Crystal Misorientation Plasticaly Formed around Fatigue Crack-Tips in CT Specimens from Pearlitie Rail Steel

The influence of the stress intensity factor on the microstructure near fatigue crack tips in rail steels was investigated by the orientation analysis using high-resolution EBSP. Large changes in crystal orientation and formation of cell structure were observed around the crack, and a plastic deformation zone was confirmed to be formed by the stress concentration at the crack tip. The size of the maximum plastic zone perpendicular to the crack was quantitatively determined based on the Kernel Average Misorientation, which is the average misorientation between all neighboring pairs of measurement points in the grain, to be 6.9, 8.2 and >50μm for Kmax = 9.55, 10.88 and 15.84 MPa·m1/2, respectively. For ΔKeff = 10.39 MPa·m1/2, a strongly plastic deformation zone of size in 1.3μm formed around the crack, and maximum plastic deformation zone was found outside the strongly plastic deformation zone. Although large misorientation (Δθ - 37.5°) was observed between the strongly plastic deformation zone and the maximum plastic deformation zone, the fluctuation of the crystal orientation inside the strongly plastic deformation zone was comparatively small, and the dislocation density was considered to have been small as well. On the other hand, cell structure was observed in the maximum plastic zone. A large difference in the development of cell structure was observed between the grain having a crack and neighbor grain, indicating that the size of the plastic deformation zone depends strongly on the grain size.