β-Type titanium alloys for spinal fixation surgery with high Young’s modulus variability and good mechanical properties

Abstract

Along with a high strength, ductility, and work hardening rate, a variable Young’s modulus is crucial for materials used as implant rods in spinal fixation surgery. The potential in this context of Ti–(9,8,7)Cr–0.2O (mass%) alloys is reported herein. The microstructural and mechanical properties of the alloys were systematically examined as a function of their chromium content, and the ion release of the optimized alloy was investigated to assess its suitability as an implant material. In terms of the deformation-induced ω-phase transformation required for a variable Young’s modulus, the balance between β-phase stability and athermal ω-phase content is most favorable in the Ti–9Cr–0.2O alloy. In addition, this composition affords a high tensile strength (>1000MPa), elongation at break (∼20%), and work hardening rate to solution-treated (ST) samples. These excellent properties are attributed to the combined effects of deformation-induced ω-phase transformation, deformation twinning, and dislocation gliding. Furthermore, the ST Ti–9Cr–0.2O alloy proves resistant to metal ion release in simulated body fluid. This combination of a good biocompatibility, variable Young’s modulus and a high strength, ductility, and work hardening rate is ideal for spinal fixation applications. Statement of significanceExtensive efforts have been devoted over the past decades to developing β-type titanium alloys with low Young’s moduli for biomedical applications. In spinal fixation surgery however, along with excellent mechanical properties, the spinal-support materials should possess high Young’s modulus for showing small springback during surgery to facilitate manipulation but low Young’s modulus close to bone once implanted to avoid stress shielding. None of currently used metallic biomaterials can satisfy these abovementioned requirements. In the present study, we have developed a novel alloy, Ti–9Cr–0.2O. Remarkably variable Young’s modulus and excellent mechanical properties can be achieved in this alloy via phase transformations and complex deformation mechanisms, which makes the Ti–9Cr–0.2O preferred material for spinal fixation surgery.