Key Factors Influencing Eutectic Si Modification in Al-Si Hypoeutectic Alloy by Trace La

The modification of eutectic Si to fiber morphology from coarse plate-like morphology is essential for producing an Al-Si hypoeutectic alloy. Furthermore, chemical modification through the addition of modifying elements, such as Na and Sr, to melt is the most widely used method in industrial production to improve microstructures. Recently, the effect of rare earth metals on the eutectic Si modification has also attracted considerable attention, especially for the economical element La. Key factors influencing eutectic Si modification in Al-Si hypoeutectic alloy by trace La are theoretically explored. The results demonstrate that the solubility of La in the primary alpha-Al phase and interaction parameter between La and Al (or Si) primarily contribute to the eutectic Si modification. When the addition level of trace La is within its solubility in the primary alpha-Al phase, La distributes in alpha-Al and eutectic Si, and the modification effect increases with the La addition level. When the addition level of trace La is greater than its solubility in alpha-Al, a ternary compound containing Al, Si, and La exists before the eutectic reaction due to the significant value of the interaction parameter between La and Al (or Si). Calculated results further prove that the composition of the ternary compound is AlSiLa due to the substantial value of heat for the formation of AlSiLa and the small value of interfacial energy between Al melt and AlSiLa. Under this condition, La distributes in alpha-Al, AlSiLa, and eutectic Si, and the La content in alpha-Al and eutectic Si almost remain constant. Thus, the modification effect almost stays unchanged with La addition. A suitable modification effect is achieved when the La addition level is around its solubility in the primary alpha-Al phase.