The slip - twinning competition in HCP metals*

Experimental observations suggest that complex history-dependent behavior arises in metals with

hexagonal crystal symmetry, and that the prominence of various deformation mechanisms varies with

strain rate. The material response across a range of conditions appears to be challenging to capture

using many standard J2-type plasticity models. These model forms are often motivated by observations

made for cubic-symmetry in which plastic response may be governed by dislocation motion of only a

single Burgers vector type.

The slip planes in HCP metals with the lowest resistance to flow – typically the basal and prismatic slip

planes – do not accommodate every deviatoric deformation. Thus, out of necessity, slip and

twinning are expected to play an important role in the deformation. This gives rise to a competition of

mechanisms to maintain compatibility and equilibrium. We study this competition between twinning

and slip.

Under a set of reasonable simplifying assumptions, we perform an explicit homogenization procedure

deriving an expression relating the macroscopic flow strength to a dimensionless variable relating the

strength of the different slip systems. Utilizing this expression, we pose a simple model for inelastic

deformation in hexagonal close-packed metals.

We apply the model to examine Kolsky bar type data in beryllium and discuss agreement and

improvement relative to existing models.