Kiyomichi Nakai

Radiation-induced amorphization and swelling in ceramics☆

Irradiation-induced amorphization and swelling of ceramics including graphite have been studied through in-situ observation of electron microscopy, electron energy loss spectroscopy and convergent beam electron diffraction. Amorphization occurs below 500 K for highly-oriented pyrolytic graphite (HOPG) and below 300 K for SiC under electron irradiation. The amorphization fluences of HOPG and SiC required for full amorphization show no flux dependence. The displacement threshold energy is determined to be 12 eV for HOPG from the electron energy dependence of the amorphization fluence which is equivalent to 1 dpa. An atomistic model for describing the irradiation-induced amorphization and swelling of graphite is proposed. A high concentration of C2 molecules and their clusters between basal planes induce abrupt swelling along the c-axis within 4 × 10−3 dpa and fade the periodical structural image at 4 × 10−2 dpa. The concentration of vacancies increases gradually up to 1 dpa at which amorphization is completed.

A study of amorphization and microstructural evolution of graphite under electron or ion irradiation

In order to clarify the mechanism of irradiation-induced phenomena, such as amorphization,swelling and microstructural evolution in highly oriented pyrolytic graphite, in situ observation through high-voltage and high-resolution electron microscopy, electron energy loss spectroscopy and convergent-beam electron diffraction has been performed under irradiation with 16–160 fJ electrons or 4.8 fJ He+, Ar+ or Xe+ ions in the temperature range from 110 to 470 K. The induced amorphization with electron irradiation accompanies swelling along the c-axis due to the accumulation of di-carbon molecules and their clusters between the basal planes, and is fully completed with excess vacancies introduced by atom displacements of around 1 dpa. Heavy-ion irradiations were also performed to understand the effects of the impact and/or the deposited energy density in cascades and the projected range of ions on the irradiation-induced phenomena. Twinning, amorphization and microcrystal evolution in the amorphized region emerge, depending on the heterogeneous damage rate with respect to the depth from the incident surface of ions. The microstructural evolution under ion irradiation is successfully analyzed in terms of the damage profile and the deposited energy density, which are directly related to the mass and the energy of projectiles.

Kinetic study of defect clusters in the MgO-Al2O3 system under electron- and/or ion-irradiation☆

Abstract The kinetic behavior of defect clusters in MgO, α-Al2O3 and MgAl2O4 under electron irradiation has been examined through in-situ observation using high-voltage electron microscopy. Irradiation with 1 MeV electrons induces interstitial type dislocation loops in MgO and α-Al2O3. No loops are observed in MgAl2O4 during 1 MeV electron irradiation, but Ar+ ions with energy higher than 1 keV introduce loops. The loop kinetics in MgO is analogous to that for pure metals. Based on the kinetics, the migration energy of oxygen vacancies and the displacement threshold energy of oxygen ions are estimated to be 2.03 ± 0.17 and 30 ± 8 eV, respectively. Large nuclei of interstitial loops and/or a large number of vacant sites control the kinetic behavior of loops in α-Al2O3. The structural vacancies and large nucleation sites make the nucleation in MgAl2O4 difficult, and they are effective for designing radiation-resistant materials.

Accumulation and annihilation processes of cascades in metallic and nonmetallic crystals under irradiation with ions and/or electrons

Abstract In situ observation has been performed under dual-beam irradiation with heavy ions and fast electrons in the HVEM-accelerator facility at Kyushu University. The objectives of the present study are to understand the structure and the accumulation process of cascade damage and the synergistic effect of electronic excitation and/or free point defects on the behavior of cascade damage in metallic and nonmetallic crystals, such as Cu, graphite, SiC, Si, Ge, Ge-20at%Si, MgO, α-Al 2 O 3 , MgAl 2 O 4 and others. The HVEM-accelerator facility has been confirmed to be extremely useful for observing the behavior of cascade damage: (1) Cascade damage is directly produced without the help from other collision cascades in Cu. (2) Most collisions themselves in Si, Ge and Ge-20at%Si induce invisible structural change through electron microscopy, but they are converted into visible ones by the impact from other collision cascades. (3) No visible cascade damage is formed in covalent crystals with light masses such as graphite and SiC and in ionic crystals such as MgO, α-Al 2 O 3 and MgAl 2 O 4 . (4) Free interstitials suppress the evolution of visible cascade damage and assist the nucleation and the growth of interstitial loops around collision cascades in metallic and nonmetallic crystals.

Accumulation process of cascades in ceramics under ion and/or electron irradiation

Dual-beam irradiation and simultaneous observation with 30 keV Xe+ ions and 250 or 1000 keV electrons have been performed for understanding the structure and the accumulation process of cascades and for attaining insight into the synergistic effect of electron and/or free point defects on the properties of cascades. Covalent crystals such as Si, Ge and Ge20 at% Si show contrast in transmission electron microscopy through the overlap of cascades, which stabilizes high concentration of point defects and induces an amorphous-like phase in covalent crystals. In ionic crystals such as MgO, Al2O3 and MgAl2O4, recombination of Frenkel pairs is of predominance and a small number of point defects are rather homogeneously distributed even when heavy ion impacts are introduced. The effects of electrons are to suppress the formation of vacancy clusters and to assist the nucleation of interstitial loops under the dual-beam irradiation.

