13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Figure 5. a) Precipitates in the aged 304 material at 700°C for 3000h and b) fracture after the impact toughness testing. At 650°C, grain boundary carbides are the main precipitates (fig. 6a) and type II cracks can form during the impact toughness testing, which can lead to the formation of intergranular cracking (fig. 6b). Relatively low impact toughness can then be obtained. Figure 6. a) Precipitates in the aged Sanicro 28 material at 650°C for 3000h and b) fracture after the impact toughness testing. Alloy 617 is a nickel based alloy and Ni3(Al,Ti) type of γ’ is the main precipitate in the grains (Fig. 7a), but grain boundary carbides can also be observed [7]. In this case type II cracks can be observed and partial intergranular cracking can occur (Fig. 7b). Since the carbide density at the grain boundaries is relatively low, the material can still show good impact toughness. a) b) 12 μm 10 μm 1 μm 10 μm a) b)
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