Novel Cross-Slip Mechanism of Pyramidal Screw Dislocations in Magnesium

Compared to cubic metals, whose primary slip mode includes twelve equivalent systems, the lower crystalline symmetry of hexagonal close-packed metals results in a reduced number of equivalent primary slips and anisotropy in plasticity, leading to brittleness at the ambient temperature. At higher temperatures, the ductility of hexagonal close-packed metals improves owing to the activation of secondary c+a pyramidal slip systems. Thus understanding the fundamental properties of corresponding dislocations is essential for the improvement of ductility at the ambient temperature. Here, we present the results of large-scale ab-initio calculations for c+a pyramidal screw dislocations in Mg and show that their slip behavior is a stark counterexample to the conventional wisdom that a slip plane is determined by the stacking fault plane of dislocations. A stacking fault between dissociated partial dislocations can assume a non-planar shape with a negligible energy cost and can migrate normal to its plane by a local shuffling of atoms. Partial dislocations dissociated on a {2-1-12} plane "slither" in the {01-11} plane, dragging the stacking fault with them in response to an applied shear stress. This finding resolves the apparent discrepancy that both {2-1-12} and {01-11} slip traces are observed in experiments while ab-initio calculations indicate that dislocations preferably dissociate in the {01-11} planes.