SiC/SiC composites were fabricated from Hi-Nicalon™ fibers with carbon, porous SiC and multilayer SiC interphases. These materials were then irradiated in the High Flux Beam Reactor with fast neutrons at 260 and 900–1060°C to a dose of l.1×1025 n/m2 corresponding to 1.1 displacements per atom (dpa). Results are presented for bend strength of both non-irradiated and irradiated materials. Within the interphases studied the multilayer SiC interphase material showed the least degradation (8-20%) in ultimate bend stress, while porous SiC underwent the greatest degradation (∼35%). The fiber matrix interphases are studied with TEM for both non- irradiated and irradiated materials. While no irradiation induced microstructural evolution of the interphase was observed, debonding of the interphase from the fiber was observed for all cases. This debonding is attributed to tensile stresses developed at the interface due to densification of the Hi-Nicalon™ fiber. Residual stress analysis of the fiber matrix interface indicates that the irradiation-induced densification of Hi-Nicalon™ and the volumetric expansion of the CVD SiC matrix cause tensile stresses well in excess of those which can be withstood by these, or any other viable SiC composite interphase.

Creep tests were conducted in torsion on Textron SCS-6™ silicon carbide (SiC) fibers during irradiation with light ions in the temperature range 450-1100°C up to doses of 0.06 dpa. The fibers, produced by chemical vapor deposition (CVD), should be representative of a SiC/SiC composite matrix produced by chemical vapor infiltration (CVI).

For temperatures between 450 and 600°C, the irradiation creep curves were characterised by long lasting strain transients during which the creep rate slowed down before reaching approximately steady state values. On a decrease in temperature the creep rate increased.

For temperatures between 900 and 1100°C, the transient creep regime was negligible. The creep rates reached constant values shortly after starting the irradiation and increased with temperature. The activation energy was E = 0.55±0.15 eV.

The results are discussed in terms of concentration and mobility of point defects and the change of these quantities with temperature.

This paper reviews our recent progress in study on the strong resistance of magnesium aluminate spinel to void swelling during irradiation, along with the related characteristic features of its radiation damage. Comparative experimental results on irradiated microstructures and mechanical properties in magnesium aluminate spinel and alpha-alumina are shown in terms of controlling factors of radiation resistance of the former crystal. It is experimentally shown that structural vacancies due to non-stoichiometry provide effective recombination sites for displaced cations to suppress the formation of interstitial loops. Decreased formation of interstitial loops enhances the further recombination of interstitials and vacancies and thereby the formation of voids.

Plan view and cross-section transmission electron microscopy was used to investigate the microstructure of magnesium aluminate spinel (MgAl2O4) following room temperature irradiation with either 430 MeV Kr, 614 MeV Xe, or 72 MeV I ions. The fluences ranged from 1×1016/m2 (single track regime) to 1×1020/m2. Destruction of the ordered spinel crystal structure on both the anion and cation sublattices was observed in the ion tracks at low fluences. At intermediate fluences, the overlapping ion tracks induced the formation of a new metastable crystalline phase. Amorphization with a volumetric expansion of ∼35% was observed in spinel irradiated with swift heavy ions (electronic stopping powers >7 keV/nm) at fluences above 1×1019/m2. These results demonstrate that swift heavy ion radiation can induce microstructural changes not achievable with conventional elastic collision irradiation at comparable temperatures.

Cross-section transmission electron microscopy was used to investigate the microstructure of polycrystalline silicon nitride (Si3N4) and aluminum nitride (AIN) following 2 MeV Si ion irradiation at 80 and 400 K up to a fluence of 4×1020 ions/m2(maximum damage of ∼ 10 displacements per atom, dpa). A buried amorphous band was observed at both temperatures in Si3N4 in the region corresponding to the peaks in the implanted ion and displacement damage. From a comparison of Si3N4specimens irradiated at different fluences, it is concluded that the amorphization is primarily controlled by the implanted Si concentration rather than the displacement damage level. Si3N4 amorphization did not occur in regions well-separated from the implanted ions for doses up to at least 3 dpa at 80 K, whereas amorphization occurred in the ion implanted region (calculated Si concentration >0.01 at.%) for damage levels as low as ∼0.6 dpa. The volumetric swelling associated with the amorphization of Si3N4, is <10%. Amorphization was not observed in any of the irradiated AIN specimens. A moderate density of small (∼3 nm) defect clusters were observed in the crystalline damaged regions of both the Si3N4 and AIN specimens at both irradiation temperatures. Aligned network dislocations were also observed in the AIN specimen irradiated to high dose at 80 K.

