High-strength and low-dwell-sensitivity titanium alloy showing high tolerance to microcracking under dwell fatigue condition

Abstract

Dwell fatigue failure of titanium alloy components has been one of the major threats to aircraft safety for the last 50 years. Numerous studies have focused on the identification of critical microstructural configurations, such as microtextured regions, rogue grain pairs and (0001) twist grain boundaries. In this work, a microstructure with a low primary α phase (αp) content was designed to avoid the known weaknesses in the Ti-5Al-7.5 V alloy to achieve high-strength and low-sensitivity to 2-min dwell loading. Conventional and dwell-fatigue tests confirmed the efficiency of such an approach. The deformation occurred primarily by planar slip in the αp grains, resulting in possible cracking along preexisting slip bands in the cycling process. Fatigue and dwell-fatigue failure was dominated by surface crack initiation at facet matching basal planes. Moreover, numerous internal microcracks were formed along basal slip bands and occasionally along basal twist grain boundaries. Load holds were found to facilitate cracking along prismatic planes, which highlights a mechanism switch between basal-dominant cracking mode and collective basal-prismatic mode. The mechanism switch suggests that the prismatic dislocation activity considerably increases due to the low temperature creep induced by load holding. Thanks to the low αp phase microstructure, the alloy shows remarkable tolerance to microcrack growth under dwell condition. Limited growth of the internal microcracks across transformed β regions was identified as a key feature of high dwell-fatigue resistance of the investigated material.