13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- As shown in Fig. 5, the presence of small amount of Al nitrides has a small effect on the impact toughness comparing with sigma phase. However, at low temperature, Al nitride can also initiate cleavage fracture as shown in Fig. 7a. This does not happen for an Al oxide that is round and has a larger size (Fig. 7b). The results show that needle type of Al nitride can create a stress concentration that is higher than the critical stress for cleavage fracture in the ferritic phase. The morphology of the second particle can also play an important role to cleavage fracture. Figure 7 Influence of precipitate and defect on cleavage in duplex stainless steel. (a). Al nitride, (b). Al oxide at -130°C. Recently, hydrogen induced stress cracking (HISC) in duplex stainless steel is concerned [22]. Diffusion of hydrogen into the material can lead to an increase in hardness, and on the other hand hydrogen can accumulate at phase and grain boundaries, which causes the formation of hydrogen pore or voids [22], and consequently the formation of stress concentration. When the hydrostatic pressure in the void reaches a critical value or a critical shear stress, the void can develop into a small crack or Griffith crack, and eventually cleavage initiation (Fig. 8a). Cleavage fracture propagates discontinuously due to the austenitic phase that acts as a hinder for crack propagation, which creates an unusual top-valley fracture (Fig. 8b). The cracks in the ferritic phase are mainly classical brittle cleavage. The herringbone pattern of fracture with the cleavage plane of {100} and two growth directions from <110> is the most common one (Fig. 8c). The situation here is similar to that due to the spinodal decomposition as discussed above. (a) (b)
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