New mechanisms of helical dislocation formation via the pinch-off process near a nano-inhomogeneity

Early experiments showed that short helical dislocations can be generated near an inhomogeneity in crystal materials under hydrostatic loading. For the first time, atomistic simulations also reveal that a nano-inhomogeneity can not only induce rows of prismatic loops but also helical dislocations under hydrostatic loading. Two new mechanisms of helical dislocation formation are discovered via molecular dynamics (MD) simulations. Both concern the pinch-off process between dislocation segments. The first mechanism is the pinch-off failure of an unclosed prismatic loop. The second mechanism is the early pinch-off between two nested, cross-slipped shear loops before evolving into prismatic ones. The configuration of the two closing-in dislocation segments is crucial. Two screw segments with opposite line directions will feel a strong attractive force, while other configurations may not. Thus, the pinch-off process is either promoted or hindered. The reduction of screw components can be viewed as a need to lower the interaction energy between dislocation loops. The stress required to sustain the formation of dislocation helix is found much lower than that needed to trigger dislocation emission. In addition to the formation mechanism, the ongoing material transport is studied and its distribution is revealed concerning the helical turns. To our best knowledge, the formation of helical dislocations near a nano-inhomogeneity in an initially dislocation-free matrix has never been studied via atomistic simulations.