Mechanical Properties of Implant Rods made of Low-Modulus β-Type Titanium Alloy, Ti-29Nb-13Ta-4.6Zr, for Spinal Fixture

Installing a spinal fixture using implant rods is one of the effective operations for spinal diseases. Most of the implant rods are made of Ti-6Al-4V ELI alloy (Ti64). However, some problems regarding the Ti64 rod have been pointed out; it contains vanadium, which is considerably toxic to the human body, and exhibits a much higher Young's modulus than that of the cortical bone. Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) developed by the authors exhibits good biocompatibility due to its nontoxic- and allergy- free elements, and has a lower Young's modulus than that of Ti64. In addition, the mechanical properties of TNTZ can be changed drastically according to various heat treatments. The mechanical properties of TNTZ rods subjected to various heat treatments have been investigated in this study. The as-solutionized TNTZ rod consists of a single beta phase. Isothermal omega phase and alpha phase precipitate in the beta phase of the TNTZ rod aged at 673 K for 259.2 ks. Only alpha phase precipitates in the beta phase of the TNTZ rod aged at 723 K for 259.2 ks. Tensile strength and 0.2% proof stress become larger in order of as-solutionized TNTZ rod, TNTZ rod aged at 723 K for 259.2 ks and TNTZ rod aged at 673 K for 259.2 ks, while elongation is smaller in order of as-solutionized TNTZ rod, TNTZ rod aged at 723 K for 259.2 ks and TNTZ rod aged at 673 K for 259.2 ks. Young's modulus become larger in order of as-solutionized TNTZ rod, TNTZ rod aged at 723 K for 259.2 ks and TNTZ rod aged at 673 K for 259.2 ks.