Studies On The Correlation Between Electrical Resistivity And Structure In Superheated And Undercooled Melts

Using resistivity measurement, Pb-Sb, Cu-Ni, Cu-Co, Cu-Sb and melts are studied. Cu-Sb and Co79.5Sn20.5melts are also investigated combining high temperature XRD, AIMD, viscosity measurement. The structure evolution in superheated and undercooled liquid, and its relationship to resistivity change have been studied on electronic level (valence electrons, free electrons). These would lay a theoretical basis to perfect metal solidification theory and provide theoretical support to develop new alloys.Firstly, using the pseudopotential corrected by data of Fermi surface, combined with experimental and hard-sphere structure factor, resistivity of liquid Pb is calculated. The validity and errors of calculation are discussed. This not only deepens our understanding of the relationship between resistivity and structure, but also is helpful to calculate the resistivity of Pb-Sb alloys. In Pb30Sb70, Pb20Sb80, and Pb10Sb90melts, temperature dependence of resistivity deviates from linear dependence during cooling. In Pb30Sb70and Pb20Sb80melts, abnormal viscosity verifies the existence of abnormal structure transition, which is attributed to the reinforcement of Peierls distortion.Secondly, local structure of Cu-Sb alloys has been studied by X-ray diffraction, ab initio molecular dynamics simulation, viscosity and resistivity measurements. Over the whole concentration range, heterogeneous coordination numbers are larger than that of homogeneous atoms, indicating preferential Cu-Sb coordination. A drop is observed in maximum position of simulated Sb-Sb partial distribution functions around Cu75Sb25, which reveals the rapid increase of Sb-Sb coordination. Structural inhomogeneity has been discussed by resistivity measurements. Pre-peaks in relation with medium-range order are observed in structure factor of Cu-20at.%Sb. Negative Warren-Cowley parameters, abnormally large activation energy of viscous flow and negative TCR, reflect the existence of chemical order, which is interpreted as Cu3Sb clusters following nano-crystal model. Around eutectic melts, main peak splitting is observed in both structure factor and simulated total PDFs, which reveals the co-existence of Cu-Sb heterogeneous and Sb-Sb clusters. This results in the smaller first neighbor radius and higher viscosity in Cu37Sb63melts. In pair distribution functions of Cu20Sb80melts, the hump around0.44nm becomes obvious below973K, which reveals the reinforcement of Sb clusters with Peierls distortion. This is consistent with the increase in viscous-flow activation energy in Cu20Sb80melts around973K.Thirdly, the resistivity behavior of undercooled liquid Cu-Ni and Cu-Co alloys had been studied by the electrodeless method, to probe the structure transition in undercooled melts during the cooling process. Over the entire concentration range, linear behavior of resistivity with temperature was obtained in Cu-Ni system. And similar results were obtained in Cu-Co system in the vicinity of liquidus temperature. However, a turning point was obvious in temperature coefficient of resistivity of Cu-Co alloys around the bimodal line, which was interpreted to be responsible for metastable liquid-liquid phase separation. During liquid phase separation process, resistivity decreased and the temperature coefficient of resistivity was larger than that of homogeneous melts. In combination with TEM and SEM studies on the as-solidified microstructure, this was interpreted as the formation of egg-type structure and concentration change in Cu-rich and Co-rich phases. These indicate that, when noble and transition melts enter into undercooled liquid states, temperature dependence of resistivity does not change.Finally, with the help of XRD, resistivity, viscosity and DSC, structure evolution of Co79.5Sn20.5alloys has been studied from superheated to undercooled liquid state. In undercooled liquid state, the number of atoms in atomic clusters rapidly increased. The microstructure and surface morphology change with undercooling of Co79.5Sn20.5alloy. Combing melts structure, EDS and morphology analysis, the growth model in undercooled melts has been discussed.