Studies On The Local Structures In Supercooled Liquid Ga-In And Ga-Sn Alloys

In the present thesis, the local structures in the liquid Ga-In and Ga-Sn alloys are investigated by using torsional oscillation viscometer, X-ray diffractometer, the ab initio molecular dynamics simulations and X-ray absorption fine structure spectroscope. The forming of the clusters in the liquid alloys was dissected and their evolution was revealed.The viscosities of the Ga-In alloy melts with different compositions are investigated by using a torsional oscillation viscometer. It is found that the viscosity of the Ga-In alloy increases with decreasing temperature in the superheated and supercooled liquid regions. And a difference in the temperature dependence of the viscosity between the supercooled and the superheated melt of Ga-In alloys is detected. Based on the concept of the melt fragility, we find that the fragility of the supercooled melt is larger than that of the superheated melt in this alloy system and both of them are beyond 1. The effect of the magnetic field on the viscosities of Ga-Sn alloy melts is also studied. The results show that the viscosity of the Ga-Sn alloy melts increases with decreasing temperature and a difference in the temperature dependence of the viscosity between the supercooled and normal Ga-Sn alloy melt is detected under the same magnetic field intensity. Furthermore, viscosity of the Ga-Sn alloy melts increases with increasing magnetic field intensity and increases more quickly under higher magnetic field intensity at one temperature. It shows that a sudden change in the liquid structures of Ga-In and Ga-Sn alloys occurs with the superheated liquid changing to supercooled liquid.The liquid structure of the Ga-In alloys is investigated by using x-ray diffraction and ab initio molecular dynamics simulations. It is observed that the liquid structure changes abruptly on the transition from superheat liquid to supercooled liquid. There are Ga-Ga covalent bonds in both superheat and supercooled liquid, and the length of the gallium covalent bond is larger in the supercooled melt. The cluster abruptly becomes much bigger and the ordering degree increases obviously near the liquidus with decreasing temperature while the nearest neighbor coordination number decreasing. The nearest neighbor interatomic distance increases and the partial coordination number of Ga and In atom increases with decreasing temperature. In addition, the nearest neighbor interatomic distance increases with increasing In content, and the interactions between atoms increase and the average coordination number increases. Dissect the clusters’ structure obtained by x-ray diffraction through X-ray absorption fine structure spectroscope. The cluster models with a center In atom in superheated and supercooled liquid are refined. The atoms rearrange and the nearest neighbor interatomic distance increases as the liquid transforms the superheat state into supercooled state, and the high-coordinated polyhedra are inclined to evolve into low-coordinated ones in the liquid. Especially, for the supercooled liquid Ga85.8In14.2 alloy, the distribution of Ga atoms around the center In atom has mirror symmetry and the symmetrical plane is xOy-plane or yOz-plane with the center In atom as the original point. The forming mechanism and evolution of the clusters in the liquid alloy are revealed. It provides a new perspective of clusters evolution for understanding the liquid-solid transition in undercooled conditions.The temperature dependence of the liquid structure of Ga98Sn2 and Ga91.6Sn8.4 alloy is studied by using x-ray diffraction. As the temperature decreases, the liquid structure of Ga-Sn alloy shows an obvious change near the liquidus, and the correlation radius of the clusters and the ordering degree show an abnormal evolution, but they increase with decreasing temperature in the superheated and supercooled region, respectively. The covalent bond structure is more significant in supercooled liquid than that in the superheated liquid. The local structure of Ga-Sn alloy melts are studied by using ab initio molecular dynamics simulations. It is also found that the ordering degree decreases with increasing temperature. The bonding forces between like atoms is stronger than the unlike atoms in the melts. The Sn-Sn covalent bond tends to break at the high temperature. Comparing with the Ga95Sn5 alloy melt, the covalent bond structure is more stable in the Ga91.5Sn8.5 alloy melt. The cluster structure is dissected by using X-ray absorption fine structure spectroscope. The coordination structures of the Sn atom in the liquid Gac91.6Sn8.4 and Ga95Sn5 are similar and the coordination structures of the Ga atom show obvious differences. But the electronic states of Sn atom in the two liquid alloys are different obviously.