The Study Of Modification Mechanism On Mg₂Si Phases In Mg-Al-Si Alloys

The modification mechanisms of Mg2Si phases in Mg-Al-Si alloy weresystematacially investigated. The effects and mechanisms of Sr on modification ofMg2Si phases were studied by adding Al-10wt.%Sr master alloy to Mg-6Al-1.5Sialloy with an experimentally methodology. Meanwhile, the Al4Sr(100)/Mg2Si(100)heterogeneous nucleation interfaces were studied using first-principles method basedon the density functional theory(DFT) to explore the properties of surfaces and thelaws of interfacial stacking, and clarify the heterogeneous nuclei potential of Al4Srparticles for Mg2Si phases in Mg alloys. Eventually, this study may providetheoretical explanations for the modification behaviour of Mg2Si phases inSr-containing Mg-Al-Si alloy. The main conclusions are summarized as follows:By adding0.5wt.%Sr, the shapes of Mg2Si phases in Mg-6Al-1.5Si alloy werechanged a lot. The shape of primary Mg2Si was changed from irregular polygons toinerratic squares, and the shape of eutectic Mg2Si was changed from Chinese script tofine fiber. According to the EDS analysis result, it can be found that an unknownparticle contaning Al and Sr appears inside the center of Mg2Si phase, which can beconcluded the compound is Al4Sr, and/or Sr-rich exsits in the particle. Themodification mechanism of Mg2Si phases is not only due to the role of heterogeneousnucleation, but also affected by the Sr segregated, which alters the growth of Mg2Siphases.The Mg2Si(100) surface relaxation indicates that the effects of relaxation aremainly localized within the top three atomic layers. For Mg-terminated surface, thespacing changes tend to convergence when the layer thickness is greater than11layers, and for Si-terminated surface, the spacing changes gradually stable when thethickness is more than9layers. The calculation results of surface energies show thatMg-terminated surface is more stable than Si-terminated surface. The electronicstructure reveals that the metallic feature/covalent bonding for Mg-terminated surface,and indicates the existence of semiconducting property/covalent bonding forSi-terminated surface. The relaxation of Al4Sr(100) surfaces are mainly occur within the top threelayers, and the relaxation effects inside Al4Sr(100) surfaces are not obvious. Thesurface energies caculating results indicate that Sr-terminated surface might be morethermodynamically stable than Al-terminated surface. The electronic structure revealsthe existance of metallic feature/covalent bonding on Al4Sr(100) surfaces.There are four types of interfacial structures for Al4Sr(100)/Mg2Si(100)interfaces. The interfacial spacing of Al-Mg interface is lower than that of Al-Siinterface, indicating that Al-Mg interface is easier to form chemical bonds than Al-Siinterface. Nevertheless, the adhesion energy of Al-Si interface is higher than that ofAl-Mg interface, indicating Al-Si interface has a higher adhesive strength. The chargedensity and charge density difference of Al4Sr(100)/Mg2Si(100) interfaces show thatpolar covalent bonds exsit in the interface. In general, the covalent bonding for Al-Siinterface is stronger than that of Al-Mg interface. The interfacial energies caculatingresults reveal that bonds are easily formed between Al atoms on Al4Sr(100) surfaceand Si atoms on Mg2Si(100) surface and the interfaces can maintain stability with lowenergy. This provides Al4Sr particles the excellent nucleation potency for the growthof Mg2Si phases from interfacial atomic structure and atomic bonding considerations.