Correlation between crystallographic anisotropy and dendritic orientation selection of binary magnesium alloys

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 , 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 to as the Al-concentration increased, and for Mg-Zn alloys, this direction changes from to or 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.

Atomistic Determination of Anisotropic Surface Energy-Associated Growth Patterns of Magnesium Alloy Dendrites

“…34,35 It has been confirmed that the position of additional atoms does not affect significantly the variation trend of the calculated surface energy with respect to the surface orientations. 36,37 The concentration of the additional elements for the magnesium alloys was chosen on the basis of their solubility in the solid or the maximum solute concentration in the Mg matrix. 36 We optimized the atomic structure of bulk materials and then constructed the surface slab model with respect to the growth orientations of the α-Mg dendrite.…”

“…36,37 The concentration of the additional elements for the magnesium alloys was chosen on the basis of their solubility in the solid or the maximum solute concentration in the Mg matrix. 36 We optimized the atomic structure of bulk materials and then constructed the surface slab model with respect to the growth orientations of the α-Mg dendrite. In terms of an n -layer surface slab model with respect to the { hkil } plane, the surface energy can be obtained via 30,33 where E slab n is the total energy per primitive slab unit cell, E bulk is the total energy per primitive bulk unit cell, S is the surface area per primitive slab unit cell, and the factor of 2 accounts for two equivalent surfaces of a particular slab model.…”

“…Remarkably, even with such difference, the predicted results on the growth pattern of the magnesium alloy dendrite agreed quite well with the experimental findings. 36,37 However, more investigation, in particular relevant atomistic simulations at elevated temperatures, should be performed to explore the exact influence of temperature on the dendritic growth pattern.…”

Utilizing Synchrotron Radiation for the Characterization of Biodegradable Magnesium Alloys—From Alloy Development to the Application as Implant Material

“…[49][50][51] Increasing contents of Al and Zn have also been reported to change the growth direction in the nonbasal plane from 1123 to 2245 . [48] Apart from alloying elements, the solidification method and cooling rate were found to have a significant influence on dendrite morphology, but not growth orientation. [41,47,52] Specifically, Shuai et al showed that slower cooling rates (3 C min À1 ) were found to result in globular dendrite morphologies, while faster cooling (12 C min À1 ) revealed sixfold symmetry for Mg-15Sn.…”

“…Specifically, a sixfold symmetry of α‐Mg dendrites in the basal plane was observed for Mg–9Al, [ 42,43 ] Mg–40Zn, [ 44 ] Mg–30Sn, Mg–30Gd, [ 45 ] Mg–30Sn, Mg–25Al, Mg–10Ba, Mg–20Y, [ 46 ] and Mg–15Sn. [ 47 ] The primary dendrite growth direction in most cases was , [ 48 ] while different authors identified either the [ 44 ] or [ 45,46 ] as second growth direction, with the difference being attributed to measurement uncertainty by some authors. [ 46 ]…”