Effects of Zr and Y additions on microstructure and mechanical properties of cast A356 alloy

In the present work, the effects of Zr and/or Y addition on microstructure and mechanical properties of cast A356 alloy were investigated using X-ray diffractometer (XRD), differential scanning calorimetry (DSC), optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The heterogeneous nucleation effect on Zr-rich phases generated by the peritectic reaction, the increased constitutional undercooling caused by Y addition and serve as surfactant led to the grain refinement. Besides, The average size of eutectic Si reduced from 8.5 μm in the base alloy to 6.2, 1.3, and 1.7 μm with the addition of Zr/Y/Zr+Y, respectively. Zr can slightly refine eutectic Si due to the limited growth space caused by grain refinement, while Y can modify the morphology of eutectic Si from plate-like into fibrous-like through high density twins formed by the IIT mechanism. The optimal mechanical properties are obtained when Zr and Y are added simultaneously with the ultimate tensile strength, yield strength, and elongation values of 104 MPa, 201 MPa, and 7.6%, respectively. The YS and UTS increase was due to grain-refining, Si modification and precipitation strengthening. The enhancement of El was mainly caused by Si modification, which reduced the possibility of crack initiation and propagation at Si/α-Al interfaces.