  Deformation twinning is generally considered to be the primary mechanism for
hexagonal close-packed (hcp) metals due to their limited slip systems. Recent
microcompression experiments on hcp metals point to intriguing strong size
effects on twinning mechanisms, indicating that pyramidal slips dominate
compression.In this work we analyze size effects on twinning due to
lattice-rotation strain in hcp single crystals. A criterion for deformation
twinning is derived at the nanoscale and tested by molecular dynamics
simulations of magnesium and titanium single crystals. The results show <c+a>
pyramidal slip dominates the compression deformation at the nanoscale,
consistent with experimental observations as well as our analysis. Our finding
clarifies the nature of size effects in deformation twinning, while also
providing an explanation for the so-called strength differential (SD) effect.
