Void Formation in Magnesium Aluminate Spinel Heavily Irradiated with Fast Neutrons.

Abstract

Single crystals of magnesium aluminate spinel (MgAl2O4) were heavily irradiated to fission neutron fluences from −1 × 1026 to −2×1027 n/m2 (E>0.1 MeV) at 658 and 1,023 K in FFTF (Fast Flux Test Facility) to investigate their microstructural evolution under heavy neutron irradiation and to confirm their previously observed resistance to void swelling. At 658 K no voids were observed up to 2.29 × 1027 n/m2, but 1/4[110] interstitial loops were formed. These loops changed their habit planes from (111) to (110) with increasing neutron fluence. At 1,023 K the 1/4[110] type of interstitial loops grew to form stacking fault netuorks composed of stacking faults on each of the six equivalent (110) planes. Tiny voids were also observed to form preferentially on or near stacking faults after 1.37×1027 n/m2. A limited number of very small cavities, contributing to a volumetric swelling of only 0.07%, were also observed in the crystal matrix after 2.17×1027 n/m2. The possible mechanisms of suppression of void formation in MgAl2O4 are discussed.