Rutile (TiO2) single crystals with (110) orientation were irradiated with 360 keV Xe2+ ions at 300K to fluences ranging from 2×1019 to 1×1020 Xe/m2. Irradiated samples were analyzed using: (1) Rutherford backscattering spectroscopy combined with ion channeling analysis (RBS/C); and (2) cross-sectional transmission electron microscopy (XTEM). Upon irradiation to a fluence of 2×1O19 Xe/m2, the sample thickness penetrated by the implanted ions was observed to consist of three distinct layers: (1) a defect-free layer at the surface (thickness about 12 nm) exhibiting good crystallinity; (2) a second layer with a low density of relatively large- sized defects; and (3) a third layer consisting of a high concentration of small defects. After the fluence was increased to 7×1019 Xe/m2, a buried amorphous layer was visible by XTEM. The thickness of the amorphous layer was found to increase with increasing Xe ion fluence. The location of this buried amorphous layer was found to coincide with the measured peak in the Xe concentration (measured by RBS/C), rather than with the theoretical maXimum in the displacement damage profile. This observation suggests the implanted Xe ions may serve as nucleation sites for the amorphization transformation. The total thickness of the damaged microstructure due to ion irradiation was always found to be much greater than the projected range of the Xe ions. This is likely due to point defect migration under the high stresses induced by ion implantation.

We have investigated radiation hardening in alumina (A1203), stoichiometric and nonstoichiometric spinel (MgO·nA1203: n= 1, 2.4) crystals at 300 K irradiated with 100 keV He+ ions by using a ultra-microhardness technique. Al203 shows a remarkable radiation hardening (35% increase in hardness) at a fluence of 5×1019 He+/m2 and saturates at 60 % of the maximum value at fluences higher than 8×1019 He+/m2. In spinel crystals, hardness increases monotonically with fluence, reaching to a saturation at a fluence of 1×1020He+/m2. Analyses of load- displacement curves indicate that both plastic and elastic hardening are responsible for the radiation hardening in A1203, and that plastic hardening is the main cause in MgO'nAl2O3. Microstructure observations and lattice constant measurements showed that point defects are mainly responsible for the radiation hardening in both A1203 and MgO·nAl2O3. The difference in the radiation hardening response is discussed in terms of the difference in recombination rate of point defects among the MgO-A1203 system ceramics.

Eight perovskites of different compositions and pre-existing vacancy contents were irradiated with 1.5 MeV Kr+ ions using the HVEM-Tandem User Facility at Argonne National Laboratory. The critical dose of 1.5 MeV Kr+ ions for amorphisation (Dc) of NaNbO3, SrTiO3 and two natural perovskites varies with composition and may decrease with Na content. The Dc values of 5 members of the solid solution series from SrTiO3 to La0.67TiO3 do not exhibit a simple relationship with vacancy content: In particular, Dc(Sr0.7La0.2TiO3) is extraordinarily high (>170 × 1014 ions cm−2) compared to those of the other (Sr, La)-perovskites in the series, which have Dc, values between 2.4 and 11×1014 ions cm−2.

In this study, we simulated gas bubble formation in glasses by in-situ ion implantation. Alkali silicate glass and Na-borosilicate glass were implanted in situ with 50 keV Xe ions at temperatures at 200°C in a Hitachi-9000 electron microscope. Bubble formation was studied by transmission electron microscopy images taken during interruptions of the ion beam after discrete implanted-ion dose steps. We present a possible mechanism of bubble formation and growth based on amorphous network structures.

