Study On The Influence Mechanism Of Solute Atoms On Heterogeneous Nucleation In Magnesium Melt

As one of the lightest structural material with high strength, high specific stiffness and good damping ability, magnesium alloys are widely applied in the aerospace, electronics industry and automobile. However, the crystal structure of magnesium alloy is close-packed hexagonal with less slip system and worse plastic deformation, which restricts their application and development. Grain refinement can effectively improve the comprehensive properties of the alloy. Carbon inoculation is considered to be the most effective and promising way to refine the grains of magnesium alloys. However, carbon inoculation is easily to be influenced by impurity elements Fe and Mn. The current research results about the influence mechanism of Fe(Mn) on carbonaceous refinement for magnesium alloys conflict with each other. In this paper, the influence mechanism of Fe(Mn) on carbonaceous refinement in magnesium alloys was experientally studied by using AZ91D alloy with different Mn content and treated by carbon inoculation and theorically studied by using First-principle calculation method.Microstructures of AZ91D alloy with addition of different Mn content and carbon-containing refiner were studied by analyzing the characterstic and morphology of heterogeneous phase. The results show that carbon refinement method can effectively refine grains of magnesium alloy with low content of Mn. While the content of Mn beyond 0.28wt.%, the Mg grains would turn to be coarser with the increasing Mn content. The heterogeneous phases in the alloy are proposed to be a composite of Al-C phase and Al-Mn phase by analyzing with SEM and EDS. And the Al-Mn particles may be adsorped on Al-C phase, leading to the change the surface structure of Al-C phase and the formation of Al-C-Mn compound, and in turn influencing the heterogeneous nucleation effect of Al-C phase and coarsing the grain.First-principle calculation based on the density functional theory was adopted to investigate the adsorption energy, stability, electronic structure and bonding of Fe(Mn) atom adsorption on Al-terminated and C-terminated Al4C3(0001) surface under 0.25ML and 0.5ML. Results show that the structure of Fe(Mn) adsorption on C-terminated Al4C3(0001) surface is more stable than that on Al-terminated surface according to the formation energy calculation. For Fe(Mn) adsorption on Al-terminated surface, Fe(Mn) is more favorable to reside at the site H1 comparing with other sites. As well, for Fe(Mn) adsorption on C-terminated surface, the structure of Fe(Mn) adsorption at site H'2 is the most stable one. By analyzing the electronic structure and bonding, it is found that the ionic bonds are mainly formed between Fe(Mn) atoms and Al-terminated surface, while the polar covalent bonds are formed between Fe(Mn) atoms and C-terminated surface. According to the interlayer spacing calculation, Al4C3(0001) surfaces are reconstructed after Fe(Mn) adsorptionAccording to first-principle study, Fe(Mn) adsorption on Al4C3(0001) surface was believed to be a kind of chemical adsorption, proving that Fe (Mn) can react with Al4C3 to form Al-C-Fe (Mn) compounds. The above calculating results are consistent with that of the experiments, showing that excess Fe(Mn) content will reduce the heterogeneous nucleation effect of Al4C3 particles. The surface structure has been reconstructed after Fe(Mn) adsorption on Al4C3(0001) surface, which in turn affect the following stacking of Mg atoms on Al4C3(0001) surface.