| Polymorphism and structural transition in liquid have become common recognition, and been paid much attention in scientific fields. Thereinto, the temperature-induced liquid-liquid structural transition has been observed and testified in different kinds of alloys by Chinese and foreign countries's researchers. For many years, some melt heat treatment techniques have been explored to change materials'structures and properties, and it has been proved to be evidently validity. So the relativities between structures and properties of solid materials and thermal history of original liquids also have been one of important research frontiers. However, up to now, the depth and scope of knowledge on liquid structure and its transition is till not enough, and fewer studies are focused on the change of solidification behaviors and solidified microstructure from the viewpoint of liquid-liquid structural transition. These insufficiencies are the breakthrough and the main content of this thesis.Because of the liquid structure and its complexity, we tried to use fractal theory and statistic method to explore the characteristic of liquid structure and from a new viewpoint to know the mechanism of temperature-induced liquid-liquid structural transition. Furthermore, by means of the free solidification and the direction solidification, pure metal and different kinds of binary alloys were selected to investigate the effect of liquid-liquid transition on nucleation, growth mechanism, microstructure defects, coefficient of solution redistribution, preferred growth, morphology of solid/liquid interface during solidification. Main conclusions are summarized as follows:Firstly, the fractal theory was implemented to study the liquid structure of In-80wt%Sn, In-49.1wt%Sn and In-20wt%Sn, and the results revealed that the liquid structure was the multi-range fractal structure, which is composed of low-dimensional fractal, high-dimensional fractal and the transition zone between the two fractals. The model of multi-range fractals in liquid structure was proposed according to the composition characteristics of two fractal structure. The fractal dimension, which is the physical quantity characterizing fractals, could also be used as a parameter sensitive to liquid structure as electrical resistance, internal friction, and viscosity. The linearity between excess entropy and low fractal dimension suggested that the fractal dimension could reveal the liquid structure to a certain extent, and established the connection between fractal dimension and traditional structural parameters. The different variation characteristics of the high-dimensional structure and the low-dimensional structure with temperature in multi-range fractals indicated that the critical phenomenon of liquid structural change was depended on the space dimension, and revealed the physics law of liquid structural change in the non-integral dimensional space. Based on hard sphere model and the size effect of ultrafine particles, the mechanism of temperature-induced liquid-liquid structural change of In-Sn alloys may be solid like clusters changed to denser packing clusters of face-adjacent tetrahedra subjected to size effect which is caused by the decrease of the size of cluster.Secondly, the rescaled range analysis was used to examine the pair distribution function of liquid In-20wt%Sn and In-80wt%Sn, and the results revealed that the probability of finding a particle at a distance r from a reference particle in liquids obeyed the Hurst's empirical law, and the probability was subject to high index fractional Brownian motion. Same to the change of mean nearest neighbor distance, the Hurst exponent also changed with temperature. However, different to traditional structural parameters, the Hurst exponent has its unique physical meanings. In the presence of temperature-induced liquid structural transition, the change of Hurst exponent suggested the probability of finding a particle departed from the fractional Brownian motion and tended to Brownian motion. The jump of the Hurst exponent was the indicator of the change from one phase to another in liquids.Thirdly, based on liquid-liquid structure transition, selecting pure Bi, hypereutectic Pb-90wt%Sn, hypoeutectic Sn-40wt%Bi and solid-solution Bi-10wt % Sb to carry out free solidification experiments, we discovered that after experienced liquid-liquid structure transition the nucleation-controlled effect on the solidification process was greatly improved, and nucleation undercooling, growth rate and acceleration were enhanced. Liquid-liquid structure transition increased the amount of twin in pure Bi, and induced the appearance of the step bunching phenomenon and the formation of dendrites near the grain boundaries. After liquid structural change dendrites were remelted for Sn-40wt%Bi,which the coefficient solute distribution is great, and coarse dendrites were changed to fine equiaxed grains, while for Pb-90wt%Sn, which the coefficient solute distribution is small, the grains were still dendrites although its size decreases. When liquid-liquid structure transition happened in Bi-10wt%Sb alloy, which solid microstructures were equiaxed grains, the morphology of grains changed from the nonhomogeneous and irregular morphology to the homogeneous and three-pointed-star morphology and the grains was refined.Fourthly, the experiments on Pb-80wt%Sn, Pb-90wt%Sn and Pb-99wt%Sn were successful with the self-designed directional solidification equipment. The results indicated that the solute non-equilibrium distribution coefficient was decreased due to the liquid structural change, and the solute on the frontier of solid/liquid interface was enriched. Moreover, cellular spacing and second-dendrite dendrite arm spacing was decreased. Effects of the liquid structural change on crystallization orientation were investigated. It is found that the melt after the liquid structural change preferably grew on the high index crystal plane with the solidification process, which was responsible for the branching of low index single crystal plane. The interface stability of Pb-80wt%Sn and Pb-99wt%Sn were decreased after liquid structural change. At the cooling rate of water quenching, the tips of Pb-80wt%Sn dendrites and Pb-99wt%Sn cellulars after liquid structural change could keep on growing into component undercooling zone, while the growth of the tips was prevented before liquid structural change and the solid/liquid interfaces in good condition were saved.
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