Brittle beta-Al5FeSi (beta-Fe) intermetallic phases directly affect the mechanical properties of Al-Si alloys; thus, it is essential to modify these phases. Scanning electron microscope analysis showed that rare earth Lanthanum (La) additions (0, 0.15, 0.30 and 0.50 wt%) had a refining effect on beta-Fe phases in Al-7Si-4Cu-0.35Mg-0.2Fe (wt.%) alloys prepared by squeeze casting. The underlying refinement mechanisms of rare earth La on the beta-Fe were discussed in terms of experimental observations, nucleation thermodynamics and growth kinetics. For nucleation thermodynamics, La additions could induce the formation of La-rich intermetallic phases Al4Cu2SiLa with orthorhombic structure. Because beta-Fe nuclei had a good lattice matching and epitaxial relationship with the Al4Cu2SiLa phases, the Al4Cu2SiLa provided energetically favorable nucleation sites for beta-Fe and greatly pro-moted the nucleation of beta-Fe in alloys. Meanwhile, the added La atoms could restrain the diffusion of atoms to the solid/liquid interface front and alter the beta-Fe interfacial structure and energy, therefore restricting the beta-Fe growth. Furthermore, beta-Fe/Al4Cu2SiLa grew into side-by-side and cross-like morphologies, and its growth be-haviors were controlled by solute concentration and anisotropy of the solid-liquid interfacial energy in a cooperative eutectic reaction. In addition, different fracture behaviors from brittle fracture to mixed ductile fracture and quasi-cleavage brittle fracture were also illustrated with increasing La addition, and these fracture behaviors were influenced by the characteristics of brittle La-rich and beta-Fe intermetallic phases. When the addition of La was increased to 0.3 wt% and 0.5 wt%, it occurred quasi-cleavage brittle fracture, which was caused by the precipitation of coarse plate-like La-rich phases.
