Microstructure of neutron irradiation-induced defects in sintered and siliconized SiC

High voltage electron microscopy (HVEM) and X-ray diffractometry were used to examine the microstructure of boron sintered SiC and reaction bonded siliconized SiC doped with natural boron and with boron enriched in 10B and 11B to study the effect of helium gas from the 10B(n,α)7Li reaction. X-ray studies show that both types of SiC exhibit a 1% swelling in the unit cell as contrasted with 3% macroscopic swelling. Analysis of the diffraction peaks produced strong evidence confirming the beta to alpha phase shift in the reaction bonded material. A Warren-Averbach analysis of the peak profiles of the irradiated 10B sintered material shows a crystallite size of 150 nm and a strain in the crystal of approx.=0.43%. Reaction bonded SiC is characterized by planar defects in the form of large loops (10-200 nm) and dislocation tangles for all irradiation levels regardless of dopant. Boron sintered SiC has many helium associated defects which appear as symmetric strain field structures of 40 to 90 nm dimensions. After annealing at 1773 K, the microstructure of reaction bonded SiC does not change significantly while the strain fields in the boron sintered SiC disappear. (orig.)