Effects of rare earth Er addition on microstructure and mechanical properties of hypereutectic Al-20% Si alloy

The microstructure evolution and mechanical properties of hypereutectic Al-20%Si alloy with Er addition were investigated in the article. The as-cast samples were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Microstructural analyses demonstrated that primary Si was significantly refined from coarse polygonal, platelet-like and star-like shape to fine blocky shape, and eutectic Si structure was modified from coarse platelet-like/needle-like structure to fine coral-like fibrous structure as the addition contents of Er is 0.5%. However, the primary and eutectic Si phases became coarser when the level of rare earth Er was up to 0.8%. The mechanical properties were investigated by tensile test with various concentration of Er. It was found that the ultimate tensile strength (UTS) and elongation (El) increased by 72.5% and 72%, respectively, due to decreasing of the size and changing of morphology on primary and eutectic Si crystals, and the change of mechanical properties corresponds to the evolution of microstructure. In addition, the modification mechanism of Er on Al-20%Si alloy was also discussed.