Studies On The Local Structures Of Alloy Melts And Their Inheritances

In the present thesis, high-tempearture X-ray diffraction, ab initio molecular dynamics simulations and X-ray absorption fine structure technique on typical binary isomorphous alloy, peritectic alloy, intermetallic alloys and ternary amorphous alloy have been performed to investigate the local structures of the alloy melts and the structure evolutions during solidification, as well as the inheritance correlation between liquid structure and solid structure.The structure of isomorphous Ag-Au alloy melts and the structure evolution with changing temperature and composition were investigated using an X-ray diffractometer. It is found that the atom packing of isomorphous Ag-Au alloy melts agrees well with the model of hard sphere packing. The coordination numbers of the melts are similar with those of the Ag-Au solid solution. For the Ag95Au5alloy melt, the mean nearest neighbor distance r1stays stable with decreasing temperature, while, r1of the eutectic melts exhibit obvious temperature dependence. The r1of Ag-Au melts at the same temperature remains stable at different compositions. For the eutectic melts under discussion, r1takes on a linear relationship with element component, and the slope of r1versus composition depends on the atomic size ratio. It shows that the cluster in the isomorphous alloy melts possesses structural stability. The different evolution behaviors of the melt structures between isomorphous and eutectic alloy systems lead to the different solidification manners.The structure inheritance of typical glass-forming alloy melt Au55Cu25Si20is studied using high-temperature X-ray diffraction and ab initio molecular dynamics simulations, with a comparison with the structure of Au81Si19alloy melt. It is found that the formation of the Au55Cu25Si20amorphous alloy is closely related to the medium-range structure in the melt. Compared with the cluster in the Au81Si19alloy melt, the addition of Cu atoms makes the cluster become loose in the Au55Cu25Si20alloy melt, the size of the cluster become small, the Au-Au and Au-Si bonds become long, and finally the r1increase. Meanwhile, the Si-Si bond shortens, and the short-range order structure clusters centered with Si atoms form the medium-range order structure. During the rapid solidification, the structural information carried by the medium-range order structure is inherited from the melt to the amorphous solid, which promotes the glass formation. The intrinsic quality of the amorphous phase formation is revealed:the centered Si atoms are restrained from agglomeration and precipitation, and the medium-range order structure depresses the nucleation and enhances the glass-forming ability.The local structure inheritance of Ag-Sn alloy melts is investigated using high-tempearture X-ray diffraction, ab initio molecular dynamics simulations and X-ray absorption fine structure technique. Through comparison of the structure of Ag50Sn50and Ag3.8Sn96.2melts, it is found that the structure is strongely influenced by the composition. The coordination number Nm around Ag atom is similar in the alloy and in pure Ag melts (Nm-10), while, during the alloying process, the local structure around Sn atoms rearranges. Sn-Sn covalent bonds were substituted by Ag-Sn chemical bonds, and the total coordination number around Sn increases by about70%as compared with those in the pure Sn melt. Changes in the electronic structure of the alloy have been studied by Ag and Sn K-edge X-ray absorption spectroscopy, as well as by calculations of the partial density of states. We propose that a leading mechanism for local structure inheritance in Ag50Sn50is due to s-p dehybridization of Sn and to the interplay between Sn-s and Ag-d electrons. During the rapid solidification of Ag50Sn50alloy melt, the Ag-Sn bond length is always shorter than the Ag-Ag one, and the numbers of the unlike atoms around Ag atom decrease obviously. The ratio of Ag:Sn changes from1:1to3:1. It indicates that the formation of Ag3Sn intermetallic phase stems from the Ag-Sn bond. By analyzing the electronic structure, it is found that the electronic structure evolution results in the change of the local structure around Ag atoms.The inheritance from the melt structure to the solid phase of the BiIn2and AgIn2intermetallic alloys is investigated using high-tempearture X-ray diffractometer and X-ray absorption fine structure technique. It is found that there are local chemical bonds (Bi-In or Ag-In) in the intermetallic alloy melts, which can promote the formation of the intermetallic clusters and dominate the structure evolution as the temperature changes. From the local structures of the melt-quenched alloy ribbons, the structures of the quenched ribbons are looser than those of the equilibrium solidified crystal alloys; the Bi-In bond in the melt-quenched alloy ribbon is stronger, and the Ag-In bond is weaker. It is also found that the intermetallic phase in the quenched alloy originates from the chemical bonds between unlike atoms in the melt. The chemical bonds preserve the chemical ordered clusters, dominate the clusters evolution, and lead to the structure inheritance effect from melt to solid.