Simulation of plastic deformation in a two-dimensional atomic glass by molecular dynamics IV

Abstract
Plastic deformation in a structurally well-relaxed two-dimensional atomic glass was simulated by a computer molecular dynamics approach. The simulation, which was carried through yielding and to substantial plastic strains, demonstrated that the principal mechanism of plastic strain production is by local partly dilatant shear transformations nucleated preferentially in the boundaries of liquid-like material separating the small quasi-ordered domains that form when the glass is well relaxed. Under imposed forward shear-strain increments, local shear transformations in atomic clusters were found to be mostly in the same direction as the applied stress. There were, however, substantial levels of shear transformations in other random directions, including many opposed to the applied stress. In all instances, however, nucleaton of shear transformations reduced the Gibbs free energy monotonically, which is governed largely by the locked-in excess enthalpies of the glassy state. At shear strains above 15%, localization of shear into bands was observed to begin. This steadily intensified and formed well-defined sharp shear bands into which all the shear strain became concentrated by the end of the simulation at a strain of 27%. A strong correlation was found between the tendency for shear localization and retained shear-induced dilatation.