S. Ohnuki

Formation of nanoscale complex oxide particles in mechanically alloyed ferritic steel

Oxide-dispersion-strengthened (ODS) ferritic steel incorporating nanoscale oxide particles was produced by mechanical alloying, and the complex oxide particles, comprised of titanium and yttrium, characterized by high-resolution transmission electron microscopy (HRTEM) and characteristic X-ray nano-analyses. The interface between the ferritic matrix and the major Ti-Y complex oxide was incoherent. From HRTEM and characteristic X-ray nano-analyses, plus the extremely limited literature data on ODS ferritic steels, it was determined that the most stable complex oxide was cubic Y2Ti2O7, which formed by a spontaneous reaction between nano-sized ceramics with a low solubility limit in the metal.

Swelling behavior of F82H steel irradiated by triple/dual ion beams

Abstract Irradiations for spallation target vessels and structural materials of fusion reactors were simulated using simultaneous triple/dual ion beams consisting of Fe 3+ , He + and H + ions or Fe 3+ and He + ions at temperatures between 470 and 600 °C to 50 dpa. The swelling of F82H (Fe–8Cr–2W–0.2V–0.04Ta–0.1C) martensitic steel was enhanced by a synergistic effect of displacement damage and the implantation of helium and hydrogen. The maximum swelling of F82H steel was 3.2% at 470 °C under a simulation of structural materials of fusion reactors, and was higher than 1.2%, which applied to a simulation of spallation target vessel. The swelling under a simulation of fusion reactor decreased with increasing irradiation temperature, however the swelling under a simulation of spallation target vessel was again increased at 600 °C by the high helium concentration. From the microstructural analysis of taking account of cavity growth process, the cause of the enhancement of swelling under a simulation of fusion reactor is thought to be gas pressure of hydrogen and helium in cavities during irradiation. The effects of 50% cold-working and carbon implantation on swelling behavior were also examined. The swelling was reduced from 3.2% to 1.4% by 50% cold-working, and to 0.5% by carbon implantation.

Radiation-induced segregation at internal sinks in electron irradiated binary alloys

Abstract Radiation-induced segregation phenomena in copper, nickel and iron based binary alloys were studied by means of a high voltage electron microscope and an energy dispersive X-ray microanalyzer. The segregation was caused during electron irradiation of undersaturated Cu-2 at% Ni alloy. In supersaturated Cu-2 at% Ag and Cu-2 at% Fe alloys precipitates were formed near surfaces and grain boundaries in the early stage of irradiation, subsequently voids nucleated at the central region of foil. On the other hand, alloying elements were depleted at defect sinks in Ni-2 at% Cu, Fe-5,13 at% Cr and Fe1 at% Mn alloys. The changes of solute concentration near sinks in these alloys indicate that the size effects lead to the segregation or depletion and under- and oversize solutes tend to migrate toward and away from the sinks, respectively.

Effect of triple ion beams in ferritic/martensitic steel on swelling behavior

Abstract The synergistic effects of displacement damage and atomic hydrogen and helium on swelling of the ferritic/martensitic steel, F82H, has been investigated. The irradiation was performed at temperatures between 470 and 600 °C to 50 dpa (displacement per atoms) under conditions of simultaneous ion beams consisting of Fe3+, He+ and H+ ions or Fe3+ and He+ ions. The swelling of F82H steel under triple beams with 18 appm He/dpa and 70 appm H/dpa was larger than that under dual beams with 18 appm He/dpa. The swelling in F82H under triple beams increased with decreasing irradiation temperature from 0.1% to 3.2%, while swelling under dual beams was between 0.04% and 0.08%. On the other hand, in the case of triple beam irradiation with a high ratio of gas/dpa, the swelling tended to increase with irradiation temperature. The swelling in ferritic/martensitic steels is significantly enhanced by the synergistic effect of displacement damage, hydrogen and helium atoms.

