The Synergistic Refinement of Primary Si in Hypereutectic Al-Si Alloys Using Ce and Er Elements

Hypereutectic Al-Si alloys are used in a number of industries; however, the large size of primary Si grains significantly limits their industrial applications. Grain refinement through the addition of modifiers has become a crucial technical approach to overcome this challenge. Rare earth elements, particularly Ce and Er, are promising modifiers, and they have a synergistic effect on the refining of Si in hypereutectic Al-Si alloys. However, the synergistic mechanism between Ce and Er is rarely reported. Thus, this study aimed to reveal the mechanism underlying the synergistic effect of Ce and Er on refining the grains in hypereutectic Al-25wt.% Si alloy. The results indicate that the synergistic doping of Ce and Er significantly reduces the size of the primary Si grains. The addition of 0.5 wt.% Ce and 1 wt.% Er to the alloy reduced the primary Si grain size to 428.51 μm, achieving an overall refinement rate of up to 55.1% compared with the alloy without Ce and Er. Microstructural analysis revealed that Ce and Er accumulated around the primary Si crystals, resulting in the formation of complex intermetallic phases. These intermetallic complex phases provided additional nucleation sites for the growth of primary Si, thereby promoting its heterogeneous nucleation and inhibiting grain growth. Furthermore, they reduced the intensity of crystal growth in the direction of the preferred growth orientation of the primary Si, thereby further inhibiting its growth. This study provides essential experimental evidence supporting the synergistic refinement of hypereutectic Al-Si alloys using Ce and Er.