  Deformation twinning in pure aluminum has been considered to be a unique
property of nanostructured aluminum. A lingering mystery is whether deformation
twinning occurs in coarse-grained or single-crystal aluminum, at scales beyond
nanotwins. Here, we present the first experimental demonstration of macro
deformation twins in single-crystal aluminum formed under ultrahigh strain-rate
($\sim$10$^6$ s$^{-1}$), large shear strain (200$\%$) via dynamic equal channel
angular pressing. Deformation twinning is rooted in the rate dependences of
dislocation motion and twinning, which are coupled, complementary processes
during severe plastic deformation under ultrahigh strain rates.
