Phase transformations and mechanical properties of biocompatible Ti-16.1Nb processed by severe plastic deformation

A coarse grained biocompatible Ti-16.1Nb (wt.%) alloy was used to study the impact of severe plastic deformation on microstructural changes, phase transformations, and mechanical properties. The starting material, showing a rather low value of Young's modulus (66 GPa), contained orthorhombic α″ martensite. Hydrostatic pressure of 4 GPa solely yields a partial transformation to the ω-phase; increasing the pressure to 8 GPa increases the volume fraction of the ω-phase and causes a concomitant increase of Young's modulus. By processing samples through high pressure torsion at room temperature, i.e. applying both hydrostatic pressure and shear deformation, a nanocrystalline structure was obtained. The samples almost exclusively contained the ω-phase and showed rather high values of Young's modulus (up to 130 GPa) and hardness (up to 4.0 GPa). The ω-phase formed during high pressure torsion revealed stability upon unloading. However, upon heating to about 500 °C the ω-phase decomposes into a phase mixture of hexagonal α and body centred cubic β phases which is still ultra-fine. Cold rolling and folding achieves a microstructure consisting of ω, α/α′ and α″ phases. Concomitant decrease of grain size and increase of defect density yield a hardness (3.3 GPa) which is smaller than that of high pressure torsion but a Young's modulus of about 100 GPa being closer to that of the initial material.