Exploration Of Kinetic Process Of Liquid Structure Transition In Binary Alloys

Compared with solid and gas state, people's knowledge on liquid structures and properties is superficial. In recent years, many respectable staggered achievements have been obtained and provide abundant phenomenology basis for further exploring the essence of liquid structure theoretically. From press induced liquid-liquid structure transition to temperature induced liquid-liquid structure transition (TI-LLST), more and more evidence suggest the existence of phase transition in liquids. All these present a challenge to our conventional picture of liquids as entities with a continuously varying averaged structure.As a new research field, people's knowledge to the TI-LLST of liquid alloy is still in phenomenological. There are more works need to be done, such as the essence and mechanism of TI-LLST of liquid alloy, the kinetics and thermodynamics during transition process, and the exploration of the transition effect on electronic level. All these are the subject of our research in this article. We select different alloys used for study, such as PbSn, InSn, PbIn and CuSn, which are belong to different kinds of alloy liquids according to the classification of Y.Marcus. New research methods have been used, such as resistivity, thermoelectricity, DTA and X-ray diffraction. It has been carried out that the isothermal experiments holding at different temperatures and the heating experiments with different heating rate. The characteristic, mechanism and essence of TI-LLST have been studied and discussed using some theories, such as kinetic theory of phase transition, thermodynamics theory and Fiber-Ziman theory. Main conclusions are summarized as follows:1. The kinetic behavior in TI-LLST process of those alloys has been investigated through continuous heating experiments and isothermal experiments. Using kinetic theory of phase transition, we found that the kinetic transition process is nucleation-growth type, and the nucleation rate is the dominant controlling factor of the TI-LLST process. There are two different kinds of transition modes: Johnson-Mehl-Avrami transition mode and self-catalysis transition mode. The microcosmic physics mechanism of the transition process have been discussed in this paper, and for the first time, the characteristic of TI-LLST (the transition speed of TI-LLST is slower than normal solid state phase transitions) have been explained.2. Based on the results of heating experiments, the apparent activation energy E_k of TI-LLST have been calculated using Kissinger equation. E_k is the energy needed to overcome the energy barrier of the structure transition, and could reflect the difficulty level of TI-LLST of different alloy melts, for example InSn80>InPb80 >PbSn61.9. Furthermore, compared with normal solid state phase transition, the E_k value of TI-LLST is much smaller.3. Through X-ray diffraction experiments, we obtain some parameters reflecting atomic structure character, such as mean nearest neighbor atomic distance, the first coordination number, size of ordering domains and the ordering degree. During the change, a sudden drop happened for both the nearest neighbor distance and the coordination number, and the size of short-range orders decreased abruptly. The calculated results of the pair correlation entropy showed ;hat the ordering degree of the melt altered obviously and revealed the discontinuous feature of the TI-LLST. Using percolation theory, the discontinuous feature has been also discussed.4. Based on the results of electronic transport properties and the calculated mean nearest neighbor atomic distance, the dependence of the density of electronic states N(Ef) and its gradient value dN(E_F)/dE on temperature were deduced with the help of Faber-Ziman theory, reflecting that the electronic structure of the melt at Fermi level also changes during the transition.