Phase Field Study On Anisotropy Of Irradiation Void And Microstructure Evolution Of Helium Bubble

Nuclear reactor pressure vessel steel is exposed to high temperature,high pressure,and high-energy particle irradiation,and the service environment is extremely harsh.The structural stability and mechanical stability of the in-service materials have become the key to the efficient and safe use of nuclear energy.The materials dissociate and cascade under the impact of high-energy particles,resulting in a large number of irradiation point defects such as vacancies and interstitial atoms.Point defect diffusion and recombination,cluster aggregation,and interaction with crystal defects(defects such as grain boundaries,precipitation,dislocations,etc.)gradually evolve to form cavities,helium bubbles and other unique radiation-damaged tissues under irradiation environments,resulting in Irradiation hardening,swelling,brittleness.Therefore,an in-depth understanding of radiation-damaged tissue is helpful to the design of materials with high radiation resistance.In this paper,the mesoscale phase field method is used to simulate the evolution of the radiation-damaged tissue,and the phase field model of the void microstructure and the helium bubble microstructure of the coupling rate theory is constructed.Coupling the grain boundary,anisotropy,and elastic stress elements in the phase field model to study the evolution characteristics of the structure of voids and helium bubbles.The main results obtained in the thesis are as follows.A void model coupled with conservative and non-conservative fields is established,and interface energy anisotropy is introduced into the model based on the characteristics of the FCC structure and obtained anisotropic characteristics consistent with the experimental results,the evolution of quadrilateral and hexagonal voids.The relationship between the minimum interface energy surface index,the anisotropy intensity factor,the angle of the grain orientation and the evolution of voids is obtained.When the minimum interface energy surface index changes,the morphology of the void will change significantly,and the voids with different morphologies of quadrilateral and hexagon will appear.The strength of anisotropy affects the strength of the selective growth trend of void.When the strength of anisotropy increases,the transformation trend of void into polygonal morphology becomes more obvious,and there is a positive correlation between the strength factor of anisotropy and the growth rate of voids in a certain range.The deflection of the space position of the void is consistent with the angle of the orientation of the crystal grains.When the angle of the orientation of the crystal grains is deflected,the void will follow the deflection at the same angle.The cross-section integral number and average radius of the void with interface energy anisotropy are obviously larger than that of the isotropic void.The hexagonal void has the largest cross-sectional integral number,the circular void has the smallest cross-sectional integral number,and the average radius of the quadrilateral shape void is greater than the hexagonal void.Using the Cahn-Hilliard model to couple the defect reaction rate theory,combined with the grain boundary model of the evolution of the Allen-Cahn equation,the effect of the interaction between the vacancy and the grain boundary on the evolution of the void structure is studied.The grain boundary absorbs the vacancies and gaps in the crystal,so that the vacancies and interstitial atoms are constantly annihilated at the grain boundary and depleted along both sides of the grain boundary.The phenomenon of grain boundaries versus vacancies leads to the concentration of voids being depleted near the grain boundaries and clustering in the direction of the grains in the depleted areas,thereby forming "void denuded zones" and"void maximum zones" along the grain boundaries.As the distance from the grain boundary to the grain increases,the vacancy concentration distribution becomes less affected by the grain boundary,so that the farther from the grain boundary,the later the void nucleation and the lower the nucleation rate.A phase field model of the co-evolution of helium bubbles and voids of FeNi and FeNiCr alloys is constructed,which is used for the co-evolution of helium bubbles and voids under stress.Binary FeNi alloy helium bubbles nucleate and grow faster,and the addition of Cr effectively delays the nucleation and growth of helium bubbles,which improves the radiation resistance of the material.After irradiation,the acceleration of irradiation promotes the nucleation and growth of helium bubbles.The interaction between helium bubbles and elasticity will also accelerate the nucleation and coarsening of helium bubbles.The applied stress will cause the local stress to concentrate on the edge and continue to accumulate,resulting in vacancies and Helium atoms oriented and diffuse to reduce the local elastic strain energy.Therefore,the voids and helium bubbles extend in the direction of stress and gradually transform from spherical to ellipsoidal.When there is a dislocation stress field,the vacancies and helium atoms will preferentially cluster near the dislocation dipole,and the dislocation stress field will be the center around this area to nucleate successively.Later,they will form the same size along the dislocation dipole.