The Effects Of Grain Boundaries On Formation Of Helium Clusters In BCC Fe

Body-centered cubic iron and Fe-based alloys are common structural materials in nuclear industry. Reduced activation ferritic/martensitic steels are good candidates for the future fusion reactors. In nuclear reactors, structural materials must endure neutron induced atomic displacement damage as well as Helium (He) effects. He as foreign elements is generated concurrently as a result of (n, a) reactions in the host material. As well as we know, structural materials are polycrystalline in real application, so studying the damage behavior of polycrystalline materials requires understanding how He atoms interact with structural defects containing a free volume, particularly vacancies and grain boundaries (GBs). Molecular dynamics (MD) simulations are performed to understand the evolution of defects near grain boundaries in a-iron.Sixty-three grain boundaries with<110> and<100> tilt axis in a-iron are structured in this thesis. Molecular dynamics simulations are performed to investigate the properties of HenVm(n=1-20, m=0,1,3,5) clusters near grain boundaries with <110> and<100> tilt axis in a-iron. We also studied how∑3<110>{121} symmetric tilt grain boundary (GB) affects point defects and defect clusters in He-doped a-iron at300K in picosecond time scales.The structure, energetics and excess volumes of thirty-eight<110> and twenty-five<100> tilt grain boundaries are systematically studied at beginning. We studied the average formation energy and potential energy of HenVm clusters in two groups of grain boundaries. Molecular statics calculations show that formation energy of HenVm cluster was lower in grain boundaries than in perfect crystal. The simulation result shows that the average formation energy and the average potential energy of clusters both in the grain boundaries and in perfect crystal was decrease with the increase of the He atom number and the vacancies in the cluster. The difference of the effects of grain boundaries on the average formation energy and the average potential energy of helium clusters were decrease with the increase of the He atom number and the vacancies in the cluster in the grain boundaries with<110> tilt axis, and the difference of that effects wasn’t obviously in the grain boundaries with <100> tilt axis.The average potential energy of He clusters is decrease with the increase of the He atom number in grain boundaries with<110> tilt axis in α-iron at300K and500K, and the difference of the effects of grain boundaries on the average potential energy of helium clusters is decrease. The average potential energy of clusters in perfect crystal is decrease with the increase of the temperature, while the changes of the average potential energy of clusters in grain boundaries are not very noticeable. Microcale He clusters are stable in the grain boundaries, so the probability of the He clusters nucleation is larger in the grain boundaries than in the crystal for nuclear materials.Molecular dynamics (MD) simulations are carried out to study the behavior of point defects or defect clusters in∑3<110>{121} symmetric tilt grain boundary. Molecular statics and Molecular dynamics calculations show that the grain boundary acts as a good sink for point defects especially for interstitial He and self-interstitial atoms (SIAs), because the formation energy of self-interstitial atoms and substitutional Helium and interstitial Helium is reduced in the grain boundary. Simulation results show that the total number of point defects is more in the grain boundary than in the reference system after the Molecular dynamics was finished. The total number and the type of point defects in two systems show the behavior of point defects was affected by the grain boundary in this study. It is found that the number of HenV (n=2,3) clusters in the grain boundary region is reduced with the increase of He concentration in the system, while the number of the HeV clusters is increased. We also observed the He clusters in the grain boundary, and the He cluster tends to evolve to a symmetrical configuration during evolution, in agreement with ab initio calculations. So we can conclude that He atoms tend to form He bubbles in the grain boundary core.There are six chapters in this paper, chapter one is introduction; chapter two is theory and concepts involved in this paper; chapter three is how to build grain boundary structure; chapter four is the property of He clusters in the grain boundaries, chapter five is the behavior of point defects or defect clusters in∑3<110>{121} symmetric tilt grain boundary; chapter six is the summary and forecasting.