  Alloying magnesium (Mg) with rare earth elements such as yttrium (Y) has been
reported to activate the pyramidal <c + a> slip systems and improve the
plasticity of Mg at room temperature. However, the origins of such dislocations
and their dissociation mechanisms remain poorly understood. Here, we
systematically investigate these mechanisms using dispersion-inclusive density
functional theory, in combination with molecular dynamics simulations. We find
that <c + a> dislocations form more readily on the pyramidal I plane than on
the pyramidal II plane in Mg. The addition of Y atoms in Mg facilitates the
dissociation of <c + a> dislocations on pyramidal II, leading to the easier
formation of the pyramidal II than pyramidal I in Mg-Y alloy. Importantly, in
pyramidal II slip plane, a flat potential-energy surface (PES) exists around
the position of stable stacking fault energy (SFE), which allows cooperative
movement of atoms within the slip plane. Alloying Mg with Y atoms increases the
range of the PES, and ultimately promotes different sliding pathways in the
Mg-Y alloy. These findings are consistent with experimentally observed
activation of the pyramidal II <c + a> slip system in Mg-Y alloys, and provide
important insight into the relationship between dislocation structure and
macroscopic enhancement of plasticity.
