Effect of Grain Size on Mechanical Properties of Irradiated Mono- and Polycrystalline $MgAl_2O_4$

The influence of the size of crystalline regions on mechanical properties of irradiated oxides has been studied using a magnesium aluminate spinel $MgAl_2O_4$. The samples characterized by different dimensions of crystalline domains, varying from sintered ceramics with grains of few micrometers in size up to single crystals, were used in the experiments. The samples were irradiated at room temperature with 320 keV $Ar^{2+}$ ions up to fluences reaching 5 × $10^{16} cm^{-2}$. Nanomechanical properties (nanohardness and Young's modulus) were measured by using a nanoindentation technique and the resistance to crack formation by measurement of the total crack lengths made by the Vickers indenter. The results revealed several effects: correlation of nanohardness evolution with the level of accumulated damage, radiation-induced hardness increase in grain-boundary region and significant improvement of material resistance to crack formation. This last effect is especially surprising as the typical depth of cracks formed by Vickers indenter in unirradiated material exceeds several tens of micrometers, i.e. is more than hundred times larger than the thickness of the modified layer.