S. Hamada

Microstructural development of austenitic stainless steels irradiated in HFIR

Abstract Microstructural developments of neutron irradiated JPCA and USPCA, which are Ti-modified austenitic stainless steels and candidate structural material for fusion reactor first wall, have been examined. The irradiation has been performed in High Flux Isotope Reactor (HFIR) at temperatures ranging between 300 and 600°C to a peak neutron fluence corresponding to approximately 34 dpa and 2500 appm helium. Microstructure of PCAs after irradiation at temperatures of 400°C and below suggests that the mutual stability of the radiation enhanced MC precipitation, and fine bubbles associated with such precipitation, has contributed to the extension of the transient regime of swelling to fluences above 34 dpa. At higher irradiation temperatures of 500°C and above, however, the conversion of some of the helium bubbles to voids has occurred at 34 dpa irradiation. MC precipitation on the radiation-induced dislocation lines is reduced at 500°C and above. This reduces the effective sink strength and increase the number of sites for helium bubble formation, which may lead to a severe reduction in the incubation regime of swelling at these temperatures. The factors controlling the stability of the MC precipitates in PCAs is discussed.

Microstructural development of PCAs irradiated in HFIR at 300 to 400° C☆

Abstract Microstructural developments were determined on solution-annealed (SA) and cold-worked (CW) JPCA and U.S. PCAs irradiated in the High Flux Isotope Reactor (HFIR) at 300 and 400°C. Irradiation produced damage levels of about 10 and 34 dpa and helium concentrations of around 580 and 2500 appm respectively. High concentrations of fine bubbles and MC precipitates, as well as Frank faulted loops, were observed in both SA and CW PCAs. Mutual stability of the MC particles and associated fine bubbles contributed to the extension of the transient regime of swelling to higher fluence. The irradiation responses of JPCA and U.S.-PCA were similar in the HFIR, despite minor compositional differences (P, B) between the two materials. Useful fusion applications of SA-PCA as well as CW-PCA in the 300 to 400° C temperature range are suggested from these data.

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.

Temperature dependence of swelling in type 316 stainless steel irradiated in HFIR

The temperature dependence of swelling was investigated in solution-annealed (SA) and 20% cold-worked (CW) Type 316 stainless steel irradiated to 30 dpa at 300 to 600°C in the High Flux Isotope Reactor (HFIR). At irradiation temperatures ⩽ 400°C, a high concentration (2 to 4 × 1023 m−3) of small bubbles (1.5 to 4.5 nm diameter) formed uniformly in the matrix. Swelling was low ( < 0.2%) in both SA and CW materials irradiated to 30 dpa. In SA 316, cavity size increased but the number density decreased with increasing irradiation temperature above 500°C. At 500°C, there was a mixture of bubbles and voids, but at 600°C, most of the cavities were voids. Maximum swelling ( ~ 5%) occurred at 500°C. By contrast, cavities in 20% CW specimens were much smaller, with diameters of 6 and 9 nm at 500 and 600°C, respectively, suggesting that they were primarily bubbles. The cavity number density in CW 316 at both 500 and 600°C (~1 × 1022 m−3) was about one order of magnitude less than at 400°C. Swelling increased slightly as irradiation temperature increased, peaking at 600°C (0.3%). These results indicate that SA 316 swells more than CW 316 at 500 and 600°C, but both SA and CW 316 are resistant to void swelling in HFIR at 400°C and below to 30 dpa.

Post irradiation tensile and fatigue behavior of austenitic PCA stainless steels irradiated in HFIR

Abstract Mechanical properties were determined on solution annealed (SA) and cold worked (CW) JPCA (Ti-modified austenitic stainless steel) irradiated in the High Flux Isotope Reactor (HFIR) at temperatures ranging from 300 to 600°C. The irradiation produced damage levels from 16 to 56 dpa and helium concentration from 1020 to 4100 appm. The improved stability of MC precipitates which formed in the matrix during irradiation prevent loss of ductility at 500°C and below. Application of solution annealed JPCA is recommended for structural components of fusion reactors to be operated at 500°C and below.

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.

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.

Compositional behavior and stability of MC-type precipitates in JPCA austenitic stainless steel during HFIR irradiation

Abstract The behavior of Ti-rich MC-type precipitates in the Japanese prime-candidate-alloy (JPCA) during irradiation in HFIR to 58 dpa at 300°C is described. Fine MC precipitates formed after irradiation to 34 dpa. The number density of MC particles decreased, however, as irradiation continued to 68 dpa. Chemical composition of the radiation produced MC phase particles was strongly dependent on precipitate size. MC precipitates produced by thermal aging prior to irradiation also showed the same size-dependent composition after irradiation. The relation between the size-dependent composition and the stability of the MC phase particles is discussed.

Microstructural evolution in austenitic stainless steels irradiated to 57 dpa in HFIR

Abstract Austenitic stainless steels with different levels of titanium and carbon — JPCA, Type 316 and two heats of low-carbon steel — were irradiated in the solution-annealing (SA) and cold-worked (CW) conditions in HFIR at 500°C to doses of about 34 and 57 dpa. Their swelling behavior was investigated, with emphasis on the microstructural evolution. Swelling was low (