Naoto Sekimura

The primary origin of dose rate effects on microstructural evolution of austenitic alloys during neutron irradiation

The effect of dose rate on neutron-induced microstructural evolution was experimentally estimated. Solution-annealed austenitic model alloys were irradiated at ≃400 °C with fast neutrons at seven different dose rates that vary more than two orders difference in magnitude, and two different doses were achieved at each dose rate. Both cavity nucleation and growth were found to be enhanced at lower dose rate. The net vacancy flux is calculated from the growth rate of cavities that had already nucleated during the first cycle of irradiation and grown during the second cycle. The net vacancy flux was found to be proportional to (dpa/s)1/2 up to 28.8 dpa and 8.4×10−7 dpa/s. This implies that mutual recombination dominates point defect annihilation in this experiment, even though point defect sinks such as cavities and dislocations were well developed. Thus, mutual recombination is thought to be the primary origin of the effect of dose rate on microstructural evolution.

Influence of details of reactor history on microstructural development during neutron irradiation

Abstract Microstructurally-oriented irradiation experiments are shown in this paper to be strongly dependent on details of reactor history that frequently are not brought to the experimenters attention. In some cases, these details can dominate the experiment so as to produce very misleading results. To aid in the design and interpretation of microstructurally-oriented experiments, a number of studies are reviewed to highlight history effects and then guidelines are presented to minimize the impact of reactor history in new experiments.

In-situ observation of cascade damage in gold under heavy ion irradiation at high temperature

Formation, stability and annihilation of vacancy clusters produced during irradiation with energetic heavy ions at high temperature have been studied using gold as a model material. The experiments were carried out using a 400 kV heavy ion accelerator/200 kV TEM linked facility. In-situ observation during Xe+ or Ar+ irradiations at temperatures between 290 and 670 K reveals instantaneous formation of defect clusters. These include vacancy loops and stacking fault tetrahedra (SFT) with some unresolved defects with dot contrast. At lower temperatures the clusters tend to form in groups, reflecting subcascade structure, while at 570 and 670 K, ungrouped clusters are dominant. Both vacancy loops and SFT have considerably shorter lifetimes during irradiation than in the absence of irradiation. Moreover, the lifetime spectrum has two components; a long component is related to SFT, while loops usually have shorter lifetimes. By interrupting irradiation at 570 and 670 K, defects annihilate and the process has two components. Since the SFT survive longer, the SFT/loop ratio is history dependent.

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.

In-situ microstructural observation of radiation damage in nickel produced by energetic heavy particles

Abstract Microstructural changes during 300 and 400 keV Ar+ irradiation in pure nickel between 300 and 773 K have been observed in-situ in an electron microscope. Some of the observations are recorded on a video tape. Various phenomena characteristic of cascade damage have been observed. Clustering of point defects is influenced strongly by the presence of point defects sinks: surfaces, pre-existing dislocations, loops and cavities. Wedge-shaped specimens are utilized to sort out the complex behavior of microstructural evolution. Of great interest is the fact that under certain conditions, metastable defect clusters with a very short lifetime are formed during irradiation at 773 K. The implication of these observations to fusion neutron damage modeling is discussed.

In Situ TEM Observation of Growth Process of Zirconium Hydride in Zircaloy-4 during Hydrogen Ion Implantation

In situ observation during hydrogen ion implantation was performed with a transmission electron microscope installed in an ion accelerator to investigate the growth process of Zr hydride in Zircaloy-4. To clarify the effect of radiation damage, some samples were irradiated with 4 MeV Ni3+ ions prior to hydrogen implantation. Growth processes of Zr hydrides accompanying the formation of dislocations were observed. The crystallographic relationship between the Zr matrix and Zr hydrides was identical with that found in previous studies: inter- and intragranular hydrides exhibiting a tendency to grow in the (112-0) direction. This growth can be attributed to a shear mechanism in which there are partial dislocations along basal planes. In specimens preirradiated with Ni ions, the growth rate of hydrides was suppressed, but the growth direction was not affected by radiation defects. It was also confirmed that the black spots induced by Ni3+ ion irradiation grew during hydrogen ion implantation, suggesting the formation of hydrides at the defects.

The effect of cascade and helium on microstructural evolution under fusion irradiations

Abstract Cascade damage and high helium production rate are major characteristic features of fusion neutron radiation effects. Experimental as well as computational studies of these two factors which have been performed by the authors are presented and discussed from the standpoint of microstructural evolution. The experimental work has been performed using a facility comprising two small ion accelerators and an electron microscope, capable of observing cascade damage during heavy ion bombardment, and of carrying out dual beam irradiations to study the effect of simultaneous injection of helium atoms with displacement cascades. It has been shown that the evolving microstructures depend strongly on the nature of the cascade, type of materials, irradiation temperature, and amount and mode of helium implantation. The implication of these microstructural studies to the radiation effects relevant to fusion reactor design is discussed.

In-situ observation of cascade damage in nickel and copper under heavy ion irradiation

In-situ observation of cascade clusters is performed to examine dynamic behavior of defect clusters in nickel and copper under 400 keV Xe+-ion irradiation at high temperature. TEM images of the cascade clusters under irradiation are recorded on videotapes, from which initial sizes and lifetimes of the cascade clusters are derived. Results are compared with previous experiments on gold. From the lifetime distribution of clusters, it is concluded that only V-clusters are formed in copper foils, while both V and I-clusters are formed in nickel foils of 50 and 100 nm in thickness.

Influence of flux-spectra differences on transmutation, dimensional changes and fracture of vanadium alloys

Abstract Density and dimensional changes can arise in vanadium alloys from radiation—induced segregation and precipitation of chromium, titanium, and other elements. Transmutation of vanadium to chromium can also lead to significant changes in density, either directly or indirectly, via its effect on precipitation. Furthermore, the swelling of vanadium and its alloys is known to be very sensitive to the chromium level. Since the transmutation rate to produce chromium is very sensitive to neutron, spectra, an apparent flux-dependence of void swelling may occur that actually arises from the spectral differences that accompany changes in displacement rate within most reactors. Transmutation and segregation can also severely impact the fracture behavior in some alloys.

The effect of titanium addition on microstructural evolution in austenitic steel irradiated with fast neutrons in FFTF

Abstract Austenitic model alloys were irradiated with fast neutrons in FFTF (Fast Flux Test Facility) to investigate fundamental mechanisms of microstructural evolution in simple austenitic steels under relatively high dose irradiation. Irradiations were performed in MOTA (Material Open Test Assembly) at 693, 793 and 873 K. Void swelling was reduced by the addition of minor elements such as titanium and carbon to a simple Fe-Cr-Ni ternary alloy at all the tested temperatures. In the titanium bearing alloys, very small cavities were detected on dislocation lines and MC precipitates. Fine dislocation loops were observed in the matrix of the irradiated materials and their size and density were not found to be affected by the solute additions.