Evolution Mechanisms Of Helium Bubbles In Nickel And Nickel-based Alloy

Structural materials in nuclear reactors are exposed to extreme harsh irradiation environment.Particularly,the nuclear transmutation reaction(n,?)due to neutron irradiation poses a serious risk of helium embrittlement to the structural materials in some specific reactors.Previous studies showed that formation and evolution of helium bubbles play a crucial role in causing helium embrittlement of metallic materials.Therefore,revealing the evolution mechanism of helium bubbles is prerequisite to understand the helium-induced degradation of structural materials,and thus to promote the comprehensive evaluation of their performance in reactors.In this thesis,a kind of nickel-based Hastelloy N alloy,which is the candidate structural materials for the molten salt reactor,and pure nickel were selected as the material samples.Using ion irradiation to introduce helium atoms and radiation damage into the materials,behavior of helium bubbles in nickel and nickel-based Hastelloy N alloy was systematically investigated in two aspects.Investigated the evolution mechanism of helium bubbles in specific environments.The evolution of helium bubbles can be affected by many factors,of which temperature and stress are two most important ones.By performing the post-irradiation annealing experiments,the evolution of helium bubbles in Hastelloy N alloy at different temperatures was investigated.The diffusion and escape behavior of helium atoms in irradiated samples were discussed,according to the variation of helium concentration profiles measured by elastic recoil detection technique.It was revealed that “Migration and Coalescence” and “Ostwald Ripening” mechanisms have governed the bubble growth in two special regions(Coalescence Zone and Ripening Zone)respectively in the irradiated Hastelloy N alloy.Literatures reported that the effect of helium embrittlement is closely associated with the evolution of helium bubbles at grain boundaries.In this thesis,by deploying the in situ heating experiment in transmission electron microscopy,the evolution of intergranular helium bubble in pure nickel was investigated.Results showed that helium bubbles at grain boundaries(GBs)performed elongation along GBs and trigged bubble coalescence when they encountered with each other during the heating at 400 oC.Due to the difference in internal pressure between the coalescing helium bubbles,two coalescence modes appeared: bubble coalescence driven by surface diffusion and by pressure gradient.Such behavior of intergranular helium bubbles under heating was termed the "Deformation and Coalescence" mechanism driven bubble evolution.Additionally,a finite difference numerical model describing the evolution of intergranular helium bubbles was developed based on the surface diffusion dynamics of helium bubbles.The calculation results reproduced the elongation behaviors of intergranular helium bubbles observed experimentally rather well.It is worth noting that stress is another key factor that governs the helium embrittlement of structure materials.Performing the small angle X-ray scattering experiment during the in situ tensile stress loading,the variation of helium bubble size,number density and volume with increasing stress in pure nickel was investigated.It was revealed that helium bubbles under uniaxial stress demonstrated the "Deformation and Coalescence" mechanism controlled evolution,similar to that of intergranular helium bubbles under heating.The underlying reasons were discussed in terms of the interaction between the sliding dislocations and helium bubbles.Investigated the interaction between helium bubbles and irradiation-induced microstructures.By performing the helium and subsequent xenon ion irradiation of the Hastelloy N alloy,the enhanced growth of both helium bubbles and dislocation loops occurred,compared with irradiation case by single ion beam.The results show that helium bubbles induced by helium pre-irradiation weakened the annihilation of Frankel defect pairs caused by subsequent xenon ion irradiation,thus promoting the synergetic evolution between helium bubbles and dislocation loops and enhancing the irradiationinduced hardening of materials.Simultaneously,it was found that xenon ion irradiation induced the formation of hexagonal closed-packed(hcp)structured precipitates with has a specific crystalline orientation with the matrix lattice.Results showed that a large number of helium bubbles formed inside the precipitates,and the sizes were significantly smaller than those outside the precipitates.Based on the discussion of competition between vacancy capture by helium bubbles and precipitates,the coupling effect between two microstructures was revealed.