A first principles investigation on the mechanism of TiC act as heterogeneous nucleation substrate of Mg phase to refine grains in AZ91

Abstract

The lattice misfit of small Miller indices crystal faces between Mg and TiC is calculated based on Bramfitt two-dimensional lattice misfit theory. The electronic structure, adhesion energy and interfacial energy of Mg(001)/TiC(111) interface are investigated by first-principle calculation. The bond characteristics between interfacial atoms are analyzed by charge density difference and electron localization function. The calculation shows that the two-dimensional lattice misfit of Mg(001)/TiC(111) interface is 4.9%, which indicates TiC is suitable to act as an effective heterogenous nucleation substrate for Mg. The band structure and density of states calculations prove that the chemical bonds in Mg bulk structure are mainly metallic bonds, while that in TiC bulk structure are mixed bonds composed of polar covalent bonds and metallic ones. At the Mg-C interface, the electrons between Mg atoms and C atoms are shared to form covalent bonds, and the charge accumulation is obvious. The bonds in the Mg-Ti interface are metallic bonds, and the charge density between Mg atom and Ti atom is uniformly distributed. Compared with the Mg-Ti interface, the Mg-C interface has a larger adhesion energy (Wad = 6.96 J/m2) and a smaller interfacial energy (γ = 1.41 J/m2). Therefore, the crystallographic orientation most likely to produce a heterogeneous nucleation is Mg(001)//TiC(111), and the interface structure is optimally formed as the Mg-C type model.