One issue faced by as-cast Al-Si alloys is low ductility due to the coarsening α-Al grains and large lamellar Si eutectic structure. While B is usually used as the grain refiner of α-Al grains, Sr can modify the Si eutectic structure. However, when adding both B and Sr to the Al-Si alloys, Sr is consumed due to the formation of Sr-B intermetallic compound. The effect of Sr as the modifier of large lamellar Si eutectic structure is therefore significantly reduced, which is referred to as poison effect of B on Sr. In a recent publication (J. Mater. Sci. Technol., 2023), Luo and colleagues developed a solution to simultaneously refine the α-Al grains and modify the large lamellar Si eutectic structure by adding B, Sr, and La to the Al-10Si alloy. In their work, they found that 500ppm La is sufficient to eliminate the poison effect of B on Sr, and the ductility and tensile strength of Al-10Si-0.02B-0.015Sr (at%) alloy are greatly enhanced due to the addition of 500ppm La. By performing CALPHAD simulation using Pandat software and PanAl database, Luo and colleagues were able to reveal the mechanism of refinement during the solidification process. They found that the Sr concentration in the liquid before eutectic reaction is the key in modifying the final eutectic structure. By adding La, B and La formed LaB6, therefore release Sr to the liquid before eutectic reaction.
Q. Luo, X. Li, Q. Li, L. Yuan, L. Peng, F. Pan, W. Ding, Achieving grain refinement of α-Al and Si modification simultaneously by La‒B‒Sr addition in Al‒10Si alloys, J. Mater. Sci. Technol., 135 (2023) 97-110. doi:10.1016/j.jmst.2022.07.009.