Y. Katano

RADIATION AND ANNEAL HARDENING IN VANADIUM.

Abstract The tensile properties at room temperature in vanadium irradiated to a fast neutron fluence of 8.2 × 10 19 n/cm 2 at about 200 °C were investigated in terms of changes in microstructure with the irradiation and subsequent heat treatment. Dislocation channeling was observed in the deformed specimens that were irradiated and subsequently annealed for 1 h at temperatures of 400 °C or below; the specimens contained small dislocation loops of the order of 10 16 /cm 3 in density. Hardening due to postirradiation annealing was found to occur in the temperature range of 180 to 600 °C. The radiation anneal hardening was considered to arise from interstitial impurity clusters. The radiation-induced hardening recovered almost completely with a l h anneal at 700 °C.

Radiation and anneal hardening in neutron-irradiated vanadium

Abstract Vanadium samples were neutron irradiated at the reactor ambient temperature to fluences in the range from 2.0 × 10 7 to 1.0 × 10 20 n/cm 2 ( E n ⩾ 1 MeV ). The radiation hardening measured at the ambient temperature increased linearly with the square root of the neutron fluence, up to a fluence of about 2.5 × 10 19 n/cm 2 , to approximately 25 kg/mm 2 for the highest fluence. The radiation-anneal hardening phenomenon was clearly observed in samples irradiated at a low fluence (2.0 × 10 17 and 1.0 × 10 18 n/cm 2 ) and the hardening was accompanied by changes in the density and size distribution of the radiation-produced defect clusters. The radiation hardening induced during irradiation to 1.0 × 10 20 n/cm 2 recovered monotonically as the annealing temperature increased. Defect clusters invisible in the electron microscope played an important role in the radiation and anneal hardening except when radiation hardening was induced at the highest fluence.

Amorphization behaviors in polycrystalline alumina irradiated with energetic iodine ions

Abstract A cross-sectional transmission electron microscopy (XTEM) technique utilized in examining the sintered alumina (Al2O3) sample irradiated by 85.0 MeV iodine ions I7+ to 2.8×10 18 m −2 reveals that almost complete amorphization takes place up to depths around 2 μm. Several grains including grains about to be fading out at depths of 2.5–3.5 μm are seen to have moved from the yet-amorphized region leaving the amorphized region behind. The amorphized regions extend up to depths around 4.5 μm for the sample irradiated to 12×10 18 m −2 . Any defect clusters cannot be observed in grains located at depths around 8 μm, where nuclear energy depositions are predicted to peak to about 1 dpa. The present results clearly demonstrate that the energy depositions through electronic process in alumina irradiated with heavy ions like present I7+ transform the crystalline phase into amorphous ones even near room temperatures, for accumulated electronic energy depositions above some 1.5–2 GGy with deposition rates above 4–5 keV/nm/ion.

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.

Damage structure in Al2O3 single crystal irradiated with He-ions

Abstract The microstructures of single crystal alumina irradiated with He-ions at 1023 to 1223 K to a dose of 1 × 10 20 He/m 2 were examined in an electron microscope. In as-irradiated samples, high densities of defect clusters with 6–7 nm in an average size were formed and these were almost independent of the irradiation temperatures. The clusters were revealed to he dislocation loops of interstitial type. In the sample annealed for 1 h at 1223 K after the He-irradiation at 1023 K, five distinguishable features appeared: high density of small cavities with 7 nm in an average size, highly coalesced cavity channels, small sized spherical precipitates of aluminium, large size aluminium islands, and dislocation loops with an average size of 70 nm. These features were revealed to be strongly dependent on the helium irradiation damage and also on the distances of damaged layers from foil surface during annealing.

Electron Irradiation Damage in Aluminum in a High Voltage Electron Microscope

Defect clusters are observed to form in aluminum during examination at about 20°C in a high voltage electron microscope operating at 1000 kV with a beam current of 0.1 µA in a 5 µm diameter spot. As the electron dose increased, the clusters grow into resolvable dislocation loops. The defect clusters are identified as vacancy loops of Frank type. When the temperature is raised gradually to 70°C during electron irradiation, defect clusters grow into larger loops and eventually disappear with 70°C irradiation. Furthermore, no electron damage is observed in the specimen irradiated at 70°C without previous 20°C irradiation. In a quenched specimen, production rate of the defect clusters is larger compared with an annealed specimen and large quenched-in loops are found to shrink during 20°C irradiation. The damage rate is discussed in relation to nucleation and growth of the loops under vacancy supersaturated environment.

Microstructure in pure copper irradiated by simultaneous multi-ion beam of hydrogen, helium and self ions

Abstract Pure copper was irradiated at 300–500°C by 5 MeV Cu ions (single beam) and Cu ions plus gas atoms (H and He) (dual beam irradiation) simultaneously. The high energy ion irradiation was carried out with the accelerator TIARA at the Takasaki-establishment of JAERI. The ions stop within a few microns from surface level and damage was formed up to this depth. The damage structure was observed as a function of the depth utilizing a focused ion beam (FIB) device. Below 300°C irradiation with a single beam produced a high density of stacking fault tetrahedra (SFT) but void formation was not observed. Large voids were observed with single beam irradiation at 500°C. In specimen irradiated with a dual beam of helium and Ni ions, the number density of voids was increased significantly. In copper irradiated with hydrogen and Ni ions, the number density of voids was not so large. Experimental results show that helium atoms promote void formation. Hydrogen atoms have less effect on void formation than helium atoms in pure copper.

Radiation-generated prismatic loops around gas bubbles in alumnium-lithium alloy

Abstract Prismatic dislocation loops were observed to emanate from a gas bubble in neutron-irradiated aluminum-lithium alloy during electron irradiation at 200°C in a 200 kV electron microscope. The loops aligned in the 〈110〉 directions were analysed to be vacancy loops on 〈111〉 plane with a/2 〈110〉 Burgers vector. In a 1000 kV electron microscope, the prismatic punching were observed to occur during electron irradiation at 50°C. The prismatic loops were considered to nucleate at the surface of a gas bubble by condensation of radiation-produced vacancies and to propagate in the 〈110〉 directions by pencil glide under vacancy supersaturated environment.

Electrical properties and phase stabilities of some ceramics irradiated by neutrons and ions

Abstract The electrical properties of silicon nitride (Si 3 N 4 ) and tetragonal zirconia polycrystals (TZP) were measured as a function of temperature in the frequency range 5 Hz to 13 MHz. Dielectric properties at 9.1 GHz were also measured by the standing-wave method. The dielectric loss tangent tan δ at 9.1 GHz of unirradiated hot-pressed Si 3 N 4 with small amounts of MgO was lower than 2 × 10 −3 up to 500°C. On the other hand, TZP with 3 mol% Y 2 O 3 (3Y-TZP) had about 10 times higher values of tan δ than Si 3 N 4 .

Swelling and nickel segregation around voids in electron-irradiated Fe-Cr-Ni alloys

Abstract The swelling in Fe-Cr-Ni alloys is related to the formation of void embryos which exhibit strain field contrast suggesting segregation of solute atoms, such as nickel. In the present work, the nickel dependencies of both the swelling and the segregation in pure and commercial ternary alloys are shown experimentally. The effect of segregation on the stabilization of void embryos is discussed. The results show that the dependence of swelling on nickel concentrations is closely related to the segregation of nickel atoms to the surface of void embryos.