ABSTRACT
This chapter presents a fundamental study into the role of interfacial energy in determining solidification structures. The effects of solidification and cooling rates on the modification and microstructural refinement process are discussed in relation to the mechanisms operating. The silicon is coarsened with strontium addition at low solidification rates, but is modified with strontium addition at high solidification rates. The chemical composition of the dendritic free zone has become hypereutectic, indicating a difficulty of silicon growth in strontium-containing alloys, and a corresponding increase in the undercooling for solidification. The strontium adsorption layer controls the silicon growth, and the critical solidification rate is therefore dependent on the strontium content. The change in silicon morphology at the solid-liquid interface by the addition of strontium and calcium has been investigated using the self-sealing unidirectional solidification method. When strontium and calcium coexist in an Al–Si alloy, the strontium and calcium react to form a Ca–Sr silicide.
