Abstract: Basal fracture due to dwell fatigue (DF) is primary cause of core material failure during aero-engine service, which highlights the importance of understanding the crack initiation mechanism. In this study, the relationship between activation of plastic slip within the [0001] basal plane orientation, local strain and crack initiation in near-α Ti60 alloys was investigated using coupled in situ and high-resolution digital image correlation (HR-DIC) methods. The results show that basal slip is preferentially activated resulting into a higher shear strain in DF compared to $\left[ 10\bar{1}0 \right]$ prismatic orientation grains. Two types of cracking induced by strong basal slip were identified in grains with medium to high basal Schmidt factor (SF): slip transgranular cracking and basal twist grain boundary (BTGB) cracking. The slip deformation and strain evolution of the two types of cracks before and after their initiation were observed for the first time on the sample surface by in-situ dwell, which provides experimental and theoretical support for their formation mechanism. In order to constitute potential locations for crack nucleation, these regions have high local and shear strains during the basal slip stage prior to crack initiation. The higher local strain in the former is attributed to the intrusion and extrusion mechanism of the slip bands, while the latter is attributed to the inconsistency of the slip deformation of the microscopic dislocations on both sides of the BTGB making the GB slip-shear. Additionally, the incoordination on both sides of the GB, coupled with the inherently low cleavage energy, makes the BTGB more susceptible to cracking compared to slip transgranular cracking. No elemental segregation or β-phase influence was observed at the grain boundary interface. This study provides new insights into the mechanisms involved basal slip-induced cracking in Ti alloys subjected to dwell loadings.

Key words: Titanium alloy, In-situ dwell fatigue, Crack initiation, Basal slip, Strain