Relation between macroscopic length change and the crystal structure in heavily neutron-irradiated ceramics

Macroscopic length changes of heavily neutron-irradiated α-Al2O3, AlN, β-Si3N4 and β-SiC were measured and analyzed, and the results were elucidated by considering the crystal structure of each material. These specimens were irradiated under the same conditions (775–1039 K, 2.8–7.3 × 1026 n/m2) in the experimental fast reactor JOYO. Alpha-Al2O3 and AlN showed large length change compared with that of β-Si3N4 and β-SiC. In α-Al2O3 and AlN, interstitial dislocation loops are inserted mainly into the (0 0 0 1) plane, and they do not interfere with each other. Therefore, new loops are induced without any restriction and anisotropic lattice stretching induces intergranular cracking. On the other hand, interstitial dislocation loops are inserted into {1 0 1̄ 0} or {1 12̄ 0} planes in β-Si3N4 and {1 1 1} planes in β-SiC. These planes of forms have several symmetrical planes, and dislocation loops on them interfere with each other to form nano-partitions. So, growth and nucleation of loops are limited, and induced Frenkel pairs only recombine with each other.