CHARACTERISTICS OF THE LOOP FORMATION PROCESS IN THE MGO-AL2O3 SYSTEM IRRADIATED WITH FISSION NEUTRONS

Magnesium aluminate spinel, MgAl2O4, has been identified to be resistant to void swelling during neutron irradiation. The microstructure controlling the swelling is investigated through characterization of the development of interstitial loops. Many clusters of 1/6[111]{111} are formed directly from damage cascades. However, the majority of these clusters are removed by the radiation-induced diffusion of structural vacancies. The surviving clusters become stable through the following process: 1/6[111]{111}-->1/4[110]{111}-->1/4[110]{101}-->[110]{110}-->1/2[110]{110}. Aggregates of six equivalent 1/4[110]{110} and/or 1/2[110]{110} loops become dominant with increasing neutron fluence and/or irradiation temperature. This process reduces the self-energy of the loops through the compositional change of stacking faults, and the rotation of and the reactions between loops, and decreases the concentration of interstitials retained in the form of stable loops which bias vacancies and induce void swelling. The conclusion is that the reduced density of stable loops during damage cascades is caused by the dominant recombination of interstitials and structural vacancies and leads to a high resistance to void swelling under neutron irradiation.