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The damage induced in cerium dioxide by swift heavy ion irradiation was studied by micro-Raman spectroscopy. For this purpose, polycrystalline sintered pellets were irradiated by 100-MeV Kr, 200-MeV Xe, 10-MeV, and 36-MeV W ions in a wide range of fluence and stopping power (up to ∼28 MeV μm−1). No amorphization of ceria was found whatsoever, as shown by the presence of the peak of Raman-active T2g mode (centered at 467 cm−1) of the cubic fluorite structure for all irradiation conditions. However, a clear decrease of the T2g mode peak intensity was observed as a function of ion fluence to an asymptotic relative value of about 45%. Similar decays were also observed for satellite peaks and second-order peaks. Track radii deduced from the decay kinetics for the 36-MeV W ion data are in good agreement with previous determinations by X-ray diffraction and reproduced by the inelastic thermal spike model for low ion velocities. However, interaction between the nuclear and electronic stopping powers is needed to describe the decay kinetics of 10-MeV W ion data by the thermal spike process. Moreover, the asymmetrical broadening of the main T2g peak after irradiation was analyzed with different theoretical models.

Progress of Ferrous Nano-Metal Project is introduced in the present paper. In the project, maximum use of copper clusters and precipitates is pursued for achieving better strength-ductility balance than that of conventional high-tensile strength steels. Fundamental aspect of clustering and precipitation of Cu in Fe-Cu alloys was studied using Optical Tomographic Atom-Probe (OTAP). It was found that Cu precipitation during aging was enhanced by plastic deformation. The observed Cu precipitation behavior was well related to the age-hardening behavior, that is, aging started at lower temperature and maximum hardness was higher for plastically deformed and aged ferrite. Aging behavior and associated tensile properties were further examined for Fe-C-Mn-Cu martensitic steel. Higher value of tensile strength times elongation was achieved in Fe-C-Mn-Cu steel than Fe-C-Mn steel. Finally, effect of Cu precipitation on grain-refinement of ferrite was studied for Fe-C-Mn-Cu steel. Ferrite grain smaller than 1 mm was obtained in both processes of strain-assisted ferrite transformation from heavily deformed austenite and dynamic recrystallization of heavily deformed ferrite. Ferrite grain size was found to decrease by addition of Cu at the both processes. It was suggested that a simple additivity rule does not hold in terms of the strengthening by grain-refinement and that by precipitation, especially at grain-size range less than 1 mm.

Single crystals of magnesium aluminate spinel (MgAl2O4) with (1 0 0) or (1 1 0) orientations and cerium dioxide or ceria (CeO2) were irradiated by 1.0 MeV and 2.5 MeV electrons in a high-fluence range. Point-defect production was studied by off-line UV-visible optical spectroscopy after irradiation. For spinel, regardless of both crystal orientation and electron energy, two characteristic broad bands centered at photon energies of 5.4 eV and 4.9 eV were assigned to F and F(+) centers (neutral and singly ionized oxygen vacancies), respectively, on the basis of available literature data. No clear differences in color-center formation were observed for the two crystal orientations. Using calculations from displacement cross sections by elastic collisions, these results are consistent with a very large threshold displacement energy (200 eV) for oxygen atoms at room temperature. A third very broad band centered at 3.7 eV might be attributed either to an oxygen hole center (V-type center) or an F2 dimer center (oxygen di-vacancy). The onset of recovery of these color centers took place at 200 °C with almost full bleaching at 600 °C. Activation energies (~0.3-0.4 eV) for defect recovery were deduced from the isochronal annealing data by using a first-order kinetics analysis. For ceria, a sub-band-gap absorption feature, which peaked at ~3.1 eV, was recorded for 2.5 MeV electron irradiation only. Assuming a ballistic process, we suggest that the latter defect might result from cerium atom displacement on the basis of computed cross sections.

Mechanical milling (MM) treatment of metallic powder is suitable for fabricating nano-crystallized materials, because milling action by steel balls enables to give a huge amount of strain with multi-directional plastic deformation to powder particles. In this study, effect of carbon on the grain refining behavior during MM treatment was investigated in high purity iron material and Fe-C materials. The powder used in this study is electrolytic pure (9 ppmC) iron and cementite (6.2 mass%C) powder particles. The powders are mixed to set the chemical composition to be Fe-(0-2)mass%C. The mixed powder is subjected to MM treatment for various times (3.6-360 ks) and then microstructure was examined by means of X-ray diffractometry, TEM observation. With MM treatment, cementite decomposes into ferrite matrix and ferritic single structure is obtained after 360 ks MM treatment. On the other hand, microstructure of ferrite develops from dislocation cells structure to fine-grained structure through dynamic continuous recrystallization (DCR). The grain size is reduced gradually with MM treatment. However the grain size after reaching steady state is different between high purity iron and Fe-C materials. The grain size after 360 ks MM treatment decreases with increasing carbon content, and nano-crystallized structure with about 15 nm grain size was obtained in the Fe-0.8 mass%C. This indicates that carbon addition enhances grain refining and is necessary for nano-crystallization by severe plastic deformation. Considering the interaction between carbon atoms and dislocations, carbon addition would assist the increment of stored dislocations which contributes to DCR. This results in the effectiveness for the formation of nano-crystallized structure in carbon added iron.

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Reflection spectra of cerium dioxide sintered samples were measured in the UV-visible range after irradiation with various heavy ions (2.4-MeV Cu, 5-MeV W, 10-MeV W, 36-MeV W, 100-MeV Kr, and 200-MeV Xe). Differential reflectance spectra of irradiated samples after subtraction of the reference sample spectrum are fit with six broad Gaussian bands centered at about 1.2, 2.2, 2.8, 4.0, 4.8, and 6.2 eV. The growth curves of most bands show a saturation behavior vs ion fluence. Reflection spectra are consistent with the UV-visible absorption spectra of electron-irradiated ceria single crystals for photon energies lower than 3.2 eV, corresponding to the optical gap. The spectra are tentatively analyzed on the basis of charge transfer bands and the 4f-5d transitions related to the Ce3+ ions in the distorted environment formed by irradiation. More insight into defect formation is given by the reflection spectra rather than the absorption ones due to the limitation in the absorption of the optical gap. These data are also discussed with reference to the radiation damage processes by electronic excitations and nuclear collisions in ceria.

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