Brannerite, UTi2O6, occurs in polyphase Ti-based, crystalline ceramics that are under development for plutonium immobilization. In order to investigate radiation effects caused by α- decay events of Pu, several brannerite compositions were synthesized: UTi2O6, ThTi2O6, CeTi2O6 and a more complex material, which is composed of Ca-containing brannerite and pyrochlore. An 1 MeV Kr+ irradiation was performed over a temperature range of 25 K to 1020 K with in-situ TEM. The ion irradiation-induced crystalline-to-amorphous transformation was observed in all brannerite samples. At room temperature, the critical amorphization dose, Dc, resulting from Kr+ ion irradiation increases in the order: Dc (CeTi2O6) < Dc (ThTi2O6) < Dc (Ca- containing brannerite) < Dc (UTi2O6) < Dc (Ca-containing pyrochlore). The critical amorphization dose for stoichoimetric brannerite increases at elevated temperature due to the effect of thermal annealing. The critical amorphization temperatures of the different brannerite compositions are: 970 K, UTi2O6; 990 K, ThTi2O6; 1020 K, CeTi2O6. The effects of structure and chemical composition on radiation resistance of brannerite-type and pyrochlore-type ceramics are discussed.

Our understanding of radiation effects in stainless steels irradiated at temperatures below annealing stage V (<573K) remains unclear. The microstructure under these conditions has been loosely described in terms of Frank loops and “black spot” damage, the latter of which is typically considered to be 4-5 nm or less in size. To further clarify the nature of these defects, 6 stainless steels heats neutron irradiated at 550K from 0.6 to 13.3 dpa have been characterized by TEM. The microstructure is comprised of a very high density of small Frank loops present in sizes as small as 1 nm and perhaps lower, which could be of both vacancy and interstitial-type. The density of the visible loops is already established at the lowest dose available, however, the microstructure continues to evolve as the dose increases. The size distributions of the Frank loops change from a narrow symmetric distribution to a much broader size distribution with a bimodal distribution beginning to form at the higher doses.

Atom probe tomography provides one of the most effective tools to characterize the solute distribution and precipitation that occurs in pressure vessel steels and associated model alloys during irradiation. The three-dimensional atom probe is able to experimentally determine the elemental identities of the atoms and their spatial coordinates with near atomic resolution so that their distribution within small volumes of the specimen can be reconstructed and analyzed. This technique together with conventional atom probe field ion microscopy has been applied to many different types of pressure vessel steels and model alloys and has revealed and characterized several different nanostructural transformations. These radiation induced or enhanced processes lead to the formation of copper-nickel-manganese-silicon-enriched precipitates, and solute segregation to dislocations, dislocation loops, nanovoids and boundaries.

Primary radiation damage in displacement cascades in metals has been studied extensively by atomistic simulation during the last decade. The variety of defect types observed in cascade simulation is not entirely consistent with experimental data. For example, experiments on copper show a very effective production of stacking fault tetrahedra (SFTs) but this was not observed systematically in cascade simulation. To clarify this and related issues, extensive simulation of displacement cascades in copper have been performed using two different interatomic potentials, a short-range many-body potential and a long-range pair potential. We have studied the damage created by primary knock-on-atoms of energy up to 20keV, i.e. below the energy range for formation of subcascades, at temperatures 100 and 600K. Special attention was paid to cascade statistics and the accuracy of simulation in the collision stage. The former required many simulations for each temperature whereas the latter involved a modification of the simulation method. The results on variety of clusters observed, e.g. SFTs, glissile and sessile interstitial clusters, and faulted and perfect interstitial dislocation loops, lead to conclusions on the effect of the potentials and the significant variation of the number of Frenkel pairs and clustering effects produced in different cascades under the same conditions.

We developed a rate-theory-based model to investigate the nucleation and growth of interstitial loops and cavities during low-temperature in-reactor irradiation of uranium-molybdenum alloys. Consolidation of the dislocation structure takes into account the generation of forest dislocations and capture of interstitial dislocation loops. The theoretical description includes stress-induced glide of dislocation loops and accumulation of dislocations on cell walls. The loops accumulate and ultimately evolve into a low-energy cellular dislocation structure. Calculations indicate that nanometer-size bubbles are associated with the walls of the cellular dislocation structure. The accumulation of interstitial loops within the cells and of dislocations on the cell walls leads to increasing values for the rotation (misfit) of the cell wall into a subgrain boundary and a change in the lattice parameter as a function of dose. Subsequently, increasing values for the stored energy in the material are shown to be sufficient for the material to undergo recrystallization. Results of the calculations are compared with SEM photomicrographs of irradiated U- 10Mo, as well as with data from irradiated UO2.

