Comparison of Silicon Phase in Al-20Si Alloys and Zn-27Al-3Si Alloys with Strontium Addition

Abstract

The evolution of the silicon phase of hypereutectic Al-20Si alloys and Zn-27Al-3Si alloys with the addition of Sr (0, 0.06, 0.1, and 0.2 wt%) was investigated to understand the appearance of spherical primary silicon in Zn-Al–Si alloys but not in Al–Si alloys. With Sr content remaining unchanged (0.2%), dendritic primary silicon was observed to grow along the preferential direction in the Al-20Si alloy, while in the Zn-27Al-3Si alloy, faceted cells or smooth spherical patterns were observed on its surface. The cooling curves showed that a 0.2% Sr content helps reduce the precipitation temperature of primary silicon (approximately 10 °C in the Al-20Si alloy and 40 °C in the Zn-27Al-3Si alloy). From EPMA mappings, Sr was found mostly dissolved into the nodular primary silicon in the Zn-27Al-3Si alloy with a 0.2% Sr content, while there was no evidence that Sr atoms aggregate within dendritic primary silicon crystals in the Al-20Si alloy with 0.2% Sr content. Multiple twins were observed in the Sr-modified Zn-27Al-3Si alloy, which can be interpreted according to the IIT and TPRE growth mechanisms, while different parallel twins were observed in the Sr-modified Al-20Si alloy. Overall, it was determined that a higher undercooling rate and the preferential adsorption of Sr in primary silicon are advantageous to obtain spherical primary silicon.