13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- analysis it is shown that the areas that look like recrystallization only have up to 5° misorientation which gives evidence for cell or subcell structures formed by creep deformation. According to the ECCI investigation there is areas and/or bands that display such cell or subcell structure, see fig.4c) and d). Further investigations are needed to confirm whether the cell or subcell structure is formed due to creep deformation in the AISI 316L material. Elevated temperature and low strain rate decrease the ductility due to grain boundary embrittlement referred as sensitization and in some cases DSA. Sensitization is when chromium diffuses to the grain boundaries creating Cr-rich precipitates [16-19]. For sensitization the ageing time must be long enough for the precipitation to take place. According to literature sensitization occurs in AISI 316 material after less than 1h up to 100h depending on alloy content at 650°C and 700°C [17, 19]. After SSRT with a strain rate of 10-4/s there are precipitates in the grain boundaries but they are too small to produce large scale embrittlement resulting in less internal cracks and larger elongation to fracture. For higher temperatures and long ageing times as in the case of SSRT tests with a strain rate of 10-6/s and 700°C (fig.5 d)), desensitization probably can occur which means that chromium diffuses back into the interfacial region where the precipitate have grown [16, 19]. The desensitization seems to increase or keep the embrittlement of the material constant, see fig.2. In fig.2 b) the tensile stress increases which could refer to a hardening coupled to DSA that explain the large decrease in ductility. As known, precipitates in the matrix play two important roles for the life of the material, the first is to interact with moving dislocation and increase the strength or hardness of the material. The second is to cause stress concentration at the precipitate during dislocation accumulation that will in turn cause initiation of cracking by either precipitate cracking self or in the matrix. 5. Conclusion After slow strain rate tensile testing at elevated temperatures and subsequent investigation of the microstructure using ECCI the following conclusions has been reached: • Damage appears at interactions between moving dislocations and precipitates due to local stress concentration. • DSA is present at all tested strain rates, but disappear at certain strain values for some deformation conditions. This is due to mismatches in the rates of moving dislocations and solute atoms at different strain intervals during the tensile deformation. • Grain boundary embrittlement is causing damage as internal cracks and lowers the elongation to fracture. Therefore it is strongly affecting the material fracture behaviour at elevated temperature and is affecting for the high-temperature performances during deformation of the tested materials. • AISI 316L show behaviour similar to creep at low strain rates (10-6/s) and high temperatures. Acknowledgements Present study was financially supported by AB Sandvik Material Technology in Sweden and the Swedish National Energy Administration through the Research Consortium of Materials Technology for Thermal Energy Processes, Grant No. KME-501. Agora Materiae and Strategic Faculty Grant AFM at Linköping University are also acknowledged.
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