13th International Conference on Fracture June 16–21, 2013, Beijing, China particles exist in Region 1, whereas phase particles can be easily observed in Regions 2 and 3. Moreover, the number density of existing phase particles in Region 3 is obviously higher than that in Region 2. Therefore, it suggests that the existence of phase particles can not cause the fatigue crack initiation, but can act as the preferential route for crack propagation. Fig. 5 shows the optical microstructure of the areas just underneath the fracture surface of the sample S3. It reveals that only a few twins can be activated in the Region 1, whereas the activated twins can be easily observed in Regions 2 and 3. Additionally, density of the activated twins in Region 3 is remarkably higher than that in Region 2. In the same way, it indirectly demonstrates that the crack initiation is can not be caused by the activated twins under a stress amplitude far less than the yield stress. After crack initiation, the propagated crack will gradually increase the stress intensity factor near the crack tip, leading to the local plastic deformation and the activation of twins. Previous work indicated that the micro cracks mainly initiate along the intense slip bands in the grain interior [12], as shown in Fig. 6. Therefore, it firmly demonstrates that the localized slip bands activated at elastic stress amplitude is the main reason for the crack initiation. 105 106 107 108 109 70 80 90 100 110 120 130 140 150 Stress amplitude, MPa Cycles to failure, Nf/cycle Runouts 105 106 107 108 109 90 100 110 120 130 140 150 Stress amplitude, MPa Cycles to failure, Nf/cycle Runouts 105 106 107 108 109 80 90 100 110 120 130 140 150 160 Stress amplitude, MPa Cycles to failure, Nf/cycle Runouts 104 105 106 107 108 109 80 90 100 110 120 130 140 Runouts Stress amplitude, MPa Cycles to failure, Nf/cycle a) b) Fatigue strength: 90 MPa c) d) Fatigue strength: 105 MPa Fatigue strength: 95 MPa S1 S2 S3 S4 Fatigue strength: 100 MPa Fig. 2. S-N curves of the as-extruded Mg-Zn-Y-Zr alloys with Y content of: a) 0wt%, b) 1wt%, c) 2wt% and d) 3wt%. -3-
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