Enhanced nucleation of primary silicon in Al-20 wt. % Si alloy with Ni-Si inoculation

Abstract

The inoculation effect of a novel Ni-32.5 wt. % Si master alloy on Al-20 wt. % Si melt was investigated. With the addition of 0.5 wt. % Ni-Si master alloy, primary Si is refined from ∼350 μm to ∼150 μm and the corresponding nucleation undercooling is reduced by 4.5 °C. Moreover, its morphologies become more regular with higher proportion of octahedrons. TEM studies reveal that inoculation introduces a high number density of nano-particles in Si grain. Careful structural analysis suggests that these nano-particles are different from any known crystal compounds containing Ni and Si. Both the high number density and the unknown crystal structures of these nano-particles are different from the heterogeneous nucleation theory. These anomalous observations imply that pre-nucleation clusters assisted nucleation mechanism is active here. The Ni-32.5 wt. % Si master alloy decomposes into local ordering clusters and triggers the formation of pre-nucleation clusters in Al-Si melt. The pre-nucleation clusters accelerate crystal nucleation and transform into the nano-particles.