The Evolution And Chemecial Stability Of Al-P Clusters In Al-Si-X-P Melts

Due to same crystal structure and similar crystal parameters, AlP is considered as an effective heterogeneous nucleus for primary Si in hypereutectic Al-Si alloys. In recent years, it was found that AlP could also refine primary Mg2Si in hypereutectic Al-Mg2Si alloys. In melt with high temperature, P exists in the form of Al-P clusters. The evolution and chemical stability of Al-P clusters in complex commercial melt have a vital effect on the formation of AlP and the final phosphorous refining performance. In this paper, the evolution of Al-P clusters in Al-Si-X-P melt during cooling and solidification process was studied; the enrichment of Al-P clusters and precipitation behaviors of AlP on γ-Al2O3 film, as well as the resulting formation of Si-rich layer in Al-Si alloy surface, were also discovered and discussed. Meanwhile, the effect of alloying elements on the chemical stability of Al-P clusters was investigated systematically by combining ab initio molecular dynamics (AIMD) calculations, experiments, thermodynamic tool (Ellingham diagram) and electronegativity theory. What’s more, the evolution of Al-P clusters and heterogeneous nuclei in Al-Si-Mg alloys was studied.(1) Evolution of Al-P cluster during solidification and its enrichment and precipitation behaviors on γ-Al2O3 surfaceThe structure of liquid Al-P alloy was investigated by ab initio molecular dynamics simulation. It was found P atoms repel each other, but tend to form AlnP (3≤n≤10) clusters in melt. While, the coordination number of Al atoms in AlnP clusters (i.e., n) is not constant, and its average number is 6.32. The structure of Al-P clusters is also very sensitive to the temperature change and evolves to a state close to the crystalline state even at a huge cooling rate (3 X 1014℃/s). During solidification process, each AlnP monomer evolves by decreasing the coordination number of Al atoms. Meanwhile, AlnP monomers would gather mutually to form larger Al-P clusters bridged by some Al atoms.It was found γ-Al2O3 film could adsorb Al-P clusters and thus facilitate the evolution and crystallization of Al-P clusters. During the cooling and solidification process of phosphorous-refined Al-Si melt, plenty of (111) orientated AlP and primary Si grains precipitated successively on γ-Al2O3 film by "γ-Al2O3/AlP/Si" successive heterogeneous nucleation mechanism, resulting the formation of Si-rich layer in the surface of Al-Si alloys. In Si-rich layer of Al-13.5Si alloy, the primary Si grains show hexagonal-plate-like morphology. While, due to the high temperature gradient in alloy surface, the top (111) plane of primary Si grains in hypereutectic Al-18Si alloy could further grow along [111] direction to form hexagonal-pyramid-like primary Si grains with new{311} exposed planes.(2) The chemical stability of Al-P clusters in Al-Si-X-P meltThe chemical stability of Al-P clusters in Al melts were investigated systematical by combining AIMD simulation, experiments, thermodynamic tool (Ellingham diagram) and electronegativity theory. According to the destructive effect on Al-P clusters, the partition map of elements’ effects on the chemical stability of Al-P clusters has been drawn, and the alloying elements in Al-Si based alloys have been divided into three groups:poisoning elements (φ<2.9, characterized by the high tendency to eliminate Al-P clusters), active elements (2.9<φ<3.6, characterized by the low tendency to eliminate Al-P clusters), and inactive elements (3.6<φ, having no effect on the chemical stability of Al-P clusters). What’s more, it was also found Si and Al atoms could enhance the chemical stability of Al-P clusters in Al-Si-M-P melt in some degree.(3) Evolution of Al-P clusters and heterogeneous nuclei of primary Mg2Si in Al-Si-Mg alloysIt was found that the adding order of Mg and Al-P alloy could affect the chemical stability of Al-P clusters and final phosphorus-modification effect. If pure Mg was added into the melt after the addition of Al-P alloy, Mg atoms can eliminate the pre-existing Al-P clusters quickly and reduce the phosphorous-modification effect. If pure Mg was added into Al-Si melt before adding Al-P alloy, Mg atoms can combine with Si atoms and reduce the activity of Mg, then lose the ability to eliminate Al-P clusters and ensure a good phosphorous-modification effect.The nuclei of primary Mg2Si in Al-xMg2Si (x=15,20 and 25) alloys added with AlP have been systematically studied, and their evolution mechanism and nucleation mechanism have also been investigated. When added into Al-Mg-Si melt, ALP particles can react with Mg atoms and evolve according to the path of "AlPâ†' Mg3P2/AlP coupled particleâ†'Mg3P2". As the increase of Mg content in the melt, the above evolution of nuclei would be accelerated. The crystal lattice misfit calculation indicates that the formation of Mg3P2 significantly reduces the lattice misfit between the nuclei and Mg2Si, improving the potency of the nuclei. It was also found the new-formed Mg3P2 in Al-Mg-Si melt is actually Al-doped Mg3P2, and the nuclei in Al-Mg-Si-Ca melt could further self-adjust to form (Al, Ca)-doped Mg3P2. The doped Mg3P2 has higher chemical stability and thus shows a strong Ca-resistant stability. What’s more, as the Mg content in melt increases, the Ca-resistant stability of the nuclei is also enhanced.