As air transport becomes less carbon intensive, green hydrogen is emerging as a promising new source of energy for powering aircraft engines. During an engine cycle, critical parts, such as turbojet injectors will be subjected to a high-pressure hydrogen gas environment.
However, it’s well known that, a long-term exposure of metal alloys to this new energy source can lead to a deterioration in their mechanical properties and a premature failure of the material, known as hydrogen embrittlement (HE), as a result of the adsorption of atomic hydrogen in their crystalline structure.
This Hydrogen embrittlement is highly dependent on the type of material and the loading conditions. Numerous works have explored the exact causes of this embrittlement, in particular based on interactions between hydrogen and metallurgical defects in the material. Several mechanisms have been identified in the literature as being at the root of this embrittlement. These include Hydrogen Enhanced Localized Plasticity (HELP), Hydrogen Enhanced Decohesion (HEDE), Adsorption-Induced Dislocation-Emission (AIDE), Hydrogen Enhanced Strain-Induced Vacancy (HESIV)…etc. Nevertheless, none of these phenomena can exclusively explain hydrogen-assisted crack initiation (HE effects), which is often the result of a synergistic effect between some of these mechanisms and the loading conditions [1],[2],[3],[4].
In order to study the embrittlement effect of hydrogen in a controlled and static way, without considering dynamic variations in hydrogen flow representative of injector operating conditions, this study focuses on the effect of hydrogen concentration on the cyclic viscoplastic behavior of Inconel 718 at ambient temperature.
Two metallurgical states of 718 are studied: Direct Aged and Inconel 718 obtained by L-PBF. Inconel 718 DA is extracted from turbine discs and is characterized by grain sizes of between 5 and 20µm. The 718 Inconel obtained by L-PBF is followed by homogenization and aging treatment to annihilate the columnar grain structure and the anisotropic microstructure characteristic of L-PBF, and to precipitate the second-phase particles of the material.
The mechanical test specimens are pre-charged with hydrogen gas. The gaseous charge is carried out in a furnace operating at a pressure of 400 bar and at a temperature of 200°C. The temperature kinetics during heating and cooling are chosen so as to limit thermal effect and H desorption following charging. Depending on the quantities of test specimens introduced into the autoclave, hydrogen dosing samples are also introduced to serve as controls and enable the hydrogen concentration introduced into the mechanical test specimens to be estimated.
Thermal desorption spectrometry was used to identify the state of H (trapped and in solution) and to quantify the concentration (quantity) of hydrogen introduced into the material.
After the specimens precharging with hydrogen gas, mechanical tests (monotonic loading and Oligo-cyclic) are carried out to evaluate the effect of hydrogen concentration on the cyclic viscoplastic behavior of the material at ambient temperature.  All this mechanical testing will help to identify the alloy’s degree of sensitivity to hydrogen embrittlement.
A comparison of the two metallurgical states will provide the initial basis for understanding of role of metallurgical heterogeneities in 718 on hydrogen diffusion and trapping, and their impact on plastic deformation mechanisms in the presence of a certain concentration of hydrogen.
The role of the microstructure and the deformation mechanisms associated with the presence of hydrogen are better explored by carrying out micromechanical tensile tests under SEM on Inconel 718 L-PBF.
References
[1]    S. Puydebois, A. Oudriss, P. Bernard, L. Briottet, et X. Feaugas, « Hydrogen effect on the fatigue behavior of LBM Inconel 718 », MATEC Web Conf., vol. 165, p. 02010, 2018.
[2]    X. Li et al., « Tensile mechanical properties and fracture behaviors of nickel-based superalloy 718 in the presence of hydrogen », International Journal of Hydrogen Energy, vol. 43, no 43, p. 20118‑20132, oct. 2018.
[3]    J. A. Lee, « Hydrogen Embrittlement », National Aeronautics and Space Administration Marshall Space Flight Center Huntsville, Alabama, NASA/TM-2016–218602, avr. 2016.
[4]    A. Behvar, M. Haghshenas, et M. B. Djukic, « Hydrogen embrittlement and hydrogen-induced crack initiation in additively manufactured metals: A critical review on mechanical and cyclic loading », International Journal of Hydrogen Energy, vol. 58, p. 1214‑1239, mars 2024.