Effect of yttrium addition on the microstructures and mechanical properties of hypereutectic Al-20Si alloy

The effects of Y additions (0.2, 0.4, 0.6, 0.8 and 1.0 wt%) on the microstructure and mechanical properties of hypereutectic Al-20Si alloy have been studied in the present work. The as-cast specimens were examined by scanning electron microscope (SEM) equipped with energy spectrometer (EDS), electron probe micro-analysis (EPMA) and X-ray diffraction (XRD). The results demonstrated that the morphology of primary Si could be refined from coarse irregular star-like and plate-like shape to fine block-like when the addition contents of Y increased to 0.8%. The average size of primary Si reduced by 62.9% from 89 µm to 33 µm and the aspect ratio decreased by 38.6% from 2.07 to 1.27. In addition, the eutectic Si structure was modified from coarse needle / flake to fine coral fibrous structure and partial particles. With the refinement and homogenization of Si phases, the optimal elongation (El) increased by 22.5% from 1.13% to 1.36% when the Y content was 0.6%. However, the ultimate tensile strength (UTS) enhanced by 47.9% from 94 MPa to 139 MPa when the addition content of Y was up to 0.8%. Moreover, the refinement and modification mechanism of Y on Al-20Si alloy was also discussed.