Effects of Sr, B, Zr and their interactions on the microstructure and mechanical properties of non-heat-treated Al-Si alloys

Sr and B are frequently incorporated into the Al-Si alloys to modify eutectic Si and refine α-Al grains. However, their combined addition may lead to the poisoning of Sr and the vanish of the modification effect. Based on a multi-scale collaborative computational strategy of phase diagrams and crystallography, this study proposes a simple approach for the simultaneous modification of eutectic Si and refinement of α-Al through merely adjusting the concentrations of Sr, B, and Zr. An orthogonal experimental design was employed to produce eight distinct Al-7Si-0.45Mg-0.3Cu-0.15Mn-xSr-yB-zZr alloys. The findings indicate that a low ratio of Sr to B in the alloy results in Sr being invalid. At this point, increasing the Zr content can effectively alleviate this phenomenon. Calculation of Phase Diagrams (CALPHAD) discloses that the interaction between Sr and B in the melt results in the consumption of Sr via the formation of SrB 6 . The incorporation of Zr results in the formation of ZrB 2 , thereby releasing Sr and preserving its modification effect. Furthermore, the grain sizes of the alloys with different contents exhibit substantial disparities. Crystallographic analysis indicates that SrB 6 , ZrB 2 , and other high-temperature phases in the samples, AlB 2 and Al 3 SiZr, are all effective nucleation site for α-Al, with the refinement effects in descending order of SrB 6 > AlB 2 > Al 3 SiZr > ZrB 2 . The optimal alloy (0.05% Sr, 0.01% B, 0.3% Zr) achieves both well-modified eutectic Si and fine α-Al, resulting in the highest tensile strength (216 MPa) and elongation (13.6%).