Application of coupled substrate aging and TiO₂ nanotube crystallization heat treatments in cold-rolled Ti–Nb–Sn alloys

Among titanium alloys, the β-type is the most indicated for orthopedic implants due to the reduced elastic modulus compared with α + β alloys. To improve osseointegration, the growth of a self-ordered titania nanotube layer onto the surface of titanium alloy implant pieces is a strategy used to accelerate bone growth. In this paper, the effects of heat treatment for titania nanotube crystallization on Ti–Nb and Ti–Nb–Sn alloys on the phase transformation, the Vickers hardness, and the elastic modulus of the substrate were investigated. TiO₂ layers were grown onto cold-rolled Ti alloy substrates by anodization, and crystallization to anatase was followed by glazing-angle high-temperature X-ray diffraction with a heating ramp of 288 K/min to 623 K, where the samples were held for up to 4 h. The dynamic of the α- and ω-phase formation/dissolution was followed by X-ray diffraction. Transmission electron microscopy was used to confirm the presence of the α- and ω-phases and their volumes and dimensions. As a result of the TiO₂ crystallization heat treatment, a continuous increase in the hardness was observed for the Ti–35Nb and Ti–35Nb–2Sn alloys, which is attributed to dissolution of α″ and the formation of ω precipitates. The same feature was observed for the elastic modulus. In the Ti–35Nb–4Sn alloy, the reverse decomposition of martensite resulted in the β phase and later in α phase precipitation. The aging of this alloy resulted in a homogeneous distribution of a high volumetric fraction of fine and dispersed α phase, which resulted in a hardness increase from 220 to 270 HV. This coupled heat treatment resulted in high hardness, low elastic modulus, and a nanotube with an anatase crystal phase.