A new look at attractive/repulsive junctions and cleavage crack formation in BCC materials

In this paper, the formation of sessile cleavage crack nuclei of the type a 〈 0 0 1 〉 from glissile dislocations of the type a 2 〈 1 1 1 〉 is considered. While an energy reduction is necessary, formation of a nucleus necessarily involves interaction forces. The propensity for cleavage crack formation in BCC crystals is explained in terms of the fact that glide dislocations inevitably interact to form microcrack nuclei. The calculated dislocation interactions are consistent with the observation that cleavage cracking in BCC alloys on {0 0 1}-type planes requires plastic deformation. Continued growth of a microcrack is examined in terms of distance between pre-existing sessile dislocations and the external force necessary to drive glissile dislocations to a common junction. Pre-existing sessile dislocations significantly alter the orientation dependence of the glide force on the glissile dislocations from that computed for glissile/glissile interactions. An important feature of this work was to provide a general framework for the computation of dislocation interactions using closed-form solutions in a computational environment.