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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.

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 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...