Effect of microstructure on ultra-high cycle fatigue behavior of Ti-6Al-4V

The fatigue behavior of Ti-6Al-4V alloy with the bimodal and basketweave microstructures has been investigated. The results show that the S-N curves of Ti-6Al-4V with both microstructures continuously decrease with increasing the number of cycles to failure and have no horizontal asymptote in the regime of 105 to 109 cycles. SEM observation of fracture surface indicates that the crack initiation sites abruptly shift from surface to interior of the specimen with decreasing the stress amplitude. It is found that the surface crack initiation mainly results from the machining traces. Nevertheless, most of the internal fatigue cracks initiate in the border region of primary α-grains in bimodal microstructure or at α-β interfaces in basketweave microstructure, which induced by inhomogeneity of microstructures and incompatibility of deformation between two phases. The size of the internal crack initiation site was evaluated and the dependence of the fatigue life on the location and size of the initiation sites is discussed.