Multi-step nucleation in Al–Si liquids catalyzed by a Ni–Si master alloy

Abstract

Multi-step nucleation catalyzed by a Ni–Si master alloy in Al–Si liquids was investigated. Instead of enhancing heterogeneous nucleation, the master alloy triggers pre-nucleation clusters before initiating stable phase nucleation. The pre-nucleation clusters manifest themselves by a high number density of nano-sized silicon particles with rhombohedral (9R, ABCBCACAB…) and hexagonal (2H, ABAB…) stacking sequences, which have higher energies than diamond-cubic phase. Thermal analysis in Al–Si alloys with different Si concentration and inoculation time shows the effect of pre-nucleation clusters varying with the amount of local ordering (NiSi) clusters. In addition, the growth of silicon crystal is changed due to the presence of pre-nucleation clusters by suppressing the twin plane re-entrant mechanism.