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Magnesium (Mg) and its alloys offer great potential for reducing vehicularmass and energy consumption due to their inherently low densities.Historically, widespread applicability has been limited by low strengthproperties compared to other structural Al-, Ti- and Fe-based alloys. However,recent studies have demonstrated high-specific-strength in a number ofnanocrystalline Mg-alloys. Even so, applications of these alloys would berestricted to low-temperature automotive components due to microstructuralinstability under high temperature creep loading. Hence, this work aims to gaina better understanding of creep and associated deformation behavior of columnarnanocrystalline Mg and Mg-yttrium (Y) (up to 3at.%Y(10wt.%Y)) with a grain sizeof 5 nm and 10 nm. Using molecular dynamics (MD) simulations, nanocrystallinemagnesium with and without local concentrations of yttrium is subjected toconstant-stress loading ranging from 0 to 500 MPa at different initialtemperatures ranging from 473 to 723 K. In pure Mg, the analyses of thediffusion coefficient and energy barrier reveal that at lower temperatures(i.e., T < ~423K) the contribution of grain boundary diffusion to the overallcreep deformation is stronger that the contribution of lattice diffusion.However, at higher temperatures (T > ~423K) lattice diffusion dominates theoverall creep behavior. Interestingly, for the first time, we have shown thatthe(101-1),(101-2),(101-3) and (101-6) boundary sliding energy is reduced withthe addition of yttrium. This softening effect in the presence of yttriumsuggests that the experimentally observed high temperature beneficial effectsof yttrium addition is likely to be attributed to some combination of otherreported creep strengthening mechanisms or phenomena such as formation ofstable yttrium oxides at grain boundaries or increased forest dislocation-basedhardening.

Atomic-scale investigation of creep behavior in nanocrystalline Mg and Mg-Y alloys