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. 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.