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Revisiting the effects of low-concentration hydrogen in NiTi self-expandable stents Fan SUN, Laurence JORDAN, Alexandre DA SILVA, Frantz MARTIN, Frédéric PRIMA

Fan Sun Laurence Jordan Alexandre da Silva Frantz Martin 1 Frédéric Prima
1 LECA - Laboratoire d'Etude de la Corrosion Aqueuse
SCCME - Service de la Corrosion et du Comportement des Matériaux dans leur Environnement : DEN/DPC/SCCME/LECA
Abstract : Superelastic NiTi is widely used in self-expandable transcatheter devices such as cardiovascular stent, neuro thrombectomy retriever and aortic valve scaffold. Hydrogen at a concentration of CH < 100 wppm is almost always contained in these devices due to the chemical surface treatments during production. The survey of hydrogen concentrations in 9 commercial stent samples show H concentrations values ranging from 6 to 55 wppm. The effects of such low-concentration hydrogen are revisited to reveal the mechanism of hydrogen-induced excessive strain-hardening and important reduction of ductility in Nitinol devices. Comparative investigation between samples charged to 8 and 40 wppm hydrogen is performed using in-situ electron backscatter diffraction (EBSD) mapping during tensile deformation and transmission electron microscopy (TEM). The results display that the influence of 8 wppm H on final material properties is insignificant. However, a concentration of 40 wppm H noticeably decreases the martensitic transformation temperature and the volume fraction of stress-induced martensite. In the non-transformed austenite zones, an intensive dislocation activity is observed after deformation, indicating a hydrogen-enhanced localized plasticity (HELP) effect. Such heterogeneous deformation mechanism leads to a microstructure composed by dislocation-glide-dominated austenite islands (15 vol.%) in the stress-induced martensite matrix (85 vol.%), resulting in very high macroscopic strain-hardening rate followed by an early failure in the 40 wppm H sample. The hydrogen segregation at low energy grain boundaries could probably be responsible for the formation of the non-transformed B2 islands. The results suggest that the NiTi stents containing less than 8 wppm hydrogen present low risk in hydrogen-related failures.
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https://hal-cea.archives-ouvertes.fr/cea-02954412
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Submitted on : Thursday, October 1, 2020 - 9:26:40 AM
Last modification on : Saturday, October 10, 2020 - 3:25:37 AM

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Fan Sun, Laurence Jordan, Alexandre da Silva, Frantz Martin, Frédéric Prima. Revisiting the effects of low-concentration hydrogen in NiTi self-expandable stents Fan SUN, Laurence JORDAN, Alexandre DA SILVA, Frantz MARTIN, Frédéric PRIMA. Materials Science and Engineering: C, Elsevier, 2021, 118, pp.111405. ⟨10.1016/j.msec.2020.111405⟩. ⟨cea-02954412⟩

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