Effect of Yttrium Addition on Microstructure and Mechanical Properties of Sr-Modified Low-Si Cast Aluminum Alloy

The effects of yttrium additions (Y) addition (0–0.12%) on the microstructure and mechanical properties of a Sr-modified Al–Si–Mg alloy were systematically investigated. The results reveal that the addition of Y results in refinement of the α-Al dendrite and eutectic Si, and promotes the fibrous structure transformation of eutectic Si. New phases Al₃Y and Y₂Al₃Si₂ were precipitated in the Y-containing alloy. Thermal analysis indicated that the inclusion of Y elements can augment the degree of undercooling, which is conducive to improving the α-Al grain nucleation rate. However, an overabundance of Y can induce coarsening of Y-rich phase, weakening the modification and refining effect of Y element on eutectic Si and α-Al dendrites. Moreover, the coarse Y₂Al₃Si₂ phases are difficult to dissolve during solution treatment, which will increase the stress concentration during the strain and potentially deteriorate the mechanical properties of the alloy. The tensile test results demonstrated that an alloy with 0.03%Y offered the optimal strength, while an alloy with 0.06%Y achieved the best elongation.