The Effect Of Molten Salt Environment On The High-temperature Helium Behaviors Of GH3535 Alloy

GH3535 alloy is considered to be the primary option of metallic structural materials for the Molten Salt Reactor（MSR）.It will be subjected to the extreme environments,i.e.high temperature,high neutron doses and corrosive coolant during the operation of MSR.Taking into account that the safe operation of MSR will mainly depend on the service performance of structural alloys,it is necessary to evaluate the behaviors of such kind of alloys in the complicated environments.We investigated the key issues in aspects of irradiation and corrosion in MSR structural alloys,and simulated the case of neutron irradiation at high temperature via helium ion irradiation technique.The evolution of microstructure and mechanical properties induced by high temperature He ion irradiation was studied.Then,the unirradiated and irradiated alloys were immersed in molten salt.The effect of molten salt environment on the high-temperature He behaviors was explored via comparing the microstructure evolution of irradiated and irradiated&corroded samples.And the effect of irradiation on corrosion properties of GH3535 alloy in the molten salt environment was also investigated.On the basis of the studies regarding the influence of moisture and oxidizing impurity and metallic ions(Fe³⁺、Cr³⁺)on the corrosion behavior of nickel based alloys,we investigated the effect Ni impurity on the corrosion behavior of GH3535alloy in FLiNaK molten salt.Compared with purer salt,large amount of fine grains were observed on the surface of the alloy immersed in salt with Ni and oxidizing impurities,which may be related with the strong losses of alloying elements and diffusion between Ni and Cr.1.2 MeV He ion irradiation was conducted on the GH3535 alloys at 650℃.TEM results reveal that He bubbles were observed at the irradiation fluence of5×10¹⁶ion/cm².Neither“black spots”nor dislocation loops were observed in TEM images of the as-irradiated sample.The number densities and sizes of He bubbles increase with increasing irradiation fluence.AFM and Nanoindentor techmiques were applied to investigate the swelling and irradiation hardening behaviors of high temperature He ion irradiated GH3535 alloy.Irradiation fluence up to 3×10¹⁷ion/cm²causes a swelling of 2.67%,which is mainly ascribed to He bubbles.He bubbles induced hardening increases with increasing irradiation dose.The irradiation hardening behavior was compared with heavy ion irradiation.The result reveals that the obstacle strength of He bubbles is less than that of the saturated heavy ion induced irradiation defects.To investigate the effect of molten salt environment on the high-temperature He behaviors of GH3535 alloy,the unirradiated and irradiated samples were immersed in the molten salt at 650 ^oC for 200 h.A unique outward migration of helium bubbles was observed in the irradiated alloy under molten salt environment,which strongly enhanced the formation of holes on the surface of the irradiated&corroded sample.Moreover,it was elucidated that corrosion induced vacancies rather than high-temperature annealing effect of molten salt is responsible for the evolution of helium bubbles formed by high-temperature helium ion irradiation.The evolution of He bubbles and heavy ion irradiation induced defects in molten salt environment was compared.It was found that dislocation loops were able to absorbe vacancies and enhance voids formation.However,He bubbles play a more important role in increasing the corrosion of GH3535 alloy,compared with small dislocation loops.The effect of high temperature He ion irradiation on the corrosion behaviors and properties of GH3535 alloy was investigated.It was revealed that there was not obvious difference in the surface morphology and element distribution of the corroded alloy with and without irradiation at the dose of 5×10¹⁵ ions/cm².While for the corroded alloy pre-irradiated with He ions at the dose of 5×10¹⁶ ions/cm²,large amount of holes were observed on the surface and the loss of Cr element was found from the bulk to the surface,which can be ascribed to the He bubbles formation.With irradiation doses increasing,the microstrains and the distribution depth of the fine grains are increased.The hardness of irradiated&corroded alloy is lower than the irradiated alloy,and higher than of the corroded alloy.It is a combined effect of irradiation and corrosion.Synchrotron XAFS techniques were applied to investigate the coordination structure of Cr element in irradiated,corroded and irradiated&corroded alloys.It was found that Cr became unstable and easier to reacted with oxidizing impurities after irradiation,which further acceralted corrosion of the alloy.