Lattice Inversion For Interatomic Potentials In Fe-He Interstitial System

Ferritic steels is considered to be one of the best candidates as the first wall materials in nuclear fusion reactors, due to its excellent resistant abitity to irradiation swelling, corrosion and degradation of high temperature strength. In fusion reactors the first walls are irradiated by the He atoms generated from neutron transmutation reactions, and aggragation of He atoms or clsuters are formed in a-Fe matrix of ferritic steels for the very low solubility of helium. This will lead to the swelling, high temperature brittleness and creep deformation in a-Fe, and the accompaning roughness will threaten the safety of fusion reactors for the obvious degradation of mechanical properties and the decreasing of service lifetime. It is very difficult to observe the atomic behaviours of helium diffusion in the a-Fe, and then the large-scale atomistic simulations based on interatomic potentials are required to describe the interstitial migration of He atoms for interpretation of the effect of heliums on a-Fe.The validity of interatomic potentials plays an important role to produce the reasonable results from the aomic-scale simulations. In the pase decades, the interaomic potentials in Fe-He interstitial system were often derived by empirical fitting to the experimental data and first-principle calculations, but no one was obtained from the information of intersttial structures. This has to make researchers doubt the calculated results from the empirical potentials. Therefore it is desirable to derive the interatomic potentials from the data source with regarding to the Fe-He interstial structures.In this paper two virtual interstial structures were presneted to perform the first-principle calculations within the large range of lattice constants. Based on the Chen-Mobius lattice inversion, the Fe-He interatomic potential was derived to reproduce the energy difference between the tetrahedral and octahedral interstial lattices of Fe4He or Fe2He. The lattice inversion indicated that there is no obvious dependence on the Fe:He ratio, and the Fe-He potentials has a good transferability. The interactions between Fe atoms are described by the Finnis-Sinclair potential within the frame of embedded atomic model, due to its wide applications in the corresponding simulations. And finally the He-He potential was produced from the first-principle cohesive energy curves of sc-He. The lattice inversion interatomic potentaisl for Fe-He interstial system were generated by the combination of three interatomic potentials.Based on the lattice inversion potentials, the phase stability were discussed by calculation the formation energies when single He defect was placed at the tetrahedral, octahedral and substitutional sites, respectively. The defect formation energies show a reasonable stable sequence in agreement with the previous potential calculations. At meantime, the high-pressure mechanical properties were also provided to help the wall materials design of fusion reactors closing to the real reaction extreme conditions. In order to describe the migration of He atoms, the energy barrier, along the shortest migration path connecting the nearest tetrahedral positions, was also calculated by the drag atom method. For different temperatures, further molecular dynamic calculations were performed to describe the diffusion of He or He-V clusters with different sizes and atomic configurations. The calculated mean-square displacements reveal that the combination of vacancy and He atoms would trap the He atoms and obviously decrease the diffusion of He atoms. The ability of He atoms diffusion will influenced by the temperature, cluster size and atomic configuration. The main reason is that the tetrahedral interstital sites surrounding the He clusters play a key role in dominating the migration and diffustion of He atoms via the low energy barrier path connecting the neighbour interstitial atomic sites. These atomic-scale simulations not only prove the validity of interatomic potentials in Fe-He interstital system based on lattice inversion, but also provide a feasible project to construct the interatomic potentials for helium in BCC metals.