13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- a b Figure 6. SEM-ECCI micrographs of a) IN718 650°C during hold and b) ALVAC718plus 700°C during hold. Both alloys show voids in front of the crack tip. The smallest voids are in the same range of size as the hardening precipitates that are clearly visible in a). The macroscopic crack path often shows a mismatch orthogonally indicating shear or grainboundary sliding along the crack growth direction. SEM ECCI pictures taken at a very high magnification of non propagating secondary cracks is shown in Fig. 6 where nanometric voids in the same size range as the clearly visible hardening precipitates Fig 6a are seen along the grain boundary. This type of voids is often seen along the crack front for longer cracks being coarser and more oxidized. The dwell time cracking of Alvac718Plus in pure oxygen atmosphere shows (Fig. 7b) more clearly the intercrystalline character of the crack growth due to the absence of δ-plates (black in Fig. 7a) in this condition. The observable environmental action manifested by growth of oxidized pores in the grain boundaries seems to be the same as for IN718. The completely flat grain boundary surface has still slip bands and microscopic pores. The fatigue cycling in oxygen atmosphere at 700°C before the start of the dwell time cycle show clear striation like arrest markings indicating a continuous crack growth mode. Studying the fracture surfaces in higher magnification show signs of nano sized voids at the tip of branched cracks. The intergranular facets giving a smooth appearance at lower mag. (Fig 7b) show signs of voids at higher magnification (Figure 8, 9). a b Figure 7. SEM fractographs showing a) boundary between pre cracking at room temperature (lower) and dwell time growth at 550°C (upper), black contrast is δ-phase b) boundary between dwell cracking (lower) and final fracture (upper)in Allvac718plus at 700°C in pure oxygen atmosphere. 100nm 150nm 25μm 150μm
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