Radiation effects on zeolite-NaY have been investigated by irradiation with a 500 keV H+ beam. The crystalline-to-amorphous transition occurs at a total dose equivalent to an ionizing energy deposition of 3×1010Gy and a displacement damage level of 0.05 dpa. The ion exchange capacity of the irradiated zeolite-NaY with 10 mN SrCl2 or 10 mN AgNO3 solutions varies with the extent of the damage to the crystalline structure. After 25 hours of exchange, the Sr concentration in the amorphous region is nearly zero. This result shows that radiation-induced amorphization can cause a significant loss in ion exchange capacity. The data also suggest that the radiation damage by proton radiation in zeolite-NaY is through both ionizing and displacement processes. The result from Ag-exchange reaction with the irradiated zeolite suggests that higher radiation dose is required to block the penetration of Ag into the supercages. The Ag concentration (∼3 at.%) in the fully damaged region suggests that the cations in the small cages are still exchangeable even when amorphization has been achieved.

The effect of ion beams on the formation of Si nanoclusters from a-SiOx films and their luminescence properties is investigated. a-SiOx films with Si content ranging from 33 to 50 at. % were deposited by Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition (ECR-PECVD) of SiH4 and O2. Prior to anneal, some samples were implanted with 380 keV Si to a dose ranging from 5.7 × 1014 cm-2 to 5.7 × 1016 cm-2. All films were rapid thermal annealed under flowing Ar environment, and hydrogenated after anneals to passivate defects and to enhance the luminescence of Si nanoclusters. For films with Si content less than 40 at. %, ion beam slightly reduces the photoluminescence (PL) intensity and induces a slight blueshift of the luminescence. For films with Si content greater than 40 at. %, ion beam greatly increases the PL intensity. Based on the effect of the ion beams dose and the ion specie, we propose that ion beams damage greatly promotes nucleation of small Si clusters from the a-SiOx matrix.

We reported previously an experimental result which shows that voids can move in neutron-irradiated copper at elevated temperature. To study the detailed behavior of voids, we carried out annealing experiments and in-situ observations in neutron-irradiated copper. Neutron irradiation was carried out in the temperature controlled capsule at KUR (Kyoto University Reactor). Neutron irradiation was performed at 300°C to damage levels between 10-4 to 10-3 dpa. The annealing temperature was 250°C for 10, 20, 30 min sequentially for neutron- irradiated copper. After annealing TEM observation was carried out at room temperature. Experimental results show that voids moved along the [110] direction. Voids moved during 250°C annealing but small stacking fault tetrahedra (SFT) were not changed. We observed 37 voids and 8 voids moved, the others vanished during annealing. The images of in-situ observation, void contrast images were recorded on VTR tape and analyzed frame by frame. At room temperature observation voids were static and were seen as white circles; however at 300°C the contrast changed to an oval shape and sometimes disappeared. After 7 sec contrast was a white circle and moved slightly and similar phenomena were observed quite frequently at 300°C. It is concluded that void moved with dynamical structural relaxation at elevated temperature.

The formation of nanovoids in Si(100) and MgO(100) by 3He ion implantation has been studied. Contrary to Si in which the voids are generally almost spherical, in MgO nearly perfectly rectangular nanosize voids are created. Recently, the 2D-ACAR setup at the Delft Positron Research Center has been coupled to the intense reactor-based variable-energy positron beam POSH. This allows a new method of monitoring thin layers containing nanovoids or defects by depth-selective high-resolution positron beam analysis. The 2D-ACAR spectra of Si with a buried layer of nanocavities reveal the presence of two additional components, the first related to para-positronium (p-Ps) formation in the nanovoids, and a second one most likely related to unsaturated Si-bonds at the internal surface of the voids. The positronium is present in excited kinetic states with an average energy of 0.3 eV. Refilling of the cavities by means of low dose 3He implantation (1×1014 cm-2) followed by annealing reduces the formation of Ps and the width of the Ps peak in the ACAR spectrum. This width reduction is due to collisions of Ps with He atoms in the voids. In MgO, p-Ps formed with an initial energy of ∼3 eV shows a final average energy of 1.6 eV at annihilation due to collisions with the cavity walls. Possibilities of this new, non-destructive method of monitoring the sizes of cavities and the evolution of nanovoid layers will be discussed.

