An atomic-scale modeling and experimental study of < c plus a > dislocations in Mg

A Kumar and BM Morrow and RJ McCabe and IJ Beyerlein, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, 695, 270-278 (2017).

DOI: 10.1016/j.msea.2017.04.027

We study pyramidal I and pyramidal II edge (and mixed) dislocations in Mg using a combination of experiment, dislocation theory, and atomic- scale modeling. With high-resolution transmission electron microscopy (HR-TEM) of a deformed Mg sample, a single 1/611 (2) over bar3 partial dislocation on the (11 (2) over bar2) plane emanating from a 10 (1) over bar2 twin boundary is observed, suggesting the possibility of a dissociation of a < c + a > dislocation into two 1/2 < c + a > partials on the (11 (2) over bar2) plane. Using first-principles density functional theory (DFT) calculations, we find that achieving this dissociation requires additional relaxations in the atomic positions normal to the slip direction. With molecular statics (MS) simulations, employing a modified embedded atom method (MEAM) potential, the full pyramidal-II < c + a > edge dislocation is shown under no stress to split into two equal value partials 1/611 (2) over bar3 + 1/611 (2) over bar3. When a resolved shear stress is applied, dislocations of edge and mixed character are glissile and the stacking fault in-between them narrows or widens depending on the sense of shear. With further analysis of this model, we show that the HR-TEM observation can be explained if one of the partials is pinned at the twin boundary. Last, with these atomic scale methods, we show for the first time that the full edge pyramidal-I < c + a > dislocation dissociates into two equal value partials of 1/620 (2) over bar3 and 1/602 (2) over bar3) Burgers vectors consistent with recent experimental observations. In contrast to the extended pyramidal-II dislocation, the extended pyramidal-I dislocations of similar edge or mixed character cannot move under an applied resolved shear since only one of the two partials is glissile.