Orbital-free density functional theory simulations of dislocations in magnesium
I Shin and EA Carter, MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, 20, 015006 (2012).
Metal plasticity is controlled by nucleation and motion of dislocations. Key metrics determining the ease of these two events are stacking fault energies (SFEs) and dislocation structures. Here we study screw and edge dislocation structures on the basal, prismatic and pyramidal planes in hexagonal-close-packed magnesium (Mg) using orbital-free density functional theory (OFDFT) in order to gain insight into plastic deformation mechanisms in Mg. The accuracy of the method is first benchmarked against the more accurate Kohn-Sham DFT (KSDFT) with emphasis on testing OFDFT's main approximations, i.e. the kinetic energy density functional and the bulk-derived local pseudopotential by comparing predicted equilibrium bulk energies, elastic constants and various SFEs. Then we compare generalized SFEs for the basal, prismatic and pyramidal slip systems calculated by OFDFT versus two mainstream counterparts, KSDFT and the classical potential embedded atom method (EAM). The latter produces spurious minima along the generalized SFE surface on the prismatic plane whereas OFDFT agrees with qualitative experimental observations. Thereafter, we optimize isolated dislocation structures within periodic cells containing a few thousand atoms. We predict that on the basal plane, the screw and edge dislocations separate into partial dislocations with widths of similar to 12 and similar to 24 angstrom, respectively. Screw dislocations on the prismatic and pyramidal planes preferentially cross-slip and dissociate on the basal plane although a local minimum exists for a dissociated prismatic screw dislocation with widths of >=similar to 5 angstrom. By contrast, the edge dislocations on prismatic and pyramidal planes are predicted to remain undissociated. Such cross-slip behavior of screw dislocations is not reproduced by EAM simulations. We propose that the propensity for screw dislocations to remain on or cross-slip to Mg's basal plane, along with the compact nature of edge dislocations on non- basal planes, is likely to be responsible for its limited ductility.
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