Interaction between nucleant particles and a solid-liquid interface in Al-4.5Cu alloy

Abstract Al3Ti and TiB2 particles were made in-situ from adding halide salts in molten Al-4.5 wt % Cu alloys. The molten halide salts also dissolved the oxide in the molten metal to ensure that the newly formed Al3Ti and TiB2 particles had intimate interfaces with the molten metal. Excess amounts of titanium were added into the melt to ensure that the wetting angles between the particles and the molten alloy were small and the particles were able to significantly reduce the grain sizes of the aluminum alloy. Such particle/matrix systems should have a negative total interfacial energy change when a particle is engulfed into the solid by an advancing solid-liquid interface, resulting in particles being engulfed by an advancing solid-liquid interface according to widely accepted theories on particle pushing. Experimental results indicate that these particles are actually pushed by the growing dendrites into the interdendritic regions, regardless of the growth rates being positive, close to nil, or even negative. It seems that the particle pushing mechanisms based on interfacial energy interactions are not applicable for these particle/matrix systems under experimental conditions of this study. Analytical analysis suggests that these particles are pushed by the growing dendrites due to fluid flow before they could get into close contact with the solid-liquid interface within a several interatomic layers where the van der Waals forces become significant. The shrinkage driven flow in the liquid of the mushy zone is sufficient to dislodge a particle from a depression and rolls the particle on the solid-liquid interface.