Nucleation and growth process of dislocation loops in electron-irradiated β-Nb-3·1 wt% Zr alloy

Abstract In order to understand the kinetic behaviour of point defects in β-Nb-3·1 wt% Zr containing small amounts of impurities, the nucleation and growth process of dislocation loops has been examined during electron irradiation in a high-voltage electron microscope. Small dislocation loops of interstitial character are formed during irradiation at temperatures from 300 to 675 K. The density of loops saturates at the beginning of irradiation, and loops grow in proportion to the cube root of irradiation time. The temperature dependence of the saturated loop density under a fixed electron flux consists of five stages. These results, together with the dependence of loop density on electron flux, are analysed in terms of the effect of impurities on the nucleation process of loops and the mobility of point defects. Two kinds of impurity, probably oxygen and nitrogen, act as nucleation sites of loops. The binding energies between a Nb interstitial and each of these impurities are estimated to be (1·1 ± 0·1) ×...

Irradiation-induced spinodal decomposition in alloys

Abstract Studies of homogeneous phase transformations induced by irradiation are aimed at developing materials for nuclear applications and understanding basic mechanisms of decomposition. Spinodal decomposition, one of the major modes of homogeneous phase separations, induced by electron irradiation is investigated with respect to the basic mechanism and to the confirmation of its inducement with a large increase in the coherent spinodal temperature. The inducement of spinodal decomposition is successfully clarified by analyses of the induced modulated structure using the theory of spinodal decomposition. The increase in spinodal temperature up to its chemical limit under irradiation is shown to be due to the relaxation of the coherent strain associated with the modulated structure. The process of relaxation of the coherent strain under irradiation is investigated through high resolution electron microscopy. Interstitials and vacancies produced by irradiation accumulate in a spatially modulated structure possessing a definite wavelength. The elastic interaction between the strain fields around point defects themselves and the coherency maintained in compositional fluctuations are associated with the spatial modulation. It is emphasized that the periodic distribution of excess point defects under irradiation contributes remarkably to the relaxation of the coherent strain without destruction of coherency between atoms, resulting in inducement of the spinodal decomposition up to the chemical spinodal temperature.

Nucleation and growth mechanism of homogeneous radiation-induced precipitates in Cu-0.18 wt% Be alloy

Abstract A theory of formation of interstitial loops during irradiation is applied to the nucleation and growth of precipitates in electron-irradiated Cu-0.18 wt% Be alloy. The dependences of density and size of the homogeneously nucleated, radiation-induced precipitates on electron flux and irradiation time are analyzed. The conclusions are that a di-MDI, (mixed dumbbell interstitial) and/or a complex of an impurity and the di-MDI acts as the nucleus of the precipitate, and the precipitates grow as a result of the high mobility of the mixed dumbbell interstitials.

Inducement process and mechanism of spinodal decomposition in electron-irradiated Au-Ni alloys

Abstract Irradiation-induced spinodal decomposition has been investigated to clarify its basic mechanism which gives an insight into developing fusion materials. The drastic expansion of spinodal region up to the chemical spinodal temperature under irradiation has been reported in many systems, and successfully explained in terms of the relaxation of the coherent strain associated with the modulated structure. In the present study the Au-Ni system is used for observing the relaxation process because of its large coherency-strain energy. The relaxation process of coherent strain is examined through observations of the spacial distribution of dislocation loops and high-resolution structure images. It is concluded that a heterogeneous distribution of interstitials and vacancies contributes remarkably to the relaxation of the coherent strain.

Effects of impurities on the kinetic behaviour of point defects in electron-irradiated β-Nb-3.1 wt% Zr alloy

Abstract In order to understand the effects of N and O on the nucleation and growth process of dislocation loops in β-Nb-Zr alloys, Nb-3.1 wt%Zr alloys doped with N and O have been examined under irradiation in a high-voltage electron microscope. The nucleation of loops takes place very early in the irradiation at temperatures from 300 to 582 K, and loops grow in proportion to the cube root of irradiation time. The temperature dependence of the saturated loop density under a fixed electron flux shows four stages in the similar temperature regions as those which appear in the undoped alloy, whilst the loop density in the lower temperature regions increases with doping by N and O. These results, together with the dependence of loop density on electron flux, are analysed in terms of heterogeneous nucleation kinetics at N and O clusters. On the basis of the analysis the binding energies between an Nb interstitial and N and O clusters are estimated to be (7.2 ± 0.8) × 10−20 and (1.1 ± 0.1) × 10−19J, respectively.