As part of a broad effort to understand the mechanisms of irradiation embrittlement in reactor pressure vessels steels, irradiation hardening and microstructural evolution in simple model Fe-0.9Cu-1.0Mn, Fe-0.9Cu and Fe alloys irradiated with 3.2 MeV protons at 300°C are compared to the corresponding changes in hardening produced by neutron irradiation over a similar dose range of 0.0004 to 0.015 dpa. In the case of the proton irradiated samples, Vickers hardness was measured at a 25 g load and the microstructures were characterized using small angle x-ray scattering (SAXS). In spite of the much higher dpa rate for protons (3 × 10−7 dpa/s compared to neutron rates of about 10−9 to 10−10 dpa/s) as well as very different primary recoil spectra, the observed hardening-dose response is very similar in both cases. The large increase in hardness in the alloys with 0.9% copper, and the SAXS data are consistent with precipitation of coherent copper-rich features accelerated by irradiation enhanced diffusion, as well as a much smaller contribution presumably from defect clusters that do not require the presence of copper.

Molecular dynamics (MD) simulations of various cluster ions impacting on solid targets were performed in order to examine the implant and damage formation processes. Ne and Ar rare gas cluster with various cluster sizes, and fullerene (C60) were impacted on diamond (001) surface. It was shown that the impact process of cluster ion depends on the cluster size. When the cluster size is small and incident energy-per-atom is high, such as Ar15 with 800eV/atom, all incident Ar atoms penetrate the surface and reside in the substrate. As the cluster size increases and the incident energy-per-atom decreases, the implant depth decreases and the profile of the displacement becomes shallower. A large cluster, such as Ar60 with 200eV/atom, shows a shallower implant depth and a higher sputtering yield than Ar15. However, Ar240 with 50eV/atom shows a shallower implant depth, but less sputtering yield than Ar60.These results suggest that there is proper cluster size and incident energy where the maximum sputtering yield is achieved.

We present the crystallization effects occurring in sputtered amorphous Carbon (a-C) thin films deposited on Si induced by post-growth low energy (0.5-1.5 keV) Ar+ ion beam irradiation (IBI). The a-C films after IBI have the form of an amorphous matrix with embedded crystalline regions. X-ray diffraction and Electron Microscopy measurements identified the crystalline phases of carbon and SiC. We study in detail the effects of ion energy and fluence on the crystallization process. It was found that low fluence (∼2×1016 ions/cm2) of ions with an optimum ion energy (∼1.5 keV) promoted the diamond formation. X-Ray Reflectivity (XRR) and Spectroscopic Ellipsometry were used to study the amorphous matrix. XRR discriminated the IBI induced surface and bulk effects through the density and the a-C surface roughness, showing surface smoothing to be more prominent for low energy IBI.

Molecular dynamics (MD) simulations of a-SiC using several Tersoff potentials have been performed and their influences on structure ordering were studied. It was found that using different potential cutoffs leads to remarkably different structures. An abrupt cutoff at 2.5 Å greatly increases the chemical ordering of a-SiC by disfavoring the formation of Si-Si bonds. In addition, annealing of SiC cascades embedded in β-SiC was simulated, and the final structures were compared. Again, much stronger topological and chemical ordering was observed in the structure modeled with the 2.5 Å potential cutoff.

This study examines the effect of implantation and annealing parameters on the formation of Ge crystallites in ion-implanted silica. Samples were implanted at room temperature or −196oC with 1.0 MeV Ge+ ions to fluences in the range from 0.6 to 3×1017 Ge.cm−2. Ge nanocrystals were not observed in samples implanted at −196°C but were observed in samples irradiated at room temperature. Nanocrystals were observed in all samples after annealing and their size distribution was found to be almost independent of the implant fluence. The distributions were also shown to be asymmetrical with depth, with smaller nanocrystals located on the near-surface side of the implant distribution.

Aluminum films containing solid Xe precipitates have been subjected to 1 MeV electron irradiation in a high-voltage electron microscope. High-resolution images have been recorded on videotape in order to monitor the changes to the system resulting from the passage of electrons through the film. Inspection of the video recordings reveals that complex, rapid processes occur under the electron beam. These include shape changes, the creation and movement of extended defects within the Xe lattice, movement of small clusters, coalescence of neighboring clusters and the apparent melting and resolidification of the Xe. An interpretation of many of the observations is presented in terms of the interaction of the nanoclusters with defects created in the aluminum by the high-energy electrons.

An extensive database of atomic displacement cascades in iron has been developed using molecular dynamic simulations. More than 300 cascades have been completed at 100K at energies between 100 eV and 100 keV, with fewer simulations at 600 and 900K. A systematic evaluation of the database has revealed an unexpected influence of PKA direction in low energy simulations. For primary knockon (PKA) directions that lie in the close-packed {110} planes, the cascade tends to be constrained to develop in only two dimensions. This planar “channeling” leads to much higher point defect survival than when a random high-index PKA direction is used to initiate the cascade. For example, the average number of stable Frenkel pair produced in 300 eV cascades is about 2.1 for cascades initiated with a [135] PKA, and 3.2 for [114] cascades. Some influence of this PKA direction effect was observed for energies up to 2 keV. The interstitial clustering behavior also appears to be affected in cascades with high defect survival.

