Dislocation motion, constriction and cross-slip in fcc metals

A Finnis/Sinclair-type interatomic potential for copper is used to examine the properties of dissociated screw dislocations in the face-centred cubic lattice. The critical stress for dislocation motion is found to be a sensitive function of the partial dislocation separation, with a lower limit at least 85% smaller than the Peierls stress. The constriction of Heidenreich-Shockley partials is modelled using an applied stress which interacts only with the edge Burgers vectors; recombination is nor observed, but there is a critical separation below which the potential energy of the dislocation rises very rapidly. The classical model of cross-slip, in which the dislocation cannot leave its dip plane unless it is fully constricted, is found to be incorrect. Instead, cross-slip is possible at all partial separations, provided the driving stress is large enough. Finally, a new mechanism for cross-slip nucleation is described.