Atomistic study of edge and screw 〈c+a〉 dislocations in magnesium

The gamma surfaces in the pyramidal I {1 -1 0 1} and II {1 1 -2 2} planes for hexagonal close packed Mg have been calculated using two embedded-atom-method potentials and by ab initio methods, and reasonable agreement is obtained for key stacking fault energies. Screw and edge textless c + a textgreater dislocation core structures and Peierls stresses at 0 K and finite temperature have been examined using the embedded-atom-method potentials. Screw textless c + a textgreater dislocations glide in the {1 -1 0 1} pyramidal plane I, and in the prism plane for larger stresses, but not in the {1 1 -2 2} plane as observed in experiments. However, the preference for pyramidal I glide correlates well with the gamma surfaces. New low energy edge textless c + a textgreater dislocation cores were found in addition to the sessile Type I and Type III cores observed in previous simulations while the Type II core was not observed. The lowest energy core is a glissile core that lies in the {1 1 -2 2} plane and has a 3 nm long {1 1 -2 1} twin embryo, rather than the sessile Type III core found in previous simulations. As the temperature increases from 0 to 300 K, the Peierls stresses in compression/tension drop from -80 MPa/+140 MPa and -140 MPa/+220 MPa for the most glissile screw and edge dislocations to -5/+2.5 MPa and -27/+5 MPa, and dislocation glide changes from kink motion to face-centered-cubic-like motion. At 300 K and under an applied stress, almost all the edge cores found at low temperature transform into a glissile core denoted IT, which glides at low stresses. Thus, at 300 K both screw and edge textless c + a textgreater dislocations were found to glide at stresses smaller than the similar to 40 MPa measured experimentally. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.