13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- Figure 6. ECCI-picture showing internal cracks in the grain boundaries (GB) in Alloy 617, strain rate 10-5/s and temperature a) 650°C, b) 700°C, c) crack tip, 650°C and b) crack tip, 700°C, tested to fracture. It is the same load direction in all the pictures. 4. Discussion As known, DSA intensifies with increasing temperature and decreasing strain rate. However, in Alloy 617, DSA is present during the whole tensile deformation at elevated temperature when using a strain rate of 10-2/s, but seems to disappear when the strain reach medium-high levels at an elevated temperature when using strain rates of 10-3/s and 10-4/s. DSA also appear again just before fracture when testing at elevated temperature using a strain rate of 10-3/s. This behaviour could be described as a mismatch of the rates of solute atoms and moving dislocation, which could differ along the stress-strain curve and therefore serrations can appear just before fracture, where the local strain rate might be higher. At room temperature the strain rate influencing the elongation in AISI 316L, when the strain rate decreases the elongation increases, also the stress level is lower at low strain rate. This is the result of the competition between hardening due to deformation and recovery due to the slow strain rate, where the recovery have more influence at slow strain rates than at higher. The recovery has a lager influence at lower deformations rates due to the fact that the moving dislocation rate decrease which affect the hardening to decrease. At high temperature the recovery probably can be an effect of creep, due to increasing diffusion rate and low deformation rate [11]. No clear microstructural signs as higher density of voids have been seen but they may be vanished due to high plastic deformation when the specimen is deformed to fracture. However, another sign that can be related to creep have been discovered, cell or subcell structure probably formed by creep deformation, from the EBSDa) b) c) d) Cracks in GB Cracks in GB Cracks in GB Precipitates Precipitates Load direction 250 μm 250 μm 500 nm 1 μm
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