Effect of Ni on eutectic structural evolution in hypereutectic Al–Mg2Si cast alloys

The aim of this work is to investigate the eutectic structural evolution of hypereutectic Al–20% Mg2Si with Ni addition under a gravity casting process. Three-dimensional morphologies of eutectic phases were observed in detail using field emission scanning electron microscopy, after Al matrix was removed by deep etching or extraction. The results show that Al–Mg2Si binary eutectic gradually evolves into Al–Mg2Si–NiAl3 ternary eutectic with the increase of Ni content, and flake-like eutectic Mg2Si transforms into rods. The ternary eutectic presents a unique double rod structure that rod-like NiAl3 and Mg2Si uniformly distribute in Al matrix. Further, the high temperature (350 °C) tensile strength of the alloy increases by 23% due to the eutectic structure evolution, and the mechanism of structural evolution was discussed and analyzed in terms of the detailed microstructural observations.