The core structure of ½(111) screw dislocations in b.c.c. crystals

Abstract

A relaxation-type calculation of the structure of the dislocation core has been made for the ½ 〈111〉 screw dislocation in b.c.c. crystals, using a variety of central-force potentials. Two stable configurations were found, corresponding to the centre of the dislocation being along either the left-hand or the right-hand type of three-fold screw axis in the crystal. These two configurations differed only in the very centre. For both configurations and for all potentials, the core structure possessed three-fold symmetry, the largest displacements being in the directions in which displacements on (211) type planes were in the twinning sense. The structure can be described by a combination of large displacements on {110} type planes, plus ‘stacking faults’, 1–2b wide on {211} type planes in the twinning sense only. An investigation of the effect of boundary conditions showed that any errors caused by incomplete relaxation were negligible, and that changing the initial dislocation position or the position of the boundaries did not affect the final core structure. Four potentials were used, similar in form but having a five-fold variation in the depth of the minimum. The core structure was similar for all of these : the only changes were in the relative magnitude of the displacements and in the ‘stacking fault’ width which varied between, approximately, 1.2b and 2b. This core structure therefore appears to be general for b.c.c. crystals, and the implications of these results on the movement of dislocations are briefly discussed.