  While deformation twinning in hexagonal close-packed metals has been widely
studied due to its substantial impact on mechanical properties, an
understanding of the detailed atomic processes associated with twin embryo
growth is still lacking. Conducting molecular dynamics simulations on Mg, we
show that the propagation of twinning disconnections emitted by basal-prismatic
interfaces controls the twin boundary motion and is the rate-limiting mechanism
during the initial growth of the twin embryo. The time needed for disconnection
propagation is related to the distance between the twin tips, with widely
spaced twin tips requiring more time for a unit twin boundary migration event
to be completed. Thus, a phenomenological model, which unifies the two
processes of disconnection and twin tip propagation, is proposed here to
provide a quantitative analysis of twin embryo growth. The model fits the
simulation data well, with two key parameters (twin tip velocity and twinning
disconnection velocity) being extracted. In addition, a linear relationship
between the ratio of twinning disconnection velocity to twin tip velocity and
the applied shear stress is observed. Using an example of twin growth in a
nanoscale single crystal from the recent literature, we find that our molecular
dynamics simulations and analytical model are in good agreement with
experimental data.
