Change in mechanical properties of biomechanical Ti-12Cr subjected to heat treatment and surface modification processing

Abstract

In recent years, metallic biomaterial applications have demanded a relatively low elastic modulus of around 30 GPa that is nearly equal to that of bone. However, in the case of spinal fixture applications, metallic materials with a relatively high Young's modulus are required to suppress spring-back by elastic and plastic deformation during implantation. Therefore, Young's modulus control by stress-induced transformation in a newly developed biomedical β-type Ti-12Cr alloy, has been proposed by the present authors. However, the relationship between the microstructure and mechanical properties of Ti-12Cr has not been fully investigated up till now. Therefore, changes in the mechanical properties of Ti-12Cr were investigated through heat treatment and the fine particle bombarding process (FPB), which is a surface modification process used in this study. Peak aging of Ti-12Cr heated at 673 K showed for around 2.4 ks. The Vickers hardness of Ti-12Cr in the peak aging condition (PA) at 673 K was around 90% (HV 524) higher than that (HV 294) in the solutionized condition (ST). Meanwhile, both the 0.2% proof stress and tensile strength of Ti-12Cr in the PA at 673 K were also around 50% higher those in the ST. However, the ductility of Ti-12Cr in the PA at each temperature reduced significantly. Therefore, a solo-solution treatment was judged to be the optimal heat treatment for Ti-12Cr with an excellent combination of strength and ductility. The Vickers hardness and Young's modulus of as-solutionized Ti-12Cr subjected to FPB increased by around 40% and 70%, respectively, at the very edge of the specimen surface, as compared to those of the unprocessed sample. Furthermore, the fatigue strength of Ti-12Cr subjected to FPB increased by around 70 MPa. The bone contact ratio of Ti-12Cr rose slightly with an increase in the implantation period from 24 to 52 weeks.