Effect of Li on the deformation mechanisms of nanocrystalline hexagonal close packed magnesium

Abstract

Addition of solutes such as lithium enhances ductility of hexagonal-close-packed (hcp) Magnesium (Mg). However, the atomistic underpinning of Li addition on individual deformation mechanisms remain unclear and is the focus of the present work. We compared the deformation mechanisms in nanocrystalline (NC) and single crystal simulation systems of pure Mg and Mg-Li hcp alloys. Five deformation modes are observed in the pure NC Mg with randomly oriented grains-one basal {0001} 1120 , one pyramidal type-I {1011} 1123 , and three twinning slip systems {1012} 1011 , {1013} 3032 , and {1011} 1012. Distributing 10 at.% Li randomly to this NC Mg decreased its compressive yield strength by 14.5%. This also increases the ductility by activating non-basal deformation modes and by reducing the plastic anisotropy. We benchmarked these results by comparing the effect of Li addition on these deformation modes in Mg single crystals. Finally, we present a formability parameter (F p) model based on unstable stacking fault energy, twin fault energy, and nucleation stress for dislocations (τ N S). Quantifying the changes in F p values for the Mg-Li alloys with respect to pure Mg in single crystal simulations explain the decrease in compressive yield strength and change in deformation mechanisms with Li additions. A sensitivity analysis study, comparing our CD-EAM results with a MEAM potential, shows that the effects of Li on the single deformation mechanisms are potential independent. Lastly, while results for Mg-10 at.% Li random alloy are presented here, similar conclusions can be drawn for other compositions of this hcp Mg-Li alloy.

Effect of Process Parameters on Formability of a AZ31 Magnesium Alloy Thin-Walled Cylindrical Part Formed by Multistage Warm Single-Point Incremental Forming

“…The CRSS of non-basal slip system is about a hundred times higher than that of basal slip, and {0001} < 1120 > dominates the slip behavior in coarse grains and a high plastic anisotropy (Barnett et al, 2004;Karewar et al, 2017). It is the reason that, although AZ31B magnesium alloy has more slip systems than FCC structures, it has poor ductility still when forming temperature is in the range of 20-150 • C. According to Von Mises, to achieve a uniform deformation in the material, at least five independent slip systems should be activated.…”

Phase Composition Effects on Dynamic Behavior and Strain Rate Sensitivity in Metastable β-Ti Alloys

“…However, the BCC β phase exhibits a total of forty-eight slip systems at room temperature, i.e., twelve (112) [111] type slip systems, twelve (110) [111] type slip systems, and twelve (123) [111] type slip systems [19]. For polycrystalline materials, at least five independent slip systems are needed for the homogenous plastic deformation of materials [20]. As a consequence, the ductility of dual-phase Ti alloys is majorly governed by the HCP α phase, which exhibits limited slip systems.…”

Atomistic modeling of grain boundary behavior under shear conditions in magnesium and magnesium-based binary alloys

“…Using the metric proposed by Bhatia et al [6], the conclusion of Reddy and Groh suggests that segregated Ca alloying elements increases the grain boundary sliding energy barrier. Within the same idea, Karewar et al [9] investigated numerically the effect of a random distribution of Li alloying elements on the ductility of magnesium-based alloys using empirical potentials. These authors revealed a decrease of the plastic anisotropy by enhancing the activity of the non-basal slip system and tension twinning ሼ101 ത 2ሽ〈101 ത 1〉.…”