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Nouveaux alliages de titane superélastiques pour la fabrication de limes endodontiques : du matériau au prototype

Abstract : The NiTi (Nitinol) is the only alloy used for the manufacture of medical devices that require a shape memory effect or superelasticity, such as orthodontic arch wires, stents, catheters and osteosynthesis staples. The superelasticity is the ability of an alloy to deform reversibly much more than a conventional alloy. This is due to a reversible stress induced martensitic transformation between two crystallographic phases: austenite and martensite. NiTi is widely used in the manufacture of endodontic files, a medical device used in dentistry. The endodontic files are used to shape the root canal system and the superelasticity of the tool is necessary to follow very bent root canals. However, the use of NiTi suffers from several drawbacks, including the recognized toxicity of nickel but also its brittle character and its difficulty to manufacture. Some metastable β titanium alloys also exhibit superelastic properties. The advantage is that they can be elaborated with only biocompatible elements (i.e. Nb, Hf, Mo, Zr & Sn). Therefore, they are promising candidates for medical applications. The objective of this thesis is to determine the potential interest of metastable β titanium alloys for the production of endodontic files. Three classes of alloys were studied: the NiTi used in the manufacture of current endodontic files; a commercially available metastable β titanium alloy, the Beta III, whose composition is Ti-11,5Mo-6Zr-4,5Sn (weight %); and original metastable β titanium alloys developed in the laboratory, Ti2448 and Ti2334 having respective compositions of Ti-24Nb-4Zr-8Sn (weight %) and Ti-23Hf-3Mo-4Sn (atomic %). In this study, the influence of composition and thermomechanical treatments on the mechanical properties and on the stress induced martensitic transformation, responsible for the superelastic behavior, was studied. Stress induced martensitic transformation and its reversibility were most deeply studied by in situ X-ray diffraction analysis under synchrotron radiation. In all medical devices, it is needed to prevent accidental failure during use, therefore a high resistance to cyclic loading is necessary to avoid fatigue fractures in the canal system. This has led to study the fatigue behavior of alloys and the changes in their mechanical and superelastic responses during cycles. Finally, the behavior of alloys once manufactured into prototypes was evaluated with the company’s equipment and procedures.
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Lorène Héraud. Nouveaux alliages de titane superélastiques pour la fabrication de limes endodontiques : du matériau au prototype. Matériaux. INSA de Rennes, 2016. Français. ⟨NNT : 2016ISAR0016⟩. ⟨tel-03190714⟩

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