13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- Figure 5. ECCI pictures show the microstructure under the fatigue crack initiation area in Fig. 3a, (a). Overview, (b). Enlarged (a). In this “fine grain” area, high plastic deformation can be found (Fig. 6a). They are mainly appear near gran boundaries and twin boundaries. This may be attributed to dislocation pile-up as reported earlier [13]. Dislocation slip bands or eventually persistent slip bands (PSB) can be observed (Fig. 6b). They interact at grain boundaries and cause the formation of damage or micro crack due to impingement cracking. This observation shows that the fatigue crack initiation in the very high cycle fatigue regime can still compare with that in the low cycle fatigue regime [14]. Figure 6 ECCI pictures show the microstructure under the fatigue crack initiation area in Fig. 3a, (a). Plastic deformation in the “fine grain, (b). Slip bands or PSB, interaction between slip band and grain boundaries. As known, grain boundary or twin boundary act always as barriers to stop the movement of dislocations carried by slip bands, which leads to the formation of dislocation piling-up. Figures 7 shows that the twin boundary has blocked the movement of dislocations carried by slip bands, which cause the formation of damage at twin boundary due to the stress concentration by the piling-up of dislocations. “no deformation” ”fine grain” (a) (b) annealing twin slip band/PSB slip band Slip band damage slip band Grain boundary
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