13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- 4. Fracture surface observation 4.1 Characters of fracture surface Fig. 4 shows the diminishing of the inter/trans granular zone in the fracture surface with the increase of the tempering temperature. The rough surface zone of region A outside of the fisheye (Fig. 4d) is considered as inter/trans granular zone by Zhao et al. [7]. Region A is clearly seen in the fracture surface of RL150 (Fig. 4a), while this zone diminishes gradually with the increase of the tempering temperature as Fig. 4b and Fig. 4c show. Under cyclic loading, materials with high brittleness is lack of sufficient number of independent slip systems to accommodate plastic deformation between contiguous grain leading to grain boundaries crack. Elevating of the tempering temperature can reduce its brittleness resulting the diminishing of inter granular fracture surface. This phenomenon also verifies the change of the microstructure with different heat treatments. Fig. 4. Diminishing of the inter/trans granular zone with the increase of the tempering temperature: (a) RL150, (b) RL180, (c) RL300, and (d) Schematic drawing of the fracture surface (A denotes the inter/trans granular zone, and B denotes the fast growth zone). 4.2 Fatigue crack initiation Fig. 5 shows the typical morphology of the fatigue crack origin for surface and interior initiation. For RL150, fatigue crack tends to initiate from the grains boundaries for VHCF as shown in Fig. 5a and Fig. 5b. Sometimes FGA was observed when the VHCF crack initiated from interior grains boundaries (Fig. 5b), and the proportion of grain boundary initiation is up to 88%. For RL180 and RL300, almost all VHCF failures originate from interior inclusions with a fisheye pattern as Fig. 5c shows. Fig. 5d shows an example of non-propagating cracks for surface inclusion. The size and the stress intensity factor (SIF) range are 29 μm and 3.65 MPa⋅m1/2 for the subsurface inclusion, while they are 11 μm and 3.28 MPa⋅m1/2 for the smaller one at the specimen surface. SIFs for both the two inclusion fall below the threshold value (5.0 MPa⋅m1/2) for this heat treatment, and fatigue crack originates from the subsurface inclusion with higher SIF which indicates that SIF is an (b) (c) (d) (a)
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