PSI - Issue 57

A. Radi et al. / Procedia Structural Integrity 57 (2024) 642–648 Achraf radi/ Structural Integrity Procedia 00 (2019) 000 – 000

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trapping behavior. In the case of HT0, the fact that the H-induced softening progressively decreases after a certain number of cycles suggests that the initially stored hydrogen around the precipitates is insufficient to ensure large hydrogen softening throughout the entire mechanical test.

4.2. Strain localization For several decades, considerable attention has been paid to the strain localization of fcc metals. Transgranular crack initiation in these metals is assumed to be mainly a consequence of the shear bands (SBs) emergence in the form of extrusion and/or intrusion on the material surface. The change in the SBs features such as inter-band spacing (I B ) and width ( w ) in presence of hydrogen gives direct implication of hydrogen effect on the dislocation-precipitate interaction. Based on the AFM and SEM measurement shown in table 2 we can observe a change of the persistent shear bands features. In what follows, the degree of the plastic strain localization, which is partially characterized by the measurements of I B is examined with and without hydrogen content for HT0 and HT4. Previous work [20] showed that I B remained equal to 2µm whatever the grain size and precipitate size variations tested in fatigue without hydrogen content. However, I B can be affected by the plastic strain amplitude; it decreases with the increase of the plastic strain amplitude, as demonstrated for austenitic Fe-36 at.% Ni-12 at.% Al alloy[22] , copper [23] and Waspaloy [1,24]. In this study, and as mentioned previously the plastic strain amplitude was kept constant for both HT0 and HT4 hydrogen free and charged samples so we observe only the effect of hydrogen on the SBs features. The inter-band spacing (I B ) decreases in the presence of hydrogen for both metallurgical states HT0 and HT4. This reduction signifies a decrease in the distance between shear bands in the presence of hydrogen. The observed decrease in inter-band spacing can be attributed to two potential mechanisms. Firstly, the activation of dislocation sources by hydrogen may lead to the production of new mobile dislocations with a predominantly edge character, contributing to the observed strain localization. Secondly, the formation of a Cottrell atmosphere, which immobilizes dislocations, could also account for the decrease in inter-band spacing. However, the latter mechanism is considered less probable due to the low frequency (f = 0.05Hz) applied in the study, which is insufficient to cause hydrogen segregation and create a Cottrell atmosphere around dislocations. Additionally, the observed softening of the flow stress, rather than hardening, further supports the less probable nature of the Cottrell atmosphere mechanism in this context. The former mechanism is more probable and join the results of the cyclic behavior where hydrogen induced a softening of the flow stress and the effective stress which were explained by the HELP mechanism. 5. Conclusion In conclusion, this study has highlighted the significant impact of hydrogen presence on the cyclic behavior of a precipitation-hardened nickel-based superalloy, depending on the aging conditions and the amount of hydrogen stored around the precipitates. The results have demonstrated that hydrogen induces a softening of the effective stress and hardening of back stress. This softening is attributed to the decrease in short-range interactions associated with the hydrogen's elastic screening effect, suggesting increased mobility in the presence of hydrogen. In the presence of hydrogen, the inter-band spacing (I B ) decreases, which may support an increase of strain incompatibility and consequently the back-stress hardening. First additional measurements made on the other shear bands features (inter band width ( w ) and dislocation density within the shear bands (ρ SB )) seem to show an influence of the presence of H on these characteristics, which suggests that hydrogen impact the dislocations-precipitates interaction. More investigations are being conducted to elucidate the mechanism behind the transition induced by hydrogen. Acknowledgements This study was carried out within the framework of the SLHYCC project (ANR-19-CE08-0027). The authors would like to thank the technical team who assisted them in conducting the experiments (L. Escher, I. Velluet, and F. Nadaud from UTC, F. Guilloret and A. Claisse from Cetim) as well as Ms. V. Borg, responsible for the FOD