Effect of phosphorus on microstructure and growth manner of primary Mg2Si crystal in Mg2Si/Al composite

The effects of phosphorus on primary, eutectic Mg2Si phases and growth manner of Mg2Si crystal in Mg2Si/Al composite with steel die and sand mould cast are investigated. The results show that, with the addition of phosphorus, the morphologies of primary Mg2Si particulates changed from dendritic or equiaxed to polygonal shape, and their sizes decrease from ∼300 μm and 200 μm to ∼20 μm and ∼50 μm, respectively. Furthermore, the morphologies of eutectic Mg2Si phases change from fibriform to short fibriform and/or dot-like, while the eutectic Si phases do not change obviously. The exact morphologies of Mg2Si crystals are identified as tetrakaidecahedron and/or octahedron after modification with P, due to V[1 0 0] is depressed. Also, growth mechanism and the formation of hopper-like crystal are discussed.