Titanium alloys have always overtopped by their excellent biocompatibility. Nowadays, Ti-6Al-4V alloy (Ti 64) is widely used as a bone substitute (hip or knee prosthesis) and the amazing properties of Ni-Ti alloys (superelastic or shape memory effect) are used to design functional devices (stents, orthodontic wires, shape memory clips, etc.) Yet, Ti64's modulus of elasticity is too high compared to bone, that limits osseointegration and decreases service life of prosthesis. Moreover, the additive elements, such as aluminum, vanadium and nickel, are known to be cytotoxic. Metastable beta titanium alloys can be elaborated with biocompatible elements only (tantalum, niobium) and can exhibit a low modulus of elasticity and superelasticity or shape memory effect. Six compositions based on the Ti 25-xNb scheme were elaborated, using the cold crucible levitation technique. The aim of this thesis consisted in establishing a relationship between the microstructures and the mechanical properties of these alloys. Depending on the niobium content, these alloys exhibit an alpha'' martensitic microstructure (associated with a shape memory effect) or a beta grain microstructure (associated with a superelastic effect). All these properties were characterized by numerous techniques including microscopy, X-ray diffraction, tensile tests and dynamic mechanical analysis. The plastic deformation mechanisms that explain their excellent cold-workability were studied by electron backscatterering diffraction and transmission electron microscopy. Two twinning systems, {112} and {332}, were highlighted and the competition between them was assessed, based on a Schmid factor analysis.
