

In this work, nickel is added in substitution to cobalt in the composition of a cast cobalt-based chromium-rich superalloy designed to obtain a double-phased {fcc matrix + TaC} microstructu...

A thermoelectric generator is a powerful system used to produce electricity by the action of heat. The development of nanostructured thermoelectric materials is widespread with the objective to improve their efficiency. However, if these materials are used at high temperatures under oxidative atmosphere (e.g., in air), they may suffer degradation in service, drastically decreasing their lifespan. This work investigates the oxidation behavior of an innovative skutterudite material made of cerium, iron, cobalt and antimony (Ce0.75Fe3CoSb12) either microstructured or nanostructured. For that purpose, several oxidation experiments are carried out under a flow of synthetic air at 650 K (15 h, 50 h and 100 h). The oxide layers formed on surface are observed, and their characteristics (chemical composition, thickness, structure) are compared to those obtained with microstructured Ce0.75Fe3CoSb12 samples. As a result, it is observed that the nanostructuring of the skutterudite materials slightly slow down the oxidation reactions in air. Consequently, the nanostructured Ce0.75Fe3CoSb12 is established to be a promising thermoelectric material for use in oxidative environments.

The skutterudite materials belonging to the CoSb3 family are widely studied for thermoelectric applications. They are typically used under vacuum, but applications under oxidizing environments are increasingly considered. The addition of ytterbium is known to enhance the thermoelectric properties of CoSb3, but the corresponding impact on the oxidation behavior of the skutterudite material is barely explored. For that purpose, the present research describes the oxidation behavior of Yb0.2Co4Sb12 under a flow of air at 800 K for 15, 50, 100, and 1000 hours. The oxidation treatment induces the growth of a surface layer made of three oxides in various amounts as a function of the oxidation time. The spinel oxide CoSb2O4/CoO·Sb2O3 and CoSb2O6 are observed from 15 hours of oxidation, whereas Sb2O4 is formed only from 100 hours of treatment. To assess the impact of ytterbium, the oxidation behavior of Yb0.2Co4Sb12 is compared to that of CoSb3 oxidized in the same experimental conditions. The results show that the low amount of ytterbium promotes the oxidation reactions of the skutterudite material. Nonetheless, the impact on the degradation of the material remains acceptable to use Yb0.2Co4Sb12 for thermoelectric applications under oxidizing environments.

International audience; Nickel is often added to cobalt-based superalloys to stabilize their austenitic structure. In this work the effects of Ni on several high temperature properties of a chromium-rich cobalt-based alloy reinforced by high fraction of TaC carbides are investigated. Different thermal analysis techniques are used: differential scanning calorimetry (DSC), thermo-mechanical analysis (TMA) and thermogravimetry (TG). Results show that the progressive addition of nickel did not induce great modifications of microstructure, refractoriness or thermal expansion. However, minor beneficial effects were noted, including reduction of the melting temperature range and slight decrease in thermal expansion coefficient. The most important improvement induced by Ni addition concerns the hot oxidation behavior. In this way, introducing several tens wt % Ni in this type of cobalt-based alloy may be recommended.

Cobalt and nickel may play antagonist roles on the microstructures and high temperature properties of cast chromium-rich superalloys containing the same tantalum and carbon molar fractions. Since such alloys may be considered for uses at temperature levels as high as 1200 °C, the whole range of base compositions was explored with six model quaternary alloys with Ni and Co contents varying from 0 to 100% of the remaining elements, the 25 wt%Cr, 0.4 wt%C and 6 wt%Ta being deducted. The interdendritic carbide network of the as-cast microstructures is made of chromium carbides and tantalum carbides for the Ni-richest alloys and of TaC only for the Co-richest ones. {170 h, 1200 °C}-exposure carried out in laboratory air led to important morphology deterioration for the two Ni-richest alloys but limited fragmentation of the TaC in the four Co-richest alloys. At the same time, the most of the alloys more or less well behaved in oxidation, except the two Co-richest alloys for which the too hindered volume diffusion of chromium did not sustain the chromia-forming behavior. Notably the Ni-free cobalt-based alloys suffered from starts of catastrophic oxidation in several surface locations. The quaternary alloy bases allowing taking benefit of the best compromise between hot oxidation resistance and possibly good mechanical behavior at elevated temperature was the model alloys with 14–28 wt%Ni.

International audience; This paper aims to investigate the thermal expansion behavior, up to an elevated temperature, of superalloys based on nickel and cobalt with various proportions and designed to be strengthened by tantalum carbides. The as-cast microstructures of these superalloys and their evolutions at two very high temperatures were also of interest. All results are discussed by considering the Ni/Co repartition in the base element position. It appears that when the Ni content is higher than the Co one: 1/the thermal expansion is slower, 2/the as-cast microstructures as well as the ones stabilized at high temperature contain not only TaC but also chromium carbides, and 3/the hardness in as-cast or aged state is lower.

