Research On Multi-scale Simulation Method Of ?-iron Radiation Damage

The population expansion and industrialization are leading to an increasing demand of energy,and therefore nuclear power will become an increasingly favored option for largescale power generation.However,in the process of energy production via fission and fusion,structural materials within nuclear reactors can sustain substantial radiation damage.Radiation damage changes the microstructure of materials,thereby degrading the performance of materials.Therefore,in order to select and design high-performance radiation-resistant materials.it is necessary to understand the effects of radiation damage on the micostructure and properties of materials.Ferritic steel is one of the structural materials in existing fission reactors and future new advanced fission reactors,and reduced activation ferritic/martensitic(RAFM)steel is a candidate for the first wall and blanket structural materials in future fusion reactor system.Body-centered cubic(bcc)or a-phase iron is the model metal and matrix material of these steels.The effect of radiation damage on the microstructure and properties of a-iron remains is still not fully understood,and therefore a-iron is selected as the research object of radiation damage.Radiation damage is inherently a multiscale phenomenon on the time and space scale,and therefore radiation damage modeling and simulation is a multiscale problem,which provides a hierarchical method of simulating material radiation damage on the wide range of space and time space from the atomistic level of radiation damage production to the engineering application.In this thesis,radiation defect production is studied by molecular dynamics method,and its accumulation is studied by kinetic Monte Carlo method in a-iron,thus a multiscale modeling method of radiation damage is established.Primary damage state obtained by displacement cascade is the input data of damage accumulation and evolution model,such as kinetic Monte Carlo method,and hence it has a significant influence on the evolution of the microstructure in materials.Therefore,the point defects and their clusters produced by displacement cascades are very important for the research of radiation damage in materials.Molecular dynamics simulations,however,do not keep track of crystal defects explicitly,and therefore crystal defects and defect-free crystal regions must be recovered from the generated particle-position datasets in a post-processing step.Defects produced by displacement cascades in materials are complex in nature,but there is still no clear answer to the question of how to analyze point defects.We first modify the equivalent sphere method of 0.3 aO(30%of the lattice constant)and the equivalent sphere method of 1/2 NN(half of the nearest neighbor distance).Then they and the Wigner-Seitz(WS)cell method together are used to analyze the molecular dynamics data of the displacement cascades,and it is found that different defect identification methods have a significant effect on the number of defects produced by the displacement cascades.In order to analyze the defect configuration,a modified equivalent sphere method of 0.3 aO can be used because it can give various interstitial configuration,but it can not give a reasonable number of Frenkel pair.The Wigner-Seitz(WS)cell method ensures that the whole domain is checked for defects and does not involve any arbitrary cutoff as an input parameter,and hence the WS method gives a reasonable number of Frenkel pair.Therefore,it is recommended that the primary damage state obtained by the Wigner-Seitz cell method be used as input data for damage accumulation and evolution models to correctly predict the evolution of microstructure in materials under irradiation.For defect cluster analysis,the choice of cutoff radius is affected by the binding energy between the alike defects.It is proposed to use the third nearest neighbor criterion for interstitials and the second nearest neighbor criterion for vacancies.The main source of uncertainty in molecular dynamics simulations is the interatomic potential.In order to improve the confidence of molecular dynamics computer simulations of displacement cascades,it is necessary to select a reliable interatomic potential,because the interatomic potential determines the main properties of simulating system.In this thesis,the repulsion part of the 'magnetic' potential is modified before the cascade simulation.Then the modified 'magnetic' potentials(MP00-GTLQ,MP30-GTLQ,and MP33-GTLQ potential)and the Mendelev-type potential(AMS04 potential)were used to study the effect of interatomic potential on displacement cascades using molecular dynamics methods.It is found that the higher cascade density promotes the recombination of defects,and the defect production efficiency has nothing to do with the displacement threshold energy.It is also found that the displacement cascade results are closely related to the nature of the interstitials predicted by the interatomic potential.Because the AMS04 potential accurately reproduces the behavior of the self-interstitials described by the ab initio calculation,the AMS04 potential is considered to be the most reliable potential to simulate displacement cascades.The first wall and blanket structural materials of the fusion reactor are exposed to the high flux high-energy neutron to produce radiation damage.In addition,large amounts of helium(He)atoms can be directly implanted by a high flux of helium escaping from the plasma or generated internally by(n,a)nuclear transmutation reactions during high energy neutron irradiation in materials.The interaction of helium atoms with radiation defects has a significant impact on the microstructure and performance of a-iron.The helium atoms,implanted or created by transmutation,are initially interstitial and randomly distributed everywhere in the iron matrix.Therefore,understanding the interaction of helium atoms with displacement cascades in materials still remains as one of the most important issues in the research and development of materials resistant to radiation under fusion conditions.The effects of cascade energy,helium atomic concentration and irradiation temperature on the interaction of interstitial helium atoms with displacement cascades in Fe-He system are studied by molecular dynamics with recently developed Fe-He potential.It is found that the presence of interstitial helium atoms does not produce additional defects during displacement peak stage,but at the end of cascades,interstitial helium atoms promote the recombination of defects at low energies,but suppress the recombination at large energies in the Fe-0.1at.%He system.In the Fe-0.1at.%He systems,interstitial helium atoms promote SIA clustering at large energies and suppress SIA clustering at low energies,while interstitial helium atoms always inhibits the clustering of vacancies.There is no effect of helium atom concentration on the number of defects during displacement peak stage.The number of defects increases with helium atom concentration at the end of cascades,but interstitial helium atoms can promote the recombination of defects at low concentrations,but suppress the recombination of defects at large concentrations.Furthermore,larger helium atom clusters can help the production of defects in the Fe-He systems with large helium concentrations at the end of cascades.Interstitial helium atoms promote clustering of SI As and vacancies at larger helium concentration.High radiation temperature can promote the production of defects at the peak time and the recombination of defects at the end of cascades,but there is no obvious tendency concerning the effect of radiation temperature on the clustering of defects and helium atoms.The results of the interaction of interstitial helium atoms with displacement cascades using molecular dynamics methods can be used as input data of damage accumulation and evolution models to predict the influence of helium atoms on the a-iron microstructure in the fusion environment.The properties and performance of structural materials in nuclear reactors are governed by the fate of defects produced by irradiation.The evolution of these defects can lead to various phenomena,such as void swelling,hardening and embrittlement,and irradiation creep.Therefore,the knowledge of defect formation and subsequent evolution is essential for understanding and predicting the effect of high-energy particles radiation on the microstructure and properties in the materials.It is important that effect of the detailed features of the primary damage state on the long-term evolution and accumulation of radiation damage.Based on the primary damage state obtained with different defect identification methods and different interatomic potentials,the effect of cascade structures on the damage accumulation has been investigated by kinetic Monte Carlo method.And the effect of different radiation dose rate on the damage accumulation has also been studied in bcc iron.There is no effect of defect configurations with different defect identification methods on the damage accumulation,because this is a same cascade process.Cascade structures obtained with different interatomic potentials significantly influence the damage accumulation.Different interatomic potentials describe different interactions between atoms,and hence lead to different displacement cascades.Radiation dose rate also influence the damage accumulation.Before the damage accumulation ratio reaches the maximum,radiation damage accumulation is independent on the dose rate,and after the maximum,the damage accumulation ratio increases with dose rate.