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Abstract
The magnesium content of stoichiometric MgAl2O4 spinel is found to decrease progressively with electron dose during irradiation at elevated temperatures in a hgh voltage electron nucroscope (HVEM). The rate of decrease vanes with the electron beam energy. Diffraction patterns from the irradiated regions suggest that the end product is cubic γ-alumina rather than the equilibrium trigonal α-alumina. Additional reflections present in the patterns indicate a long-range ordering of the aluminium ions consistent with a true lattice parameter of 23.70 Å, a three-fold increase over the tabulated value of 7.90 Å for γ-alumina. A mechanism for the (spinel← γ-alumina) transformation is proposed. Microcracking is observed in some irradiated regions as a consequence of the 6.5% volume contraction associated with the transformation.