Energetics and stability of screw dislocations in magnesium: A first-principles study

Abstract

Magnesium (Mg), as the lightest structural metal, suffers from poor ductility and formability. Although in literature, the cross-slips in Mg have been identified as the key factors that are decisive for ductility, there are still many controversies and ambiguities in these cross-slip processes, e.g., energy, stability, etc. In this study, we propose a force correction method to eliminate the surface effects, which thus effectively enhances the efficiency/accuracy of the first-principles DFT calculations of screw dislocations. Based on this method, both a and c+a screw dislocations are calculated with convergence carefully examined. Our results show that a dislocation on the Basal plane has a lower energy than on the Prism plane, by 62 meV/b. The Prism core is energetically unstable since no barrier appears in the Prism-Basal cross-slip path. However, the energy landscape around the Prism core is extremely flat, where the tiny configurational forces could be below the convergence criterion, making the Prism core obtainable in the atomistic simulations. Then for the c+a dislocation, the ground state is revealed to dissociate on the Pyramidal II plane. Since multiple metastable c+a core structures are found on the Pyramidal planes, the energy difference during the c+a cross-slip is thus within a range of 8–19 meV/b. All these findings here provide the basis and guidance for understanding and enhancing the ductility in Mg and Mg alloys.