Cross-section transmission electron microscopy was used to investigate the microstructure of polycrystalline silicon nitride (Si 3 N 4 ) and aluminum nitride (AIN) following 2 MeV Si ion irradiation at 80 and 400 K up to a fluence of 4×10 20 ions/m 2 (maximum damage of ∼ 10 displacements per atom, dpa). A buried amorphous band was observed at both temperatures in Si 3 N 4 in the region corresponding to the peaks in the implanted ion and displacement damage. From a comparison of Si 3 N 4 specimens 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 Si 3 N 4 , 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 Si 3 N 4 and AIN specimens at both irradiation temperatures. Aligned network dislocations were also observed in the AIN specimen irradiated to high dose at 80 K.

A temperature controlled instrumented capsule has been irradiated in the RB* position of the high flux isotope reactor (HFIR) at the Oak Ridge National Laboratory in order to perform in situ thermal conductivity measurements of monolithic ceramics and composite materials during neutron irradiation. This determination of thermal conductivity utilizes a temperature gradient technique whereby two thermocouples measure the absolute temperature along a cylindrical sample which is constrained to one-dimensional thermal conduction. The heat source is the intrinsic gamma heating of the core region of the HFIR. This paper provides an overview of the experiment and gives preliminary results on the degradation in thermal conductivity of a few ceramic specimens measured as a function of irradiation temperature (200–700°C) and fast neutron fluence up to ∼3.4×1025 n/m2 (E>0.1MeV).

Neutron irradiation at low temperatures (100 to 275 degrees C) to 0.5 displacements per atom causes significant embrittlement and changes in the subsequent room temperature tensile properties of V-4Cr-4Ti. The yield strength and microhardness at room temperature increase with increasing irradiation temperature. The tensile properties at room temperature show large increases in strength and a complete loss of work hardening capacity with no uniform elongation. Embrittlement, as measured by an increase in the ductile-to-brittle transition temperature, increases with increasing irradiation temperature, at least up to 275 degrees C. It is shown that this embrittlement is not due to pickup of O or other interstitial solutes during the irradiation.

Single crystal silicon carbide (SiC) has been 2 MeV silicon ion irradiated in various irradiation temperature and ion flux ranges to measure the effect of these parameters on the critical dose for amorphization. The temperature and flux range for which amorphization was observed ranged from 80 to 400 K and 0.066 to 3 × 104 dpa/s, respectively. The critical dose, Dcrit was found by locating the depth of the boundary between partially crystalline and fully amorphous material using dark field TEM from samples prepared in cross section. This depth was compared to the damage profile as calculated using the TRIM-96 code. The temperature dependence of Dcrit is found to agree well with previously reported values, though new evidence suggests a defect species becoming mobile in the 250-300 K range. Also of significance is that Dcrit was dependent on flux at 340 K, ranging from 0.79 displacements per atom at the lowest ion flux to ∼0.6 dpa at the highest flux level. The dose rate dependence of Dcrit, is compared with a chemical rate theory model previously described by the authors. It is seen that the dose rate dependence is substantially weaker than theorized. An extrapolation of the measured dose rate dependence is also compared with recent data on fast neutron amorphized SiC.

An experimental investigation of the in situ electrical conductivity of Wesgo Al995 polycrystalline alumina at approximately 450 °C has been performed at the high flux beam reactor at Brookhaven National Laboratory. The measured radiation induced conductivity (RIC) was about 10−8 S/m at an ionizing dose rate of 6000 Gy/s. No evidence for permanent radiation induced electrical degradation was observed for an applied electric field of 147 V/mm up to a dose level of ≈1.8 displacements per atom. The effect of neutron irradiation on the electrical properties of two mineral insulated cables was also investigated. The RIC in the MgO insulation of a coaxial and a triaxial cable was measured to be in the range of 6–20 ×10−8 S/m at an ionizing dose rate of ≈ 6000 Gy/s.

Experimental details, raw data, method of analysis and results are presented for the low-temperature, low-dose HFBR-V1 through V4 irradiation experiments conducted at ORNL on V-4Cr-4Ti specimens (US Fusion Program Heat No. 832665). Four separate capsules were irradiated in the V-15 and V-16 In-Core Thimbles of the High Flux Beam Reactor at the Brookhaven National Laboratory to doses of 0.1 or 0.5 dpa at temperatures between 100 and 505 C. Testing included microhardness, electrical resistivity, tensile properties, and Charpy impact properties.

