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Oxygen/nitrogen-assisted degradation of the mechanical behavior of titanium alloys exposed at elevated temperature

Abstract : Titanium alloys are increasingly used in the manufacture of intermediate/high temperature structural components. In service, these components are exposed to elevated temperatures (T> 300°C up to 650°C) and oxidative/nitriding atmospheres, typically the air. However, titanium and titanium alloys are particularly prone to substantial oxygen and nitrogen solubility (ex: up to 33% at. for O and 9.6 % at. for N in the alpha phase at 650°C). Therefore, this chemical solubility leads to element partition during high temperature oxidation/nitridation and thus to the formation of an oxygen/nitrogen-affected diffusion zone (ONADZ) for depths at least one order of magnitude thicker than the oxide layer. Due to the brittleness of the ONADZ, this zone strongly impairs the mechanical integrity of Ti-alloys structural components subjected to mechanical loading and thermal exposure, especially for cyclic stresses [1-2]. The ONADZ fails in a brittle manner and transverse cracks develop on the whole gauge section, acting as severe notches in service conditions. Therefore, assessing the local mechanical behavior of the ONADZ and predicting its early cracking in various stress amplitude/temperature ranges are central problematics to better implement microstructure-based fatigue criteria in structural design of Ti-alloys components. In the present study, the mechanical behavior of the ONADZ is investigated using tensile testing of ultrathin pre-oxidized specimens. Ultrathin specimens exhibit a substantial surface/volume ratio that is interestingly appropriate for the study of surface reactivity, i.e. oxidation, nitridation. 65-µm thick specimens were prepared using a precision Jig polishing technique detailed in ref [3]. Preoxidation treatments at 750°C with various high temperature exposure durations have been performed in order to generate different ONADZ thicknesses. Room temperature (RT) tensile testing of the pre-oxidized specimens paired with a two-scale digital image correlation (DIC) technique. This allows an investigation of: (i) the macroscopic stress/strain response of the composite structure made of a non-affected core region surrounded by the ONADZ, (ii) irreversible deformation/cracking events at the microstructure scale as a function of the ONADZ thickness. RT tensile tests on pre-oxidized Ti6242 specimens demonstrated the feasibility of such micromechanical testing adapted to Ti alloys. Increasing the pre-oxidation time at a given temperature leads to a considerable drop of the ductility, the 750°C-24h pre-oxidized specimen being fully brittle. Fracture surface observations outline the extension as a function of the pre-oxidation duration and the brittle behavior of the ONADZ (Figure 1). Figure 1: Fracture surface of pre-oxidized specimens after RT tensile testing: a) non-oxidized specimen, b) 750°C-2h pre-oxidation treatment, c) 750°C-24h pre-oxidation treatment. References: [1] C.E. Shamblen, T.K. Redden, Air contamination and embrittlement in titanium alloys, The Science, Technology, and Application of Titanium, Pergamon Press, (1968) 199–208. [2] T.A. Parthasarathy, W.J. Porter, S. Boone, R. John and P. Martin, Life prediction under tension of titanium alloys that develop an oxygenated brittle case during use, Scripta Mater. 65 (2011) 420–423. [3] D. Texier, D. Monceau, J.-C. Salabura, R. Mainguy, E. Andrieu, Micromechanical testing of ultrathin layered material specimens at elevated temperature, Mater. High Temp. 33 (2016) 325–337.
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Damien Texier, Q Sirvin, Vincent Velay, Mehdi Salem, Daniel Monceau, et al.. Oxygen/nitrogen-assisted degradation of the mechanical behavior of titanium alloys exposed at elevated temperature. Ti2019: The 14th world conference on titanium, Jun 2019, Nantes, France. 4 p., ⟨10.1051/matecconf/202032106004⟩. ⟨hal-02359251⟩



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