Modification mechanism of Te and morphology evolution of primary Mg2Si in an Al-20Mg2Si alloy.

In the present work, the modification effect of Te and the corresponding growth process of Mg₂Si were clarified. With 0.5 wt% Te addition, the dendritic primary Mg₂Si transformed to polygons, accompanied by a reduction in size from over ∼165 μm-∼18 μm. However, 2.0 wt% Te addition resulted in degradation of modification effect, and the morphology of Mg₂Si changed to dendrite. Three-dimensional morphology of primary Mg₂Si indicated that Te altered the morphology of primary Mg₂Si from dendrites to polyhedrons surrounded by {111} and {100}. The modification mechanism of Te can be attributed to the heterogeneous nucleation and the doping of Te during the growth process of primary Mg₂Si, which greatly contributed to dimension decrease and morphology evolution of primary Mg₂Si respectively. With Te corporation, the relative surface energy between the {100} and {111} (r) drops, contributing to the exposure of {100}. Our work shows that the solute concentration under non-equilibrium solidification plays an important role in determining the final morphology of Mg₂Si. The local segregation of Mg and Si solutes results in the degradation of dendrite arms along certain directions during the initial stage of crystal growth. Combined with the modifying effect of Te, which alters the surface energy of the crystal, the final crystal morphology deviates from its equilibrium state. Our study demonstrates that ultimate morphology and dimensions of primary Mg₂Si can be determined simultaneously with the addition of certain modifiers, which can be achieved by regulating the growth and nucleation of Mg₂Si at the same time.