The Origins of Primary Silicon Particles in Hypereutectic Aluminium-Silicon Alloys

Castings of hypereutectic aluminium-silicon alloys were produced in order to examine the mechanisms of primary silicon particle formation. In castings made using cold moulds, primary silicon particles are shown to originate from transient eutectic colonies which form as the molten metal comes in contact with the mould walls and are subsequently disrupted by local reheating, releasing eutectic silicon particles to serve as nuclei for primary silicon particles. Impurity elements also serve as heterogeneous nucleating agents for primary silicon particles. It is shown that small additions of strontium greatly reduce the influence of these elements.