Synergistic influence of La and Zr on microstructure and mechanical performance of an Al-Si-Mg alloy at casting state

The evolution of microstructure and mechanical properties of Al-7Si-0.4Mg alloys doped with La/Zr/La+Zr was investigated by experimental characterization and thermodynamic calculation. The combination of growth restriction effect by La-rich intermetallics and inoculation effect by peritectic Zr-rich cores led to the most dramatic grain refining. Furthermore, eutectic silicon was observed to be refined from avg. 9.8 µm in length within original alloy to avg. 5.8, 8.7, and 6.4 µm under the effect of La/Zr/La+Zr additions, respectively. The length reduction in eutectic silicon was attributed to the shrunk growth space caused by grain refinement and the presence of La-rich phases, as well as the impeded effect of several microns La-rich phases distribute directly on the growth front of Si phase. The strength and ductility were both elevated in the alloys at the casting state alloyed with La, Zr, or La+Zr. Particularly, the simultaneous introduction of La+Zr led to the optimal combination properties with a yield strength of 103 ± 3.4 Mpa, an ultimate tensile strength of 189.2 ± 4.6 Mpa, and fracture elongation of 6.1 ± 0.8%. The enhancement of yield strength was related to the strengthening from grain refining and dispersed precipitates. The improved ultimate tensile strength was ascribed to the higher and more stable strain-hardening rate, resulting from the combined influence of refined microstructures (e.g. grain and eutectic Si), as well as the emergence of La-rich and/or Zr-rich intermetallics. The elevation of fracture elongation was intimately correlated with the refining in eutectic silicon, which relieved the nucleation and development of crack along the interface between silicon and Al matrix.