Novel Cross-Slip Mechanism of Pyramidal Screw Dislocations in Magnesium

TL;DR

This paper reveals a novel slip mechanism of pyramidal screw dislocations in magnesium, showing that stacking faults can migrate non-planarly, which challenges traditional views on slip plane determination in hexagonal metals.

Contribution

It introduces a new understanding of dislocation slip behavior in magnesium, demonstrating non-planar stacking fault migration and dissociation in planes different from the conventional slip plane.

Findings

Dislocations dissociate on {2-1-12} planes but migrate in {01-11} planes.

Stacking faults can assume non-planar shapes with negligible energy cost.

The slip behavior explains experimental observations of slip traces in magnesium.

Abstract

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.