Mechanical properties and microstructure of α-alumina and magnesium aluminate spinel irradiated with He ions

Mechanical properties of α-alumina, stoichiometric- and nonstoichiometric-magnesium aluminate spinel single crystals were examined by using ultra-microhardness technique. The samples were irradiated with 100 keV He⁺ ions at temperatures of 300-870 K and to fluences up to 2 × 10²⁰ He⁺/m². Apparent hardness, ΔH, in α-alumina increases with fluence in three stages, while that of spinel crystals increases monotonically with fluence. We have also evaluated elastic modulus, plastic and elastic energies, and plastic and elastic indentation depths through the analysis of load-displacement curves. These analyses showed that plastic and elastic hardening are responsible for the variation of ΔH of α-alumina, and that plastic hardening is the main cause of hardening in spinel crystals. Corresponding TEM observations suggested the importance of point defects and/or 'invisible' defect clusters for radiation hardening compared to 'visible' dislocation loops. The relationship between microstructure and mechanical properties is given for various ceramics.