High-R Notched HCF performance of macrozones in Ti-6Al-4V

Abstract

The hypothesis is examined that the macrozones that can occur in large Ti-6Al-4V forgings, in combination with the effects of stressed volume and sustained loads, can result in a debit in high cycle fatigue (HCF) performance, which would be of concern in jet engine applications. Here, the effect of placing a notch root in hard vs. soft oriented macrozones, i.e. $\left\{0002\right\}$ parallel or perpendicular to the loading direction, respectively, were examined. The deformation features associated with crack initiation were analysed. A significant ($10\times$) variation in life was observed, as low as 5.6 × 10⁶ cycles in hard macrozones compared to 58.6 × 10⁶ cycles in soft macrozones, at a peak notch root stress of 912 MPa and load ratio $R$ of 0.5. Local macrozones were found neighbouring the initiation facets, which were subsurface. Compared to plain fatigue, LCF or dwell fatigue, the initiating faceted grains possessed low dislocation density, which were predominantly of basal $〈a〉$ character.