Anisotropy in hexagonal close-packed structures: improvements to crystal plasticity approaches applied to magnesium alloy

Abstract

Due to its polarity, twinning in strongly textured hexagonal close packed (HCP) structures can be maximized or minimized under particular loading conditions. The resulting anisotropy can be dramatically demonstrated for magnesium with a  fibre, for example. The stress–strain behaviour from compression loading parallel to the fibre produces a ‘parabolic’ stress–strain curve, but a ‘sigmoidal’ curve when loaded normal to the fibre. When modelling anisotropy in HCP structures with crystal plasticity, contemporary researchers usually fit hardening parameters to only these two extreme cases, i.e., maximized or minimized twinning activity, presuming that the same parameters would interpolate the correct behaviour under any other transitional stress direction. A comparison with experiments presented in this paper demonstrates that this assumption is not fully accurate, whether using the phenomenological Voce hardening model or the dislocation density based hardening model in the VPSC (visco-plastic self-consistent) framework. This indicates that slip-twin interactions are not properly captured in these models. Through a simple phenomenological implementation, we show that dislocation transmutation by twinning is an important aspect of slip-twin interactions that improve the predictability of the above crystal plasticity models for HCP structures.

Keywords:

• crystal plasticity
• deformation twinning
• magnesium alloys
• microstructure
• EBSD

Acknowledgments

The authors gratefully acknowledge the funding support through the DOE Southern Regional Center for Lightweight Innovative Design award DE-FC26-06NT42755 to carry out a portion of this research work. This work has benefited from the Lujan Neutron Scattering Center at LANSCE, which is funded by the US Department of Energy, Office of Basic Energy Sciences. Los Alamos National Laboratory is operated by Los Alamos National Security LLC under DOE Contract No. DE-AC52-06NA25396.