SIMULATIONS OF THE ATOMIC-STRUCTURE, ENERGETICS, AND CROSS SLIP OF SCREW DISLOCATIONS IN COPPER

Using nanoscale atomistic simulations it has been possible to address the problem of cross slip of a dissociated screw dislocation in an fee metal (Cu) by a method not suffering from the limitations imposed by elasticity theory. The focus has been on different dislocation configu rations relevant for cross slip via the Friedel-Escaig (FE) cross-slip mechanism. The stress free cross-slip activation energy and activatio n length for this mechanism are determined. We show that the two const rictions necessary for cross slip in the FE cross-slip mechanism are n ot equivalent and that a dislocation configuration with just one of th ese constrictions is energetically favored over two parallel Shockley partials. The effect of having the dislocation perpendicular to a free surface is investigated. The results are in qualitative agreement wit h transmission electron microscopy experiments and predictions from li near-elasticity theory showing recombination or repulsion of the parti als near the free surface. Such recombination at the free surface migh t be important in the context of cross slip because it allows the crea tion of the above-mentioned energetically favorable constriction alone . In addition we observe a strong preference for the partials to be in a glide plane parallel to the surface step. We have performed simulat ions of two screw dislocations of opposite signs, one simulation showi ng surface nucleated cross slip leading to subsequent annihilation of the two dislocations. It was possible to monitor the annihilation proc ess, thereby determining the detailed dislocation reactions during ann ihilation.