Revisiting the effects of low-concentration hydrogen in NiTi self-expandable stents
Résumé
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. Correspondingly, hydrogen (8 wppm and 40 wppm) is charged to NiTi samples at two thermal-treatment states (500 °C-annealed and 800 °C-annealed) for comparative studies. The effects of hydrogen are revisited to clarify the hydrogen-induced changes of the operating deformation mechanism and the subsequent reduction of ductility in NiTi of both states. In-situ investigations at mesoscopic length scale (50–500 μm) are performed using electron backscatter diffraction (EBSD) mapping during tensile deformation of the 800 °C-annealed samples. The deformation microstructure and the operating deformation mechanism are further studied at microscopic length scale (<5 μm) by transmission electron microscopy (TEM) for both 500 °C and 800 °C annealed samples. The results display that the influence of 8 wppm H on final material properties is insignificant in 500 °C and 800 °C annealed samples. However, for both, a concentration of 40 wppm H noticeably decreases the martensitic transformation temperature and the macroscopic ductility of the samples. In the 800 °C-annealed sample, in-situ EBSD observes that the propagation of martensite band (MB) has finished at ε = 0.1 in the 8 wppm H sample while the propagation seems interrupted in the 40 wppm H sample until the same strain. Inside and surrounding the MB, island-like non-transformed austenite zones are formed representing 14.8area% of the observation zone. In both 500 °C and 800 °C annealed samples, TEM observations found extensive dislocation activity in specific regions of the samples after being charged to 40 wppm hydrogen. Thermal desorption spectroscopy (TDS) confirms that a hydrogen redistribution has happened in the 40 wppm hydrogen during the homogenization process. The change of local operating deformation mechanism from martensitic transformation to dislocation slip is thought to be responsible to the changes of mechanical properties of hydrogen-charged NiTi. The effect is further discussed in relation to the redistribution of hydrogen due to trapping at structural defects. In general, the results suggest that the influence of 8 wppm hydrogen is insignificant on the mechanical properties of NiTi, whereas the NiTi containing 40 wppm hydrogen may present risks of hydrogen-related mechanical failure within the strain amplitude range of a stent crimping process.
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