A zirconia based ceramic is foreseen as an inert matrix fuel for burning excess plutonium in light water nuclear reactors. For reactor safety reasons the behaviour of volatile fission products such as cesium and iodine must be studied since a retention of fission products is favourable for licensing the studied inert matrix fuel. In this study, implantation of Cs and I was performed into polycrystalline (Zr0.85, Y0.15)O1.925 samples. The implantation depth was selected on the basis of the ability to observe by Rutherford backscattering spectroscopy (RBS) the behaviour of Cs and I after treatment. With a 1 MeV incident energy, the ions are implanted at a depth of 200 nm as predicted by TRIM. After implantations full quantification of I and Cs concentration profiles was performed by RBS. The implantation profiles are measured as a function of sample temperature during stepwise heating programs. It is interesting to observe retention of Cs and I at relatively high temperature (e.g. for 2 h, below 900 K for Cs and below 1400 K for I). This behaviour is likely to be due to the size and interactions of these species in the zirconia solid solution.

The results of molecular simulations of a-recoil nucleus displacement cascades in zircon (ZrSiO4) are presented. The local temperature and pressure are found to increase dramatically during the cascade. The structure of the cascade tracks is amorphous. Its shape has been analyzed in terms of disordered and distorted cations. SiO2 nanophase are found to exist in the tracks consistently with what is observed in the experiments.

The structure of a completely amorphous zircon was determined by time-of-flight neutron diffraction at Argonne's Intense Pulsed Neutron Source (IPNS). The sample of metamict zircon (ZrSiO4), initially doped to 8.85 weight percent 238pu, had been completely amorphized by alpha-recoil damage since its synthesis in 1981 at the Pacific Northwest National Laboratory (PNNL). The measured diffraction structure factor, S(Q), indicated a completely amorphous sample, with no signs of residual zircon microcrystallinity. The pair distribution function obtained indicated that the structure was that of an oxide glass, retaining the Si–O, Zr–O, and O–O bond lengths of crystalline zircon.

Pyrochlore is an important nuclear waste form phase for actinide immobilization. Two synthetic pyrochlores, Gd2Ti2O7 and Gd2Zr2O7 were irradiated at various temperatures (25 K to 1073 K) by different ion species (1.5 MeV Xe+, 1.0 MeV Kr+, and 0.6 MeV Ar+). The titanate pyrochlore amorphized at relatively low doses (0.5 ∼ 0.6 dpa). Temperature dependence of the amorphization dose for titanate pyrochlore was measured, and the critical temperatures for amorphization were 1300 K, 1100 K and 950 K by 1.5 MeV Xe+, 1.0 MeV Kr+ and 0.6 MeV Ar+, respectively. The higher critical temperature for the heavier ion irradiation is consistent with an amorphization mechanism by which the heavier ion produces a larger cascade. The zirconate pyrochlore, Gd2Zr2O7, showed a strong amorphization “resistance”. Gd2Zr2O7did not become amorphous under 1.0 MeV Kr+ and 1.5 MeV Xe+ irradiation. After prolonged irradiation (up to 7 dpa) even at a temperature of 25 K, no amorphization was observed. The irradiated zirconate pyrochlore showed abundant dislocations as observed by TEM. The pyrochlore structure of Gd2Zr2O7 transformed to the fluorite structure after irradiation. The diffraction patterns of irradiated Gd2Zr2O7 showed the existence of short-range ordering of cations. The large difference between these two pyrochlores emphasizes the strong effect of chemical composition on radiation-induced amorphization.

Three different zeolites (analcime, natrolite, and zeolite-Y) were irradiated with 200 keV and 400 keV electrons. All zeolites amorphized under a relatively low electron fluence. The transformation from the crystalline-to-amorphous state was continuous and homogeneous. The electron fluences for amorphization of the three zeolites at room temperature were: 7.0× 1019 e/cm2 (analcime), 1.8×1020 e/cm2 (natrolite), and 3.4×1020 e/cm2 (zeolite-Y). The different susceptibilities to amorphization are attributed to the different channel sizes in the structures which are the pathways for the release of water molecules and Na+. Natrolite formed bubbles under electron irradiation, even before complete amorphization. Analcime formed bubbles after amorphization. Zeolite-Y did not form bubbles under irradiation. The differences in bubble formation are attributed to the different channel sizes of the three zeolites. The amorphization dose was also measured at different temperatures. An inverse temperature dependence of amorphization dose was observed for all three zeolites: electron dose for amorphization decreased with increasing temperature. This unique temperature effect is attributed to the fact that zeolites are thermally unstable. A semi-empirical model was derived to describe the temperature effect of amorphization in these zeolites.

