13th International Conference on Fracture June 16–21, 2013, Beijing, China 5.0x106 1.6x107 4.0 x107 1.0x108 2.9x108 1.0x1010 Fig. 4 Evolution of surface irreversible deformation under fatigue loading σa =174 MPa Detail observations of surface irreversible slip bands or PSB under Scanning Electron Microscope (SEM) are given in Fig. 5. PSBs are found in 5 grains, where other grains remain undeformed at all. As described in f.c.c material, PSB formed by fatigue is localized irreversible plastic deformation. PSB is significantly changing the roughness of the original surface. At extrusion area, PSB lies to the mass of matrix out of the surface. At observation site, there are two main orientations of PSB at different grains whose PSB orientations have angle close to 90°. Even in similar orientation, the PSB have clear end at grain boundary. It suggests that PSB is anchored at grain boundary. PSB could active slip system in neighbor grain by enforce the stress concentration at inter boundary, but not go across the grain boundary. In the EBSD observation, same slip band direction always in the similar oriented grain (see slip band in red grain and orange grain). When neighbored grains have big difference in crystalline orientation, direction of slip band in different grain intents to be perpendicular (see slip band in red grain and blue grain). Unlike straight PSBs find in the f.c.c. crystalline, irreversible slip bands in b.c.c. are formed less individual.
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