Investigations On Local Structure Of Molten Alloys By Experimental And Ab Initio Methods

X-ray diffraction is an important method for investigating the structures of liquid alloys at high temperatures. But this method projects the three-dimensional atomic configurations to one-dimension, and only gives the total structure factors or pair correlation functions. Therefore, the comprehensive description of the structure of molten alloys needs other complementary methods to obtain partial structure parameters, among which ab initio molecular dynamics simulation has been found in recent twenty years is an important one. The structures of liquid Fe-Si and Fe-C alloys, which are fundamental systems in iron and steelmaking, are investigated by using both the θ-θ X-ray diffraction and ab initio molecular dynamics simulation method. The relationship of structure parameters with composition in Fe-Si liquid alloys and the local structure characteristic of liquid Fe-C eutectic alloy are studied in this dissertation. Furthermore, by using ab initio molecular dynamics simulation method and combining it with the existing experimental results;we also investigate the structures, dynamics properties and electronic structure of liquid GaSb and InSb, as well as the structure changes of liquid GaSb with temperature and pressure. The main contents of this paper are as follows.Having a great of difficulties in experiments, the partial structure parameters and the electronic structure are still left unknown for liquid Fe-Si alloys. In this dissertation, we have investigated the liquid structures of eleven Fe-Si alloys in the whole concentration range, and systemically given the change of the structure parameters with composition. Based on these results, we suggest that the structure of liquid Fe-Si alloys should be divided into four subintervals separated by the compounds Feo.75Sio.25, Fe_0.50Si_0.50 and Fe_0.285Si_0.715. It is the first time to investigate the electronic structure of liquid Fe-Si alloys by using ab initio method. The calculated results indicate that the Fe-Si bonds, which are stronger than Fe-Fe and Si-Si ones, are resulted from the hybridization of Fe (3d) and Si (3p) orbits. According to the analysis of electronic structure, we arrive at the conclusion that the Si-Si bonds exist in molten Fe-Si alloysas Si content beyond about 70 at.%.Early experimental investigations on molten Fe-C alloys mainly focused on the total structure parameters, and did not give the characters of local structure of molten Fe-C alloys. In this dissertation, we have investigated the structure of molten Fe-C eutectic alloy, and given its partial structure parameters. The results indicate that the heterogeneous bonds are stronger than the homogeneous bonds in molten Fe-C eutectic alloy. The nearest-neighbors of C atoms nearly consist of the Fe atoms, while the correlation between C atoms is at much more large distance. By comparing the structure parameters of liquid Fe-C eutectic alloy with those of Fe3C crystalline states, we find that the local structure of the first shell Fe atoms around the C atom is analogous between them. The calculated electronic density of states for liquid Fe-C eutectic alloy indicates that the origins of the strongest Fe-C bonds are the hybridization of Fe (3d) and C (2p) orbits.The previous experimental results showed that the relationship between physical properties and liquid structures of GaSb (/-GaSb) and InSb (/-InSb) is different from the simple liquid metals. However, the existing experimental results of the liquid structures of GaSb and InSb can not interpret this phenomenon. We have investigated the structural, dynamical and electronic properties of liquid GaSb and InSb by using ab initio molecular dynamics simulation. The calculated results indicate that the local structures of Ga (In) and Sb atoms in liquid GaSb and InSb are analogous with those in pure element liquids, and the Sb-Sb correlation is also similar to its crystalline states. The calculated electronic density of states show that both the liquid GaSb and InSb are metallic, but still preserved some characters of crystalline states.We have also performed ab initio molecular dynamics simulation of structural change of liquid GaSb with temperature. The calculated results indicate that the structural change of liquid GaSb with temperature is nonuniformly. The clusters of Sb atoms have the open structure at low temperature, and then turn into a more close-packed structure at high temperature;on the other hand the Ga-Ga is not sensitive to the temperature. All the changes of the static and dynamical structuralparameters of liquid GaSb with temperature have the nonlinear behavior. These calculated results are consistent with the experimental results of viscosity.The recent high temperature and high pressure X-ray experiments show that the liquid structure of GaSb under pressure up to 20 GPa contracts nonuniformly different from the simple liquid. We have performed ab initio molecular-dynamics simulation of liquid GaSb up to 20.0 GPa. The simulated results suggest that the structural change of liquid GaSb in the range of 1.8-20.0 GPa should be divided into three stages: 1.8-5.4 GPa, 5.4-10.0 GPa and 10.0-20 GPa. A further analysis shows that the local structure of Ga clusters is not sensitive to pressure, while the local structure of Sb clusters changes obviously under pressure, i.e., transforming from opened structure to close-packed structure under high pressure. Therefore, we think that the rearrangement of Sb atoms under pressure plays a crucial role in the structure change of liquid GaSb.