Simulations of the atomic structure, energetics, and cross slip of screw dislocations in copper

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Abstract
Using nanoscale atomistic simulations it has been possible to address the problem of cross slip of a dissociated screw dislocation in an fcc metal (Cu) by a method not suffering from the limitations imposed by elasticity theory. The focus has been on different dislocation configurations relevant for cross slip via the Friedel-Escaig (FE) cross-slip mechanism. The stress free cross-slip activation energy and activation length for this mechanism are determined. We show that the two constrictions necessary for cross slip in the FE cross-slip mechanism are not equivalent and that a dislocation configuration with just one of these 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 with transmission electron microscopy experiments and predictions from linear elasticity theory showing recombination or repulsion of the partials near the free surface. Such recombination at the free surface might be important in the context of cross slip because it allows the creation 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 simulations of two screw dislocations of opposite signs, one simulation showing surface nucleated cross slip leading to subsequent annihilation of the two dislocations. It was possible to monitor the annihilation process, thereby determining the detailed dislocation reactions during annihilation.