Improvement In Fatigue Strength of Ti-29Nb-13Ta-4.6Zr Alloy with Keeping Low Modulus for Biomedical Applications

A biomedical beta-type titanium alloy, Ti-29mass%Nb-13mass%Ta-4.6mass%Zr alloy (TNTZ), that is composed of non-toxic and allergy-free elements and exhibits low Young's modulus of around 60GPa after solution treatment, has been developed by the authors. In practice, fatigue strength is one of the most important properties for metallic biomaterials. Therefore, in order to improve fatigue strength of TNTZ, various thermomechanical treatments were conducted previously. According to our previous studies, fatigue strength could be improved up to the same level as a practical titanium alloy, Ti-6mass%Al-4mass%V ELI alloy, by aging treatment at 723K for 259.2ks after cold rolling because of a large amount of alpha phase precipitation. However, Young's modulus was increased above 80GPa by this treatment. In this case, short time aging is expected to be one of the effective techniques to improve fatigue strength and keep low Young's modulus simultaneously because the amount of precipitate is smaller than the above-mentioned treatment. Further, omega phase is appropriate as a precipitated phase because this phase increases tensile strength more greatly than alpha phase, although too much precipitation of this phase drastically decreases elongation. Therefore, short time aging at 573K after cold rolling was conducted to introduce a suitable amount of omega phase precipitation, and then the effects of the short time aging on Young's modulus, tensile and fatigue properties of TNTZ were investigated in this study. As the results, TNTZ aged at 573K for 3.6ks and 10.8ks shows good balance between tensile strength and elongation. In the case of 3.6ks aging, the tensile strength can be improved, but fatigue strength is not improved. On the other hand, in the case of 10.8ks aging, not only tensile strength but also fatigue strength is improved. This fatigue strength is the highest value among TNTZ having low Young's modulus below 80GPa.