【作者】 余瑾；

【导师】 祖方遒； 丁厚福；

【作者基本信息】 合肥工业大学 ， 材料学， 2009， 博士

【摘要】 对液体结构和性质的深入认识是诸多领域争取科技进步的基础,已成为凝聚态物理、材料学、生命科学、冶金及化学等领域共同关注的探索对象。然而,人们对液态物质结构和性质的认知相对于固态和气态而言要肤浅得多。可喜的是,近年来液态领域的研究取得了不少阶段性成就,为人们从理论上进一步探索液态物质的结构本质提供了丰富的现象学依据。由衍射技术和计算机模拟研究揭示出的种种拓扑及化学短程序结构极大地丰富了液体结构短程有序的物理内涵。压力诱导非连续液-液结构转变的直接实验证据打破了液体结构连续渐变的传统观念;合金温度诱导液-液结构转变的发现必将对凝固微观机制的认识及新材料的研发和加工产生深远的影响。本文以探索合金液态结构转变普适性为目的,通过采用电阻法和DSC分析法,选择二元合金PbIn、SnZn和三元合金PbSnBi为研究对象,并就液-液结构转变可逆性对凝固行为的影响进行了研究,主要内容、结论及创新点归纳如下:1、为克服传统电阻测试装置容易发生重力偏析的缺点,设计了新型的样品池及测试结构,并搭建了四电极电阻测量装置,可实现对固态和液态物质电阻率的高精度连续测量。此外,通过硬件设计组装及软件编程,建立了电阻率测试计算机数据采集系统,成功实现了数据的自动采集。2、通过对纯金属Pb、Zn、In、Bi和Sn等在固、液态电阻率随温度变化特性的测量研究,探索了电阻法研究金属熔体液-液结构转变现象。并验证了该电阻测试装置测试电阻的有效性,以及对液态物质状态变化的灵敏性。3、通过对二元合金PbIn、SnZn的研究,发现PbIn合金熔体在连续变温诱导下存在不可逆液-液结构转变,而SnZn合金熔体在连续变温诱导下则不存在液-液结构转变现象,SnZn等合金熔体的这种“没有现象”,恰恰是一种重要现象,因为这提示人们一个重要结论,并非所有合金熔体都会发生温度诱导的非连续液-液转变。4、过去对液态物质结构及性质的研究,多集中于单元和二元系统,而未见涉及三元液态物质的研究。本文尝试探索了三元合金(PbSnBi)不同成分熔体性质的温度行为,以探讨其结构随温度变化的可能性及其规律。研究表明,PbSnBi三元合金熔体在连续升降温过程中存在液—液结构转变现象。对PbSnBi合金熔体进行多次热循环研究发现,第一次热循环升降温过程中所发生的液-液结构转变为不可逆逆结构转变;发生液-液结构转变与否与合金成分有关,有些成分的PbSnBi合金熔体在第二次热循环过程中发生完全可逆液-液结构转变,有些则不发生液-液结构转变。合金熔体结构转变的可逆性与合金熔体液相区上方存在的不可逆的微观不均匀区、可逆的微观不均匀区和均质熔体区的结构和性质有关。另外,研究发现随着升降温度速率的变化,合金熔体发生结构转变的开始温度也随之变化。5、根据PbSnBi合金熔体的以上结果,本文进一步探索了熔体发生结构转变对于凝固行为及组织的影响规律。对于发生可逆液-液结构转变的合金熔体而言,其空冷凝固行为表现出明显的可逆性。在空冷条件下,其初生相析出温度及过冷度与熔体的热历史无关,其空冷或铜模冷却凝固金相组织对其热历史均不敏感,而当合金熔体发生不可逆液-液结构转变时,其冷却凝固行为与其热历史有关,即发生液-液结构转变后的凝固组织明显细化,其凝固组织的显微硬度也呈现一定规律性。并根据晶体生长理论对液—固结构依存关系进行了分析与讨论。更多还原

【Abstract】 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.更多还原