Computer Simulation Of Point Defects And Edge Dislocation In BCC Transition Metals

Metal materials usually exist in polycrystalline form. Point defects and dislocations exist at a certain concentration in all materials. Point defect results in the increase of the material electrical resistivity, crystal volume and thermal expansion coefficient. In addition, diffusion of point defect is the dominant mechanism of atomic transport or diffusion in metals. Line defect (the dislocation) mobility is responsible for plastic deformation of materials, and directly influences intensity, cracks, diffuse and phase transition etc. In semiconductors dislocations affect the electronic properties. Therefore, the study of point defects and dislocations in crystal are central problem of materials, metallurgy, solid physics and even solid chemistry at all times. The people expect to improve the performance of materials or to provide a direction of theory for material designs by their speciality. In this paper, with modified analytic embedded-atom method (MAEAM), the strain energy and the equilibrium core structure of a [100] edge dislocation have been simulated and calculated on atomistic scale in BCC transition metals: Mo and Fe as prototype; Further, the strain energy of a [100] edge dislocation, the distribution of the atomic energies and the interactions of dislocations and point defects are investigated and discussed. The main results can be concluded as follows:(1) For a [100] edge dislocation in BCC metals Fe and Mo, both of the equilibrium structures of the dislocation core show two principal characteristics: one is that the dislocation core has a very narrow effect region which is consistent with the general character of the BCC metals, and the other is that the relatively large relaxation occurs in the tensile region of the crystal. We further analyze the coordinates of atoms and find the dislocation core structure has a C_2v point group symmetry. These structure characteristics are also reflected in the energy. That is, the distribution of energy has very good symmetry in the [100] direction, which is consistent with C_2v symmetry of structure. Calculated strain energy E_s per unit length of the dislocation is a linear function ln(R/2b) while radial distance R≥1.5b～2b, thus the corresponding core radius is determined to be 1.5b≤r_c≤2b. From intercept of linear fitting those data corresponding to the outside of the dislocation core of a [100] edge dislocation in Fe and Mo, we determined the dislocation core energy E_core are about 7.356eV/nm and 16.334eV/nm respectively.(2) Considered interatomic relaxation, the interactions of a [100] edge dislocation and point defects in BCC metal Fe have been investigated. We can see that, the closer the vacancy to the dislocation line, the lower the relaxed formation energy of the vacancy. Especially when the separation distance between the vacancy and dislocation line is less than two lattice constants, the formation energy is the lowest. This means that the edge dislocation has a trend to trap the vacancy.(3) Although elastic theory can be used to calculate the elastic strain energy E_e, it can not define the dislocation core range r_c and energy E_core. The MAEAM modeling used in present work can calculate the position and energy of each atom in all dislocation areas, then open out the microscopic state of dislocation core, and attempt to cover the shortages of the elastic theory.