Pre-nucleation clusters mediated crystallization in Al-Si melts

Triggered by exogenous local ordering structures, pre-nucleation clusters appear in Al-Si melts and significantly alter the crystallization process. In absence of potent nucleating substrates (in high-purity melts), they delay silicon crystal nucleation, otherwise (in commercial-purity melts), they enhance nucleation and growth as well as change the growth mechanism of silicon. This makes the silicon grain size a competition result of nucleation and growth processes and some empirical rules of classical nucleation unreliable, e.g., easy nucleation no longer means grain-refined microstructures. These phenomena are further explained by characterizing the remains of these pre-nucleation clusters in solidified alloys.