About the 1996 TMS Annual Meeting: Tuesday Afternoon Sessions (February 6)

RADIATION MATERIALS SCIENCE IN TECHNOLOGY APPLICATIONS II: Radiation Effects in Ceramics

Program Organizers: L. K. Mansur, Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6376; C. L. Snead, Jr., Applied Technologies Division, Brookhaven National Laboratory, P.O. Box 5000, Upton, NY 11973-5000

Tuesday, PM Room: Grand H

February 6, 1996 Location: Anaheim Marriott Hotel

Session Chairperson: Y. Chen, Department of Energy, Division of Materials Sciences, ER-131, 19901 Germantown Road, Germantown, MD 20874-1290

2:00 pm Invited

FLUENCE AND TEMPERATURE DEPENDENCE OF ION-BEAM-INDUCED AMORPHIZATION IN [[alpha]]-SiC: W. J. Weber, Pacific Northwest Laboratory, P.O. Box 999, Richland, WA 99352; N. Yu, Los Alamos National Laboratory, P.O. Box 1663, MS K-762, Los Alamos, NM 87545; L. M. Wang, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131

Single crystals of [[alpha]]-SiC have been irradiated at 173, 300, and 373 K with 360 keV Ar+ ions using the facilities within the Ion Beam Materials Laboratory at Los Alamos National Laboratory. The fluence dependence of the amorphization process was investigated as a function of temperature using in situ Rutherford backscattering spectroscopy in channeling geometry. The disordered fraction increased sigmoidally with ion fluence, consistent with a defect accumulation process. The rate of disordering decreased with increasing temperature. Post-irradiation HRTEM and Raman spectroscopy confirmed the amorphous nature of the damage state. Nanoindentation indicated decreases in hardness and elastic modulus with increasing disorder.

2:30 pm

EVIDENCE FOR IONIZATION ENHANCED DIFFUSION IN ION-IRRADIATED CERAMIC INSULATORS: S. J. Zinkle, Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6376

Cross section transmission electron microscopy was used to investigate radiation-induced microstructural changes in polycrystalline specimens of alumina (Al203), magnesium aluminate spinel (MgAl204) and silicon nitride (Si3N4) following irradiation at room temperature and 650deg.C with ions of varying mass and energy (1 MeV H+ to 4 MeV Zr). The density of interstitial dislocation loops decreased and the mean loop size increased with decreasing mass of the bombarding ion. In addition, cavity formation could be induced in some specimens at irradiation temperatures as low as 20deg.C, which corresponds to a homologous irradiation-temperature of only 0.12 Tm. Irradiation of silicon nitride with 3.6 MeV Fe ions at 20deg.C produced amorphization in the Fe-implanted regions, but amorphization did not occur in specimens that were simultaneously irradiated with 1 MeV He ions. Possible physical mechanisms responsible for these effects will be discussed, with particular emphasis on ionization enhanced diffusion. Research sponsored by the Office of Fusion Energy, U.S. Department of Energy, under contract DE-AC05-840R21400 with Lockheed Martin Energy Systems.

2:50 pm

IN SITU BEAM CHARACTERIZATION OF RADIATION DAMAGE IN YTTRIUM STABILIZED ZIRCONIA INDUCED BY 400 keV XENON IONS: Nina Yu, K. E. Sickafus, Michael Nastasi, Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545

Single crystals of cubic yttrium stabilized zirconia (YSZ) were irradiated with 400 keV Xe ion beams at cryogenic temperatures and room temperature. Radiation damage accumulation in YSZ crystals as a function of irradiation dose was in situ monitored with Rutherford backscattering spectrometry in conjunction with ion channeling techniques using a 2 MeV He ion beam. It was observed that the amount of lattice disorder in the peak damage region increased with the increasing dose and finally saturated at about 70% of the random level up to a dose of 3x10¹⁶Xe/cm^2. Cross-section transmission electron microscopy was used to determine the microstructures of irradiated YSZ crystals.

3:10 pm

THE MECHANICAL AND STRUCTURAL RESPONSE OF SPINEL FOLLOWING 12 MeV Au ION IRRADIATION: Ram Devanathan, Ning Yu, K. E. Sickafus, Michael Nastasi, Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545

Magnesium aluminate spinel (MgAl204) is a promising candidate for insulating and structural applications in fusion-reactors because of its radiation resistance. We have studied the structure-property relationships in MgAl204 subjected to high-energy ion irradiation. Single-crystals of spinel were irradiated with 12 MeV Au³⁺ at 100 K. The projected range of the implanted Au ions, determined by TRIM, was 1.5 um. The microstructure of the irradiated samples was studied using cross-sectional transmission electron microscopy, while the mechanical properties were determined using the nano-indentation technique. For each dose, the Young's modulus and nano-hardness were determined for three different indentation depths. The changes in these mechanical properties with dose will be discussed in light of the observed structural changes. These results will be compared with the findings of our earlier study of spinel subjected to 400 keV Xe ion irradiation.

