Core structure of screw dislocations in body-centred cubic metals: relation to symmetry and interatomic bonding

In this paper we discuss both the crystallographic aspects that govern the general features of the cores of 1/2(111) screw dislocations in bcc metals and the role of interatomic bonding that is specific to given materials. This analysis is carried out by comparing the results of two atomistic calculations of dislocations in molybdenum, one performed using many-body central force potentials and the other bond-order potentials that include the angular dependence of interatomic interactions. In both cases the core spreads into three {110} planes of the [111] zone but in one case it is unique and invariant with respect to the (101) type diad, a symmetry operation of the bee lattice, and in the second case two distinct configurations exist that are related by the diad operation. Which of the structures is found depends on interatomic interactions and it is shown that the gamma surface for {110} planes can be used to predict the type of the core spreading. We then demonstrate that both core structures may lead to very similar responses of the dislocation to applied stresses since the strained crystal loses the original symmetry, in particular the corresponding (101) type diad, and thus the distinction between the two types of core vanishes. Finally, we discuss the generality of these concepts when analysing dislocation cores in materials with other structures than bcc.