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

Abstract

In the present study, primary Mg₂Si with different morphologies and sizes over a regime of 5–70 μm 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.