Reduced oxygen Hi–Nicalon™ fiber reinforced composite SiC materials were densified with a chemically vapor infiltrated (CVI) silicon carbide (SiC) matrix and interphases of either `porous' SiC or multilayer SiC and irradiated to a neutron fluence of 1.1×1025 n m−2 (E>0.1 MeV) in the temperature range of 260 to 1060°C. The unirradiated properties of these composites are superior to previously studied ceramic grade Nicalon fiber reinforced/carbon interphase materials. Negligible reduction in the macroscopic matrix microcracking stress was observed after irradiation for the multilayer SiC interphase material and a slight reduction in matrix microcracking stress was observed for the composite with porous SiC interphase. The reduction in strength for the porous SiC interfacial material is greatest for the highest irradiation temperature. The ultimate fracture stress (in four point bending) following irradiation for the multilayer SiC and porous SiC interphase materials was reduced by 15% and 30%, respectively, which is an improvement over the 40% reduction suffered by irradiated ceramic grade Nicalon fiber materials fabricated in a similar fashion, though with a carbon interphase. The degradation of the mechanical properties of these composites is analyzed by comparison with the irradiation behavior of bare Hi–Nicalon fiber and Morton chemically vapor deposited (CVD) SiC. It is concluded that the degradation of these composites, as with the previous generation ceramic grade Nicalon fiber materials, is dominated by interfacial effects, though the overall degradation of fiber and hence composite is reduced for the newer low-oxygen fiber.

The unirradiated tensile properties of wrought GlidCop AL25 (ITER grade zero, IGO) solutionized and aged CuCrZr, and cold-worked and aged and solutionized and aged Hycon 3HP{trademark} CuNiBe have been measured over the temperature range of 20-500{degrees}C at strain rates between 4 x 10{sup {minus}4} s{sup {minus}1} and 0.06 s{sup {minus}1}. The measured room temperature electrical conductivity ranged from 64 to 90% IACS for the different alloys. All of the alloys were relatively insensitive to strain rate at room temperature, but the strain rate sensitivity of GlidCop Al25 increased significantly with increasing temperature. The CuNiBe alloys exhibited the best combination of high strength and high conductivity at room temperature. The strength of CuNiBe decreased slowly with increasing temperature. However, the ductility of CuNiBe decreased rapidly with increasing temperature due to localized deformation near grain boundaries, making these alloy heats unsuitable for typical structural applications above 300{degrees}C. The strength and uniform elongation of GlidCop Al25 decreased significantly with increasing temperature at a strain rate of 1 x 10{sup {minus}3} s{sup {minus}1}, whereas the total elongation was independent of test temperature. The strength and ductility of CuCrZr decreased slowly with increasing temperature.

Room temperature electrical resistivity measurements have been performed on vanadium alloys containing 3-6%Cr and 3-6%Ti in order to evaluate the microstructural stability of these alloys. A nonlinear dependence on Cr and Ti concentration was observed, which suggests that either short range ordering or solute precipitation (perhaps in concert with interstitial solute clustering) has occurred in V-6Cr-6Ti.

The unirradiated tensile properties of CuCrZr produced by two different vendors have been measured following different heat treatments. Room temperature electrical resistivity measurements were also performed in order to estimate the thermal conductivity of these specimens. The thermomechanical conditions studied included solution quenched, solution quenched and aged (ITER reference heat treatment), simulated slow HIP thermal cycle ({approximately}1{degrees}C/min cooling from solutionizing temperature) and simulated fast HIP thermal cycle ({approximately}100{degrees}C/min cooling from solutionizing temperature). Specimens from the last two heat treatments were tested in both the solution-cooled condition and after subsequent precipitate aging at 475{degrees}C for 2 h. Both of the simulated HIP thermal cycles caused a pronounced decreases in the strength and electrical conductivity of CuCrZr. The tensile and electrical properties were unchanged by subsequent aging in the slow HIP thermal cycles caused a pronounced decrease in the strength and electrical conductivity of CuCrZr. The tensile and electrical properties were unchanged by subsequent aging in the slow HIP thermal cycle specimens, whereas the strength and conductivity following aging in the fast HIP thermal cycle improved to {approximately}65% of the solution quenched and aged CuCrZr values. Limited tensile and electrical resistivity measurements were also made on two new heats of Hycon 3HP CuNiBe. High strength but poor uniform and total elongations were observed at 500{degrees}C on one of these new heats of CuNiBe, similar to that observed in other heats.

The TRIST-ERI capsule was assembled and irradiated in a large Removable Beryllium (RB{star}) position of the High Flux Isotope Reactor (HFIR) during this reporting period. Irradiation began on March 8, 1996, was completed on June 20, 1996, during operating cycles 344, 345, and 346. This report describes the thermal operation of the capsule.