The microstructures of reduced-activation ferritic/martensitic steels, 9Cr-2WVTa and 9Cr-2WVTa doped with 2% Ni, irradiated at 400oC up to 12 dpa in the High Flux Isotope Reactor (HFIR), were investigated by transmission electron microscopy. Specimens were tempered at two different temperatures in order to investigate the effects of tempering on microstructural evolution during irradiation. Before irradiation, the lath width of Ni-doped 9Cr-2WVTa was somewhat narrower and the dislocation density tended to be higher compared with 9Cr-2WVTa. Dislocation density of specimens tempered at 750°C was lower than that tempered at 700°C. In all steels, precipitates on grain and/or lath boundaries were mainly M23C6, and there were a few TaC along dislocations in the matrices. Irradiation-induced cavities were observed in all the steels. The cavity number density of the Ni-doped 9Cr-2WVTa was higher than that of 9Cr-2WVTa due to the higher concentration of helium; however, swelling in each steels was < 0.1% because cavity sizes were so small. There was no difference of cavity number density between the steels tempered at 700°C and 750°C, but the mean size of the cavities in the steels tempered at 750°C was larger than that tempered at 700°C. Irradiation-induced a0<100> and (a0/2)<111> type dislocation loops were observed in all steels; number density and mean diameter of a0<100> type loops was higher and larger than that of (a0/2)<111> type loops. There was a tendency for the number density of loops in Ni-doped 9Cr-2WVTa to be slightly higher than that in 9Cr-2WVTa. In addition, the mean size of loops in the steels tempered at 750°C was larger than for those tempered at 700°C, while there was not much difference of number density between them. In the steels doped with Ni, irradiation-produced precipitates, identified as M6C(η)-type carbide, were found in the matrices. In this experiment, the change in tensile properties and the δDBTT of the 9Cr-2VWTa-2Ni was greater than for 9Cr-2WVTa. The microstructural evidence suggests that these differences in mechanical behavior are related to the formation of radiation-induced precipitates and cavities in the Ni-doped alloy; further analysis of the data using barrier hardening models is in progress.

Changes in bulk composition are known to affect both radiation-induced segregation and microstructural development, including void swelling in austenitic stainless steel. In this work, three alloys (designations corresponding to wt%) have been studied: Fe-18Cr-8Ni alloy (bulk composition corresponding to 304 stainless steel), Fe-18Cr-40Ni (bulk composition corresponding to 330 stainless steel), and Fe-16Cr-13Ni (bulk composition corresponding to 316 stainless steel). Following irradiation with high-energy protons, the change in hardness and microstructure (void size distribution and grain boundary composition) due to irradiation was investigated. Increasing the bulk nickel concentration decreases void swelling, increases matrix hardening, and increases grain boundary chromium depletion and nickel enrichment. The analysis shows that decreases in lattice parameter and shear modulus due to radiation- induced segregation (RIS) correlate with decreased void swelling and a decreased susceptibility to irradiation assisted stress corrosion cracking (IASCC). Traditional thinking on IASCC assumed RIS was a contributing factor to cracking. It may, however, be that properly controlled RIS can be used to mitigating cracking.

Medium-range order has been observed in ion-implanted amorphous silicon, suggesting a paracrystalline structure for this material. The origin of a paracrystalline structure may be due to an energy spike phenomenon. To evaluate the influence of energy spikes on a particular process, we have attempted to calculate the characteristic energy in a spike. However, the observed depth dependence of amorphous structures in as-implanted silicon is puzzling. To explain this, we simulated the depth distribution of cascade events in a particular energy range. We found a great increase of point defect concentration and cascade events as the depth increases. This result could explain the experimental depth dependence.

Grain boundary compositions and near-boundary microstructures have been measured in complex Fe-Cr-Ni alloys after neutron irradiation at intermediate temperatures where nanometer-scale damage promotes structural integrity problems in nuclear reactor systems. Radiation-induced segregation (RIS) and dislocation loop microstructures have been determined as a function of irradiation dose up to 13 dpa and at temperatures near 280°C. The most significant effect on RIS was the grain boundary structure (low-energy special boundaries versus high-energy random boundaries) and composition (enrichment of Cr and Mo) before irradiation. Grain boundary character distribution did not change with irradiation and only high-energy boundaries exhibited significant radiation-induced changes. The initial grain boundary composition in mill-annealed stainless steels was difficult to remove during subsequent irradiation and retarded the development of Cr- and Mo-depleted regions.