Breakdown of the Schmid law in homogeneous and heterogeneous nucleation events of slip and twinning in magnesium

During the past two decades, twinning and slip in hexagonal close-packed structures have been extensively studied using molecular dynamics. However, the simulation methods and corresponding results have rendered different conclusions regarding the active twin modes and their mechanisms for nucleation and growth. The nucleation mechanisms for twinning in hexagonal close-packed polycrystalline materials are known to depend strongly on grain boundary orientations, but little is known of the exact mechanisms that occur. The variability in the experimental behavior of single crystals reported in early literature may result from the extreme sensitivity of twinning and slip to heterogeneities in the crystals and their complex dislocation cores. Therefore, both the boundary conditions and loading directions are likely to have profound effects on the mechanisms of deformation captured by molecular dynamics simulations. In an effort to rationalize the inconsistencies reported in literature and guide future molecular dynamics studies, we have performed a comprehensive molecular dynamics study on magnesium that encompasses effects of crystal orientation, boundary conditions, initial defects, and interatomic potentials. The general trends of the results support theories which advocate heterogeneous nucleation for both twinning and slip. In fact, the behavior of perfect crystals with periodic boundary conditions deviated substantially from previous experimental observations. Additionally, strong non-Schmid effects were identified and consistently correlated to non-Schmid stresses. Deviations from Schmid's law were strikingly reduced by introducing defects such as free surfaces and voids, but twinning was still influenced by non-Schmid stresses. Twin–twin interactions led to secondary twinning and nanovoids, which encouraged stress localization and damage.

► Homogeneous twinning and slip nucleation was found to depend on non-Schmid stresses.
► Tension twins nucleated following prior slip at stress concentrations.
► Secondary twinning was observed as one twin variant passed through another.
► {1121} twin embryos nucleated easily but detwinned rather than propagating.