  Deformation twinning is an important deformation mechanism in a variety of
materials, including metals and ceramics. This deformation mechanism is
particularly important in low-symmetry hexagonal close-packed (hcp) metals such
as Magnesium (Mg), Zirconium (Zr) and Titanium (Ti). Extension twins in Mg, Zr
and Ti can accommodate considerable plastic deformation as they grow. Thus, the
rate and the mode of twinning greatly influences the mechanical behavior
including strength and ductility. Herein, we study deformation twinning in
terms of nucleation, twinning mode and variant selection as a function of
strain rate in Mg single crystal (considered as a model material). We show that
twin variant selection is sensitive to the loading rate, with more twin
variants nucleating at the dynamic strain rates. Low Schmid factor twin
variants (one of them being a double extension twin variant) were also found at
the dynamic strain rates. Further at high strain rates, the first twins
generated do not thicken beyond a critical width. Instead, plasticity proceeds
with nucleation of second generation twins from the primary twin boundaries.
The rates of area/volume fraction evolution of both generations of twins are
found to be similar.
