Heterogeneous Nucleation Of α-Mg, Mg₂Si In Mg Alloys And Development Of Relevant Master Alloys

The influence of Si-P master alloy on the microstructure of Mg-Si alloy, and influence of Al-C master alloy on that of Mg-Al alloy, were studied by using X-ray diffraction (XRD), optical micrograph (OM), scanning electron microscope (SEM), and electron probe microanalysis (EPMA), etc. The grain refinement and modification process were optimized by considering various parameters. The heterogeneous nuclcation mechanism of α-Mg and Mg₂Si in Mg alloys was investigated to clarify the grain refinement and modification.Mg-8Si alloy is reinforced by in situ Mg₂Si particles. However, its microstructure often shows a very coarse and nonhomogeneous three-level character. That is, Mg₂Si primary dendrite is surrounded by α-Mg halo, which is then surrounded by Mg₂Si+Mg eutectic structure. Mg₂Si primary dendrite is very developed with interdendritic angle of 90° and secondary dendrite spacing d₂ of about 17 μm. During its growth, solute redistribution occurs and Si is continuously absorbed into the solid-liquid interface. As a result, the Si concentration is increasingly down ahead of the interface until the growth of Mg₂Si dendrite is ultimately stopped for lack of Si. Subsequently, α-Mg particles begin to nucleate and grow as halos surrounding primary Mg₂Si. Finally, eutectic reaction occurs in the rest melt to form Mg2Si+Mg eutectic structure with irregular lamellar or bacillary eutectic Mg₂Si distributed in a continuous matrix of eutectic Mg.Si-10P master alloy has a significant modification effect on the microstructure of Mg-8Si alloy. An optimization of modification effect occurs as 0.5% Si-P master alloy is added into the Mg-8Si alloy melt at 800℃. After modification, the developed Mg:Si dendrite transits to small polygon of about 15 μm. An assistant addition of 0.5% Ce can improve the P modification effect and leads to further refinement of Mg₂Si polygon size to only 8 μm. With Mg₂Si particle size refinement and...