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The complex modification of primary Mg2Si in an Al–20Mg2Si alloy by simultaneous addition of yttrium (Y) and antimony (Sb) was investigated in the present work. It was found that the combined addition of 0.5 wt% Y–Sb had a more significant modifying effect than the single addition of an equivalent amount of Y or Sb. After modification, coarse Mg2Si dendrites were changed into fine polyhedra with an average size decreasing from 86 to less than 18 μm. It has been demonstrated that Sb participated in the formation of Mg3Sb2, which acts as a heterogeneous nucleus of primary Mg2Si and hence significantly refines their sizes. Meanwhile, Y could obviously adsorb and poison the preferred growth along the ＜100＞ directions of primary Mg2Si crystals, and hence not only changed their final morphology to a truncated octahedron but also reduced their sizes. Furthermore, the skeleton-type growth process of the truncated octahedral primary Mg2Si in the Al–20Mg2Si alloy co-modified by 0.5 wt% Y–Sb was also revealed. Our study provides a new insight into the design of more efficient modifiers by the combined addition of a refiner (e.g. Sb) and a growth inhibitor (e.g. Y), which is critical to tailor new light-weight alloys with high strength and toughness. [ABSTRACT FROM AUTHOR]

In this work, primary Mg2Si crystals shaped by {100} and {111} facets were successfully fabricated in Al–Mg–Si melts via the adsorption and poisoning of the Ca element. The modifier Ca plays an important role in determining the morphologies of primary Mg2Si, which can be transformed from dendrites (0 wt%) to crystals with a combination of eight {111} and four {100} facets (0.08 wt%), then to truncated cubes (0.1 wt%), and finally to cubes fully bounded by {100} facets (0.2 wt%) with increasing Ca content. The growth processes of cubic primary Mg2Si have been revealed. As the growth rates of {100} surfaces are diminished by adsorption of the Ca element, primary Mg2Si tends to form perfect cubes with exposed {100} facets. Moreover, by prudently adjusting the concentration of the Ca element, complex polyhedral crystals enclosed by {111} and {100} surfaces were designed and prepared. The results offer valuable insights into designing the modification effect of different modifying elements, which are critical to promoting the industrial application of light alloys with improved morphology and refined size of microstructures. [ABSTRACT FROM AUTHOR]

The morphology evolution of primary Mg2Si in the Al–20Mg2Si–0.1Ca alloy was investigated by a mould temperature-controlled solidification technique at various solidification cooling rates. It is found that by lowering the pre-set mould temperature, i.e. increasing the cooling rates, the influence of external factors, i.e. the adsorption of Ca atoms on {100} facets, on the morphology evolution of primary Mg2Si can be strengthened, leading to primary Mg2Si with varying morphologies. With the decrease of mould temperature from 600 to 400 °C and then to 200 °C and finally to room temperature, primary Mg2Si transformed from a mixture of an equiaxed-dendrite and an octahedron to a truncated octahedron and then to a truncated cube and finally to a cube; in contrast, primary Mg2Si still remained dendritic in the Ca-free Al–20Mg2Si alloy. Moreover, the skeleton-type growth process of the truncated cubic primary Mg2Si in the modified alloy was identified, where growth steps with some hillocks were observed for the first time. Our study offers a simple and low-cost method to control the morphology and size of primary Mg2Si crystals in the Al–Mg–Si alloy, which is beneficial to the design of lightweight Al–Mg–Si alloys with high strength and toughness. [ABSTRACT FROM AUTHOR]

The surface energies of Mg2Si (100) and (111) facets doped with modifying elements (Sb, Te, Sn, P and Bi) are calculated using first-principles methods, aimed at investigating the effects of the incorporation of modifying elements on the morphological evolution of Mg2Si. Based on the results of surface energy, the equilibrium shape of pure Mg2Si is predicted to be an octahedron surrounded by {111} planes. With the incorporation of Te, Sn, P and Bi, the ratio between the surface energies of the (100) and (111) planes (r) drops compared with that of pure Mg2Si, promoting the extension of the (100) facets and the contraction of the (111) facets on Mg2Si grains. Accordingly, the trend in morphological evolution for Mg2Si is from the initial octahedron composed of {111} planes to a truncated octahedron surrounded by {111} and {100} planes. The results also show that Te and P are more effective in promoting the appearance frequency of (100) facets on the Mg2Si crystal and can be used as candidate modifiers to realize the artificial manipulation of the morphology of Mg2Si into a truncated octahedron. The findings here provide new insights for understanding the morphological evolution of Mg2Si induced by the incorporation of modifying elements, which contributes to guiding the development of new high-temperature-resistant light alloys. [ABSTRACT FROM AUTHOR]

Primary Mg2Si with a truncated octahedral morphology and size over a range of 8–15 μm were prepared by simultaneously adding Ca and Sb into the Al–Mg–Si melt. It is revealed for the first time that CaSb2 can act as the nucleus of primary Mg2Si during solidification, which changes the morphology and refines the size of primary Mg2Si significantly. It is useful for improving the mechanical properties of the Al–Mg–Si alloy. The skeleton-type growth processes of truncated octahedral primary Mg2Si crystals were identified by observation of the three-dimensional (3-D) morphologies of the crystals extracted from the Al–Mg–Si alloy. The content of Ca and Sb plays an important role in determining the morphologies of primary Mg2Si, which can be transformed from coarse dendrites to defective truncated octahedra and finally to the coexistence of perfect truncated octahedra and flat truncated octahedra with increasing the content from 0 to 0.2 and then to 0.5 wt%. Our study provided a one-step less-cost method to change the morphologies and sizes of primary Mg2Si, which is important to tailor new light-weight alloys with high strength and toughness. [ABSTRACT FROM AUTHOR]