13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Figure 4. Strain contouring mappings obtained from EBSD analysis (a). Alloy 690 with Δσ=135 MPa, N=5.45x108 cycles, (b). Alloy 690 with Δσ=140 MPa, N= N=2.77x108 cycles. Table 2. Maximum and average strain contouring from EBSD analysis of three materials MFS Alloy 690 Ti6Al4V Average 4,42 1,77 1,15 Maximum 36,46 9,35 3,76 MFS material, which has the highest strain localization (Table 2), has a hard martensitic phase and a soft ferritic phase. The soft phase could undertake cyclic plastic deformation at a stress even well below the bulk yield strength. This is because the applied stress can still be higher than the yield strength of the soft phase. The phase will thus undertake a cyclic plastic deformation, which can then cause a plasticity exhaustion and consequently formation of damage. SNDFCO is therefore formed for fatigue crack propagation. MAS material, which has a hard martensitic phase and a soft austenitic phase, has the similar situation. As known, strain localization can be related to accumulation of dislocations. 3.2 “Fine grains” in fatigue crack initiation area Figure 5 shows the ECCI pictures under the fatigue crack initiation area (quasi-cleavage area) in Fig. 3a. Two types of microstructures can be identified. One is the “no or less deformation” area where is near the sample surface and center. Another is the area where “smaller or fine grains” can be observed (Fig. 5a). Actually, the area has high plastic deformation (Fig. 5b). “fine grains” have formed in some areas, but large plastic deformation can still be seen in other areas. This indicates that the formation of “fine grains” is a cyclic plastic deformation process, or formation of dislocation subcells. The size of “fine grains” depends on the stress concentration in the area and number of cycles. This phenomenon can explain the formation of FGA in the VHCF material reported earlier [11]. High stress concentration around the inclusion after giga numbers of cyclic loading can cause local dislocation initiation and movement, which cause the formation of dislocation subcells, and consequently the formation of fine grains. (a) (b) Slip bands
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