Effect of neutron irradiation on hardening in MgO crystals

Using a nanoindentation technique, hardness and Young’s modulus were determined in both nominally pure MgO and lithium-doped MgO crystals, before and after neutron irradiation in the dose range ${10}^{15}–{10}^{19}\hspace{0.5em}{\mathrm{n}/\mathrm{c}\mathrm{m}}^2.$ The resulting defects were monitored by optical-absorption spectroscopy. The concentrations of single oxygen vacancies and higher-order point defects involving oxygen vacancies increase with neutron dose. A constant value of $(290\mathrm{}\pm 15)$ GPa for the Young’s modulus was measured in all the crystals, indicating that the elastic properties are not influenced by either impurities or defects produced by irradiation. Hardness increases with neutron dose and is independent of the presence of lithium in the crystal. Neutron-irradiated crystals contain oxygen vacancies, higher-order point defects, and interstitials, whereas thermochemically reduced (TCR) crystals contain oxygen vacancies. Comparison between a neutron irradiated and a TCR MgO crystal containing similar concentrations of oxygen vacancies, shows that 70% of the hardening by neutron irradiation is produced by interstitials, 30% by oxygen vacancies, and a negligible amount by higher-order point defects.