A Molecular Dynamics Study on Local Lattice Instability at a Dislocation Nucleation and Motion.

Abstract

To clarify the mechanism and mechanics of dislocation nucleation and motion in the atomic scale, a molecular dynamics simulation of tension along [001] direction is conducted on a nanoscopic specimen of single crystalline nickel. After showing perfect elastic behavior, the specimen plastically deforms at the applied strain of 0.084 by the passage of partial dislocations nucleated at the surface of the specimen. Detail observation of the dynamic process of a dislocation nucleation reveals that two successive instable behaviors of local lattices take place at the nucleation site in the homogeneously deformed body. The first instable behavior is the collapse of local lattices in the transverse direction to bring about the deformation concentration at the nucleation site. The second is the instable shear of local lattices on a slip plane in the deformation concentrated area. The nucleation and glide of a partial dislocation is recognized as the migration of atoms on the slip plane as the result of these instable behaviors. In order to elucidate the onset condition of the instable behaviors of local lattices, the positive definiteness of the elastic stiffiness coefficients, Bijkl, of all local lattices are investigated. That is, the instability criterion proposed by Wang and Yip is adopted to the evaluation of the local instability. The results show that the minor determinants of Bijkl related with the compliances of lattice for the transverse collapse and the slip deformation become negative at the onset of each instable behavior. Thus, the local lattice instabilities bring about the dislocation nucleation and motion and can be clarified by the negative definiteness of local Bijkl.