13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- a b Figure 4. SEM-ECCI micrograph showing a) the crack tip region and b the damage zone in front of the crack tip at higher magnification with stringers of pores. a b Figure 5. SEM-ECCI picture of the damaged zone in IN718 at 550°C temperature close to the crack showing a) the hardening precipitates along with nano sized pores in an area with high plastic deformation and b) pores in a grain boundary with no plastic deformation. events caused by unloading between each hold time are difficult to distinguish along the crack front. The crack front seems to have the character of a spherical discontinuous process zone (fig.2) with branched intergranular cracks rather than a single crack moving through the microstructure. Crack growth could then consist of environmental oxygen induced intercrystalline growth of cracks along sensitized paths in different directions and an interlinking between embrittled areas showing ligaments with more severe slip. The ligaments are severely plastically deformed to a level that is very close to dynamic recrystallization with characteristic pores shown as black dots in Fig. 4b, Fig. 5 indicating a future crack path. The growth of the main crack is also influenced by interaction with δ-plates acting either as a crack path or as a crack stopper (fig 4a) depending on orientation. The fact that the δ-phase is ductile makes it less probable as a crack path but the interface between δ and matrix is often acting as a crack [10] path through severely oxidized pores in the interface as observed in this work. Non propagating branched cracks are stopped due to blunting and/or geometrical reasons like going into mode II. Residual stress built up during growth manifested by a closure of the crack along the front also plays a role for the path taken by the crack. 2.5μm 250nm 0.5μm 250nm
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