Phase stability of oxide dispersion-strengthened ferritic steels in neutron irradiation

Abstract Oxide dispersion-strengthened ferritic steels were irradiated by neutrons up to 21 dpa and studied by microstructural observation and microchemical analysis. The original high dislocation density did not change after neutron irradiation, indicating that the dispersed oxide particles have high stability under neutron irradiation. However, there is potential for recoil resolution of the oxide particles due to ballistic ejection at high dose. From the microchemical analysis, it was implied that some of the complex oxides have a double-layer structure, such that TiO2 occupied the core region and Y2O3 the outer layer. Such a structure may be more stable than the simple mono-oxides. Under high-temperature irradiation, Laves phase was the predominant precipitate occurring at grain boundaries α phase and χ phase were not observed in this study.

Void formation and microstructural development in oxide dispersion strengthened ferritic steels during electron-irradiation

ODS ferritic steels (13Cr–0.5Ti–0.2Y2O3) were prepared by the mechanical alloying method followed by the hot extrusion and several heat treatments including recrystallization. ODS steels with different heat treatment and a ferritic/martensitic (F/M) steel for the reference were irradiated to 12 dpa at 670–770 K in HVEM. After recrystallization, the dislocation density decreased with increasing grain size, however, the oxide particles did not show any obvious change in the size and the number density. During the electron-irradiation the microstructure was relatively stable, i.e. oxide particles showed good stability and the dislocation density remained almost constant. A limited void formation was observed in the specimens, and the suppressive effect due to dislocations with high number density was confirmed. From these results, the behavior of microstructure and the limited void formation in ODS steels have been discussed.

Void swelling and segregation of solute in ion-irradiated ferritic steels

Void formation and radiation induced segregation were investigated through the interaction between defects and solute atoms in pure iron, Fe-13wt%Cr and Si or Ti doped alloys by 200 keV C+ ion irradiation up to 118 dpa at 798 K. Microstructural observation was carried out by transmission electron microscopy, energy dispersive X-ray microanalysis and electron energy loss spectroscopy. The ferritic alloys exhibited significant resistance to void swelling. In Fe-Cr and Fe-Cr-Si alloys, the irradiation produced the precipitates consisting chiefly of chromium and implanted carbon, and chromium was enriched at grain boundaries and voids. In the Fe-Cr-Ti alloy, Ti-rich precipitates-were formed, and chromium was depleted from grain boundaries. These facts suggest that the solute atom-defect interaction which controls void formation and segregation is affected by the presence of chromium and other alloying elements.

Effect of hydrogen accumulation on mechanical property and microstructure of V–Cr–Ti alloys

Abstract Pure vanadium, V–5Cr, V–5Ti and V–5Cr–5Ti alloys doped with different amounts of hydrogen were tested under tension and observed under TEM in order to clarify the interaction between hydrogen and vanadium. Hydrogenated samples tested under tension showed a typical hydrogen-embrittlement behavior. TEM observation suggests that the embrittlement is caused by hydrogen diffusion at the triple point of grain boundaries, hydride formation and the subsequent generation of dislocations and micro-cracks.

Study of cavity formation in 316 stainless steels by means of HVEM/ ion-accelerator dual irradiation

Abstract A new dual irradiation system which links a 1300 kV HVEM and 300 keV ion accelerator has been developed. Using this system, dual irradiation studies can be made under in-situ observation. At low doses, the dislocation loop structures were strongly affected by helium of the order of: preinjection irradiation, dual irradiation and simple electron irradiation. The effect on cavity formation showed that helium injected during dual irradiation enhances the cavity nucleation without the suppression of their subsequent growth. This study is the first report on dual irradiation by electrons and helium ions.

Electron irradiation effect on phase transformation in TiNi shape memory alloy

Abstract The sensitivity of phase transformation in TiNi shape memory alloy to electron irradiation has been investigated by means of HVEM at room temperature. In the present study, the phase transformation of this alloy finally went to the amorphous phase. The amorphization process can be categorized according to the following three types. (1) The B2 phase transformed to the amorphous phase successively with the increase of dose. (2) The martensitic phase transformed to the B2 phase at the beginning of irradiation. The transformation to the amorphous phase progressed successively. (3) The martensitic phase transformed to the amorphous phase directly. The amorphization progressed irregularly. The critical dose for the amorphization in all three processes has been investigated. The critical doses for the amorphization were quite sensitive to the micro-chemical composition; the dose of the stoichiometric composition was relatively high.