Distribution of internal crack initiation sites in high-cycle fatigue for titanium alloys

In order to clarify how the internal crack initiation site distributes according to microstructure in high-cycle fatigue, alpha-beta or near alpha titanium alloys with various microstructure were subjected to fatigue test in liquid nitrogen. The internal crack initiation occurred longer than 10(5) cycles at 77 K. The initiation site was a facet or its aggregate which originated from a grain transgranular cracking in each material. In equiaxed a structures, the location of initiation site was obviously restricted to near the specimen surface (i.e. 10-200 mu m deep from the specimen surface). In acicular alpha structures, the location was more or less near the specimen surface and more interior cases were also seen, which were associated with a larger initiation site size. In elongated alpha structure, on the contrary, the location was scattered from the vicinity of specimen surface to the center in fracture surface. The size of the internal crack initiation sites was evaluated, and the location of the sites was discussed from the viewpoints of the cyclic stress and their size.