POINT-DEFECT HARDENING IN MGO.3AL2O3

Vickers hardness in spinel increases by 23% after neutron irradiation to a fluence of 8.3 x 10(22) n/m2 at 100-degrees-C. Annealing at high temperatures above 100-degrees-C gradually decreases the hardness. Above 500-degrees-C, the hardness is reduced to almost the same value as that of unirradiated material. The hardness of spinel, irradiated at 470-degrees-C to a fluence of 2.4 x 10(24) n/m2, is unchanged both after irradiation and after annealing up to 1000-degrees-C. The length change during annealing was also measured and is similar to the hardness change. Frank dislocation loops with a density of 3 x 10(14)/cm3 are induced by neutron irradiation at 470-degrees-C but they apparently do not affect the hardening in spinel. Thus, point defects are concluded to act as obstacles against dislocation movement. The yield stress measured at 1400-degrees-C is also unchanged after irradiation. It is believed that not only the point defects but also the loops are annihilated by annealing at 1400-degrees-C.