First-principles study of $\langle c+a \rangle$ dislocations in Mg

We use first-principles density functional theory to study the generalized stacking fault energy surfaces for pyramidal-I and pyramidal-II slip systems in Mg. We demonstrate that the additional relaxation of atomic motions normal to the slip direction allows for the appropriate local minimum in the generalized stacking fault energy (GSFE) curve to be found. The fault energy calculations suggest that formation of pyramidal-I dislocations would be slightly more energetically favorable than that for pyramidal-II dislocations. The calculated pyramidal-II GSFE curves also indicate that the full pyramidal II dislocations would dissociate into the Stohr and Poirier (SP) configuration, consisting of two $\frac{1}{2}\langle c+a \rangle$ partials, $\frac{1}{6}[11{\bar2}3] + \frac{1}{6}[11{\bar2}3]$ , but the pyramidal-I GSFE curves, while also possessing a local minimum, would not dissociate into the same SP configuration. We report observation of these partials here emanating from a $\{10{\bar1}2 \}$ twin boundary. Using MD simulations with MEAM potential for Mg, we find that the full pyramidal-II $\langle c+a \rangle$ dislocation splits into two equal value partials $\frac{1}{6}[11{\bar2}3] + \frac{1}{6}[11{\bar2}3]$ separated by ~22.6 $Å$. We reveal that the full pyramidal-I $\langle c+a \rangle$ dislocation dissociates also into two equal value partials but onto alternating $(30{\bar3}4)$ and $(30{\bar3}2)$ planes with $\frac{1}{6} [20{\bar2}3]$ and $\frac{1}{6} [02{\bar2}3]$ Burgers vectors separated by a 30.4 $Å$ wide stacking fault. When a stress is applied, edge and mixed dislocations of the extended pyramidal-II dislocation can move on their glide plane; however, pyramidal-I dislocations of similar character cannot.

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