Crystallographic aspects of dislocation emission from a crack tip in an fcc metal

摘要

Dislocations are observed to emit from crack tips in atomic simulations of fcc metals as Shockley partials. The crystallographic anisotropy associated with emission as partials relative to perfect dislocations is discussed. The stress intensity needed to emit a dislocation is derived in terms of both the anisotropic and the isotropic versions of a Peierls-like criterion posed by Rice et al for dislocation emission and which invokes a critical emission force phi(lc). By systematically changing the orientation of the crystal relative to the crack coordinates, the behaviours of atomic models of embedded-atom method nickel are observed to change from emission to crack extension in a way that is quantitatively consistent with the combined predictions of the Rice et al. and Griffith criteria using a constant emission force phi(lc), and the relevant surface enthalpies. Furthermore, using the value of phi(lc), that best represents the single-crystal results, the predictions are also found to agree favourably with the behaviour of a double-ended crack on a grain boundary where one end emits dislocations while the other extends in a brittle manner. Finally, we explore the emission of trailing dislocations and find that the second and third dislocations require somewhat higher stress intensities to emit but, because the K needed to extend the crack is also increased, the emission of the first dislocation remains the critical step. The origins of phi(Ic) are discussed, and it is suggested that, although phi(Ic), is often associated with the unstable stacking-fault energy it may not, in general, be adequately defined solely in terms of the unstable stacking-fault energy.