Experimental And Molecular Dynamics Simulation Investigation Of Ion Irradiation Effects On Nanocrystalline SiC

Silicon carbide（SiC）is an important nuclear engineering material,which has many promising applications in future nuclear energy systems.The reactor neutron irradiation will cause atomic displacements and transmutation products such as H and He,leading to structural damage and mechanical degradation,which utterly influences the lifetime of materials and the security of reactors.Nanocrystalline SiC（nc-SiC）is anticipated with improved radiation resistance,which is attributed to the large fraction of grain boundaries that could absorb irradiation-induced point defects.To date,the research on the radiation damage effects on nc-SiC is still in the preliminary exploration stage,and the understanding of many key issues is still unclear.In this study,the nc-SiC with different grain size,amorphous and single crystal SiC samples were irradiated with Xe,Kr and H ions at high and room temperatures,and the grazing-angle incidence x-ray diffraction（GIXRD）and transmission electron microscopy（TEM）were used to study the irradiation-induced microstructural changes.Meanwhile,the nanoindentation technique and molecular dynamics（MD）simulation are combined to investigate the influence of grain morphology（grain size and structure）and radiation damage on the mechanical properties of the nc-SiC.The main contents are as follows:（1）The amorphous SiC samples were irradiated with Xe and Kr ions at high temperatures to study the irradiation-induced grain nucleation and growth.It is found that the threshold temperature for grain nucleation in amorphous SiC is between 550and 700 K,which is higher than that of the critical amorphization temperature.The irradiation-induced crystallization of SiC at the low temperatures（≤900 K）shows no dependence on the irradiation temperature.The nucleation and growth rates are mainly determined by the ion species and energy.The lighter the mass of the impinging ions,the higher the nucleation rate,and the lower the growth rate.The results of nanoindentation test and MD simulation suggest that the hardness of nc-SiC decreases with increasing grain size,which shows an inverse Hall-Petch relationship.It is mainly attributed to the reduced number of intact Si-C bonds with grain refinement.The grain boundaries significantly suppress the nucleation and growth of indentation-induced dislocations.Preferential amorphization at grain boundaries occurs in nc-SiC during nanoindentation,which is due to the lower threshold energies for atomic displacements at grain boundaries.（2）The nc-SiC samples with different grain size were irradiated with Xe ions at700 K.After irradiation,grain growth is observed for the nc-SiC with an average grain size smaller than 8 nm,while the nc-SiC with larger grains exhibits good irradiation stability.For the larger SiC grains,most atomic displacements caused by irradiation are expected to be created in the grain interiors,which leads to the increased defect concentrations in grain interiors and decreased crystallinity.The nc-SiC with dense stacking faults（SF）exhibits super-hardness of～53 GPa,whereas the hardness decreases significantly after the Xe irradiation at 700 K.Although the nano-layered SF structure within grains is well retained after Xe irradiation,the invisible defect clusters are created by irradiation which could cause the hardness to decrease.The MD simulation results suggest that the hardening/softening mechanisms of twin boundary（TB）,extrinsic stacking fault（ESF）and intrinsic stacking fault（ISF）are similar.The planar defects suppress the indentation-induced dislocation nucleation and block dislocation propagation,which leads to the hardening;however,the dislocations slip along the planar defects and nucleate at planar defects,which leads to the softening.The hardening and softening mechanisms of these planar defects compete and couple with each other during nanoindentation,which determines the hardness of SiC materials.The blockage of planar defects to dislocation motions decreases after irradiation,which causes the hardness to decrease.（3）The single crystal SiC and nc-SiC with dense SF within grains were irradiated with H₂⁺molecular ions at room temperature.The nc-SiC is amorphized at a comparable dose to that for single crystal SiC,suggesting no enhanced irradiation resistance of nc-SiC.Different from the experimental results of Si ion and electron irradiations reported in literatures,the migration of pointed defects created by H irradiation shows no dependence on the nano-layered SF structure.This may be caused by the formation of stable Si-H and C-H bonds,which immobilizes the point defects in SiC at room temperature.The uniaxial tension of single crystal and nc-SiC containing cavities（voids and He bubbles）were performed using MD simulation.The brittle-to-ductile transition occurs in SiC with cavities when the pressure in He bubbles increases.It is because that the deformation mechanism transfers from the bond breaking to intensive dislocation activities.During the tension process,the tensile stress tends to accumulate around the cavities,which results in preferred cracking in the area with a larger tensile stress.For the nc-SiC with dense SF,the microcracks preferentially initiate and propagate at the grain boundaries,which leads to intergranular fracture.With the formation of nano-sized He bubbles,the nc-SiC still fails in intergranular fracture mode.The He atoms diffuse easily into grain boundaries when bubble pressure increases,which causes reduction in grain boundary strength and degradation to tensile properties.In summary,this work unveils the changes in grain size and microstructure of nc-SiC caused by ion irradiation at high temperatures,clarifies the influence of grain morphology and irradiation damage on the mechanical properties of nc-SiC,and preliminarily explores the damage effects of H and He on nc-SiC.The research results will provide experimental and theoretical basis for the application of nc-SiC materials in advanced nuclear reactors.