Role of irradiation spectrum in the microstructural evolution of magnesium aluminate spinel

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Abstract
In-situ observations of the nucleation and growth of defect clusters in MgAl2O4 were made in transmission electron microscope-accelerator facilities using concurrent irradiation with ions and electrons to obtain insight into the role of the irradiation spectrum. Various kinds of 30 or 300 keV ions (He+, O+, Mg+, Ar+ and Xe+) and 200 keV or 1 MeV electrons were used to provide a wide range of nuclear and ionizing stopping powers as well as a various effects of displacement cascades. Dislocation loops were formed both inside and outside the electron beam at 870 K under concurrent irradiation with 30keV ions and 1 MeV electrons. We observed a variety of microstructures that formed during concurrent irradiation with 300 keV ions and 200 keV electrons. These include preferential formation of cavities, suppression of dislocation loop formation and preferential formation of loops at the periphery of the focused electron beam. We discuss the irradiation spectrum effects in terms of the ionizing, displacive and subthreshold nuclear stopping powers, the localized energy density within displacement cascades and the flux distribution of the focused electron beam. Cation diffusion from the inside to the outside of the electron beam, which is probably caused by the subthreshold nuclear and ionizing stopping powers, plays an important role in microstructure evolution during concurrent irradiation.