Mechanical Properties of Thermomechanically-Processed Metastable Beta Ti-Nb-Zr Alloys for Biomedical Applications

Structural, physical, chemical, and biological surface characterization of thermomechanically treated Ti-Nb-based alloys for bone implants

Metastable near-beta Ti-21.8Nb-6Zr and Ti-19.7Nb-5.8Ta (at%) alloys were subjected to a thermomechanical treatment comprising cold rolling (CR) with a true strain of e = 0.3 and post-deformation annealing (PDA) in the 500-900°C temperature range to ensure the superelastic behavior which is important for bone implants. It was found that PDA resulted in formation of about 1-2 μm-thick oxide layer on the Ti-Nb-Zr and Ti-Nb-Ta alloy samples; the layer was mainly composed of TiO₂ , in rutile and anatase modifications. The structure, the phase and chemical compositions, and some surface-sensitive properties of the alloys were compared to those of Ti-50.7Ni and Ti-Grade2 reference materials. These surface layers (especially that of the Ti-Nb-Zr alloy) demonstrated a promising combination of high cohesion strength (load causing surface layer fracture is over 25 N), hardness (∼12 GPa), and hydrophilicity (contact angle ∼40°). Surface modification by controlled oxidation during air annealing increases corrosion resistance and enhances in vivo osteoinductive properties of Ti-Nb-Zr alloys by changing the surface microrelief, increasing the surface wettability, and improving the mechanical characteristics, thus laying the foundation for the development of medical implants with prolonged service life. So, it was confirmed that the same thermomechanical treatment, which creates conditions for the superelastic behavior of the bulk metal (CR: e = 0.3 + PDA = 500-700°C for 1 hr), would also create a strong, protective and biocompatible layer on the implant surface.