Neutron irradiation of sapphire for compressive strengthening.: I. Processing conditions and compressive strength

Sapphire suffers a dramatic loss of c-axis compression strength at elevated temperatures. Irradiation of sapphire with fission-spectrum neutrons to an exposure of ∼1022 neutrons/m2 in the core of a 1 MW fission reactor increased the c-axis compression strength by a factor of ∼3 at 600 °C. Strength was similarly improved when 99% of slow neutrons (⩽0.1 eV) were removed by 10B and Cd shields during irradiation. Annealing at 600 °C for 10 min changed the yellow–brown color of irradiated sapphire to pale yellow, but had no effect on compressive strength. Annealing irradiated sapphire at 1200 °C for 24 h reduced the compressive strength to its baseline value. Transmission electron microscopy suggests that fast-neutron-induced displacement damage inhibits the propagation of r-plane twins which are responsible for the low compressive strength. When irradiated with 10B and Cd shielding, sapphire that was not grown in iridium crucibles is safe for unrestricted handling after 1 month.