Kentaro Asakura

R&D of low activation ferritic steels for fusion in japanese universities

Abstract Following the brief review of the R&D of low activation ferritic steels in Japanese universities, the status of 9Cr-2W type ferritic steels development is presented. The main emphasis is on mechanical property changes by fast neutron irradiation in FFTF. Bend test, tensile test, CVN test and in-reactor creep results are provided including some data about low activation ferritic steels with Cr variation from 2.25 to 12%. The 9Cr-2W ferritic steel, denoted as JLF-1, showed excellent mechanical properties under fast neutron irradiation as high as 60 dpa. As potential materials for DEMO and beyond, innovative oxide dispersion strengthened (ODS) quasi-amorphous low activation ferritic steels are introduced. The baseline properties, microstructural evolution under ion irradiation and the recent progress of new processes are provided.

Creep Rupture Strength and Microstructure of Low C-10Cr-2Mo Heat-Resisting Steels with V and Nb

The new ferritic heat-resisting steels of 0.05C-10Cr-2Mo-0.10V-0.05Nb (Cb) composition with high creep rupture strength and good ductility have already been reported. The optimum amounts of V and Nb that can be added to the 0.05C-10Cr-2Mo steels and their effects on the creep rupture strength and microstructure of the steels have been studied in this experiment. The optimum amounts of V and Nb are about 0.10 pct V and 0.05 pct Nb at 600 °C for 10,000 h, but shift to 0.18 pct V and 0.05 pct Nb at 650 °C. Nb-bearing steels are preferred to other grades on the short-time side, because NbC precipitation during initial tempering stages delays recovery of martensite. On the long-time side, however, V-bearing steels have higher creep rupture strength. By adding V to the steels, electron microscopic examination reveals a stable microstructure, retardation during creep of the softening of tempered martensite, fine and uniform distribution of precipitates, and promotion of the precipitation of Fe2Mo.

Irradiation creep of low-activation ferritic steels in FFTF/MOTA☆

Abstract Irradiation creep behavior of low-activation steels, developed as structural materials for fusion reactors (JLF series steels), was investigated to obtain a fundamental understanding of these alloys under fast neutron irradiation in FFTF. (2.25–8)Cr(1–2)W bainitic steels and 12Cr-2W ferritic steels showed superior creep resistance to type-316 stainless steels under fast neutron irradiation up to 520°C. At temperatures below 460°C the creep strain increased with increasing Cr content up to 7 Cr, and further increments of Cr content up to 12% reduced the creep strain. At temperatures between 460 and 600°C, 7–8 Cr ferritic steels showed the largest creep strain. Swelling-enhanced creep, near the peak swelling temperature of 410°C, was also observed. The 9Cr-2W ferritic steel JLF-1 presented excellent properties, suggesting it as a leading candidate alloy for structural components of fusion reactors.

Microstructural stability of reduced activation ferritic/martensitic steels under high temperature and stress cycling

Abstract Reduced activation ferritic/martensitic steels are leading candidates for blanket/first-wall structures of the D-T fusion reactors. In fusion application, structural materials will suffer cyclic stresses caused by repeated changes of temperature and electromagnetic forces according to reactor operation scenarios. Therefore, creep–fatigue behaviors are extremely important to qualify reduced activation steels as fusion structural materials. In this work, microstructural stability of reduced activation ferritic/martensitic steels under various external stresses, such as constant stress cyclic stress, was studied. The materials used are JLF-1 steel (9Cr–2W–V,Ta) and JLS-2 steel (9Cr–3W–V,Ta). The microstructure inspection by means of transmission electron microscopy (TEM) and scanning electron microscopy (SEM) was performed following creep rupture tests, fatigue and creep–fatigue tests at elevated temperatures. In order to examine precipitation morphology in detail, the improved extracted residue and extracted replica methods were applied. From the microstructural observation of creep rupture-tested specimen, intergranular precipitates such as M 23 C 6 and Laves phase coarsened by applying the static stress.

