The studies we have conducted have shown that metastable Ti-Nb-based beta titanium alloys can exhibit the same specific properties as Ti-Ni alloys, namely superelasticity and shape memory effect. In addition, the elements that make up this family of alloys are perfectly biocompatible unlike Ni in Ti-Ni which is known to be very allergenic. It has also been shown that these specific mechanical properties are modular according to chemical composition. Thus, several original compositions of alloys of the Ti-Nb-X system were developed by fusion in magnetic levitation. The objective was to study the effect of adding addition elements (O, N or Si) on the specific mechanical properties of binary Ti-Nb alloys. In these new alloys, the martensitic transformation between the beta mother phase (austenite) and the alpha'' daughter phase (martensite), responsible for these specific properties, has been extensively studied. The characteristic temperatures of this transformation were determined in particular during dynamic mechanical analyses carried out in the laboratory. In addition, synchrotron X-ray in situ tensile tests (ESRF Grenoble) were carried out to study the martensitic transformation under constraint because real quantification is very problematic with conventional X-ray sources used in laboratories. Such a characterization proved to be very effective and it should be noted that it was our team that mounted the in situ traction device on the ID31 line of the ESRF as well as its load/discharge control mode. It is planned that this device will be put in the catalogue of offers of the ESRF. As a result, the experiments we have carried out have never before been conducted on titanium alloys with martensitic transformation under stress.
Nanoindentation has also been used to measure the grain-scale shape memory (SM) effect and superelasticity (SE) on different titanium alloys. Measurements of these unconventional properties were obtained from the study of load-displacement curves. The amplitude of the SE and SM effect was characterized by height and working ratios determined by the study of nanoindentation curves as well as AFM profiles made under an atomic force microscope. These mechanical measurements were correlated with EBSD (backscattered electron diffraction), which made it possible to obtain reverse pole figure maps providing information on both mechanical responses and crystallographic orientations of micrometric grains.
These superelasticity and shape memory properties are particularly appreciated for the manufacture of endodontic files.
As a result, a collaboration was recently initiated with the Micro-Méga Company (world leader in the field of endodontic file manufacturing) as part of a CIFRE thesis. These new alloys are intended to replace very quickly the biomedical alloy Ti-Ni (Nitinol®) controversial because of the bad reputation of nickel.
