13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- For the material containing sigma phase, both the crack initiation and propagation energies are low, which lead to a low impact toughness. Figure 5 Force versus time curves from the instrumental impact toughness testing for SAF 2507 material with different conditions The different cleavage behavior from different precipitates depends on the nature of precipitates and fracture mechanisms. Sigma phase has a P42/mnm structure and is brittle. They precipitate mainly along grain or phase boundaries. These particles become easily cracking during plastic deformation. The cracking usually occurs usually through the particle (Fig. 6a). These sharp microcracks become then stress raisers or Griffith cracks that provide a local stress concentration that initiate cleavage fracture in the ferritic phase (Fig. 6b). High amounts of sigma phase will increase number of microcracks or Griffith cracks formed during plastic deformation, the hardness of the ferritic phase is also increased. This leads to an easier cleavage fracture with smaller cleavage plans (Fig. 6c). Figure 6 Influence of sigma phase on cleavage in duplex stainless steel at RT. (a). Cracking of sigma phase, (b). Cleavage fracture in the material with about 1% sigma phase, (c). Cleavage fracture in the material with about 10% sigma phase. (a) (b) (c) σ
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