Clustering on Magnesium Surfaces – Formation and Diffusion Energies

Abstract

The formation and diffusion energies of atomic clusters on Mg surfaces determine the surface roughness and formation of faulted structure, which in turn affect the mechanical deformation of Mg. This paper reports first principles density function theory (DFT) based quantum mechanics calculation results of atomic clustering on the low energy surfaces {0001} and {1011}. In parallel, molecular statics calculations serve to test the validity of two interatomic potentials and to extend the scope of the DFT studies. On a {0001} surface, a compact cluster consisting of few than three atoms energetically prefers a facecentered-cubic stacking, to serve as a nucleus of stacking fault. On a {1011}, clusters of any size always prefer hexagonal-close-packed stacking. Adatom diffusion on surface {1011} is high anisotropic while isotropic on surface (0001). Three-dimensional Ehrlich-Schwoebel barriers converge as the step height is three atomic layers or thicker. Adatom diffusion along steps is via hopping mechanism, and that down steps is via exchange mechanism.Hexagonal metals in a bulk form are widely used in nuclear engineering 1 and transportation industry (magnesium and magnesium alloys are the lightest structural materials) 2 . Mg and Mg alloys exhibit low strength and poor deformability because of the easy twinning. Nanometer-sized hexagonal materials show significantly high strength, 5 to 10 times of the bulk strength 3, 4 . However, deformation twin quickly propagates across the sample once it occurs, corresponding to the burst of plastic deformation 5 . Planar defects such as twin boundaries, stacking faults, and interphase interfaces can impede the motion and propagation of dislocations and twins, thus strengthening materials [6][7][8][9][10] . To realize planar defects in Mg and Mg alloys, it is essential to understand atomic clustering processes during material processing (synthesizing and aging). For example, alloying with appropriate solutes alloying is very promising in tailoring mechanical properties of Mg alloys 11 . Shin and Wolverton 12 calculated solute-vacancy binding in Mg alloys using principles density function theory, and Ganeshan et al. performed first principles density function theory (DFT) calculations of self-diffusion in hcp Mg 13 . Understanding of solute clustering processes in Mg alloys has provided insight in modifying the dispersion and morphology of precipitates.Apart from clustering processes in the bulk Mg, planar defects in nanostructured Mg and Mg alloys can also be introduced during synthesizing, such as sputtering via physical vapor deposition techniques [14][15][16][17][18][19][20] . Atomic clustering on surfaces during a growth process of thin films or nanostructures dictates the structure and their mechanical properties. The clustering processes depend on formation and diffusion energies of adatom or atomic clusters. If the formation energy of an adatom or an atomic cluster in a fault stacking is lower than in a normal stacking, stacking faults may form. In addition, the diffusio...

Branching of Titanium Nanorods

“…The thermodynamically preferred { } surface is surrounded by six { } surfaces, and the next thermodynamically preferred surface is beyond { }. Indeed, prior experiments have reported the top surface of the nanorods as typically { } [ 27 , 29 , 40 ]. Our XRD and TEM characterizations—as presented later—also confirm that this is the case.…”

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“…HCP crystal structures are anisotropic, and surface diffusion is also anisotropic [ 28 , 29 , 30 ]. As a result, HCP nanorods generally are anisotropic, with a width to thickness ratio being substantially different from 1:1, when the incidence angle of the deposition flux is fixed.…”

Visualization and validation of twin nucleation and early-stage growth in magnesium

“…Atomistic simulations were performed for Mg using empirical interatomic potential developed by Liu et al 39 to explore the twin nucleation process. A recent study indicates that both Liu’s potential and Sun’s potential can accurately reproduce the formation energies of twin boundary and PB interface 40 . However, Liu’s potential is much closer to the density functional theory (DFT) calculations of surface energies, and also recreates the dependence of surface energy on normalized atom density, while the Sun potential struggles to recreate the DFT values 40 .…”

Visualization and Validation of Twin Nucleation and Early-stage Growth in Magnesium

“…Atomistic simulations were performed for Mg using empirical interatomic potential developed by Liu et al 35 to explore the twin nucleation process. A recent study indicates that both Liu's potential and Sun's potential can accurately reproduce the formation energies of twin boundary and PB interface 36 . However, Liu's potential is much closer to the density functional theory (DFT) calculations of surface energies, and also recreates the dependence of surface energy on normalized atom density, while the Sun potential struggles to recreate the DFT values 36 .…”