Density Functional Theory Study Of Hydrogen,Helium And Other Transmutants Behaviors In 3C-SiC Under Neutron Irradiation

Cubic silicon carbide(3C-SiC)materials have been considered as one of the most promising candidates for structural materials in future nuclear fusion reactors,and for cladding and matrix materials in advanced fission reactors,due to the excellent properties of thermal stability,high thermal conductivity,high strength,corrosion resistance and low neutron absorption cross-sections.Under the high-energy neutron environment of nuclear fusion and fission reaction systems,3C-SiC materials suffer not only irradiation damages but also transmutation reactions,which produces plenty of irradiation defects and transmutants in the materials.The main transmutants are hydrogen,helium,lithium,beryllium,boron,magnesium,aluminum and phosphorus.The interaction between irradiation defects and transmutants often affects the microstructure of 3C-SiC,causing the formation of bubbles and cavities,and further resulting in the hardening and fracture of the materials,which ultimately influences the performance of the materials.Therefore,studying the interaction between irradiation defects and transmutants in 3C-SiC materials is important for understanding the micro mechanism of performance degradation,and evaluating the service performance of 3C-SiC materials in nuclear reactors.By using density functional theory,we systemically investigate the behaviors of transmutants and irradiation defects in 3C-SiC,the interaction of irradiation defects and defect clusters with transmutants and their effects on the microstructure evolution of the materials.By investigating the effect of irradiation defects on the energetics and kinetics of hydrogen in 3C-SiC,it is found that both self-interstitial atoms and vacancies can act as trap sites for hydrogen.Self-interstitial carbon and silicon atoms can trap up to six and five hydrogen atoms,respectively.A silicon vacancy can trap only four hydrogen atoms,while a carbon vacancy can trap up to eight hydrogen atoms with two hydrogen molecules formation.The accumulation of hydrogen in a vacancy can result in the instability of atoms around the vacancy,which may result in the growth of vacancy-hydrogen clusters to bubbles or blisters.In addition,a carbon or silicon vacancy significantly increases the diffusion energy barrier of hydrogen,which can effectively promote the hydrogen retention in 3C-SiC.Our results are helpful for understanding the effect of irradiation defects on hydrogen retention,the nucleation and growth of hydrogen bubbles in the materials.For the stability of the irradiation-induced vacancy clusters and their influence on helium behaviors,it is found that the vacancy clusters containing only carbon vacancies are most favorable to form,while the binding strength of vacancy clusters containing both carbon and silicon vacancies is the strongest.The bonding strength of helium atoms at interstitial sites is very weak,thus it is difficult to form interstitial helium clusters.The vacancy and vacancy clusters can provide with free space for helium atoms to aggregate.The binding energies of vacancy clusters with helium atoms increase almost linearly with the ratio of helium to vacancy,n/m.Moreover,the diffusivity of helium will be significantly decreased by vacancy and vacancy clusters,which will promote the aggregation of helium atoms in the material.The results are beneficial to understand the formation of helium bubble and cavities,and their effects on the microstructure evolution of materials.Finally,the interactions of irradiation defects with lithium,beryllium,boron,magnesium,aluminum and phosphorus transmutants are systematically investigated,which is compared with the interactions of irradiation defects with hydrogen and helium.Hydrogen,helium,lithium,and beryllium atoms are energetically favorable to occupy interstitial sites,while boron,magnesium,aluminum,and phosphorus atoms preferentially occupy substitutional sites.The interactions of boron,magnesium and aluminum pairs are attractive.The binding strength of transmutants with vacancies and self-interstitial atoms is very strong.Hence,vacancies and self-interstitial atoms may be trapping centers for transmutants.Both the electron interactions between point defects and transmutants,and the local lattice distortion caused by the interactions between point defects and transmutants contribute to the binding of point defects and transmutants.The results are helpful to understand the formation mechanism of transmutantion precipitates and its effect on the mechanical properties of 3 C-SiC.