A beta-titanium alloy, which has good biocompatibility and low Young’s modulus, is expected to use for biomedical applications. In this study, in order to investigate the near-threshold fatigue crack propagation in a beta-titanium alloy (Ti-29Nb-13Ta-4.6Zr; TNTZ) with low Young’s modulus, stress intensity factor decreasing tests were conducted under the force ratios from 0.1 to 0.8 in air at room temperature. After testing, crack profiles were observed by scanning electron microscopy, and microstructures around crack profiles were analyzed using electron backscatter diffraction to discuss the mechanism of fatigue crack propagation. The crack growth rate in the solution-treated TNTZ followed by aging were constantly higher at comparable stress intensity range levels, and its threshold stress intensity ranges were lower compared to the only solution-treated TNTZ. This is attributed to the reduction of the opening stress intensity factor resulting from the formation of the alpha-phase by aging. However, the effect of microstructure on fatigue thresholds in TNTZ was disappeared by eliminating the crack closure phenomenon.
