Study Of The Temperature Dependence Of Electronic Transport Properties In Alloy Melts And The Correlation With Solidification

The knowledge on nature of liquid structures and properties is the foundation for a future development of science and technology in many fields; therefore, it has been becoming a common concern for condensed physics, materials science, life science, chemistry, metallurgy and so on. Although the knowledge on liquid structures and properties is more superficial than that of solid and gas, many respectable staggered achievements have been obtained in recent years, which provide abundant phenomenology basis for further exploration of the character of liquid structure from the theory point of view. The topological and chemical short-range orders, revealed by various diffraction method and computer simulations, enrich the intrinsic physical meaning on short-range order in liquid structure. The direct evidence of pressure-driven discontinuous liquid-liquid structure transition (L-LST) in liquid phosphorus breaks the traditional concept that liquid structure changes continuously and gradually with temperature and pressure changing. The discovery of temperature-induced L-LST in binary liquid alloys will influence deeply on understanding of solidification mechanism, development of the new materials and its processing technology.In the present paper, the binary alloys PbIn, SnZn and the ternary alloy PbSnBi are chosen as the investigation object by the electric-resistivity and DSC analysis method. Based on the experimental results, the characteristics, rules and mechanism of L-LST are also analyzed theoretically in this paper. Furthermore, based on the prior study and from the new point view of L-LST, the effect of L-LST on solidification behavior and microstructure is investigated.The results are summarized as follows:I .In order to overcome the shortcoming of gravity segregation, an new sample room have been designed. Meanwhile a four-electrode electric-resistivity measuring equipment has been built. And the computer data acquisition system which was designed by our self have taken place of manual work successfully, and therefore the measuring efficiency and accuracy of the system have been improved.II .The electric-resistivity behaviors of pure metals Pb, Zn, In, Bi and Sn have been studied. And the effectiveness of the four-electrode electric-resistivity measuring equipment and sensitivity to the state of the substance have been proved in this paper.III.Research results show that an irreversible L-LST exists in the binary PbIn alloy melts, but no L-LST of temperature induced takes place in the binary SnZn alloy melts. It indicates that L-LST of temperature induced does not happen in any alloy.IV. In the past, the study work on the structure of liquid substance only focus on binary alloy but ternary alloy. So in this paper the ternary PbSnBi alloy has been studied. It has been found that the L-LST exists in the ternary PbSnBi alloy melts during continuous heating and cooling. The L-LST happening during the first cycle heating and cooling process is an irreversible L-LST. The alloy composition determine whether the L-LST exist or not. In some compositions of the ternary PbSnBi alloy melts the absoluteness reversible L-LST will proceed during the second cycle heating and cooling process. But in some others the L-LST will not take place. The reversibility of the L-LST happening in an alloy melts is related to that the liquid area of an alloy melts can be divided into three portion -the irreversible heterogeneous area under a microscope, the reversible heterogeneous area under a microscope and the homogeneous area. In addition, the beginning temperature of the L-LST will change with the vary of rates of the temperature.V. When the alloy melts take place a reversible L-LST during heating and cooling process, its solidification behavior show reversibility under air cooling, meanwhile the precipitation temperature of primary phase and degree of supercooling is not related to the thermal history, and also its microstructure under air cooling or copper moulds cooling is insensitive to the thermal history. However, When the alloy melts take place a irreversible L-LST during heating and cooling process, its solidification behavior is related to the thermal history. After L-LST, its microstructure is obviously fine and the hardness shows regularity. Finally, the relationship of the solid and liquid structure has been discussed with the theory of crystal growth.