Because magnesium aluminate spinel (MgAl2O4) shows a strong resistance to void swelling during neutron irradiation at elevated temperatures, it is a candidate material for specialized applications in proposed fusion reactors. During implantation at 25°C with 2 MeV Mg+ ions to ∼2.8 × 1021 Mg+/m2, dislocation loops are formed at midrange depths (∼0.5 - 1.0 μm) on {110} and {111}. The microstructurc in the implanted ion region (∼1.5 - 2.0 μm) is shown in cross-section in Fig. 1. Within this implanted ion region, small features (4 - 10 nm diam.) were observed in dark field (DF) images using a spinel 222 reflection (Fig. 2). No evidence was found in electron diffraction patterns to suggest these features are (hexagonal) metallic Mg. However, in an earlier study, similar features in Al+ implanted spinel were identified by parallel electron energy loss spectrometry (PEELS) as metallic Al colloids. Phase identification of metallic Al within this spinel by electron diffraction is complicated because the lattice parameter of spinel (0.8083 nm) is almost exactly twice that of aluminum (0.4049 nm) and the phases are oriented cube-on-cube.
Relative Stability of Silicon Self-Interstitial Defects
