The Study Of Helium Induced Damage Behavior In Hastelloy N Alloy

The molten salt reactor(MSR)system is one of the Generation IV reactors and has attracted much attention because of its high efficiency,inherent safety and reliability.However,MSR has to face high temperature and strong fluoride molten salt corrosion.Such extreme service environment poses severe challenges to its structural materials.The nickel-based Hastelloy N alloy developed by the researchers from Oak Ridge National Laboratory is considered as a primary structural material for MSR due to its high-temperature mechanical properties and excellent corrosion resistance to fluoride salts.It is unfortunate that nickel atoms have a large neutron absorption cross-section and they are easy to undergo transmutation reactions with neutrons to produce helium atoms.Especially at high temperature condition,helium bubbles formed by the diffusion and agglomeration of helium atoms will cause the embrittlement of the alloy,which can greatly degrade the service performance of the alloy.On the one hand,the evolution of helium bubbles has a strong dependence on temperature.On the other hand,helium bubbles and displacement damage defects will exist simultaneously under actual working conditions.The interactions between them can greatly degrade the performance of the alloy.At present,there are few studies on the temperature effect of helium bubbles evolution and the interactions between helium bubbles and displacement damage defects.Considering that the structural material in the reactor is a very important safety barrier for the reactor,the deterioration of the structural material performance caused by helium damage will greatly affect the safety operation of the reactor.Hence,it is necessary to carry out the research on on the temperature effect of helium bubbles evolution and the interactions between helium bubbles and displacement damage defects in Hastelloy N alloy.In this thesis,the Hastelloy N alloy was used as the research object and defects were introduced in the material by using ion irradiation.The helium induced damage behavior of Hastelloy N alloy was systematically studied from the following two aspects: On the one hand,the evolution behavior of helium bubbles in Hastelloy N alloy during annealing was studied by performing the post-irradiation annealing experiments.The results of in situ TEM annealing experiments showed that it is difficult to grow for intragranular helium bubbles in Hastelloy N alloy samples annealed at 400 °C and500 °C.Since the grain boundary is a source of vacancies,helium bubbles are easy to nucleate and grow here.The nucleation and growth of a helium bubble was found at the grain boundary of the sample annealed at 500 °C.Due to the limitation of dislocation lines and chromium atoms,the helium bubbles in the sample annealed at 600 °C did not diffuse by random Brownian motion.These helium bubbles coalesen at their initial positions through the suppressed migration and coalescence mechanism.The results of post-irradiation isochronous annealing experiments showed that the migration and coalescence and Ostwald ripening mechanisms dominate in the coarsening of helium bubbles in samples annealed at 800 °C and 900 °C,respectively.The number density of dislocation loops in sample annealed at 900 °C is reduced due to the defects recovery,whereas the migration and coalescence mechanism induced by the diffusion of matrix atoms can cause the formation of some new small dislocation loops,which leads to the number density of dislocation loops in sample annealed at 800 °C increased.In addition,it is found that the size of the helium bubbles is determined by the ratio of the number of vacancies to helium atoms in these helium bubbles.On the other hand,Hastelloy N alloy were successively irradiated by two beams of ions at 650 °C to study the interaction of helium bubbles with dislocation loops and the evolution process of helium bubbles under continuous collision cascade.TEM results showed that in the sample irradiated by xenon ions and helium ions successively,not only the displace damage induced by xenon and helium ions could produce dislocation loops,but also the selfinduced growth process of helium bubbles could also produce a large number of dislocation loops.Therefore,the number density of dislocation loops in this sample was approximately 3 times than the sum of that in the two single beam irradiation samples.In addition,the nanohardness increment of this sample was nearly 30% larger than the combined value of the nanohardness increment of the two single beam irradiated samples,indicating that the synergistic effect of helium bubbles and dislocation loops affected the change of the nanohardness of Hastelloy N alloy.In the sample irradiated by helium ions and xenon ions successively,the growth of helium bubbles and the resolution of helium atoms from helium bubbles existed at the same time and they were in a mutually competitive relationship.The former predominateed in the sample where the degree of displacement damage induced by xenon ions was smaller(3 dpa)whereas the latter played a key role in the sample where the degree of displacement damage induced by xenon ions was larger(10 dpa).In addition,both the ballistic recoil resolution and the damage assisted re-solution maked it easier for helium atoms to dissolve from small helium bubbles.