Dimensional Changes and Microstructure of In-pile Crept Ferritic-Martensitic Steel

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Pressurized tube specimens of Japanese Ferritic-Martensitic Steel (JFMS) were irradiated in Fast Flux Test Facility (FFTF) Materials Open Test Assembly (MOTA) to 37.5 dpa at 680 and 793 K. Diametral creep strain following irradiation at 680 K was 0.11% even at zero hoop stress and slightly increased with increasing the hoop stress. The creep strain following irradiation at 793 K increased from zero to 0.23% with the increase in hoop stress from zero to 86 MPa. As for the void swelling, nothing was recognized. The martensitic phase in JFMS was stable after the irradiation at 680 K, but this was not the case at 793 K where a considerable recovery of dislocation structures was found. Following irradiation at 680 K, a high density of fine spherical G phase precipitates which were accompanied by the strain fields around them were observed, while large Mo-rich Laves phase particles were observed following irradiation at 793 K. Analysis of dislocation Burgers vectors revealed that dislocations having Burgers vector of type a⟨100⟩ were observed only after the irradiation at 680 K and that a large anisotropy was indicated in a⟨100⟩ type of Burgers vector populations for a stressed specimen, but no significant anisotropy was observed in the Burgers vectors of a specimen irradiated without hoop stress. The average diameter of small precipitates in the specimen irradiated at 680 K under stress was significantly larger than that without stress. Finally, the observed dimensional changes were attributed to both the precipitation-induced volume expansion and the Stress-Induced Preferential Absorption (SIPA) irradiation creep for 680 K irradiation and Climb-Controlled Glide (CCG) irradiation creep at 793 K.

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