13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Figure 3. Cleavage fracture and elongated dimples from the impact tested samples after ageing at 700°C for 3000h, a) AISI 304 and b) Sanicro 28. 4. Discussion As shown previously, ageing at 650°C and 700°C will lead to precipitation of different intermetallic phases with different morphologies. Since these materials are hard, stress concentration can form around the particles during creep or other deformation process, which leads the formation of local cracking or fracture [15]. Four types of cracks can be identified as shown in Fig. 4. I. Cracks form at the particles within the grains. Due to stress concentration, this type of crack can induce cleavage fracture if the matrix is hard or brittle. II. Cracks form at the particles at the grain boundaries. This may cause intergranular cracking if the matrix is tough or cleavage fracture if the matrix is brittle. III. Cracks form along the needle shaped particle. This type of crack can create a high stress concentration, which can induce cleavage fracture if the matrix is not tough enough. IV. Cracks form outside of grain boundary due to the shift of the cracking particles. This can cause both intergranular cracking and cleavage cracking. Figure 4. Precipitation induced cracks, a) AISI 304 aged at 650°C for 3000 hours and b) Sanicro 28 aged at 700°C for 3000 hours. For the 304 material, needle shaped sigma phase can precipitate during ageing at 700°C for 3000h (Fig. 5a), which can form type III cracks during the impact toughness testing and lead to the formation of step type cleavage (Fig. 5b). This leads to low impact toughness. a) b) 16 μm 20 μm 10 μm 10 μm a) b)
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