Morphologically templated nucleation of primary Si on AlP in hypereutectic Al-Si alloys

AlP has been widely used as an effective heterogenous nucleus for primary Si phase in hypereutectic Al-Si alloys, but the morphological correlation between AlP and primary Si is still confusing. In the present work, the morphologies of AlP crystals were studied comprehensively by experimental observation and theorical prediction. It is found that AlP collected from an Al-0.03P melt could be divided into two categories: spinel twin crystals and non-twin crystals. During the nucleation process, these two kinds of AlP crystals triggered morphologically templated nucleation of primary Si phase, resulting in the formation of hexagonal primary Si twin and octahedral non-twin crystals, respectively. As such, the percentage of primary Si twin crystals in the experimental Al-18Si alloy was also increased obviously after the morphologically templated nucleation via AlP. The morphologically templated nucleation also eliminated the dendritic growth of primary Si phase and the formation of hopper structures inside primary Si, forcing primary Si to maintain to be faceted solid crystals through layer-by-layer growing mechanism. The insight into morphologically templated nucleation offers a new view in understanding the mechanism of heterogeneous nucleation of primary Si phase on AlP nuclei.