Several groups have irradiated single crystals of synthetic and natural zircon (ZrSiO4) with a wide range of ions (from He to Bi) over a wide range of temperatures. The results of these studies show that amorphization in zircon is controlled by a variety of parameters and is a more complex process than previously thought. The critical dose for amorphization increases with temperature, similar to other materials. However, the critical displacement dose (in dpa) for amorphization is significantly higher at lower temperatures (below 500 K) for very heavy ions, such as Pb and Bi. This unusual dependence on the damage energy density has not been observed previously in other materials. Possible explanations for this behavior in terms of amorphization mechanisms are discussed. In addition, there is a significant difference in the temperature dependence of the critical dose in synthetic and natural zircons, which suggests that the impurities in natural zircons may affect the kinetics of recovery processes.

Damage accumulation and recovery process have been investigated in single crystal SrTiO3 irradiated with 1.0 MeV Au2+ using in-situ Rutherford Backscattering Spectrometry (RBS) in Channeling geometry. Samples were irradiated at a temperature of 200 K with ion fluences ranging from 5.0×1013 2.5×1014Au2+/cm2 (0.22 – 1.10 dpa at damage peak). Subsequent isochronal annealing experiments were performed to study damage recovery processes up to a maximum temperature of 870 K. At an ion fluence between 2.0–2.5×1014Au2+/cm2 (0.88 – 1.10 dpa), the implanted region, which is just below the surface, becomes amorphous. The recovery processes occur over a broad temperature range, and the damage created by low ion fluences, 5.0×1013 − 1.0×1014 Au2+/cm2, is almost completely recovered after annealing at 870 K.

A study of vacancy defect formation is carried out using a density functional theory molecular dynamics simulation methodology. The objective is to study the effects of electronic excitations in crystalline and non-crystalline silica polymorphs. We present an initial study focusing on the formation energetics of distinct oxygen vacancy defect states in α-quartz.

Sn centers are point defects easily formed by X-ray irradiation of Sn-doped silica. Point defects are a tool for assessing the substitutional character of the dopant in Sn-doped SiO2 glasses with different Sn content (SnO2/SiO2 ratio from 0.01 to 10.0 % w/w) and prepared by two sol-gel methods differing in tin precursors (tin tetraterbutoxide and tin dibutyldiacetate). The former method produced transparent crack-free monolithic glasses of Sn-doped silica with maximum tin content of 0.1 % (SnO2/SiO2 % w/w). The latter method is more efficient and produced transparent glasses with a tin content of 1%.

Nd:YAG laser (λ,=532 nm, τ=8 ns, Φ =30 J/cm2, v=10 Hz) induced transformations in Fe81B135Si35C2metallic glass were characterized by transmission and conversion electron Mossbauer spectroscopy, hysteresis loops and ac susceptibility measurements. The radiation-driven changes were further compared with alpha particle beam irradiation effects (W=2.8 MeV, Φ=1016and 1017 cm−2) in the same amorphous material. Pulsed laser irradiation of Fe81B135Si3.5C2 metallic glass was found to induce magnetic anisotropy, surface oxidation, onset of crystallization, slight increase in the saturation magnetic moment and changes in the imaginary susceptibility component. Alpha particle beam irradiation of Fe81B13.5Si3.5C2 metallic glass caused partial crystallization of the bulk of the specimen, total crystallization of the surface, as well as dose-dependent changes in the saturation magnetic moment, hysteresis and real susceptibility component.

Synthetic ZrSiO4, HfSiO4, and ThSiO4 single-crystal specimens were irradiated by 800 keV Kr+ ions, and the microstructural evolution was observed in-situ in a transmission electron microscope. All three compounds were found to become amorphous up to temperatures in excess of 600°C. Using a new model, the activation energies for annealing were found to be in the range of 3.1 to 3.6 eV for these compounds. At temperatures above 600°C, the orthosilicates were observed to decompose into the component oxides (e.g., tetragonal ZrO2 + amorphous SiO2 in the case of zircon). A single-crystal zircon specimen was also irradiated with a pulsed picosecond Nd:YAG laser operated at 355 nm, and the resulting microstructure was investigated by optical absorption, SEM, AFM, and TEM techniques.