Effect of Ce Addition in Hypereutectic Al-Si Alloys During Rapid Solidification

Abstract

This work explores the rapid solidification of hypereutectic Al-40 wt.% Si alloys with and without Ce addition to attain modification of primary Si. Al-40Si and Al-40Si-1.5Ce powders were produced by impulse atomization with liquid cooling rates ranging from ~10³ to ~10⁵ K. The microstructures of both alloys showed an Al-rich halo surrounding the primary Si-phase. This was not part of the predicted Gulliver-Scheil solidification paths. The presence of this structure suggests solidification of both alloys deviated from local equilibrium. In both alloys, increasing cooling rate was associated with the formation of less elongated primary Si presenting a more rounded surface. Additionally, alloying with Ce improved the distribution of primary Si in the microstructures of the smallest powders. Quantitatively, the microstructures of both alloys were found to increase in primary Si and halo content as cooling rate increased. The use of Ce also led to a measurable decrease in eutectic content in Al-40Si-1.5Ce with respect to the unmodified alloy. Similarly, primary Si and halo volume percent also increased as a result of Ce addition. Ce addition led to the formation of finely dispersed intermetallics within the eutectic structure. While four different intermetallics are predicted by the Gulliver-Scheil solidification path of Al-40Si-1.5Ce, only AlCeSi and possibly CeSi were identified. Rapid solidification thus suppresses the formation of some intermetallics.