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International audience; The aircraft industry is always looking for improved efficiency through higher in-service engine temperatures and lighter structures. Titanium-based alloys are good candidates for such applications because of their high specific strength. However, when exposed to high-temperature oxidizing environments, a large amount of dissolved oxygen can be found in such alloys beneath the growing oxide scale, possibly leading to embrittlement. Consequently, evaluating the oxidation resistance of these alloys is essential. With this aim, long-term oxidation tests were carried out on Ti6242S alloy between 500 and 650 °C to study the effect of temperature, surface preparation and microstructure on oxide scale and oxygen dissolution. While increasing the temperature from 560 to 625 °C led to accelerated oxidation kinetics, surface preparation had no noticeable effect on mass variations and oxygen diffusion profiles. Regarding microstructure, when comparing Ti6242S samples having similar α-phase fraction but very different microstructures (fineness and morphology), there wasn’t any significant effect found on mass change and oxygen diffusion after 1 kh at 650 °C.

Aircraft industry always looks for higher in-service temperatures and lighter structures. With a high specific strength, Ti-based alloys are good candidates for such applications. However, when exposed to oxidizing environments at high temperatures, they undergo large oxygen dissolution while forming an oxide scale, which can greatly affect their mechanical properties. Then, evaluating the oxidation resistance and mechanical behavior of such alloys is essential. In this aim, long term oxidation tests were performed under laboratory air between 500 °C and 625 °C on two Ti-based alloys: β21s, exhibiting a fully β microstructure supposed to dissolve lower amount of oxygen and nitrogen, and Ti6242S, with an α/β microstructure. The oxidized samples were characterized using XRD, Raman spectroscopy, SEM-EDS and micro-durometer. As for the mechanical behavior, tensile tests were performed at room temperature on not aged and on oxidized samples. While larger mass variations were obtained at 500 and 560 °C and up to 997 h at 625 °C for β21s, its mass variations became lower than those of Ti6242S for longer durations at 625 °C. Nevertheless, β21s exhibited thicker micro-hardness affected depths and underwent larger mechanical property modifications compared to Ti6242S.

International audience; Titanium-based alloys are increasingly used in aircraft industry due to their good mechanical properties and low density. To reach better efficiencies, the engine design is constantly evolving and the temperature of the combustion chamber is always increasing. Then, surrounding parts are exposed to higher working temperatures and consequently have to cope with more severe oxidation phenomena. For Ti-based alloys, this leads to the formation of an external oxide scale and a large oxygen dissolution in the metal, which may be detrimental for ductility. As nitrogen is the major element of the atmosphere, it seems relevant to study its effect on the oxidation behavior of Ti-6Al-2Sn-4Zr-2Mo-0.1Si titaniumbased alloy used for these applications. Thermogravimetric analyses were carried out at 650 °C for 100 h in synthetic air (20%O2–80%N2) and in a mixture of 20%O2–80%Ar. Results showed that nitrogen decreases the oxidation kinetics by slowing down the oxide scale growth but also by limiting the oxygen dissolution inthe alloy.

Les correlations entre les cinetiques de croissance et les evolutions morphologiques et microstructurales des couches de NiO formees lors de l’oxydation du nickel de haute purete ont ete etablies a partir d’essais d’oxydation isothermes et anisothermes. La complexite des mecanismes de transport qui entrent en jeu lors de la croissance de telles couches est demontree et un modele de croissance est propose prenant en compte la diffusion des especes cationiques en volume et par courts-circuits), le transport de l’oxygene et les reactions interfaciales.

The morphology and microstructure of NiO scales grown during oxidation of high-purity nickel were investigated. The oxidation tests performed between 450 and 1200°C in pure oxygen at atmospheric pressure were stopped at given scale thicknesses ranging from 1 to 70 μm. The specific specimen preparation and test procedure used in this work led to very good test reproducibility. The morphological and microstructural evolution of NiO scales are described by temperature-thickness maps. The scale morphology can be faceted or cellular. In addition, the growth of NiO platelets was observed in a specific temperature-thickness range. The microstructure of oxide scales can be simplex compact, simplex porous, or duplex. An important effect of substrate orientation was also observed, independent of the test temperature and scale thickness.

The oxidation kinetics of high-purity nickel were studied between 500 and 1200°C, in pure oxygen at atmospheric pressure, for average oxide-scale thicknesses of 1, 5, 10, and 30 μm. In the overall temperature range studied, a decrease in the parabolic rate constant kp with increasing scale thickness was observed. Depending on temperature and oxide-scale thickness, growth kinetics can be interpreted as a mixture of parabolic- and cubic-growth kinetics. Possible correlations between growth kinetics and microstructures of the oxide scales were investigated. From this set of experimental data, oxidation-kinetics models were tested. In particular, the effect of grain-boundary diffusion on NiO-growth kinetics was discussed. The correlations between growth kinetics and oxide microstructures appear to be more complex than usually reported.

