On the energetics of dislocation emission from a crack tip in nickel containing hydrogen
- PDF / 2,594,107 Bytes
- 15 Pages / 576 x 792 pts Page_size
- 86 Downloads / 226 Views
A method that determines the work done in shearing atom pairs straddling the slip plane, O, during emission of dislocations from a crack tip in an atomic model is presented. The model is based on an EAM-type potential for nickel. The dislocations are emitted as partials, and the disregistry, A, across the slip plane is found to be fit accurately by a simple arctan function of position for each partial. The width of the partials is also found to remain essentially constant as they are emitted and move away from the crack tip. Rice's unstable stacking energy is extracted from the — A curves for the atom pairs along the slip plane and is observed to vary somewhat, particularly near the crack tip. In addition to the 4>(A) at points on the slip plane, the total work done on the entire slip plane is determined as a function of the dislocation position in the spirit of the Peierls approach. The derivative of this total work with respect to dislocation position leads to the lattice resistance, ar. The first partial dislocation to be emitted experiences a maximum in ar at about 0.2 nm from the crack tip, and several contributions to the overall resistance can be identified including the creation of a new surface at the tip as emission occurs, the creation of stacking fault as the dislocation glides away from the tip, and a small but discernible periodic component with a period related to the lattice. A string of hydrogen interstitials is introduced at various locations in the lattice and its effect on A, the $ — A curves along the slip plane, and the lattice resistance is examined. A substantial effect on the unstable stacking energy results as the dislocation passes an interstitial on the slip plane, but the effect of an interstitial on the resistance to dislocation emission expressed in terms of the maximum ar 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 additional observations including dislocation pinning on the interstitials.
I. INTRODUCTION Several postulates of conditions needed for an atomically sharp crack tip to emit a dislocation have been made and among them are the suggestions by Kelly, Tyson, and Cottrell,1 Rice and Thomson,2 and, most recently, Rice 3 and Schoeck.4 These postulates address the competition between dislocation emission from a crack tip and crack extension and, in so doing, provide a model of the mechanisms controlling toughness. Tests of such models are made primarily on the basis of their success in sorting materials into two categories, intrinsically ductile or intrinsically brittle. However, beyond an ability to make gross distinctions, somewhat more substantial goals concern predictions of ductile versus brittle response in a single material as influenced by extrinsic variables such as temperature, stress state, environment, and composition. A difficulty is often encountered in capturing such details from these models when the competition between emission and extension is evaluated more or le
Data Loading...
