To cite this version:A. Ramarolahy, Philippe Castany, F. Prima, P. Laheurte, Isabelle Péron, et al.. Microstructure and mechanical behavior of superelastic Ti-24Nb-0.5O and Ti-24Nb-0.5N biomedical alloys.Journal AbstractIn 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 Ti24Nb-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.

Mechanisms 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 compositions of metastable b titanium alloys. Analyses of diffraction patterns acquired under load and after unloading for each cycle were used to determine the characteristics of the potential mechanisms of deformation in both alloys. The Ti-24Nb-0.5N alloy exhibits the conventional behavior of superelastic b titanium alloys. Synchrotron X-ray diffraction (SXRD) experiments proved that superelasticity is exclusively due to the occurrence of a stress-induced martensitic (SIM) transformation from the b phase to the a 00 phase. The evolution of volume fraction of a 00 martensite corresponds exactly to the variation of the recovery strain of the cyclic tensile curve. Conversely, the Ti-24Nb-0.5O alloy displays a nonconventional behavior. SXRD experiments showed a huge ability of the b 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 b phase simultaneously continues to deform elastically. The superelasticity of this alloy is unexpectedly due to a combination of a high elastic deformability of the b phase and a reversible SIM transformation. In both alloys, the lattice parameters of the a 00 martensite evolve similarly in accordance with the initial texture of the b phase and the crystallography of the transformation.

Ti-Nb based alloys are well known to their good mechanical properties, shape memory effect, superelasticity, as well as good biocompatibility. The Ti-24Nb (at%) binary alloy presents a shape memory behavior and low elastic modulus. Our study is focused on the improvement of their mechanical properties by adding a third alloying element (oxygen, nitrogen or silicon). Addition of 0.5 at% of N or O modifies drastically the mechanical behavior of Ti-24Nb alloy that exhibits superelastic behavior instead of shape memory one. On the other hand, addition of 0.5 at% of Si increased yield strength of the Ti-24Nb shape memory alloy.

In this paper, results concerning newly developed Ti-Nb based superelastic alloys containing interstitial O or N element are presented. These elements were added because of their strong influence on the alpha” martensitic transformation and also because of their solid-solution strengthening effect. Microstructures and mechanical behavior of the alloys were investigated by different methods and compared with a binary Ti-Nb alloy taken as reference. On the other hand, in-situ synchrotron X-ray diffraction analyses during tensile tests have been specifically performed to characterize precisely the stress-induced martensitic transformation in such alloys.