Yukio Miwa

Embrittlement of reduced-activation ferritic/martensitic steels irradiated in HFIR at 300°C and 400°C

Abstract Miniature tensile and Charpy specimens of four ferritic/martensitic steels were irradiated at 300°C and 400°C in the high flux isotope reactor (HFIR) to a maximum dose of ≈12 dpa. The steels were standard F82H (F82H-Std), a modified F82H (F82H-Mod), ORNL 9Cr–2WVTa, and 9Cr–2WVTa–2Ni, the 9Cr–2WVTa containing 2% Ni to produce helium by (n,α) reactions with thermal neutrons. More helium was produced in the F82H-Std than the F82H-Mod because of the presence of boron. Irradiation embrittlement in the form of an increase in the ductile–brittle transition temperature (ΔDBTT) and a decrease in the upper-shelf energy (USE) occurred for all the steels. The two F82H steels had similar ΔDBTTs after irradiation at 300°C, but after irradiation at 400°C, the ΔDBTT for F82H-Std was less than for F82H-Mod. Under these irradiation conditions, little effect of the extra helium in the F82H-Std could be discerned. Less embrittlement was observed for 9Cr–2WVTa steel irradiated at 400°C than for the two F82H steels. The 9Cr–2WVTa–2Ni steel with ≈115 appm He had a larger ΔDBTT than the 9Cr–2WVTa with ≈5 appm He, indicating a possible helium effect.

Synergistic effects of hydrogen and helium on microstructural evolution in vanadium alloys by triple ion beam irradiation

Abstract In fusion materials, irradiation of 14 MeV neutrons produces He and H atoms at a high generation rate. The objective of this study is to clarify synergistic effects of He and H on microstructural evolution in pure vanadium and two vanadium alloys including candidate ternary alloy V–5Cr–5Ti. The specimens are irradiated with 12 MeV Ni 3+ ions at 873 K with simultaneous implantation of 1 MeV He + and 350 keV H + ions. Helium and hydrogen implantation ratios are independently controlled at two different He/dpa and H/dpa rates. Dual beam irradiation with heavy ions and He ions or H ions, and single beam irradiation experiments are also performed. Triple beam irradiation strongly enhances growth of cavities and swelling in pure vanadium compared with those under dual beam irradiation with He and single beam irradiation, whereas simultaneous implantation of H without He does not affect cavity growth and swelling. In V–5Cr–5Ti alloy, no cavities are detected without implantation of He. However, Ni, He and H triple beam irradiation is found to enhance swelling. These results are discussed in terms of dislocation and cavity evolution in the irradiated vanadium alloys.

Microstructural study of irradiated isotopically tailored F82H steel

Abstract The synergistic effect of displacement damage and hydrogen or helium atoms on microstructures in F82H steel irradiated at 250–400 °C to 2.8–51 dpa in HFIR has been examined using isotopes of 54 Fe or 10 B . Hydrogen atoms increased slightly the formation of dislocation loops and changed the Burgers vector for some parts of dislocation loops, and they also affected on the formation of cavity at 250 °C to 2.8 dpa. Helium atoms also influenced them at around 300 °C, and the effect of helium atoms was enhanced at 400 °C. Furthermore, the relations between microstructures and radiation-hardening or ductile to brittle transition temperature (DBTT) shift in F82H steel were discussed. The cause of the shift increase of DBTT is thought to be due to the hardening of dislocation loops and the formation of α′-precipitates on dislocation loops.

Development of a triple beam irradiation facility

Abstract A triple beam ion irradiation facility has been developed to study the synergistic effects of displacement damage, helium and hydrogen atoms on microstructural changes of materials under irradiation environments simulating a fusion reactor. The system consists of a vacuum chamber and three beamlines, which are connected with each electrostatic accelerator. Samples can be irradiated in the wide temperature range from liquid nitrogen to 1273 K in the chamber by replacing two kinds of sample stages alternatively. An austenitic stainless steel was simultaneously irradiated with triple beam of nickel, helium and hydrogen ions at 573–673 K using this facility and TEM observations were carried out from a cross sectional view normal to the incident surface. It was shown that the number density of dislocation loops decreased in the region where hydrogen and helium were deposited in comparison with ones in the region where only displacement damage was induced to a similar damage level.

