The Synergetic Effect of He and Kr Irradiation on Helium Bubble Evolution in SiC/SiC Composite: Combining In-Situ TEM Observation with MD Simulation

SiC fiber reinforced SiC matrix composites (SiC/SiC composites) are considered as one of the promising structural materials for liquid type breeding blanket in the fusion energy system. The transmutation gas production rate in fusion reactor is significantly higher compared with fission reactors. In order to evaluate the synergetic effects of helium and irradiation on SiC/SiC composites, in-situ ion irradiation and TEM observation combined with MD simulation were utilized. A comprehensive experiment matrix including Kr irradiation, He pre-implantation with sequential Kr irradiation as well as simultaneous dual beam at 1073 K to 16 dpa and 2400 appm He (150 appm/dpa) was performed using 800 keV Kr and 50 keV He ions.<br/>Combining in-situ TEM observation and MD simulation, the impact of relative concentration ratio of vacancies and helium atoms on the bubble evolution was elucidated. Bubble nucleation does not occur without helium atoms. When only helium atoms are introduced, limited bubbles can nucleate and will quickly reach saturation. When vacancies are introduced after helium implantation, a number of helium bubbles immediately nucleate at first and then the bubble density reduce with increasing irradiation dose. When introducing helium atoms and vacancies simultaneously, the He_nV_m cluster develop into helium bubbles continuously.<br/>In addition, the direct evidence for the competition of irradiation-assisted bubble growth and re-dissolution was in-situ observed in SiC matrix for the first time. Vacancies introduced by irradiation are dispensable to the nucleation and growth of bubbles, while the attendant collision cascades and thermal spikes can result in the re-dissolution of helium atoms in bubbles. The competition between these two processes leads to the growth and shrinkage of bubbles.