ON THE ENERGETICS OF DISLOCATION EMISSION FROM A CRACK-TIP IN NICKEL-CONTAINING HYDROGEN

A method that determines the work done in shearing atom pairs straddli ng the slip plane, PHI, during emission of dislocations from a crack t ip in an atomic model is presented. The model is based on an EAM-type potential for nickel. The dislocations are emitted as partials, and th e disregistry, DELTA, across the slip plane is found to be fit accurat ely by a simple arctan function of position for each partial. The widt h of the partials is also found to remain essentially constant as they are emitted and move away from the crack tip. Rice's unstable stackin g energy is extracted from the PHI - DELTA curves for the atom pairs a long the slip plane and is observed to vary somewhat, particularly nea r the crack tip. In addition to the PHI(DELTA) at points on the slip p lane, the total work done on the entire slip plane is determined as a function of the dislocation position in the spirit of the Peierls appr oach. The derivative of this total work with respect to dislocation po sition leads to the lattice resistance, sigma(r). The first partial di slocation to be emitted experiences a maximum in sigma(r) at about 0.2 nm from the crack tip, and several contributions to the overall resis tance can be identified including the creation of a new surface at the tip as emission occurs, the creation of stacking fault as the disloca tion glides away from the tip, and a small but discernible periodic co mponent with a period related to the lattice. A string of hydrogen int erstitials is introduced at various locations in the lattice and its e ffect on DELTA, the PHI - DELTA curves along the slip plane, and the l attice resistance is examined. A substantial effect on the unstable st acking energy results as the dislocation passes an interstitial on the slip plane, but the effect of an interstitial on the resistance to di slocation emission expressed in terms of the maximum sigma(r) is small and then only if it is confined to a region very near the crack tip. The significance of these results is discussed together with some addi tional observations including dislocation pinning on the interstitials .