path and the crack growth seems to be assisted by recrystallisation sometimes with twins or formation of voids and cracking oxide at the crack tip. Although the crack growth is considered intercrystalline it can be divided into three different growth modes. a Figure 2. SEM-ECCI micrographs showing cross sections of specimens with intergranular branched cracks and clear signs of slip and b) 650°C with intergranular cracking and le pronounced plasticity. The first one is the growth along grain boundaries where the slip on one or two slip systems homogeneous slip and a narrow zone structure and sometimes twins and recrystallisation structure nicely shown in ECCI mode in the SEM Figure 3. a) Figure 3. SEM-ECCI micrographs showing a) the plastically damaged zone with recrystallisation close to the crack surface and b) deformation twins. The third growth mode is cracking along interface between a δ-plate and the crack. of these events but the nanometric oxidized pores present at crack tips with a smallest size in the order of the hardening precipitates could trig state and dislocation structure contributing factors 13th International Conference on Fracture June 16 -3- path and the crack growth seems to be assisted by both slip, plasticity induced dynamic sometimes with twins or formation of voids and cracking oxide at the crack tip. lthough the crack growth is considered intercrystalline it can be divided into three different b ECCI micrographs showing cross sections of specimens with hold time The first one is the growth along grain boundaries where the mating grains are subjected to planar Figure 1a. The second growth mode is along grains with heavier homogeneous slip and a narrow zone close to the fracture surface with heavy non b) showing a) the plastically damaged zone with sub grains δ-phase often oxidized into a sponge like structure i plate and the crack. It is thus difficult to say anything about the order of the hardening precipitates could trigger the onset of intercrystalline failure dislocation structure contributing factors Figure 4, 5, 6. The separate 25μm 5μm June 16–21, 2013, Beijing, China both slip, plasticity induced dynamic hold time at a) 550°C with intergranular branched cracks and clear signs of slip and b) 650°C with intergranular cracking and less grains are subjected to planar . The second growth mode is along grains with heavier close to the fracture surface with heavy non planar dislocation sub grains and beginning often oxidized into a sponge like structure in the It is thus difficult to say anything about the time-sequence ger the onset of intercrystalline failure and the stress separate dwell time cracking 20μm 0.5μm
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