Crack initiation mechanisms of micro-textured Ti60 alloys with different dwell sensitivities subjected to fatigue and dwell fatigue loading

In this work, Ti60 alloys with different micro-texture intensity were designed to elucidate the effect of micro-texture on fatigue life and dwell sensitivity and the difference in crack initiation mechanism for low-cycle fatigue (LCF) and dwell fatigue (DF). It was found that micro-texture region (MTR) significantly reduced LCF and DF lifetimes and increased dwell sensitivity compared to no-MTR samples. However, micro-texture does not change the characteristics of the crack initiation grains, and the fracture initiation region facet of all samples matches the (0001) basal plane and is related to basal slip. Therefore, the [0001] orientation domains for crack initiation in both fatigue states are discussed, and it is found that LCF tends to nucleate along the basal slip bands or basal twist grain boundaries (BTGBs) of the high SF grains, whereas DF nucleation is independent of the basal SF value, and the mechanism of initiation of hard oriented grain cracking is also included. In situ further demonstrated that hard grains can activate basal slip as well as cracking, and the possibility of soft/hard grain pairs generating dislocation pile-up stresses leading to cracking of hard grains was determined by theoretical calculations, providing experimental support and evidence for the soft/hard grain model. Based on these findings, criteria for the angle (θ) of the c-axis with respect to the loading axis and the basal SF range are proposed to determine the differences between LCF and DF with respect to the mechanisms of hard grain cracking and BTGB crack initiation.