Atomistic simulation of kink-pairs of screw dislocations in body-centred cubic iron

The nudged elastic band method is used to calculate the activation paths connecting jumps between the two degenerate states A and B of the screw dislocation core in body-centred cubic (bcc) iron. Kink-pairs are found to be involved in all such jumps. By comparing the activation energies, it is shown that the jump from state A to state B on the {101} plane is not the same as that from B to A, and whichever is easier depends on the direction of the applied stress. This asymmetry results in a characteristic zigzag pattern of slip, and forms the basis of an explanation for pencil glide at elevated temperatures. The activation energy of the rate-determining jump is found to decrease with increasing stress, but the predicted flow stress is about three times greater than the experimental value at all temperatures.