Molecular Dynamics Simulations of Helium Atoms Clustering in bcc Iron

Helium atoms are known to have a significant impact on materials used in fission and fusion reactors. In particular, the presence of helium atoms can change the mechanical properties and degrade the lifetime of reactors. In order to develop the helium-resistance materials, the underlying interactions between helium atoms and matrix atoms have to be understood. In the past, atomistic simulations have contributed a lot to the basic energy states of helium atoms in different defects of materials. Especially, insights into the formation energies of helium atoms at simple defects have been calculated by ab initio and molecular dynamics. The substitional helium has lower formation energy than the tetrahedral and octahedral interstitial helium atom. Additionally, the binding energies of helium with small HenVm cluster have also been found to be all positive. However, all these simulations have been performed without showing the kinetic formation process of such helium defects. The interactions of defect with larger number of helium atoms are also not explored in detail. In the first part of this thesis, a new relaxation-fitting method has been developed for cross potential of Fe-He system based on more energy data of helium-defect clusters from ab initio calculations and by including the migration energy of single free helium atom in the fitting process. Besides, the recently fitted magnetic potentials of Fe have been used to describe the Fe-Fe interactions for such cross potential. The new fitted Fe-He potentials have predicted well not only the properties of simple helium defect, but also the properties of helium clusters and diffusion process of helium atoms. Based on the new Fe-He potentials, the interactions between helium atoms and Fe with the effect of single vacancy have been simulated utilizing the classical molecular dynamics (MD) method in the single bcc Fe lattice at room temperatures by helium atoms up to 18. The binding energies of helium atom with HenV cluster are computed. By increasing number of helium atoms, the binding energy decreases first, then slowly increases up to n = 4. It is then almost constant between n = 5 to 15 but increases strongly at n = 16 to form a peak. This last increase is related to the athermal self-interstitial atom (SIA) emission in the form of dumbbell which relaxes the restrained system to He16V2 cluster. The second binding energy peak occurs at n = 18 by the formation of the second SIA to form He18V3 cluster. Close inspection of the local configuration shows the formed SIA combines with the helium-defect cluster. Calculation of the pressure of the helium-defect cluster shows local peak normal stress and shear stress values up to 9 GPa and 4 GPa, respectively. The local configurations of helium-defect cluster suggest that with increasing helium content, some symmetrical structures can be formed. By increasing the number of helium atoms inserted in single bcc Fe lattice, the clustering process of the for