3:30 pm BREAK

3:50 pm

RADIATION DAMAGE IN ILMENITE-GROUP MINERALS: Jeremy N. Mitchell, Ning Yu, K. E. Sickafus, Michael Nastasi, Thomas N. Taylor, Kenneth J. McClellan, Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545; Gordon L. Nord, USGS, Reston, VA 22092

The radiation damage response of the rhombohedral oxides ilmenite (FeTiO3) and geikielite (MgTiO3) were studied using the in situ and ex situ ion irradiation facilities at Los Alamos National Laboratory. The MgTiO3 crystals studied were grown synthetically, whereas the FeTiO3 sample is a natural crystal collected from the Adirondack Mts., NY. In our experiments, single crystals of these materials were implanted with Ar and Xe at varying doses and the surface composition and structure were monitored using ion channeling combined with Rutherford backscattering spectrometry (RBS). RBS and ion channeling measurements of these crystals implanted at varying doses at -100deg.C indicate that FeTiO3 becomes fully random at -lx10^l5 Ar cm^-2 and MgTiO3 approaches a fully random level at a dose of 2x10^l5. Analysis of the FeTiO3 crystal using XPS suggests that the implantation process may have oxidized the target. Thus, the poor radiation resistance of FeTiO3 may be due to chemical and crystallographic changes caused by implantation. In this presentation we will also explore the effects of self ion irradiations of FeTiO3. The behavior of MgTiO3 holds promise for the use of this material and other titanates in nuclear reactor applications.

4:10 pm

REDOX REACTIONS IN TRANSITION METAL CATION-BEARING OXIDES AND SILICATES: KEY TO UNDERSTANDING THERMODYNAMICS OF METAL COLLOID FORMATION? R. F. Cooper, N. Yu, J. N. Mitchell, K. E. Sickafus, Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545

Self-cation implantation into oxides has been demonstrated to result in the internal formation of metal colloids: e.g., Mg⁺ or Al⁺ implantation into MgAl204 results in the formation of colloidal Al⁺ in the region at which the implant species comes to rest [e.g., Evans et al., Mat. Res. Soc. Symp. Proc. 373, (1995)]. The result clearly suggests that the relative oxidation potentials of cation components is critical in such reactions. We are addressing this question directly through self-ion implantation experiments in Fe2⁺-doped spinel ([Mg0.999Fe0.001]Al2O4) and in natural olivine ( [MgO 88' FeO 12]2SiO4). Redox potentials make specific predictions about the chemical/structural reactions that should occur. Implantation of oxygen into olivine produces backscattering spectra that is directly correlated with that of dynamic chemical oxidation. Implantation of "inert" species may produce a similar reaction. Mg⁺ implantation in either solid results in metal colloid formation. We are presently evaluating the nature of these colloids by analytical electron microscopy.

4:30 pm

THERMAL CONDUCTIVITY DEGRADATION OF GRAPHITES IRRADIATED AT INTERMEDIATE TEMPERATURE: L. L. Snead, T. D. Burchell, Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6376

Several graphites and graphite composites (C/C's) have been irradiated in the temperature range of 200 to 800deg.C to determine the absolute reduction in thermal conductivity for a dose range between 0.01 to 1.0 dpa. The unirradiated room temperature thermal conductivity of these materials varied from 114 W/m- K for H-451 isotropic graphite, to 670 W/m-K for unidirectional FMI-lD C/C composite. As expected, the amount of thermal conductivity degradation is a strong function of irradiation temperature with a substantially higher reduction occurring for the lower temperature irradiations. As an example, the saturated residual thermal conductivity for those materials irradiated in the lower temperature region was approximately 20% of the original thermal conductivity measured at the irradiation temperature while approximately 50% of the thermal conductivity was retained for the elevated temperature irradiations. Data from these measurements will be compared to a design algorithm which gives the thermal conductivity of high conductivity graphite as a function of irradiation temperature and displacement level. Research sponsored by the Office of Fusion Energy, U.S. Department of Energy, under contract DE-AC05-840R21400 with Martin Marietta Energy Systems.