Neutron irradiation of sapphire for compressive strengthening.

Sapphire suffers a dramatic loss of c -axis compression strength at elevated temperatures. Irradiation of sapphire with fission-spectrum neutrons to an exposure of ∼10 ²² neutrons/m ² 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 10 B 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 10 B and Cd shielding, sapphire that was not grown in iridium crucibles is safe for unrestricted handling after 1 month.