Modification of Mg2Si in Mg-Si alloys with yttrium

The modification of in situ formed Mg2Si in Mg-Si alloys with yttrium (Y) was investigated. As-cast microstructure analysis of Mg-5 wt.% Si alloys indicates that all the primary Mg2Si Crystals are surrounded by a layer of alpha-Mg dendritic halos, which is again surrounded by the eutectic Mg2Si + Mg structures. When the Y content is 0.1 or 0.4 wt.%, the sizes of primary and eutectic Mg2Si do not significantly reduce in Mg-5 wt.% Si alloys. When the Y content is 0.8 wt.%, the mean size of primary Mg2Si decreases from more than 100 mum to about 30 mum or less; furthermore, the Chinese script type Mg2Si particle also exhibits modified morphology as a fine fiber. When the Y content is further increased to 1.2 wt.%, the primary Mg2Si becomes coarser again and even larger than that in the unmodified alloy; however, the eutectic Mg2Si still exhibits modified morphology as a fine size. The size and amount of Mg-Y point compounds are increased with the increasing of Y content in the alloys. In addition to distributing in alpha.-Mg phase, the Mg-Y point compounds are also present in primary Mg2Si dendritic crystal. In the unmodified alloy, the coarse primary Mg2Si is formed by the preferred growth that occurs at the tips of branches; however, when the alloy is modified with 0.8 wt.% Y, the preferred growth manner of the primary Mg2Si was depressed, while another type of growth, isotropic growth manner, is enhanced in the modified alloy. (C) 2004 Elsevier B.V. All rights reserved.