Atomistic Study Of Hydrogen Interaction With Intrinsic Defects In BCC Iron

The permeation and retention of hydrogen isotopes in materials is a critical issue concerning both economics and safety for realizing the controlled nuclear fusion.Reduced activation ferritic/martensitic(RAFM)steel is considered to be the main candidate for the structural material of the future fusion first wall and blanket.In fusion reactors,the radiation of neutrons with 14 MeV high energy will produce irradiation damages(defects),which may exhibit a significant effect on the behavior of hydrogen isotopes diffusion,permeation and retention in materials.We chose body-centered-cubic iron(a-Fe)as research object and employed atomistic calculation and simulation methods,including molecular statics calculations,molecular dynamics simulations and genetic algorithm,to study the diffusion properties of hydrogen and the interaction between hydrogen with different intrinsic defects.These calculation and simulation results are of help to understand the diffusion,permeation and retention behavior of hydrogen isotopes in RAFM steels as well as the effect of diffe’rent intrinsic defects.In addition,our results can also provide important input parameters for larger scale simulations.Based on different types of defects,the contents of the dissertation are consisted with four parts as follow.(1)Hydrogen diffusion properties in perfect a-Fe and the effect of point defects.Firstly,we determined that hydrogen diffusion in bulk a-Fe as single atom by molecular statistics calculations.Then we employed molecular dynamics simulations and obtained hydrogen diffusivities at different temperatures and diffusion energy barriers at different temperature ranges by the mean squared displacements method,which are consist with density functional theory calc ulations and experiments.Further,we studied the effect mechanism of self-interstitial atoms and vacancies on the hydrogen diffusion.We found that both kinds of point defects restrain the hydrogen diffusing but the effect weakens as the temperature rises.Especially,self-interstitial atoms exhibit negligible effect when the temperature is higher than 550 K.(2)The distribution and migration mechanism of hydrogen atoms around an edge/screw dislocation in a-Fe.Firstly,we employed molecular statistic calculations to study the stress and hydrogen binding energy distribution around the 1/2(111){110}edge dislocation and 1/2(111)screw dislocation,analyzing the correlation between the hydrogen binding and stress.Based on the hydrogen binding energy distribution,we predicted the possible hydrogen migration paths near dislocation cores.Further,the nudged elastic band(NEB)method was applied to calculate the hydrogen migration energy barriers along different migration paths near dislocations cores.Our calculation results reveal that neither edge nor screw dislocations can provide fast diffusion pipe for hydrogen atoms in a-Fe.In the meantime,our calculations also predict the migration mechanisms of hydrogen atoms at different temperatures near dislocation cores.Furthermore,we conducted molecular dynamics simulations to investigate the diffusion process of the hydrogen atom at dislocation cores,which give the trajectory of the hydrogen atoms and verify the prediction of NEB calculations.(3)The formation mechanism of vacancy dislocation loop and the interaction between hydrogen atoms and dislocation loops in a-Fe.Genetic algorithm was applied to search the energy minimization configuration of vacancy clusters.Molecular statics calculations and dynamics annealing relaxation were employed to calculate the formation and binding energies of vacancies and 3D vacancy or vacancy-hydrogen clusters as well as 2D vacancy or vacancy-hydrogen clusters on {111},{110} and{211} planes.Our calculations show that vacancies prefer to gather along 3D directions while vacancy-hydrogen clusters prefer to gather along 2D directions,especially on {211} planes.Therefore,vacancy dislocation loop is difficult to form in general,however,when vacancies trap hydrogen atoms and form vacancy-hydrogen clusters,the clusters prefer to gather on {211} planes since the direction of hydrogen binding sites at vacancies,thus,form the vacancy dislocation loop with the slip direction along the(100)direction and habit plane of the {211} plane.Our results consist with the experiment and reveal the formation mechanism of the vacancy dislocation loop.Further,we studied how dislocation loops trap self-interstitial atoms,vacancies and hydrogen atoms.We found that hydrogen atoms are strongly binding by the(100)vacancy dislocation loop and lower the system energy.In addition,the hydrogen atoms will enhance the ability of the<100>vacancy dislocation loop trapping vacancies and weaken the ability of the<100>vacancy dislocation loop trapping self-interstitial atoms,thus,promote the growth of the(100)vacancy dislocation loop.(4)Hydrogen atoms trapped by vacancy clusters and the behavior of hydrogen atoms in voids of α-Fe.Firstly,based on the energy minimization configuration of vacancy clusters,dynamics annealing relaxation molecular statics calculations were conducted to study how vacancies cluster trap hydrogen and how hydrogen atoms influence the growth of vacancies clusters.The hydrogen atoms trapped by vacancy clusters occupy the octahedral interstitial sites on the surface of the clusters.We obtained the relation between the saturation number of trapped hydrogen atoms and the size of vacancy clusters.It is interesting that the hydrogen atoms weaken the ability of vacancy clusters trapping vacancies,thus,suppress the growth of vacancy clusters.Further,molecular dynamics simulations were employed to study the behavior of hydrogen atoms in voids.Hydrogen atoms occupy the octahedral interstitial sites on the surface of voids while hydrogen molecules exist in voids.As the hydrogen number increasing,hydrogen molecules in voids atomize and diffuse into α-Fe lattice,which result in voids collapse.The dissertation studies the diffusion properties of hydrogen in α-Fe and hydrogen interaction with different intrinsic defects,including different intrinsic defects trapping hydrogen atoms,the effect of intrinsic defects on the hydrogen migration mechanism,and the effect of hydrogen atoms on the formation and evolution of different intrinsic defects.The results can provide basic reference from atomistic scale for the research of hydrogen isotopes diffusion,permeation and retention in RAFM steels.