(1¯012) Twinning nucleation mechanisms in hexagonal-close-packed crystals

Mechanisms for (1¯012) twinning in hexagonal-close-packed crystals at an atomic scale were studied using topological analysis and atomistic simulations. Two twinning mechanisms were found: a normal-twinning mechanism in which a stable twin nucleus is created by simultaneous nucleation of multiple twinning dislocations; and a zonal-twinning mechanism in which a stable twin nucleus is created by simultaneous nucleation of a partial dislocation and multiple twinning dislocations. The twinning direction, dependent on the ratio of lattice parameters c/a  , is along [101¯1] when c/a<3, but along the opposite direction when c/a>3. Atomistic simulations, using density function theory for Mg, Zr and Zn and an empirical potential for Mg, were performed to study the kinetics and energetics associated with the two twinning mechanisms. The results show that the zonal-twinning mechanism is energetically favorable relative to the normal-twinning mechanism, because the zonal dislocation has a smaller Burgers vector.