Modification Mechanism of Rare Earth Eu on Eutectic Si in Hypoeutectic Al-Si Alloy

The effect of Eu (Europium) element on eutectic Si in Al–7Si alloys was studied by scanning electron microscopy (SEM), thermal analysis, electron backscattering diffraction (EBSD), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to elucidate modification mechanism of Eu on eutectic Si. The results indicate that the addition of Eu changes the morphology of eutectic Si from coarse plate-like to fine fibrous. With the increase in Eu content, the number of eutectic cells gradually decreased and the size of eutectic cells increased. Meanwhile, the eutectic nucleation temperature (TN) and the minimum temperature (TMin) decreased, which is caused by the poisoning of AlP particles by Eu. The segregation of phosphorus was found at the center of coarse Al2Si2Eu particle, and the "phosphorus layer" was found at the boundary between the Al2Si2Eu particle and matrix. The length direction of plate-like eutectic Si was <110>, and its outer surface was {111} plane. Few parallel twins were observed in unmodified plate-like eutectic Si. However, the growth of Eu-modified fibrous eutectic Si was isotropous. Multiply twins were often observed in fibrous eutectic Si after modification. The adsorption of Eu was observed along the <112>Si growth direction of eutectic Si and at the intersection of two {111}Si twins, verifying the existence of the poisoning of the TPRE mechanism and IIT mechanism.