Ti-Nb-(Zr,Ta) superelastic alloys for medical implants: Thermomechanical processing, structure, phase transformations and functional properties

The aim of this project is to develop a new class of orthopaedic implant materials that combine the excellent biocompatibility of pure titanium with the outstanding biomechanical compatibility of Ti-Ni-based shape memory alloys. The most suitable candidates for such a role are Ti-Nb-Zr and Ti-Nb-Ta near-beta shape memory alloys. Since this class of materials was developed quite recently, the influence of thermomechanical treatment on their structure and functional properties has not as yet been the subject of any comprehensive study. Consequently, this project is focused on the interrelations between the composition, the microstructure and the functional properties of superelastic Ti-Nb-Zr and Ti-Nb-Ta alloys for biomedical application. The principal objective is to improve the functional properties of these alloys, more specifically their superelastic properties and fatigue resistance, through optimization of the alloys' composition and thermomechanical processing.;It is shown in this thesis that the structure and functional properties of Ti-Nb-based shape memory alloys can be effectively controlled by thermomechanical processing including cold deformation with post-deformation annealing and ageing. It is also shown that the formation of nanosubgrain substructure leads to a significant improvement of superelasticity and fatigue resistance in these alloys. The influence of ageing on the &ohgr;-phase precipitation kinetics and, consequently, on the functional properties of Ti-Nb-Zr and Ti-Nb-Ta alloys is also observed.;Based on the results obtained, optimized regimes of thermomechanical treatment resulting in a best combination of functional properties are recommended for each alloy, from the orthopaedic implant materials standpoint.;An original tensile stage for a low-temperature chamber of an X-ray diffractometer is developed and used in this project. A unique low-temperature (-150...+100°C) comparative in situ X-ray study of the transformations' features and crystal lattice evolution is performed under strain-controlled conditions. The lattice parameters of beta- and &agr;"-phases calculated across the whole testing temperature range allow us to conclude that the higher the temperature, the lower the &agr;"→beta transformation strain. It is found also that loading at low temperatures results in &agr;"-phase formation and reorientation, while application of the load during heating changes the transformation sequences. The observed reversible beta-phase X-ray line widening and narrowing during temperature scanning are the direct result of appearance and disappearance of microstresses caused by reversible thermoelastic martensitic transformation.