2.2.3 Microstructure observation methods
After fatigue tests, the microstructure of the material was characterized by a scanning electron microscope (SEM, JEOL 7800F). The detection mode was SEI (secondary electronic imaging) mode, and the acceleration voltage is set to 15 kV, respectively. The SEM observation area was concentrated on the fracture surface and specimen surface. Especially, we carried out higher magnifications for the regions near the crack initiation sites, which is focused on the crack initiation behavior of the material. On the other hand, the EBSD (electron backscatter diffraction) technique was applied to determine the grain orientation of the material. The area detected is the cross-section of the material, and the diffraction direction is perpendicular to the cross-section.