Segregation and diffusion behaviours of helium at grain boundaries in silicon carbide ceramics: first-principles calculations and experimental investigations

Abstract

Under a severe neutron irradiation environment, helium behaviour has become a significant concern due to helium-induced degradation of mechanical properties in SiC structural materials. Experimental observations showed that helium prefers to aggregate at grain boundaries (GBs) in SiC ceramics. However, the interaction mechanisms between helium and GBs have remained unclear. By performing first-principles calculations, we studied helium segregation at six symmetric tilt GBs and helium diffusion along and across the GBs. The solution energies of helium at the GBs are much lower than that of helium in grains, indicating preferential helium segregation at the GBs. The negative segregation energies of helium decrease linearly with the excess volume of GBs and the relative charge densities of the interstitial sites, which reveals that helium easily segregates at the GBs having open structures and low charge density interstitial sites. The interstitial helium atoms at GBs are difficult to form clusters due to weak interactions. The diffusion energy barriers of helium along the GBs Σ9(221)[11¯0] and Σ27(552)[11¯0] in the direction of tilt axis [11¯0] are much lower than that of helium in the grains. The two GBs can significantly enhance the helium diffusivity, serving as fast diffusion channels for helium. The calculation results are analysed and compared with our experimental data.