抄録
In cryogenic high cycle fatigue for titanium alloys and nitrogen-strengthened austenitic steels, subsurface crack initiation apparently occurs without the existence of any defects like inclusion and pore. Microstructural origin of subsurface crack initiation site and deformation behavior for those alloys were investigated, and the mechanism of subsurface crack initiation was discussed. Subsurface crack initiation site is consisted of facets which are assigned their microstructure. Subsurface crack initiation is caused by a microcracking related with microstructural inhomogeneity. A principal slip system is dominant deformation mode and dislocation motion is planar. Even at lower stress level, piled-up dislocations of short range order are blocked or sharply localized at grain boundaries. Namely, the stress concentration at grain boundary locally occurs. Then, the stress concentration might produce the microcracking. On the other hand, the size of subsurface crack initiation site depends on the maximum stress range. Therefore, subsurface crack initiation mechanism is considered that a microcracking due to localized deformation grows or coalesces until a critical crack size and is chosen for the main crack.
