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Surface steps and deformation microstructure in cold-worked SUS316 stainless steels irradiated to 4 and 35 dpa (displacements per atom) were examined after being deformed by uniaxial tensile stress at 320 C at a slow or fast strain rate. Dislocation channeling was the predominant mode of deformation near the surface at the slow strain rate. Twinning was dominant at the fast strain rate whereas twinning and nanotwin formation occurred in the locally stressed area at the slow strain rate. Deformation heterogeneity measured using the spacing of coarse surface steps induced by dislocation channels increased with increasing dose from 4 to 35 dpa. Grain boundary separation occurred when dislocation pileups and high normal stress on the grain boundary plane coexisted, which likely was a precursor of intergranular cracking without any environmental factor.

The evolution of microstructures and microchemistry was examined by transmission electron microscopy in coldworked SUS 316 stainless steel components irradiated in a pressurized water reactor to 1-73 dpa at 565-596 K. Homogenous nucleation of dislocation loops, helium bubbles and 0 precipitates was detected. The dislocation loops consisted of a high density of Frank loops and black dots. The black dots are considered to be small Frank loops, some fraction of which could be vacancy-type. The size distribution showed a double peak, which remained unchanged up to 73 dpa, suggesting that the dislocation structure was saturated under a balance of nucleation and disappearance. The helium bubbles were extremely dense and fine, resulting in swelling of less than 0.1%. Such bubble structure was formed under irradiation with a high He/dpa ratio. The 0 precipitation was detected at doses higher than 4 dpa with an increasing density for higher doses. The measured radiation hardening was almost explained by these visible microstructural features. Radiation-induced segregation at grain boundaries was confirmed to continuously develop up to 73 dpa whereas no significant segregation could be detected at Frank loops.

The evolution of microstructures and microchemistry was examined by transmission electron microscopy in coldworked SUS 316 stainless steel components irradiated in a pressurized water reactor to 1-73 dpa at 565-596 K. Homogenous nucleation of dislocation loops, helium bubbles and 0 precipitates was detected. The dislocation loops consisted of a high density of Frank loops and black dots. The black dots are considered to be small Frank loops, some fraction of which could be vacancy-type. The size distribution showed a double peak, which remained unchanged up to 73 dpa, suggesting that the dislocation structure was saturated under a balance of nucleation and disappearance. The helium bubbles were extremely dense and fine, resulting in swelling of less than 0.1%. Such bubble structure was formed under irradiation with a high He/dpa ratio. The 0 precipitation was detected at doses higher than 4 dpa with an increasing density for higher doses. The measured radiation hardening was almost explained by these visible microstructural features. Radiation-induced segregation at grain boundaries was confirmed to continuously develop up to 73 dpa whereas no significant segregation could be detected at Frank loops.

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