The Formation and Characterization of the Primary Mg2Si Dendritic Phase in hypereutectic Al-Mg2Si Alloys

The formation of a primary Mg2Si dendritic phase in hypereutectic Al-Mg2Si alloys is investigated using melt-spinning techniques. The results indicate that, at the initial stage, the primary Mg2Si crystal rapidly grows in a non-faceted manner, forming a smooth dendritic or fluted structure. As the undercooling of the process is varied, the growth behavior transitions from non-faceted to faceted. Subsequently, Mg2Si crystals grow preferentially along the [100] direction producing growth hillocks and forming stable branches. The morphology of the primary Mg2Si crystal exhibits angular facets. Furthermore, if V-[100]/V-[111] = root 3 the crystal grows as a perfect octahedral dendrite (the dendrite is composed of octahedrons), and if V-[100]/V-[111] < root 3 the crystal grows with imperfect octahedral subunits. Lastly, the growth process of the primary Mg2Si dendrites is discussed.