Abstract
Crystals of stoichiometric magnesium aluminate spinel, pre-implanted with 1000 ppm helium, have been bombarded by intense 1 MV electron beams to simulate fusion reactor irradiation damage. The spinel proved to be much more resistant to conventional lattice structural damage than either MgO or Al2O3, no dislocation loops or voids being nucleated after doses exceeding 10 and 30 dpa on the respective cation and anion sublattices at temperatures in the range 800–1300 K. Discontinuities in the material, e.g. specimen surfaces, or inhomogeneities in the flux intensity of the irradiating beam however produced segregation of the ionic species, non-stoichiometry, precipitation of metal and non-equilibrium phases, volumetric contraction, fracture and surface erosion. Degradation rates increase rapidly (2.1 eV activation energy) with temperature. The damage process is interpreted as an internal electrolysis induced by charge segregation derived from the dynamic motion of secondary electrons. Fast neutron damage cannot be simulated because electron irradiation is unable to nucleate dislocation loops, in stoichiometric spinel.
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