Influence of Crystallographic Orientation on Twin Nucleation in Single Crystal Magnesium

Barrett, C. D., Tschopp, M. A., El Kadiri, H., & Li, B. (2011). Influence of Crystallographic Orientation on Twin Nucleation in Single Crystal Magnesium. TMS 2011 Proceedings. TMS 2011, San Diego, CA.

Experimental plasticity on single crystals has found substantial non-Schmid effects in both twinning and non-basal slip in pure magnesium. Kelly and Hosford, in their seminal work for instance, observed profuse {1012} twins in orientations where they were supposed to be prohibited by plain strain compression, while {1011} twins were triggered rather than prismatic and pyramidal slip systems although under the most favorable loading orientations for these slip modes. This deviation from Schmid's law was attributed to the strong sensitivity of both twinning and slip to small inhomogeneities in the lattice including existing dislocations, twin embryos and free surfaces. Most molecular dynamics simulations, however, use free surfaces or heterogeneities to help trigger twinning so the effect of slip on twin nucleation and vise-versa has been shrouded. This has motivated us to investigate the effect of crystal loading orientation on homogeneous slip and twin nucleation using molecular dynamics with periodic boundary conditions. These simulations will allow us to appreciate the propensity and nature of twin nucleation when pre-existing defects are absent. Analyses of deformation mechanisms and stress-strain responses shows that homogeneous dislocation nucleation on the basal slip system is correlated with the highest Schmid resolved shear stress, while homogeneous nucleation of tensile twins did not always correlate with the highest Schmid resolved shear stress.