COMPUTER-SIMULATION OF THE STRUCTURE AND MOBILITY OF TWINNING DISLOCATIONS IN HCP METALS

The properties of twinning dislocations in [1011BAR}, {1012BAR}, {1121BAR} and {1122BAR} twin boundaries in hexagonal-close-packed metals have been investigated by atomic-scale computer simulation. Care has been exercised (by use of the concept of bicrystal structure maps) to ensure that all possible stable interface structures have been modelled and the work extends the research reported earlier by us [6] in several ways. First, we have now treated the important case of the {1011BAR} twin. Second, the dependence of dislocation energy on the atomic structure of the core has been investigated. Third, the mobility of these interfacial dislocations has been examined by computing the critical resolved shear strain for glide. It has been found that the twinning dislocations corresponding to the {1012BAR} and {1121BAR} twins observed in practice are highly glissile, whereas those for the {1011BAR} and {1122BAR} modes are not. These effects are deduced to be related to the atomic structure of the core (and, in particular, to its width rather than height), and are found to be consistent with the nature of deformation twinning reported for the h.c.p. metals. For {1011BAR} twinning, however, the dislocation of lowest energy and highest mobility does not correspond to the mode observed in practice, implying that twin nucleation may possibly be a controlling factor in that case.