Controlling the morphology of eutectic silicon in multi-element hypoeutectic Al–Si alloys can significantly enhance their strength and elongation. The combined addition of modified eutectic silicon is expected to achieve simultaneous regulation of size and morphology, yet research in this area remains insufficient. This study focuses on modifying the morphology of eutectic silicon using La and Sb additives in gravity-cast multi-element hypoeutectic Al–Si alloys and reveals the neck fuse mechanism of La and Sb on eutectic silicon. Due to the minimum two-dimensional lattice mismatch of 13.91 pct between AlSb and Si at the \({(001)}_{\text{AlSi}}\) and \({(100)}_{\text{Si}}\) interfaces, which exceeds 12.0 pct indicates that AlSb will not act as a heterogeneous nucleation site for silicon. While La attaches to the reentrant corner crystal plane, altering the silicon morphology by inducing twinning and reducing stacking faults. Additionally, La and Sb accumulate at the solid–liquid interface and adsorb onto branch necks, hindering silicon diffusion. This accumulation increases the equilibrium solubility of Si in the aluminum matrix, resulting in the dissolution of eutectic silicon branches. Meanwhile, La and Sb enrich the branches, forming a silicon solid solution with a lower melting point, which causes the necks of eutectic silicon branches to melt and fuse, thereby refining the silicon size. The transformed morphology from block to round granular eutectic silicon, affects the fracture mechanism from transgranular cleavage to a mix of transgranular toughness and cleavage, due to the refinement of the alloy’s microstructure. Therefore, the tensile strength of the alloy increases from 221.04 to 232.06 MPa, and the elongation improves from 2.11 to 3.58 pct, respectively.
