Study On Irradiation Effect Of GH3535 Welded Joint

GH3535 high temperature nickel-based alloy was selected as the main structural material for molten salt reactor due to its high-temperature strength and excellent corrosion resistance to molten salts.Welding is necessary for molding structural materials.The weld and heat-affected zones（HAZs）have an inhomogeneous microstructure and performance compared with the base metal due to repeated heat cycle during welding.Thus,the welded joint is often regarded as the weak link of structural materials.Irradiation damage is a major limitation that impacts the safety and integrity of the structural materials in reactors.As a weak link of structural materials,the irradiation of welded joints should be paid more attention.There is a lack of high-temperature neutron irradiation data of GH3535 welded joints under molten salt reactor condition,and the irradiation effects of the welded joints of GH3535 alloys have not been studied extensively,especially the high-temperature helium effect.Given that the microstructure of weld metal and HAZ is different from that of the base metal,it is essential to study the evolution of helium bubbles and the corresponding mechanical properties in welded joints for its safety service.In this study,the irradiation effect of GH3535 welded joint was studied by He ions and heavy ions（Fe,Xe ion）.The irradiation properties and damage mechanism of weld,HAZ and base metal were discussed from the mechanical properties and microstructure evolution of irradiated samples.The evolution of macroscopic behavior of welded joints after irradiation was analyzed,and the evolution mechanism of irradiation defects in weld,HAZ and base metal was discussed.The relationship between the evolution of microscopic defects and their macroscopic behavior was established to provide basis and data for evaluating the irradiation properties of GH3535 alloy welded joints.In the study of mechanical behavior of the welded joint after irradiation,it is found that the hardening behavior of weld,HAZ and base metal is consistent at room temperature regardless of He ion irradiation or heavy ion irradiation.The hardening behavior of weld,HAZ and base metal increases with the increase of irradiation dose,and then tends to saturation.At high temperature,mechanical properties have different evolution behavior.In terms of the study of hardening behavior after helium ion irradiation,the hardness of the weld and HAZ after irradiation increased progressively with increasing dose at 650℃,whereas that of the base metal gradually became saturated.At a dose of 1×10¹⁷ ions/cm²,the hardening degree of the three regions was ranked as follows:weld（92.9%）>HAZ（91.3%）>base metal（72.7%）.In addition,the hardening degree of the weld and the base metal decreases gra dually with the increase of the irradiation temperature from 650,750,and 850℃ at 2×10 ¹⁶ ions/cm² of He ion irradiation.The reduction of the hardening of weld is much higher than that of the base metal.For heavy ion irradiation,there is different in irradiation hardening from the He ion irradiation with the temperature increases.Under the high temperature,the hardness of base metal and weld is higher than that at room temperature.Especially for the base metal,its hardening degree is almost 3 times that of the weld.In addition,the micro-pillar compression test method was applied to evaluate the heavy ion irradiation hardening of GH3535 alloy welded joints,the relationship between the stress change during compression and the microstructure defect characteristics of the sample was successfully established.The evolution mechanism of microstructure defects produced by irradiation in welded joints was analyzed.The relationship between the irradiation microdefects and the macroscopic hardening is also discussed.Firstly,the effect of He ion irradiation was studied.The intrinsic dislocations in the welded joints play an important role on the nucleation and growth of helium bubbles.The dislocations not only serve as rapid diffusion channels,but also provide more nucleation points for helium bubbles,resulting in the higher bubble density and larger size in the weld and HAZ than that in the base metal under 1×10¹⁷ ions/cm²He ion irradiation at 650℃.The different evolution of helium bubbles is one reason for the different hardening behavior of the GH3535 alloy welded joint.Secondly,the strength of dislocations and carbide interface sink is temperature dependent.As temperatures rise,the dislocations and carbide interface become more capable of trapping helium atoms and interstitial atom.At 850℃,the existence of high-density dislocation lines and nano-carbides in the weld absorbs helium atoms and interstice atoms more effectively.So,helium bubbles in the weld are almost distributed on dislocation lines and carbide interfaces.The average density of helium bubbles and the average density and size of dislocation loops in the weld are lower than those in the base metal.This is also the main reason for that the hardening degree of the weld（36%）is much less than that of the base metal（70%）.The evolution mechanism of helium bubbles in the base metal was also discussed under the He ion irradiation at 650,750 and 850℃.The helium atom diffusion controlled by the self-interstitial/He replacement mechanism and the helium bubble migration controlled by the surface diffusion jointly affect the evolution of helium bubbles.Furthermore,bubble-loop complexes were observed at 750 and 850℃,and the shape of helium bubbles at 850℃ is octahedron structure.With the temperature increasing,the helium bubbles prompted the growth of dislocation loops,suggesting that the interaction between the helium bubbles and the dislocation loops is dependent on the irradiation temperature.In the study of the irradiation effect of multi-energy Fe ions,it was found that only dislocation loops and dislocation lines were observed in the base metal after 8dpa irradiation damage under RT（Room temperature）irradiation.Under the irradiation at700℃,a large number of precipitates were formed in the base and weld metal,making its hardness increase abnormally compared with RT temperature.The existence of dislocation lines in the weld absorbed the interstitial solute atoms generated by the displacement damage and inhibited the nucleation of precipitates in the matrix.This is the key reason why the hardness of the weld metal is lower than that of the base metal.