VOID FORMATION IN MAGNESIUM ALUMINATE SPINEL HEAVILY IRRADIATED WITH FAST-NEUTRONS

Single crystals of magnesium aluminate spinel (MgAl2O4) were heavily irradiated to fission neutron fluences from similar to 1X10(26) to similar to 2X10(27) n/m(2) (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 x 10(27) n/m(2), 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 networks 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 1.37X10(27) n/m(2). A limited number of very small cavities, contributing to a volumetric swelling of only 0.07%, were also observed in the crystal matrix after 2.17x10(27) n/m(2). The possible mechanisms of suppression of void formation in MgAl2O4 are discussed.