In-situ X-radiographic study of nucleation and growth behaviour of primary silicon particles during solidification of a hypereutectic Al-Si alloy

Real-time observation of structure evolution in a hypereutectic Al-Si(-Cu) alloy under near-isothermal melt solidification condition was conducted by using in-situ micro-focus X-radiography. The nucleation, growth rate and morphological development of primary Si particles (PSPs) were studied at different cooling rates. It is found that an increase of cooling rate increases nucleation rate, reduces the crystal growth and extends the nucleation temperature range of primary Si particles, which all help to refine primary Si particles. The minimum nucleation undercooling also increases with increasing cooling rate. Based on the experimental findings, it is proposed that Si atom clusters act as the nuclei of Si crystals. In addition, cooling rate does not alter the branching growth mechanism of PSPs but only shorten the branch length and reduce the development of side plates. Higher cooling rate may increase the peak growth velocity of PSPs due to the higher nucleation undercooling. Further, electron backscatter diffraction (EBSD) characterization of the post solidified in-situ sample confirms that twinning in primary Si occurs in the nucleation stage or during the growth process, and the growth of twined branches and plates follows the twin plane re-entrant edge (TPRE) mechanism. Twinning during growth may facilitate the branching.