High dose neutron irradiation damage in alpha alumina
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T. E. Mitchell Los Alamos National Laboratory, Center for Materials Science, Los Alamos, New Mexico 87545
F.W. Clinard, Jr. and G.F. Hurley Los Alamos National Laboratory, Materials Science and Technology, Los Alamos, New Mexico 87545 (Received 3 January 1991; accepted 7 June 1991)
Bulk samples of single crystalline and polycrystalline alpha alumina have been neutron-irradiated in the Experimental Breeder Reactor-II (EBR-II) to doses of 1026 n/m 2 at temperatures of 925 K and 1100 K. The samples were found to swell macroscopically between 3% and 6%, depending on the temperature of irradiation and the form of the material. The damaged microstructures were investigated via transmission electron microscopy in order to understand the origin of the macroscopic swelling. In both single crystals and polycrystals the damage consists of a high density of dislocations containing predominately b = 1/3(1011) dislocation loops on the (0001) planes coexistent with a high density of voids, which are aligned along the c-axis in this rhombohedral material. The established theory of void formation in metals is utilized to explain the formation of voids in alumina. The polycrystalline samples were extensively microcracked, and this is thought to be due to anisotropic swelling of the grains which in turn leads to stresses and fracturing at the grain boundaries.
I. INTRODUCTION Ceramic solids have been identified as prime candidates for a variety of nuclear applications.1 Foremost is the major use of ceramics in all currently proposed fusion reactor concepts. Included are applications as structural first wall members, magnetic insulators, and laser windows. Such materials will be required to maintain structural and electrical integrity in the synergistic environment consisting of high doses (10 2 3 -10 2 7 n/m 2 ) of fast neutrons at temperatures from below room temperature to ~1000 K. Due to the great interest in the study of monatomic metals for nuclear technology, a general understanding of the irradiation damage process in metals has been attained. The process of radiation damage in ceramics, however, is fundamentally more complex than in metals because ceramics are multi-atomic solids with ionic or covalent bonding character. The consequences of these considerations manifest themselves in three basic ways. First, displacement of atoms can be nonstoichiometric due to differences in threshold displacement energy and mass of the atoms making up the solid. This could lead to a preponderance of a particular defect species, thereby altering the evolving microstructure. Second, the defect kinetics will differ for each of the diffusing species and can result in extended defect growth being controlled by the slowest moving species. Third, crystallographic unit cells are often large in ceramic systems, giving rise to large perfect Burgers vectors and complicated stacking 2178
J. Mater. Res., Vol. 6, No. 10, Oct 1991
http://journals.cambridge.org
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sequences. Such crystallographic considerations can lead to a hig
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