Microstructure evolution and modification mechanism of the ytterbium modified Al–7.5%Si–0.45%Mg alloys

Abstract The effects of ytterbium (Yb) on the microstructure and eutectic solidification behavior of Al–7.5%Si–0.45%Mg alloys were investigated. Samples with different contents of Yb were quenched just after the commencement of eutectic arrest. Microstructures of the as-cast and quenched samples were examined by optical microscopy (OM) and scanning electron microscopy (SEM). The microstructural observations show that the addition of Yb causes a structural transformation of eutectic silicon from a coarse plate to a fine flake-like and some branched morphology. In addition, the formation of Yb-containing phases at higher content of Yb results in a deterioration of modification. Furthermore, the results of thermal and quenching analysis reveal that the addition of Yb reduces the temperatures of eutectic nucleation and growth. The growth rather than nucleation of eutectic is thought to be the primary cause of modification, and the corresponding undercooling caused by the presence of Yb plays an important role in the refinement of eutectic silicon.