Effect of dose rate and precipitates on irradiation damage structures

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Electron irradiations were performed in three different dose rates such as 2x10^<-3>,2.9x10^<-4> and 1.7x10^<-4> dpa/sec by using High Voltage Electron Microscopy (HVEM) to investigate the effect of dose rate and helium atoms on damage structures in SUS304 stainless steels.Specimens were prepared from commercial SUS304 stainless steel, which composition was Fe-18.2Cr-8.2Ni-1.3Mn. They were heat treated at 1323K (solid solution treatment) for thirty minutes and at 893K (sensitization treatment) for twenty four hours. Helium atoms were implanted by accelerator (accelerating voltage was 70 keV) at room temperature before electron irradiations. Electron irradiations were carried out in HVEM at 573 and 673K up to 7 dpa.Voids were recognized except for the solution-treated specimen in high dose rate at 573K.The void number density increased and the void mean size decreased as the damage rate increased. The void number density and the void mean size increased with increasing temperature. The grain boundary segregations, such as nickel enrichment and chromium depletion, were recognized in the specimens at 673K irradiation. The amount of segregations increased as the damage rate decreased. The void number density of solution-treated specimens were higher than that of sensitized specimens. The fine precipitates in the matrix of sensitized specimens might be suppressed void nucleation.In the case of helium implanted specimens, no cavities were observed before irradiations. During irradiations, helium bubbles ranging under 5 nm were formed in the high density. The cavity number density decreased and the mean size increased as the dose rate decreased. The number density and the mean size were lower than those of no helium specimens.Fe-Cr-Mn alloys were irradiated to investigate the effect of precipitates on damage structures. There were great differences of irradiated structures among the matrices with different precipitates.