Morphology and size control of octahedral and cubic primary Mg2Si in an Mg-Si system by regulating Sr contents

In the present study, primary Mg2Si with different morphologies and sizes over a regime of 5-70 mu m have been prepared. By precisely controlling the content of Sr in Mg-4Si alloys, we have obtained octahedral and cubic primary Mg2Si of different average size. Morphologies of primary Mg2Si were identified though field-emission scanning electron microscopy (FESEM) in three-dimensional space. The modifier Sr plays an important role in determining the morphologies of primary Mg2Si, which can be transformed from equiaxed-dendrite to octahedron and finally to a cube shape with increasing Sr content. The 2D nucleation growth pattern of primary Mg2Si modified by Sr element has been revealed for the first time. An adsorption model has been applied to explain the influence of Sr on the growth kinetics of Mg2Si. It is proposed that Sr atoms preferentially adsorbing on {100} facets of Mg2Si crystal is the key reason for the morphology evolution. Our study demonstrates an effective, one-step and cheap method to control the morphologies and sizes of primary Mg2Si crystals in metallic melts, which is critical to achieve industrial application of light alloys with high strength and toughness.