A collaborative DOE/Monbusho series of irradiation experiments is being implemented to determine, in situ, the effects of irradiation on the electrical resistivity of ceramic materials. The first experiment, TRIST-ER1, has been designed to irradiate 15 Al{sub 2}O{sub 3} test specimens at 450{degrees}C in an RB position of the High Flux Isotope Reactor (HFIR). Each test specimen is located in a sealed vanadium subcapsule with instrumentation provided to each subcapsule to measure temperature and resistance, and to place a biasing voltage across the specimen. Twelve of the specimens will be biased with 200 V/mm across the sample at all times, while three will not be biased, but can be if so desired during the irradiation. The experiment design, component fabrication, and subcapsule assembly have been completed. A three cycle irradiation, to a fast neutron (E>0.1 MeV) fluence of about 3x10{sup 25}n/m{sup 2} ({approx}3 dpa in Al{sub 2}O{sub 3}), is expected to begin early in March 1996.

An experimental investigation of radiation induced electrical degradation (RIED) has been performed at the High Flux Beam Reactor (HFBR) at Brookhaven National Laboratory. In this study (the third in a series of experiments at the HFBR) the effects of neutron irradiation on the electrical conductivity of Wesgo AL995 polycrystalline alumina has been investigated at approximately 450{degrees}C. The capsule design used in this study is very similiar to a design used in the first two experiments in this series with some improvements made to a design used in the first two experiments in this series with some improvements made in the cable terminations. A guard ring configuration was used on the disk shaped sample. Triaxial mineral insulated cable was used as the data lead from the sputter deposited guard ring and central electrode of the sample, and coaxial mineral insulated cable was used as the sample power lead. No evidence for REID was observed in this series of experiments to a dose level of {approx}1.8 dpa. The effect of neutron irradiation on the electrical properties of two mineral insulated (MgO) cables was also investigated.

Tensile testing was carried out on SS-3 tensile specimens punched from 0.762-mm-thick sheets of the large heat of V-4Cr-4Ti and small heats of V-3Cr-3Ti and V-6Cr-6Ti. The tensile specimens were annealed at 1000{degrees} for 2 h to obtain a fully recrystallized, fine grain microstructure with a grain size in the range of 10-19 {mu}m. Room temperature tests at strain rates ranging from 10{sup {minus}3} to 5 x 10{sup {minus}1}/s were carried out in air; elevated temperature testing up to 700{degrees}C was conducted in a vacuum better than 1 x 10{sup {minus}5} torr (<10{sup {minus}3} Pa). To study the effect of atomic hydrogen on ductility, tensile tests were conducted at room temperature in an ultra high vacuum chamber (UHV) with a hydrogen leak system.

The unirradiated tensile properties of two different heats of Hycon 3HP{trademark} CuNiBe (HT Temper) have been measured over the temperature range of 20-500{degrees}C for longitudinal and long transverse orientations. The room temperature electrical conductivity has also been measured for both heats. Both heats exhibited a very good combination of strength and conductivity at room temperature. The strength remained relatively high at all test temperatures, with a yield strength of 420-520 MPa at 500{degrees}C. However, low levels of ductility (<5% uniform elongation) were observed at test temperatures above 200-250{degrees}C, due to flow localization adjacent to grain boundaries. Fission neutron irradiation to a dose of {approximately}0.7 dpa at temperatures between 100 and 240{degrees}C produced a slight increase in strength and a significant decrease in ductility. The measured tensile elongation increased with increasing irradiation temperature, with a uniform elongation of {approximately}3.3% observed at 240{degrees}C. The electrical conductivity decreased slightly following irradiation, due to the presence of defect clusters and Ni, Zn, Co transmutation products. The data indicate that CuNiBe alloys have irradiated tensile and electrical properties comparable or superior to CuCrZr and oxide dispersion strengthened copper at temperatures <250{degrees}C, and may be suitable for certain fusion energy structural applications.

A series of slow cycle and rapid cycle anneals was carried out in the large heat of V-4Cr-4Ti alloy (heat 832665). Also, a differential scanning calorimetry (DSC) study was initiated on the samples of the same alloy. The recovery and recrystallization phenomena of V-4Cr-4Ti in slow cycle annealing were quite different from that observed in rapid cycle annealing. The large driving force for recrystallization due to rapid heating resulted in the first nuclei appearing after only 1 minute of 1000{degrees}C. There was a two-stage hardness reduction; the first stage involved recovery due to cell formation and annihilation of dislocation, and second stage was associated with the growth of recrystallization nuclei. This is consistent with results obtained from the DSC in which there was a broad exothermic peak from {approx}200 to 800{degrees}C due to recovery followed by a sharp exotherm associated with recrystallization. The activation energy for recrystallization for V-4Cr-4Ti, which was determined at 576 {+-} 75, kJ/mole is significantly higher than that for pure V, and is thought to be related to Ti and Cr in solid solution.