The oxidation of high purity polycrystalline nickel was studied at 700 °C under flowing oxygen. For 1 μm thick oxide scale, the microstructure is cellular. For 3 to 30 μm thick oxide scales, rounded oxide platelets are observed on the scale surface. There is a large effect of substrate orientation. The scale is duplex, i.e. composed of an external scale of columnar grains and an inner subscale of equiaxed microstructure. The ratio external scale thickness/internal scale thickness is constant (≅ 2) and does not depend on total oxide scale thickness. The interface between internal scale and external scale corresponds to the initial surface of the metal. To explain these microstructural features, the mechanisms of scale growth proposed in the literature are critically reviewed. The relative position of the duplex oxide scale and the initial surface of the metal, and the relative growth of the outer and inner scales permits to conclude that NiO growth does not proceed only by the outward diffusion of cationic species in a dense scale. The duplex scale formation seems to be associated with the growth of the inner scale by inward transport of oxygen.

The isothermal oxidation of Pd-modified Ni aluminide coatings was studied as a function of Po2 and temperature (900–1200 °C). A kinetic transition was observed between 900 and 1000 °C. Grazing incident x-ray diffraction, thermogravimetric analysis, x-ray photoelectron spectroscopy, scanning electron microscopy/energy dispersive spectroscopy, and secondary ion mass spectrometry analyses are consistent with the growth of δ-alumina or α-alumina below or above this transition temperature. Moreover, because Po2 was established before specimen heating, an effect of heating rate was observed and analyzed. More importantly, no kinetic transition was observed for sand-blasted specimens oxidized at low Po2. Thus conditions for the direct growth of an α-alumina scale could be determined from the reported results.

Le comportement en oxydation isotherme des divers materiaux utilises pour la fabrication des sous-couches pour barrieres thermiques (systemes superalliage base-nickel/sous-couche/zircone partiellement stabilisee) a ete caracterise en fonction de la temperature (900 a 1100°C), de la vitesse de montee en temperature (12, 20 ou 60 K/min) et de l'atmosphere (pressions partielles d'oxygene de 10 -5 , 1 ou 10 5 Pa). Cela nous a permis de definir les conditions de pre-oxydation permettant de controler l'oxyde forme sur la sous-couche avant le depot de la zircone. Pour les aluminiures de nickel modifies palladium ou platine comme pour les sous-couches MCrAlY, une transition cinetique est observee lors d'oxydations courtes (6h) en dessous de 1000°C (entre 1000 et 1100°C pour (Ni,Pt)Al). L'analyse thermogravimetrique combinee aux analyses chimiques (XPS, SIMS, MEB-EDS) et structurales (DRX) montrent que cet accroissement des cinetiques d'oxydation a 900°C est du a la formation d'alumine de transition (θ ou δ). La transition peut etre supprimee en combinant un etat de surface sable (aluminiures) ou rectifie (MCrAlY) et une oxydation sous vide primaire d'air (10 Pa).

The oxidation kinetics of high-purity nickel were studied between 500 and 1200°C, in pure oxygen at atmospheric pressure, for aûerage oxide-scale thicknesses of 1, 5, 10, and 30 μm. In the oûerall temperature range studied, a decrease in the parabolic rate constant kp with increasing scale thickness was observed. Depending on temperature and oxide-scale thickness, growth kinetics can be interpreted as a mixture of parabolic- and cubic-growth kinetics. Possible correlations between growth kinetics and microstructures of the oxide scales were inûestigated. From this set of experimental data, oxidation-kinetics models were tested. In particular, the effect of grain-boundary diffusion on NiO-growth kinetics was discussed. The correlations between growth kinetics and oxide microstructures appear to be more complex than usually reported.

International audience

The isothermal oxidation of bond coats composed of vacuum plasma sprayed (VPS) MCrAlY (NiCoCrAlYTa and CoNiCrAlY) or palladium modified nickel aluminides (NiPd + APVS) was studied in several oxygen partial pressures (105, 1 and 10-5 Pa), with two heating rates (20 and 60K/min) at different temperatures (900, 1000 and 1100°C). For MCrAlY coatings, Arrhenius plots of the parabolic rate constants show a kinetic transition below 1000°C. This could be linked to a transition from Al2O3 to Cr2O3 scale growth. Lower oxygen partial pressures induce lower parabolic rate constants at 900°C. This leads to the assumption that scales grown at low oxygen partial pressures are still formed of alumina at 900°C. Nevertheless, these results could not be confirmed by chemical analysis (EDS, XPS). The two tested heating rates show no influence on the oxidation kinetics of both MCrAlY coatings. In the case of aluminide, for low oxygen partial pressures, the parabolic kinetics are reduced of one order of magnitude (for PO2 =105 to 1 Pa) and correspond to a thinner scale of a-alumina. Also, the heating rate modifies the parabolic kinetics (i.e. after the transient stage) and the total weight gains for all oxidation temperatures, with higher parabolic rate constants after heating at slower rate.

International audience

The oxidation of high purity nickel was studied between 600 and 1200°C for scale thickness between 1 and 30 µm. At or above 1100° C, the scale growth kinetics are strictly parabolic. The scales are...