Abstract
Both synchrotron X-ray tomography and EBSD characterization revealed that the preferred growth directions of magnesium alloy dendrite change as the type and amount of solute elements. Such growth behavior was further investigated by evaluating the orientation-dependent surface energy and the subsequent crystallographic anisotropy via ab-initio calculations based on density functional theory and hcp lattice structure. It was found that for most binary magnesium alloys, the preferred growth direction of the α-Mg dendrite in the basal plane is always langle 11bar{2}0rangle , and independent on either the type or concentration of the additional elements. In non-basal planes, however, the preferred growth direction is highly dependent on the solute concentration. In particular, for Mg-Al alloys, this direction changes from langle 11bar{2}3rangle to langle 22bar{4}5rangle as the Al-concentration increased, and for Mg-Zn alloys, this direction changes from langle 11bar{2}3rangle to langle 22bar{4}5rangle or langle 11bar{2}2rangle as the Zn-content varied. Our results provide a better understanding on the dendritic orientation selection and morphology transition of magnesium alloys at the atomic level.
Highlights
Magnesium alloy is one of the most promising structural and functional materials due to lightweight, efficient recyclability, high specific strength and stiffness[1,2,3]
The growth kinetics of the dendrite are highly dependent on the local solute and energy dissipation at solid/liquid interface, the dendritic orientation selection mechanism is primarily determined by the thermodynamic factor based on the anisotropic surface energy associated with the fundamental lattice structure in light of the crystallographic theory[10,16,18,29,37]
The orientation-dependent surface energy was determined via ab-initio calculations at equilibrium energy condition based on the density functional theory (DFT) and the hcp lattice structure
Summary
Mg-11.5 at.%Al Mg-7.0 at.%Al Mg-6.2 at.%Al Mg-3.1 at.%Al Mg-1.6 at.%Al Mg-1.6 at.%Ba Mg-1.6 at.%Sn Mg-1.6 at.%Ca Mg-1.6 at.%Y Mg-2.0 at.%Zn Mg-1.3 at.%Zn Mg-0.7 at.%Zn. The orientation selection behavior of binary magnesium alloy dendrite was investigated, with particular attention focused on the influence of both type and amount of the additional elements Based on both synchrotron X-ray tomography experiment and theoretical ab-initio calculations, the underlying mechanism determining the growth tendency or orientation selection of magnesium alloy dendrite was investigated in terms of the surface energy related crystallographic anisotropy based on the hcp lattice structure. For these currently studied binary magnesium alloys, including Mg-Al, Mg-Ba, Mg-Sn, Mg-Ca, Mg-Y and Mg-Zn, it was found that the preferred growth direction of the α-Mg dendrite in the basal plane is always 1120 and independent on the additional elements, whereas that in non-basal planes changes with the amount of the additional elements. These theoretical results agree quite well with that found in experiments, and confirm that the addition of the Zn element effectively promotes the DOT behavior for both Mg-Zn and Al-Zn alloys
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