Microstructure and mechanical behavior of superelastic Ti-24Nb-0.5O and Ti-24Nb-0.5N biomedical alloys

International audienceIn this study, the microstructure and the mechanical properties of two new biocompatible superelastic alloys, Ti-24Nb-0.5O and Ti-24Nb-0.5N (at.%), were investigated. Special attention was focused on the role of O and N addition on α″ formation, supereleastic recovery and mechanical strength by comparison with the Ti-24Nb and Ti-26Nb (at.%) alloy compositions taken as references. Microstructures were characterized by optical microscopy, X-ray diffraction and transmission electron microscopy before and after deformation. The mechanical properties and the superelastic behavior were evaluated by conventional and cyclic tensile tests. High tensile strength, low Young's modulus, rather high superelastic recovery and excellent ductility were observed for both superelastic Ti-24Nb-0.5O and Ti-24Nb-0.5N alloys. Deformation twinning was shown to accommodate the plastic deformation in these alloys and only the {332}〈113〉 twinning system was observed to be activated by electron backscattered diffraction analyses

New method to characterize a machining system: application in turning

Many studies simulates the machining process by using a single degree of freedom spring-mass sytem to model the tool stiffness, or the workpiece stiffness, or the unit tool-workpiece stiffness in modelings 2D. Others impose the tool action, or use more or less complex modelings of the efforts applied by the tool taking account the tool geometry. Thus, all these models remain two-dimensional or sometimes partially three-dimensional. This paper aims at developing an experimental method allowing to determine accurately the real three-dimensional behaviour of a machining system (machine tool, cutting tool, tool-holder and associated system of force metrology six-component dynamometer). In the work-space model of machining, a new experimental procedure is implemented to determine the machining system elastic behaviour. An experimental study of machining system is presented. We propose a machining system static characterization. A decomposition in two distinct blocks of the system "Workpiece-Tool-Machine" is realized. The block Tool and the block Workpiece are studied and characterized separately by matrix stiffness and displacement (three translations and three rotations). The Castigliano's theory allows us to calculate the total stiffness matrix and the total displacement matrix. A stiffness center point and a plan of tool tip static displacement are presented in agreement with the turning machining dynamic model and especially during the self induced vibration. These results are necessary to have a good three-dimensional machining system dynamic characterization

Improvement of Superelastic Performance of Ti-Nb Binary Alloys for Biomedical Applications

International audienceRecently, beta-Ti-based alloys consisting of non-cytotoxic elements and possessing a low elastic modulus attracted considerable attention for biomedical applications. In addition to low elastic modulus, these alloys must also present a high strength level required to endure stresses. However, it is not easy to find a thermomechanical route in order to achieve low elastic modulus and high strength simultaneously. In this study, we show that severe cold-rolling deformation followed by a short aging treatment on Ti-Nb binary alloys, in order to produce ultrafine grains and/or omega phase, is an effective way to improve both strength and superelasticity. High stress (900 MPa), low modulus (35 GPa), and high recoverable strain (2.5%) are obtained using this route. The obtained results on mechanical properties are explained in relation with microstructure evolution during thermomechanical processing

A thermo-mechanical treatment to improve the superelastic performances of biomedical Ti-26Nb and Ti-20Nb-6Zr (at%) alloys

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Influence of a short thermal treatment on the superelastic properties of a titanium-based alloy

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Superelastic property induced by low-temperature heating of a shape memory Ti–24Nb–0.5Si biomedical alloy

International audienceIn this study, we have explored a new way to obtain a superelastic titanium-based alloy. Contrary to the classical method consisting to obtain the β microstructure directly by water quenching from the high temperature β-phase domain, this way consists in obtaining first a shape memory alloy displaying a fully α'' martensitic microstructure and then a superelastic behavior after applying a low temperature heating

Comparison of two matching techniques for UHF RFID tags

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In situ synchrotron X-ray diffraction study of the martensitic transformation in superelastic Ti-24Nb-0.5N and Ti-24Nb-0.5O alloys.

International audienceMechanisms of superelasticity were investigated by in situ cyclic tensile tests performed under synchrotron X-ray radiation on Ti-24Nb-0.5N and Ti-24Nb-0.5O compns. of metastable β titanium alloys. Analyses of diffraction patterns acquired under load and after unloading for each cycle were used to det. the characteristics of the potential mechanisms of deformation in both alloys. The Ti-24Nb-0.5N alloy exhibits the conventional behavior of superelastic β titanium alloys. Synchrotron X-ray diffraction (SXRD) expts. proved that superelasticity is exclusively due to the occurrence of a stress-induced martensitic (SIM) transformation from the β phase to the α'' phase. The evolution of vol. fraction of α'' martensite corresponds exactly to the variation of the recovery strain of the cyclic tensile curve. Conversely, the Ti-24Nb-0.5O alloy displays a non-conventional behavior. SXRD expts. showed a huge ability of the β phase to deform elastically until 2.1%. Surprisingly, a reversible SIM transformation also occurs in this alloy but starts after 1% of applied strain that corresponds to the yield point of the stress-strain curve. Although the SIM transformation occurs, the β phase simultaneously continues to deform elastically. The superelasticity of this alloy is unexpectedly due to a combination of a high elastic deformability of the β phase and a reversible SIM transformation. In both alloys, the lattice parameters of the α'' martensite evolve similarly in accordance with the initial texture of the β phase and the crystallog. of the transformation

Elaboration et caractérisation d'alliages de type Ti-Nb-X (X = O, N) pour des applications biomédicales Synthesis and characterisation of Ti-Nb-X (X = O, N) alloys for biomedical application

Dans cette étude, trois alliages de titane β-métastables de composition Ti-27Nb, Ti-24Nb-0.5N et Ti-24Nb-0.5O ont été élaborés par fusion. Ces trois alliages présentent des propriétés superélastiques lors des essais de traction. Des essais de traction in-situ sous rayonnement synchrotron nous ont permis de monter que cette superélasticité est due à une transformation martensitique réversible β → α” bien connue pour deux alliages alors que celui contenant de l'oxygène présente un comportement moins conventionnel. Les températures caractéristiques (MS, MF) de la transformation martensitique β (austénite) vers α” (martensite) et celles (AS, AF) de la transformation inverse α” vers β ont aussi été déterminées par des essais mécaniques dynamiques. Ces températures caractéristiques augmentent linéairement avec la contrainte externe et cette augmentation suit la relation de Clausius Clapeyron. <br> Ti-Nb based alloys are well known to their good mechanical properties, shape memory effect, superelasticity, as well as good biocompatibility. Our study is focused on the improvement of their mechanical properties by adding alloying element such as oxygen or nitrogen. Superelasticity was drastically improved by addition of a few amount (0.5 at %) of oxygen or nitrogen. Martensitic transformation between the β parent phase (austenite) and α” product phase (martensite), responsible for the superelastic property, has been extensively studied by Dynamic Mechanical Analysis (DMA) and in-situ tensile test under X-ray synchrotron diffraction