Takao KAWAI's research while affiliated with Kobe University and other places

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.

TiB-reinforced Ti-3Al-2.5V matrix composites, in which TiB whiskers are oriented parallel to the direction of heat extrusion, were fabricated via mechanical alloying and hot isostatic pressing (HIP). To investigate the near-threshold fatigue crack propagation in TiB-reinforced Ti-3Al-2.5V matrix composites, stress intensity factor K-decreasing tests were conducted for disk-shaped compact specimens having two different orientations of TiB whiskers at force ratios from 0.1 to 0.8 under ambient conditions. The crack growth rates, da/dN, for the composites incorporating TiB whiskers oriented perpendicular to the direction of crack growth were constantly lower than those obtained in the case where the orientation was parallel at the same stress intensity range ΔK, while the threshold stress intensity range, ΔKth, was higher. This effect can be explained by the increase in the degree of roughness-induced crack closure resulting from the perpendicular TiB, because fatigue cracks preferentially propagated across the boundaries between the matrix and the TiB in certain regions. In contrast, the effective threshold stress intensity range, ΔKeff,th, for composites was unaffected by the TiB orientation at low force ratios.