Molecular Dynamics Study Of Al And Fe Surface Self-diffusion

Surface diffusion of atoms plays an important role in many materials associated phenomena, such as crystal growth, catalysis and so on. Aluminum and iron are widely used in modern industry. Diffusion of surface atoms is the important factor affecting the properties of materials, and it is one of the important factors for the change in the surface structure under dynamic conditions, so the study of aluminum and iron surface diffusion process is of great significance. As progress of experimental techniques, people can directly observe the movement of surface atoms by experimental means. However, limited by the time resolution, vacuum conditions and other restrictions, the study of the surface atomic diffusion mechanisms are still difficult. Computer simulations can overcome these difficulties, so that people can directly track the motion path of atoms at the surface.With tremendous progress of the theoretical methods and computer capability, people begin to use computer simulation methods, including such as first-principles, classical molecular dynamics and so on, to study the physical nature of the atoms diffusion in the metal surface. But there are still many unresolved issues in this field, including the establishment of the reasonable physical model, systematic study of different system and compared with the experimental results. For this reason, the theory study of the metal atom diffusion on the surface is particularly important to explore reasonable physical models and simulation methods and accumulate valuable data for the study of the future. In this paper, using molecular statics and molecular dynamics simulation, surface diffusion of aluminum and iron were studied.Firstly, the stability of three low index surfaces of Al and Fe were studied respectively. Surface energy, surface relaxation, formation energy of vacancy and self-adatom were calculated. The results show that, with the increasing of the compactness of the surface atoms arrangement, the surface energy and the degree of surface relaxation decrease, and the formation energy of point defects increases, that is to say, the surface structure stability increases.Secondly, the migration energy of point defects diffusion on Al and Fe surface was respectively obtained by calculating the diffusion curve of various transition mechanisms. The results show that, for the Fe(100), Fe(110) surface and three Al surfaces, the migration energy barrier of point defect on the outermost surface by the first neighbor hopping mechanism is the lowest. However, for the Fe(111) plane, the migration energy barrier of point defect by the exchange mechanism is lower.Finally, dynamics simulations for point defect diffusion on Al and Fe surfaces were carried out at different temperatures, respectively. The optimal diffusion mechanisms and migration energy on various surfaces were obtained, and the results show that, the diffusion mechanism occurs most frequently on three Al surfaces and Fe(110) surface is the simple hopping mechanism along the direction of the first neighbors. However, on the Fe(100) and Fe(111) surfaces, point defects prefer to diffuse by exchange mechanism. Relative to the vacancy, the migration energy for adatom diffusion on three Al surface occurs at lower temperature, the jump frequency is higher and the migration energy is lower. The migration energy for adatom diffusion on the Al and Fe close-packed surfaces are much lower than that on other surfaces. No matter what mechanism point defects diffuse by on Al and Fe surfaces, atoms migrate predominantly along the direction with the closest atom density.