Investigation On Liquid Structure Of Al-Cu And Al-Ni Binary Alloy Systems

The liquid structures of Al-Cu binary alloy system are investigated by ab initio molecular dynamics (AIMD) simulation in this paper. We analyze the structures of Al-Cu binary alloy system and compare it with Al-Ni binary alloy system, besides we focus on the structure of Al-Cu and Al-Ni binary thin melts to explore the nature of medium-range order (MRO).In the liquid structure of Al-Cu binary alloys, the chemical short-range order (CSRO) of Al-Cu atoms is the predominant CSRO, which result from the strong interaction of hetero atoms.CSRO of Al-Cu present the strongest at the alloy composition of Al50Cu50.The most Voronoi polyhedron around Al atoms and Cu atoms are both icosahedral-like clusters. The fraction of icosahedral-like clusters around Al atoms decreases as Cu content rises, icosahedral-like clusters around Cu atoms transform each other as Cu content rises.The coordination number of Al atoms is mainly 12 and 13.When Cu content is low, the coordination number of Cu atoms is mainly 10 and 11, it transforms to 11 and 12 as Cu content rises.Comparing the liquid structure of Al-Cu and Al-Ni alloys, we find that hetero-interaction between Al and Ni atom is stronger than that in Al-Cu, corresponding to the result of thermodynamic enthalpy of mixing. As the Cu content rises,second coordination of Cu-Cu dominates at the Cu10% content in Al-Cu alloys, yet it raises to Ni30% content in Al-Ni alloys. CSRO of Al-Ni present strongest at the alloy composition of Al75Ni25. Analysis of partial coordination number shows that Ni prefers to coordinate with Al atom than Cu. The coordination number of Al in Al-Ni alloys is mainly 11 or 12, yet it is mainly 12 or 13 in Al-Cu alloys. The coordination number of Cu in Al-rich alloy is mainly 10 or 11, yet it is mainly 9 or 10 in Al-Ni alloysThe liquid structure of Al-Cu and Al-Ni thin melts shows that second coordination of Cu-Cu and Ni-Ni both domimate in the melts and Ni-Ni presents stronger than Cu-Cu at the same composition. The CSRO strength of hetero-atoms indicates in the order of Al92Ni8>Al95Ni5>Al92Cu8>Al95Cu5.Voronoi polyhedron around Cu atoms and Ni atoms shows maximum difference at the content of<0360>- like polyhedron. The content of<0360>-like polyhedron in Al92Ni8 and Al95Ni5 melts are 21.5% and 28%, yet they are 14.5% and 12.3% in Al92Cu8 and Al95Cu5 melts.The structure factor(S(Q))of Al92Cu8, Al95Cu5,Al92Ni8, Al95Ni5 shows prepeaks are located at the position of 1.5A"1.The prepeaks in the partial S(Q) are around 1.8A-1-2A-1, they reflect the size of MRO is around 3.90A-4.50A. The height of prepeaks in the partial S(Q) shows Al95Cu5＞Al92Cu8, Al95Ni5>Al92Ni8, MRO is more stronger at the 5% composition due to almost all second coordination of Cu-Cu and Ni-Ni. By summarizing the distance and connection manner of Cu-centered and Ni-centered clusters, we consider MROs in Al-Cu and Al-Ni melts are the clusters which are formed by point connection and line connection of Cu-centered atoms and Ni-centered atoms. Combine with the Voronoi polyhedron, we infer that MRO is mainly formed by point connection and line connection of<0360>-like and<0280>-like polyhedrons.