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Alloying magnesium (Mg) with rare earth elements such as yttrium (Y) has beenreported to activate the pyramidal <c + a> slip systems and improve theplasticity of Mg at room temperature. However, the origins of such dislocationsand their dissociation mechanisms remain poorly understood. Here, wesystematically investigate these mechanisms using dispersion-inclusive densityfunctional theory, in combination with molecular dynamics simulations. We findthat <c + a> dislocations form more readily on the pyramidal I plane than onthe pyramidal II plane in Mg. The addition of Y atoms in Mg facilitates thedissociation of <c + a> dislocations on pyramidal II, leading to the easierformation of the pyramidal II than pyramidal I in Mg-Y alloy. Importantly, inpyramidal II slip plane, a flat potential-energy surface (PES) exists aroundthe position of stable stacking fault energy (SFE), which allows cooperativemovement of atoms within the slip plane. Alloying Mg with Y atoms increases therange of the PES, and ultimately promotes different sliding pathways in theMg-Y alloy. These findings are consistent with experimentally observedactivation of the pyramidal II <c + a> slip system in Mg-Y alloys, and provideimportant insight into the relationship between dislocation structure andmacroscopic enhancement of plasticity.

Origins and dissociation of pyramidal dislocations in magnesium and its alloys