Multi-Dimensional Study of the Early Slip Activity on Crack Initiation in a Near α Titanium Alloy

During service of gas turbine engines, high cycle fatigue of titanium is a leading cause of component failure highlighting the need for better understanding of the crack initiation mechanism to predict initiation sites. In this study, the relationship between plastic slip activity and fatigue crack initiation was investigated in a near-a titanium alloy using cyclic four-point bending at up to 90% of the proof stress. Detailed surface characterization demonstrates that plasticity at such low stress levels is dominated by basal slip. Two types of crack mode were seen among four short cracks. Transgranular cracking parallel to basal slip traces was observed within three primary a grains and one case of intergranular cracking was seen along the boundary between primary a grain pairs, which have their c-axes aligned nearly parallel to each other but with mis-aligned prismatic planes. Detailed 3D analysis highlights out-of-plane Burgers vector activity for the observed basal slip associated with crack initiation, consistent with the classic surface roughening mechanism. The plane of the short crack was identified to be a basal facet and lattice rotation around the c-axis close to the crack plane suggests additional prismatic slip activation during multi-step crack formation. Statistical evaluation highlighted that grains combining a moderately high Schmid factor for basal slip, high resolved tensile stress along the c-axis and the Burgers vector being orientated strongly out-of-surface plane favour crack initiation. Based on those observations a new parameter involving these three geometrical factors has been developed that predicts surface crack initiation sites.