Effects of precipitation morphology on toughness of reduced activation ferritic/martensitic steels

Reduced activation ferritic/martensitic steels (RAFs) are leading candidates for structural materials of D-T fusion reactors. It is reported that 9Cr–2W–V, Ta steel (JLF-1), one of the RAFs, has superior phase stability, swelling resistance and mechanical properties against high-fluence neutron irradiation. Recently 9Cr–xW–V, Ta steels (x=2.5, 3.0 and 3.5, JLS-series hereafter) were developed for use at higher temperatures. In this work, JLF-1 and JLS-series were thermal-aged at 823 and 923 K. Charpy impact tests were performed before and after thermal-aging. Microstructural features were observed using transmission electron microscope with energy dispersive X-ray spectrometer. From the results of Charpy impact tests, the ductile to brittle transition temperature was found to increase both by thermal-aging and by increasing tungsten content. This behavior was consistent with microstructural evolution of intergranular precipitates such as M23C6 and Laves phase coarsening.

Effects of neutron irradiation on microstructural evolution in candidate low activation ferritic steels

Abstract Fe-(2.25-12)Cr-2W-V, Ta low activation ferritic steels (JLF series steels) were developed in the fusion materials development program of Japanese universities. Microstructural observations, including precipitation response, were performed after neutron irradiation in the FFTF/MOTA. The preirradiation microstructure was stable after irradiation at low temperature (

Microstructural developments in Fe-Cr-W low activation ferritic steels under dual beam charged particle irradiation

Abstract Chrominum-molybdenum ferritic steels have a better resistance to void swelling than austenitic steels and were considered for use as first wall structural materials in future fusion reactors. One of the major alloying elements, Mo, is deleterious for the reduction of radioactivity; reduced activation steels are under development, where Mo is replaced by W. In this work, four kinds of reduced activation ferritic steels were dual or single ion irradiated to 125 dpa. In the dual ion irradiated steels with a 7–12% Cr content, the dimensional stability of the martensite phase was superior to the other phases. The cavity microstructures of bainitic steels with 2.25% Cr content were affected by the He dpa ratio (and by the He injection rate) even when they had a high damage level (up to 125 dpa). Hitherto, the low activation steels developed by replacing Mo by W suggest excellent stability of microstructures and thus a high resistance to swelling.

Mechanical property changes in ferritic steels by 14 MeV neutron bombardment

Abstract Mechanical property changes induced in seven kinds of 9 Cr-2 Mo type mono- and dual-phase steels by 14 MeV neutron irradiation were investigated by means of post-irradiation mechanical property tests on miniaturized samples. These results are discussed in terms of microstructural evolution. Micro-Vickers hardness tests and micro-tensile tests were performed. The fluence dependence of the mechanical properties showed a trend for irradiation hardening at room temperature. The irradiation hardening can be reasonably understood by the dispersion hardening from radiation-induced complex defects. Specimen size effects on RTNS-II irradiation were also investigated. Differences in measured mechanical properties were found to depend on specimen size and to be amplified with increasing neutron fluence.

A new type of twin in an ain crystal

Abstract Plate-like crystals of aluminium nitride, prepared by a sublimation method at 1800 °C, are examined by means of transmission electron microscopy. Each crystal of about 100 μm in size is divided by a twin boundary. When the crystal is thin, it is (10 1 1) plane with an angle 63° to the (1 1 00) crystal surface, but as the thickness is increased it bends and changes to (11 2 2) plane, which is perpendicular to the surface. A growth mechanism is proposed in the conjunction with the bending of the twin boundary and the morphological aspects of the crystal.

Low fluence neutron irradiation response of ferritic stainless steels

Five kinds of ferritic stainless steels were irradiated at 300 K, 473 K and 673 K with 14 MeV neutrons to a fluence of 1.3 × 1018 n/cm2 and were irradiated with fission neutrons at about 600 K to a fluence of 5 × 1018 n/cm2. Micro-bulge tests, micro-Vickers hardness tests and electron microscopy were used to characterize the radiation damage. Many radiation induced defect clusters were observed and the temperature dependence and the fluence dependence of these defect clusters suggest they are mostly vacancy type. Corresponding to the microstructural changes irradiation softening was observed.