Irradiation Damage in Sintered SiC Irradiated by He Ion

SiC is an important nuclear structural material used in advanced nuclear energy systems. He atoms can be produced by (n, α) transmutation reaction. Due to low solubility of He atoms in SiC, He atoms prefer to nucleation on grain boundaries or vacancies produced by neutron irradiation, resulting in He embitterment effects. He ions supplied by the 320 kV multidisciplinary research platform for highly charged ions of the Institute of Modern Physics, Chinese Academy of Sciences, irradiated sintered SiC at room temperature to 1 000 ℃, with its fluences from 1015 cm-2 to 1017 cm-2. After irradiation, the sample was annealed at different temperatures (up to 1 500 ℃). A combination of transmission electron microscopy, Raman spectroscopy, nanoindenter and thermal conductivity was used to characterize irradiation damage. The threshold fluence for helium bubble formation in the sintered SiC is lower than that in single crystal SiC. It is related to grain boundaries that provide nucleation center for He bubbles. What’s more, morphology and size of helium bubbles depend on irradiation temperature and annealing temperature. Nanosized bubbles are homogeneously distributed in the damaged layer when the irradiation temperature is 600 ℃ and below. However, twodimensional platelets on the (0001) planes are formed during 750 ℃ irradiation, resulting in material dimensional instability, such as surface exfoliation. Platelets exhibit high pressure states, indicating the very high ratio of He to vacancy in platelets. Irradiationinduced defects and elemental segregation were also investigated by scanning electron microscopy. Lots of defect clusters can be formed by He ion irradiation. Most of observed defect clusters are Frank loops with b=1/2〈0001〉. Tangled dislocations are formed after 1 500 ℃ annealing. Meanwhile, the growth of helium bubbles can emit selfinterstitials, resulting in the formation of interstitialtype dislocations. Dense stacking faults on (0001) plane are formed. Carbon enrichment occurs on grain boundaries and cavities, which can affect the structural stability in corrosion surroundings, such as increase in liquid PbBi eutectic corrosion. Raman spectroscopy confirms the dissociation of Si—C bonds, to form C—C bonds after high fluence He irradiation. Irradiation hardening increases initially when the irradiation dose is below 0.2 dpa, and decreases finally with irradiation dose. It is related to lattice disorder that can inhibit dislocation motion. The thermal conductivity of sintered SiC decreases after He ion irradiation, which is caused by electron migration inhibited by lattice defects. The research results are important for understanding He irradiation into sintered SiC.