Effect of carbon and nitrogen on grain boundary segregation in irradiated stainless steels

SUS304 stainless steels with carbon contents of 0.052%, 0.019% and 0.004% and SUS316L stainless steels with nitrogen contents of 0.095%, 0.032% and 0.003% were irradiated with 12 MeV Ni ions at 573 K to a dose of 1 dpa at 1 μm depth. Microstructure and grain boundary chemical composition were investigated using a transmission electron microscope with a field-emission-gun (FE-TEM) at the probe size of 0.5 nm. The number density of dislocation loop was higher as the carbon content was higher and was almost independent of nitrogen content. With increasing carbon and nitrogen content, the degree of Cr depletion and Si/Ni segregation was decreased. Both carbon and nitrogen suppressed the Cr depletion and Si/Ni segregation. The suppression effect of carbon was larger than that of nitrogen.

Swelling of F82H irradiated at 673 K up to 51 dpa in HFIR

Abstract Reduced-activation ferritic/martensitic steel, F82H (8Cr–2W–0.2V–0.04Ta–0.1C), and variants doped with isotopically tailored boron were irradiated at 673 K up to 51 dpa in the high flux isotope reactor (HFIR). The concentrations of 10B in these alloys were 4, 62, and 325 appm during HFIR irradiation which resulted in the production of 4, 62 and 325 appm He, respectively. After irradiation, transmission electron microscopy (TEM) was carried out. The number density of cavities increased and the average diameter of cavities decreased with increasing amounts of 10B. The number density decreased and the average diameter increased with increasing displacement damage. Swelling increased as a function of displacement damage and He concentration.

Development of a miniaturized hour-glass shaped fatigue specimen

Abstract Diametral strain-controlled push–pull fatigue tests with zero mean strain were carried out with miniaturized hour-glass shaped specimens of an austenitic stainless steel in solution annealed condition at room temperature. The specimens had a diameter of 1.25 mm at the minimum cross section and a total length of 25.4 mm. The number of cycles to failure (Nf) was equal to or slightly greater than that obtained with standard size specimens. Nf was also revealed to be rather insensitive to the specimen load axis offset, indicating that the requirement of the specimen alignment to the load axis was not very severe for the miniaturized specimen.

Effect of minor elements on irradiation assisted stress corrosion cracking of model austenitic stainless steels

Abstract A low impurity Fe 18Cr 12Ni (HP) and its heats doped with Si and C (HP + Si and HP + C) were irradiated to 6.7 × 10 24 n/m 2 ( E ≫ 1 MeV) at 513 K. The slow strain rate tensile (SSRT) tests were carried out at a constant strain rate of 1.7 × 10 −7 s −1 in high purity, 573 K water. Scanning electron microscopy on the fracture surface revealed that HP and HP + Si failed mainly by the intergranular stress corrosion cracking (SCC), while the major failure mode in HP + C was the transgranular SCC. All alloys exhibited radiation hardening. HP + Si exhibiting the smallest hardening showed uniform elongation of 17%, while HP and HP + C did not. Transmission electron microscopy was also carried out. Frank loops and unidentified small clusters were formed in HP and HP + C, while only small clusters were observed in HP + Si.

Tensile behavior of F82H with and without spectral tailoring

Abstract The effects of neutron spectrum on tensile properties of the low-activation martensitic steel F82H (8Cr–2WVTa) was examined using a thermal neutron shield to tailor the neutron spectrum for steels irradiated in the high flux isotope reactor (HFIR). The yield stresses of spectrally tailored specimens irradiated in HFIR to 5 dpa at 300°C and 500°C are on trend lines obtained from unshielded irradiation in HFIR. No significant effect of the neutron spectrum on tensile properties could be detected.

Characterization of 316L(N)-IG SS joint produced by hot isostatic pressing technique

Abstract Type 316L(N) stainless steel of the international thermonuclear experimental reactor grade (316L(N)-IG SS) is being considered for the first wall/blanket module. Hot isostatic pressing (HIP) technique is expected for the fabrication of the module. To evaluate the integrity and susceptibility to stress corrosion cracking (SCC) of HIPed 316L(N)-IG SS, tensile tests in vacuum and slow strain rate tests in high temperature water were performed. Specimen with the HIPed joint had similar tensile properties to specimens of 316L(N)-IG SS, and did not show susceptibility to SCC in oxygenated water at 423 K. Thermally sensitized specimen was low susceptible to SCC even in the creviced condition. It is concluded that the tensile properties of HIPed SS are as high as those of the base alloy and the HIP process caused no deleterious effects.