  Magnesium (Mg) and its alloys hold great potential as an energy-saving
structural material for automative, aerospace applications. However, the use of
Mg alloys has been limited due to poor ductility and formability. Poor
mechanical properties of Mg alloys origin from the insufficient number of slip
systems, and deformation twinning plays an important role to accommodate
plastic deformation. Here, we report a comprehensive experimental and modeling
study to understand crystal size effect on the transformation in deformation
modes in twin oriented Mg single crystals. The experiments reveal two regimes
of size effects: (1) single twin propagation, where a typical "smaller the
stronger" behavior was dominant in pillars {\le} 18 {\mu}m in diameter, and (2)
twin-twin interaction, which results in anomalous strain hardening in pillars >
18 {\mu}m. Molecular dynamics simulations further indicate a transition from
twinning to dislocation mediated plasticity for crystal sizes below a few
hundred nanometers. Our results provide new understanding of the fundamental
deformation modes of twin oriented Mg from nano-scale to bulk, and insights to
design Mg alloys with superior mechanical properties through dimensional
refinement. This subsequently can materialize into more utilization of Mg
alloys as a structural material in technologically relevant applications.

Materials Science