Optimization of Cr content of metastable β-type Ti-Cr alloys with changeable Youngs modulus for spinal fixation applications

Metallic implant rods used in spinal fixtures should have a Youngs modulus that is sufficiently low to prevent stress shielding for the patient and sufficiently high to suppress springback for the surgeon. Therefore, we propose a new concept: novel biomedical titanium alloys with a changeable Youngs modulus via deformation-induced ω phase transformation. In this study, the Cr content in the range of 10-14 mass% was optimized to produce deformation-induced ω phase transformation, resulting in a large increase in the Youngs modulus of binary Ti-Cr alloys. The springback and cytotoxicity of the optimized alloys were also examined. Ti-(10-12)Cr alloys exhibit an increase in Youngs modulus owing to deformation-induced ω phase transformation. In this case, such deformation-induced ω phase transformation occurs along with {3 3 2}_β mechanical twinning, resulting in the maintenance of acceptable ductility with relatively high strength. Among the examined alloys, the lowest Youngs modulus and largest increase in Youngs modulus are obtained from the Ti-12Cr alloy. This alloy exhibits smaller springback than and comparable cytocompatibility to the biomedical Ti alloy Ti-29Nb-13Ta-4.6Zr.