The skutterudite CoSb3 is a well-known thermoelectric material widely used to produce electricity from heat. This material is commonly employed under vacuum but in a near future it is expected to be used at high temperatures under oxidative atmospheres, e.g. in air. The lifetime of the material may be affected by the oxidative environment, considerably limiting the use of the thermoelectric equipment. The present research describes the degradation mechanisms of CoSb3 from oxidation experiments carried out under a flow of synthetic air at 800 K. The skutterudite material is progressively oxidized after 15 h, 50 h, 100 h and 1000 h of treatment, producing three oxides on the CoSb3 surface (CoSb2O4/CoO·Sb2O3, Sb2O4 and CoSb2O6). These three oxides have different growth kinetics and they are produced in various amounts as a function of the oxidation time. Next, as a solution for an appropriate oxidation protection, this work explores the chemical vapor deposition (CVD) process named pack cementation to synthesize a protective coating on CoSb3. This process produces a surface layer made of aluminum antimonide (AlSb) and cobalt aluminide (Al9Co2). The oxidation experiments carried out on the coated CoSb3 highlight the protective properties of this innovative surface layer. The coating is a protective barrier against oxygen that keeps the CoSb3 substrate unaffected by the flow of air at 800 K for 1000 h. Consequently, pack cementation is an efficient process to synthesize a protective surface layer that makes CoSb3 usable under oxidative environments.

Involving a particularly strong carbide-former metallic element, the tantalum carbides are potentially very stable at elevated temperatures in term of volume fraction and morphology. The TaC phase represents a major strengthening way to allow cast chromium-rich superalloys resisting mechanical stresses at elevated temperatures. They are exploited in recent high performance cobalt-based superalloys but seemingly not in nickel-chromium refractory alloys. Earlier studies showed that the stability of TaC in Ni-Cr alloys is not so good as in the Co-Cr ones, and they evidenced that chromium carbides may compete with TaC in the formation of the carbides population. A possible way to optimize the presence of TaC in Ni-Cr alloy may consist in rating the chromium content to an ideal value but preliminary knowledge about the TaC dependence on the Cr content is compulsory. The aim of this work is precisely the investigation of the effect of the content in chromium on the appearance and stability of the TaC phase in Ni-Cr alloys, by the means of thermodynamic calculations and real experiments in parallel. A global system Ni(bal.)-xCr-0.4C-6Ta compositions (with x varying from 0 to 50 wt%) was chosen. Thermodynamic calculations were performed to know the theoretic metallurgical states inside the considered x range. These theoretic results being dependent on the suitability of the used database, real experiments of verifications were also carried out for a selection of six alloys (x = 0, 10, 20, 30, 40 and 50 wt%). The alloys prepared by respecting these compositions were cast and isothermally exposed at high temperature (1400 and 1510 K), then subjected to metallographic characterization. For the used database the calculated results showed that no TaC should never appear whatever their Cr content, while TaC were really observed in the as-cast and aged versions of the alloys containing 20 wt%Cr and more, but never alone since chromium carbides were systematically also present. When the Cr content in the alloy is too low, the TaC are rare or even no present. This allowed concluding first that the database used for the calculations needs serious improvements, followed by tests with, as first criteria of quality, a good correspondence with the present experimental results. Second, the presence of Cr in quantity high enough is surprisingly compulsory to obtain TaC carbides in quantity high enough, but it is no possible to avoid the appearance of chromium carbides. Obviously, other ways than Cr adjustments must be found to obtain TaC in nickel-based alloys as the single carbide phase and in quantity high enough to achieve high mechanical properties at high temperature.

International audience; Titanium carbides feature among the MC carbides which tend to crystallize with a script-like shape recognized to be favorable to mechanical strength at high temperature for equi-axed cast refractory alloys. Recent results demonstrated that script-like TiC can be successfully obtained in Co-based alloys while they are less stable than chromium carbides in nickel-based alloys. The present work aims to study whether the addition of cobalt to nickel-based alloys may stabilize TiC. This double investigation, by thermodynamic calculations and by the elaboration of a series of nickel-based alloys with an increasing part of cobalt, demonstrates that the progressive enrichment in cobalt tends favoring the presence of TiC instead chromium carbides. To obtain a majority of TiC, script-like shaped and in significant quantity, cobalt must be more present than nickel in the alloys. But this enrichment in cobalt deteriorates the good resistance against hot oxidation.

The electrochemical behavior of pure Co, pure Sb, and CoSb3 has been investigated over a large anodic potential range (0 to 40 V) in two acids, i.e., oxalic acid or sulfuric acid at different concentrations (0.01 to 1 M). Potentiodynamic polarizations performed on CoSb3 plates reveal the possible formation of a passive layer between 1 and 3 V (vs SCE), on a passivation plateau. The oxidation of CoSb3 at 2 V in sulfuric and oxalic acids results in the growth of an anodic conversion layer. This coating is mainly made of a porous layer of amorphous antimony oxides due to dealloying of CoSb3. In the specific case of oxalic acid, rods of crystallized oxalates are tangled between the oxide sheets.

