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Abstract: Pre- and post-deformation microstructures and tensile properties of the newly-developed oxide dispersion strengthened (ODS) ferritic steels, K1 (Fe–19Cr–0.3W–0.3Ti–0.3Y2O3) and K4 (Fe–19Cr–4Al–2W–0.3Ti–0.3Y2O3) for nuclear fusion reactors and Generation IV nuclear fission systems were investigated. The nano-oxides of the K1 were cubic pyrochlore Y2Ti2O7, while those of K4 were mainly perovskite AlYO3, which was confirmed by transmission electron microscopy and X-ray diffraction (XRD) analyses. K1 was found to have much higher strength than K4 and the difference in the nano-oxides of K1 and K4 can account for this difference. Most of the extracted residues in the K1 and K4 identified by XRD consisted of M23C6 carbides. [Copyright &y& Elsevier]

Abstract: In the present work, microstructures of dispersoids in two Al-added high-Cr ODS ferritic steels (including the commercial MA956, and another ODS steel with a similar composition recently developed in Kyoto University) were studied. The study with transmission electron microscopy (TEM) of both the thin-foil specimens and the extraction replica specimens showed that the nano-sized oxides in the two ODS steels consist mainly of a complex Y–Al–O compound (YAlO3) in a perovskite structure (i.e. YAP), while other oxide particles with different chemical composition or crystalline structures are in significantly lower number density. The comparison between the two kinds of ODS steels shows that the size distribution and the number density of the oxide particles significantly depends on the conditions of fabrication. Mechanisms underlying the processes were discussed. [Copyright &y& Elsevier]

Abstract: Bent specimens of A533B steel (0.16wt% Cu) were irradiated at 290°C to 1dpa with 6.4MeV Fe3+ ions. Calculated tensile stresses at the irradiated surface were set to 0, 250, 500 and 750MPa. The specimens were subjected to hardness measurements, transmission electron microscopy (TEM) observations and three-dimensional atom probe (3DAP) analysis. The radiation-induced hardening decreased with increasing stress to 500MPa which was near the yield strength. TEM and 3DAP results showed that well-defined dislocation loops and solute clusters were formed. The diameter of dislocation loops increased and the number density decreased when the stress was applied, whereas the diameter and number density of solute clusters decreased. The hardening was mainly attributed to solute cluster formation. Application of tensile stress would control hardening by suppressing the solute cluster nucleation and growth. [ABSTRACT FROM AUTHOR]

Abstract: A dual-ion irradiation method is one of the most popular techniques for the research of microstructural evolution of materials under fusion environmental conditions. The application of oxide dispersion strengthened (ODS) ferritic steels is anticipated for realization of high temperature operation of fusion reactors above 600°C. In this research, the investigation of the microstructural evolution of high corrosion resistant ODS steels was performed using the ion irradiation methods. Dual ion-irradiation experiments were performed at the DuET facility, Kyoto University using 6.4MeV Fe self-ion and 1MeV helium ion. The irradiation temperatures were 300 and 500°C. The dose, dose rate, and helium over dpa ratio were 20dpa, 5×10−4 dpa/s, and 15appm/dpa, respectively. The post-irradiation experiments were performed using SEM, TEM and the nano-indentation methods. Significant microstructural evolution of grains and oxides were not observed after the single-ion irradiation at 20dpa, 500°C. Fine helium cavities were observed in the dual-ion irradiated 19Cr–2W–4Al ODS steel. Fine cavities of approximately several nanometers diameter were densely and uniformly formed in dual-ion irradiated range. [Copyright &y& Elsevier]

The effects of neutron irradiation on the microstructural features and mechanical properties of 309L stainless steel RPV clad were investigated using TEM, SEM, small tensile, microhardness and small punch (SP) tests. The neutron irradiations were performed at 290 °C up to the fluences of 5.1 × 1018 and 1.02 × 1019 n/cm2 (>1 MeV) in Japan Materials Testing Reactor (JMTR). The microstructure of the clad before and after irradiation was composed of main part of fcc austenite, a few percent of bcc δ-ferrite and small amount of brittle σ phase. After irradiation, not only the yield stress and microhardness, but SP ductile to brittle transition temperature (SP-DBTT) were increased. However, the increase in SP-DBTT is almost saturated, independent of the neutron fluence. Based on the TEM observation, the origin of irradiation hardening was accounted for by the irradiation-produced defect clusters of invisible fine size (<1–2 nm), and the shift of SP-DBTT was primary due to the higher hardening and the preferential failure of δ-ferrite. The embrittlement of the clad was strongly affected by the initial microstructural factors, such as the amount of brittle σ phase, which caused a cracking even in an early stage of deformation. [Copyright &y& Elsevier]

Abstract: The effects of He on the fracture behavior of reduced-activation ferritic/martensitic steels, including oxide dispersion-strengthened (ODS) steels and F82H, was determined by characterizing the microstructural evolution in and fracture behavior of these steels after He implantation up to 1000appm at around 550°C. He implantation was carried out by a cyclotron with a beam of 50MeV α-particles. In the case of F82H, the ductile-to-brittle transition temperature (DBTT) increase induced by He implantation was about 70°C and the grain boundary fracture surface was only observed in the He-implanted area of all the ruptured specimens in brittle manner. By contrast, no DBTT shift or fracture mode change was observed in He-implanted 9Cr-ODS and 14Cr-ODS steels. Microstructural characterization suggested that the difference in the bubble formation behavior of F82H and ODS steels might be attributed to the grain boundary rupture of He-implanted F82H. [Copyright &y& Elsevier]