**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 1/6**11 (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 1/6**11 (2) over bar3** + 1/6**11 (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 1/6**20 (2) over bar3** and 1/6**02 (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.
**

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