Primary carbides may be important for the high temperature strength of polycrystalline Cr-rich cast alloys. TaC are among the best carbides for this role. Their presence depends on the base elements of the alloys. This dependence is here studied in the case of a series of Cr-rich alloys based on Ni and/or Co and containing Ta and C in equal molar fractions. Real alloys were cast and exposed at 1400 K and 1510 K, and their as-cast and aged microstructures were characterized. In parallel thermodynamic calculations using Thermo-Calc and a home-made database were carried out. It appears that TaC is the single carbide present in the alloy stabilized at high temperature only if the Co content is higher than the Ni one. Discrepancies appeared between calculations and the experimental results, showing that the used database must be improved. The experimental part of this work provides microstructures data which can be used to test databases and to enrich them if necessary.

The refractory materials required for waste-to-energy boilers endure severe working conditions, such as exposure to heat and hot oxidation / corrosion. Thanks to their high temperature properties cobalt-based alloys may respond to these properties requirements. In this work two model alloys based on cobalt and rich in chromium were elaborated by casting and samples were prepared by cutting and polishing. These samples were exposed, one to a hot complex gaseous mixture particularly aggressive reproducing the atmosphere in WtE boilers in service (presence of water vapor, di-oxygen, carbon di-oxide, hydrogen chloride), and the other to synthetic ashes, both for more than two hundreds hours. After test the samples were characterized by X-ray diffraction and SEM observations. On sample exposed to the complex gas stream a {10 to 15μm}-thick oxide scale formed on the surface of the sample exposed to the gas mixture. It involved all the elements of the alloy and it obviously developed both inwards and outwards as suggested by the position of the oxidized carbides.

International audience; Tantalum 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.

Hafnium carbides have recently demonstrated high interest to strengthen equi–axed polycrystalline superalloys. Notably chromia–forming Ni–based alloys reinforced by HfC appeared as being both oxidation–resistant and creep–resistant at temperatures higher than 1000 °C. Since high contents in Hf are required for achieving such high performances, it can be interesting to investigate possibilities of partial replacement of this expensive and rather rare element by tantalum. In this work two levels of partial substitution were tested. Two alloys were fabricated by foundry and subjected to creep and oxidation tests at 1100 °C. Despite the presence of Ta instead a part of Hf, the strengthening carbide network is exclusively – at least principally – composed of MC carbides which demonstrated good stability at 1100 °C. The creep resistance stays very good, only few lower than the high performance HfC–reinforced reference alloy. The isothermal oxidation and resistance against oxide scale spallation at cooling are as good as the ones earlier observed for the reference alloy. This shows that the two alloys – which are cheaper than the remarkable HfC–reinforced reference alloy – are capable of good creep–resistance and very good oxidation behavior at this particularly high temperature for superalloys not protected by thermal barrier coating.

International audience; 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 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.

Three compositions of the {Ni–25Cr–0.25 or 0.50C–1 or 2Ti}-type were studied by thermodynamic calculations for anticipating their melting temperature ranges and their microstructures at 1200 °C. In parallel the corresponding alloys were synthesized by classical foundry and subjected to Differential Thermal Analysis and exposures at 1200 °C followed by metallographic characterization. Calculations and experiments are globally consistent concerning the melting range temperatures, the microstructures in the as-cast state and after aging at 1200 °C. Good agreement was also found concerning the carbides fractions and the matrixes’ chemical compositions.

International audience

International audience; Some superalloys for service at high temperature under stresses are strengthened by tantalum carbides. Their creep resistance depends on the quantity of TaC and this is the reason why it is often important to control the volume fraction of these carbides in the microstructure. Metallographic preparation followed by electron imaging and surface fraction measurements by image analysis is a frequent way for that. Another possibility is to deduce the mass fraction of TaC, and after their volume fraction, from the chemical composition of the matrix when the alloys are only double-phased, on the {matrix + TaC} type. In this work three alloys-chemically designed to be made exclusively of matrix and TaC-were elaborated and isothermally exposed to an elevated temperature for a duration long enough to allow the alloys being at their thermodynamic equilibria. The chemical compositions of the alloy and of its matrix were measured and the results allowed evaluating their TaC mass fractions which were converted in volume fractions. The obtained TaC fractions were compared to results issued from thermodynamic calculations. Good agreement was found for the three alloys, and this allowed to exploit the used software and thermodynamic database to explore further the microstructures at the same high temperature, notably to know the conditions on the Co, Ni, Cr, Ta and C contents to keep the {matrix + TaC} structure and to avoid any possible partial melting.