Development of thermo-mechanical processing for fabricating highly durable β-type Ti–Nb–Ta–Zr rod for use in spinal fixation devices

The mechanical strength of a beta titanium alloy such as Ti–Nb–Ta–Zr alloy (TNTZ) can be improved significantly by thermo-mechanical treatment. In this study, TNTZ was subjected to solution treatment, cold caliber rolling, and cold swaging before aging treatment to form a rod for spinal fixation. The {110}β{110}β are aligned parallel to the cross-section with two strong peaks approximately 180∘∘ apart, facing one another, in the TNTZ rods subjected to cold caliber rolling and six strong peaks at approximately 60∘∘ intervals, facing one another, in the TNTZ rods subjected to cold swaging. Therefore, the TNTZ rods subjected to cold swaging have a more uniform structure than those subjected to cold caliber rolling. The orientation relationship between the αα and ββ phases is different. A [110]β[110]β//[121]α[121]α, (112)β(112)β//(210)α(210)α orientation relationship is observed in the TNTZ rods subjected to aging treatment at 723 K after solution treatment and cold caliber rolling. On the other hand, a [110]β[110]β//[001]α[001]α, (112)β(112)β//(200)α(200)α orientation relationship is observed in TNTZ rod subjected to aging treatment at 723 K after cold swaging. A high 0.2% proof stress of about 1200 MPa, high elongation of 18%, and high fatigue strength of 950 MPa indicate that aging treatment at 723 K after cold swaging is the optimal thermo-mechanical process for a TNTZ rod.