Molecular Dynamics Simulation of Dislocation Nucleation in a FCC Crystal and Characterization of Preceding Local Strain Concentration by Lattice Instability Criterion.

Abstract

A molecular dynamics (MD) simulation was conducted on a thin wire of a single crystalline nickel subjected to tensile deformation in the [001] direction. When the applied strain, εzz, is smaller than 0.116, the crystal deforms uniformly and homogeneously, being perfectly elastic. A sudden drop in the applied load takes place at the moment of εzz=0.116 by a partial yielding. Dynamic process of the local strain concentration and the dislocation nucleation was analyzed through the measurement of a resolved shear strain, γrss, on every lattice plane in the direction of atom migration due to the partial dislocation : (1) When εzz reaches 0.116 at the first step of the present MD simulation, a high strain concentration region appears on the (010) surface, where the magnitude of γrss is larger than 0.13. (2) γrss increases during the relaxation time for the arrangement of atoms under a constant global strain of εzz=0.116, which is caused by the relaxation of the atomic potential energy. (3) A partial dislocation nucleates at the region when γrss reaches about 0.20. (4) The dislocation begins to glide on a (III) plane where γrss is most concentrated. It is noteworthy that the local strain increases during the relaxation of atomic arrangement before the dislocation nucleation, and that its concentration proceeds to nucleate the dislocation. In order to clarify the mechanism of the strain concentration on a smooth surface of the perfect crystal and the magnitude of the critical resolved shear strain : γrss=0.13, the crystal was subjected to an ideally homogeneous deformation, and its lattice instability criterion was evaluated based on the Born's concept. It was found that this homogeneously deformed lattice begins to be unstable in the direction of the resolved shear strain when γrss just reaches 0.13 being equal to the aforementioned crystal. Thus the localization of the resolved shear strain is attributed to the local lattice instability.