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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; Oxide-alloy interface of Ti6242S Ti-based alloy was investigated by STEM-EELS technique, after oxidation in both N2-free and synthetic air atmospheres at 650 °C for 1000 h. The chemical shift of Ti-L2,3 edge and its specific fine structure along the oxide-alloy interface were used as fingerprint to distinguish the different compositions of titanium nitrides and oxynitrides. TiN, Ti2N and TiNxOy were identified in the sample oxidized in synthetic air at the oxide-alloy interface. Moreover, a decreasing in the oxidation state of Ti oxides was found along with Sn segregation at the oxide-alloy interface for the sample oxidized in N2-free atmosphere.

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.

At high temperatures under oxidizing environments, titanium-based alloys form an oxide scale and dissolve large amount of oxygen in their metallic matrix. Oxygen dissolution is a cause of embrittlement. Nitrogen is a secondary oxidant, which also dissolves in titanium during oxidation in air. Oxidation experiments of Ti-6Al-2Sn-4Zr-2Mo-0.1Si titanium-based alloy at 650 °C for 1000 h in synthetic air (20%O2- 80%N2) and in a mixture of 20%O2-80%Ar, showed that nitrogen reduces both oxide scale growth and oxygen dissolution. Atom probe tomography revealed that nitrogen effect is due to the formation of an interfacial layer of nitride Ti2N but also to the formation of a nitrogen rich a-Ti-based solid solution, which both act as difiusion barriers for oxygen because of their low oxygen solubility.