H. Kawanishi

In Situ Studies of the Effects of Ion Beams on Materials Using the Electron Microscope Ion Beam Interface

To investigate microstructural evolution by cascade damage produced by energetic heavy particles, we have built a facility which is capable of observing damaged structure introduced by heavy ions in situ in an electron microscope. A 400 kV Cockcroft-Walton type heavy ion accelerator with a Danfysik 911A heavy-ion source has been combined with a 200 kV electron microscope. Heavy ion beams of energies up to 400 keV can bombard the specimen in the microscope with an incident angle of 45 degrees to the electron beam axis. This paper describes the outline of the facility and some of the recent results mainly concerned with heavy radiation damage of materials which are relevant to fast breeder and fusion reactor development. For example, we have investigated microstructural evolution of SUS 316 stainless steel as a function of dose, dose rate and temperature. The topics will also include observation of short-lived clusters of point defects during irradiation in nickel and direct comparison of self-ion damage in aluminum with electron damage caused by an electron beam within the 200 kV microscope. Some of these results have been discussed, compared with cascade simulation and defect kinetics calculations. The experimental results may be useful to establish correlation between neutron and ion damage through microstructural evolution modelling.

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.

Direct comparison of electron and self-ion damage in aluminum as a fusion neutron simulation study

Abstract The microstructural changes in aluminum both by electron and self-ion irradiations have been observed in-situ within the same grain of the specimen. Electron irradiation is found to produce denser but smaller loops than Al + ion irradiation. Defect accumulation near dislocations and loop denuded zone near grain boundaries are observed in the case of Al + ion irradiation but not in the case of electron irradiation. The relation of microstructural changes with dose (dpa) is remarkably different between the two irradiations. This indicates an inadequacy of the unit of dpa as a correlation parameter. In addition, the effects of irradiation have been investigated on a quenched specimen which contains vacancy type dislocation loops. These loops shrink continuously by electron irradiation, whereas they crumble by Al + ion irradiation.

The effect of helium on the microstructural evolution in PCA as studied by dual beam irradiation

Abstract Prime Candidate Alloy (PCA) for the near term fusion reactor is irradiated with dual ions to examine the microstructural changes under high helium production rate. Cavity density is found to increase with increasing He/dpa ratio. Few cavities are observed in the specimens irradiated with heavy ions only. He/dpa ratio does not change cavity diameter significantly. It is also found that helium has strong effects on the radiation induced precipitate fomation.

Synergistic effects of helium and other variables on microstructure change in neutron-irradiated Fe-Ni-Cr alloys doped with 59Ni

Abstract Microstructural data have been described on Fe -15 Cr - xNi - yP alloys in both solution-annealed and cold-worked conditions irradiated in the Fast Flux Test Facility (FFTF) reactor at 638 and 873 K to 5.2 and 8.8 dpa, respectively, where x = 25 to 45 and y = 0.001 to 0.04. The irradiation was carried out on each alloy condition exhibiting two variants in the ratio of 59 Ni isotope. TEM examinations have been conducted to study the role of He/dpa ratio, Ni content, P level, thermal-mechanical condition and irradiation temperature on the development of neutron-induced microstructure. The results obtained show that prior-cold working, higher Ni content and higher P level usually suppress swelling. Higher He generation rate does not always enhance swelling. Phosphide was precipitated only at 873 K.

In-situ observations of heavy-ion damage in stainless steel

Abstract The microstructural evolution has been investigated in SUS316 stainless steel irradiated directly inside an electron microscope with 300 and 400keV Ar + ions. It is ascertained that there is a dose rate dependence of microstructural evolution at room temperature. The temperature dependence of nucleation of interstitial loops was investigated experimentally and discussed with the aid of a simple kinetic model. Some qualitative interpretation of the experimental results have been proposed.

14 MeV neutron irradiation of vanadium alloy (V-1 at.% B)☆

Vanadium and vanadium containing 1 at.% 10B have been irradiated at 333 and 423 K with 14 MeV neutrons up to 7.8 × 1020n/m2 at the RTNS-II facility in LLNL. About 1.1 appm helium is produced in V-1 at.% 10B at this dose by thermal neutrons from the walls of the target room of the facility. A homogeneous distribution of defects has been observed by transmission electron microscopy in both irradiated materials irrespective of the irradiation temperature. Although the defect size (< 5 nm) is found to be less changed by helium, the number density of the defect is remarkably increased in V-1 at.% 10B during the irradiation. Dislocation loops are formed preferentially around dislocation tangles, especially at the 423 K irradiation. The dislocation loops have been determined to be of interstitial-type.

Microchemical evolution in PCA under dual ion irradiation

Abstract The Prime Candidate Alloy (PCA) for a near term fusion reactor is irradiated with dual ions to investigate the microchemical evolution in this alloy under fusion irradiation conditions. Radiation induced solute segregation or depletion, depending on the atomic size factor, is observed by EDS analysis on cavities and grain boundaries. At 830 K, grain boundary migration is induced by irradiation and results in a row of cavities and Ni concentration peaks at the original position of the grain boundary. At 830 and 870 K, irradiation causes the precipitation of a Ni, Ti, Si and Al rich phase and the decrease of Ni concentration in the matrix, leading to another peak in the swelling variation with temperature.

Microstructure of Fe16Ni15Cr alloys irradiated in JOYO

Abstract Fast neutron irradiations were conducted on a set of model alloys with compositions of Fe-16Ni-15Cr-X ( X = Ti , Nb and/or C) to study the effects of Ti, Nb and/or C additions on microstructural evolution during irradiation. The irradiations were carried out by using a fast breeder experimental reactor JOYO at 673 K to 0.8 and 8.1 × 10 24 n / m 2 ( E > 0.1 MeV ) and followed by TEM observations. Void swelling for these alloys was estimated to be very low, indicating that it remains in a transient region. C and Ti were found to suppress void nucleation effectively. Nb and C, on the other hand, enhanced nucleation for voids with an increase in void sizes. Pre-existing dislocations were concluded to provide preferential sites for void nucleation regardless of Ti, Nb and/or C additions. The irradiations produced faulted loops of interstitial-type on (111) planes. The morphology of these faulted loops was characteristically altered by the additions of C, Ti and/or Nb.

Microstructural investigation of helium and lithium effects in V-Ti alloys irradiated in FFTF by means of 10B-doping

Abstract To study the effect of helium and lithium on cavity formation for V and V-Ti alloys in a fission reactor, V, V-5Ti and V-20Ti (both doped and undoped with 0.014 10 B) were irradiated at 365°C to 14 dpa (below-core) and at 406/520°C (level-2) to 42.5 dpa in the FFTF/MOTA. The He/dpa ratio was estimated to be about 22 at 365°C and 5 at 406 and 520°C. TEM observations following irradiation indicated that helium and lithium had little effect on swelling in the specimens. Cavity growth was assisted by helium in V-Ti alloys, though the number density for cavities was rather low compared to that of undoped alloys. Agglomeration of small dislocation loops, designated as “rafts”, was the main feature observed in V-5Ti, whereas tangling of dislocations was predominant in V-20Ti. Halos containing many cavities were formed around boride precipitates only in V-20Ti-0.014 10 B at 520°C, and arose from the effect of lithium.