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 phosphor-us, the morphologies of primary Mg2Si particulates changed from dendritic or equiaxed to polygonal shape, and their sizes decrease from similar to 300 mu m and 200 mu m to similar to 20 mu m and similar to 50 mu m, 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-[100] is depressed. Also, growth mechanism and the formation of hopper-like crystal are discussed. (c) 2006 Elsevier B.V. All rights reserved.