Microstructure,mechanical Properties And Irradiation Damage Behavior Of SiC Joints Bonded By Calcium Aluminum Silicon Base Glass-ceramic

Silicon carbide（SiC）ceramics are considered to be a promising material for nuclear structures because of their low density（3.2 g/cm³）,high strength,excellent thermal stability,high thermal conductivity（490 W/m K）,low induced activity,and excellent irradiation resistance.However,the intrinsic brittleness and poor processability of SiC ceramics make it difficult to fabricate large size components.Therefore,it is inevitable to use joining techniques during practical application.In this study,a series of glass-ceramic solders were designed and prepared to achieve a reliable joining of SiC ceramics for nuclear applications.Firstly,the thermophysical properties of glass-ceramics were characterized to select a suitable solder,and then the microstructure evolution mechanism of the SiC ceramic joints bonded by the glass-ceramic solder was analyzed and the joining process parameters were optimized.Finally,He ion irradiation experiments were conducted to reveal the microstructure evolution and mechanism.of the obtained SiC joints under irradiationLi₂O-CaO-Al₂O₃-SiO₂（LCAS）glass-ceramic solders with different Li₂O content were also prepared by the melt-quench method,and the effect of Li₂O content on various properties of glass-ceramic was investigated.The results showed that Li₂O is present in the glass as a glass network outer body,and the increased non-bridging oxygen content can significantly reduce the characteristic temperatures of the glass,such as glass transition temperature T_g,softening temperature T_s,crystallization temperature T_p and melting temperature T_m.It was found that the addition of Li₂O also significantly reduced the crystallization activation energy of the glass,and the crystallization mechanism changed from surface crystallization to bulk crystallization.With the increase of Li₂O content,the precipitation of Li Al Si₂O₆ and CaSiO₃ was promoted and the precipitation of CaAl₂Si₂O₈ was inhibited.The CTEs of the pristine glass increased with the increase of Li₂O content,but the increase of Li Al Si2O6 precipitation with low CTE decreased the CTE of the formed glass-ceramic after crystallization heat treatment,which can be regulated in the range of8.5×10^-6/℃ to 2.8×10^-6/℃.In addition,the results of He ion irradiation damage of SiC ceramic and three crystalline phases were simulated by SRIM program,showing that the peak irradiation damage of the three crystalline phases was slightly higher than that of SiC ceramics,and the peak concentration of He was significantly lower than that of SiC ceramics.The wetting behavior of four prepared solders on SiC ceramics was investigated by the sessile drop method,and it was found that the addition of Li₂O and the increase in content significantly reduced the wetting spreading temperature of the glass,but increased the final wetting contact angle of the glass solder on SiC ceramics.By simulating the joining thermal cycle,and it was found that the glass solder with a Li₂O content of 3 wt.%LCAS3 was the best match to the CTE of SiC,with a difference of only 6.1%.The wettability of LCAS3 solder on SiC ceramic can be significantly reduced by surface preoxidation process,and the wetting angle is decreased from 62°to 33°.LCAS3 glass solder was used to join the O-SiC ceramics,and the results showed that the main crystalline phases in the glass-ceramic interlayer were striped CaAl₂Si₂O₈ and eutectic structured Li Al Si₂O₆/CaSiO₃.Through the optimization of the joining process parameters,the maximum shear strength of the joint was 127 MPa at 1240℃ for 10 min.He ions irradiation experiments with 300keV at 400℃ were performed on the joints under the optimum process obtained.The results showed that,with the increase of irradiation fluence from 2.5×10¹⁶ ions/cm² to 3×10¹⁷ions/cm²,the irradiation damage of SiC ceramics was higher than that of glass-ceramic seam,but their interfacial bonding was still good.The defects and He bubbles in the peak area of irradiation damage inside SiC at 950 nm from the surface,leading to gradual amorphization and microcracking,while the distribution of irradiation damage and He bubbles inside the glass-ceramic was more uniform and dispersed,showing better resistance to irradiation.The nano-hardness of SiC ceramics and glass-ceramic tended to increase and then decrease with increasing irradiation fluence,while the modulus of elasticity tended to decrease with increasing irradiation fluence,which have a good synergy in the magnitude of change.The residual stresses on the SiC ceramic side of the joint under different irradiation conditions were measured based on Raman spectroscopy,and it was found that the maximum stress on SiC shifted from compressive to tensile stresses as the irradiation fluence increased.