偏晶合金凝固过程及微合金化的影响

发布时间：2019-05-28 02:14

【摘要】：偏晶合金十分广泛,其主要特点是,单一均匀的偏晶合金熔体在冷却过程中将发生液-液相变,生成两个互不混溶的液相。如凝固形成少量相以微小粒子形式弥散分布于基体中的复合材料,则许多偏晶合金具有优异性能,在工业上拥有广阔的应用前景。然而,偏晶合金液-液相变过程十分复杂,影响因素繁多,在常规的凝固条件下,极易形成相偏析严重乃至两相分层的凝固组织。这限制了该类合金的工业开发与应用。开展偏晶合金凝固过程及其组织控制研究对推动偏晶合金材料工业应用具有重要意义。微合金化对金属和合金凝固过程与组织具有重要影响,但迄今为止,有关微合金化条件下偏晶合金凝固行为的研究鲜有报道,本论文采用实验与模拟计算相结合的研究方法,研究微合金化条件下偏晶合金凝固行为,探索微合金化对偏晶合金凝固组织的影响及用微合金化法控制偏晶合金凝固过程及组织的可行性。主要研究工作及成果如下:实验研究了微量元素对偏晶合金凝固组织的影响,考察了微量元素Bi对Al-Pb合金、微量元素Sn对Al-Pb(Bi)合金凝固组织的影响。发现微量元素Bi、Sn可以显著细化Al-Pb(Bi)偏晶合金凝固组织;随着微量元素添加量的增加,弥散相液滴的平均尺寸呈现先减小后保持不变的规律;弥散相体积分数越大,微量表面活性元素对偏晶合金中弥散相粒子的细化效果就越好,表明微量元素Bi可以作为Al-Pb合金的表面活性元素,微量元素Sn可以作为Al-Pb(Bi)合金的表面活性元素。建立了微量表面活性元素作用下偏晶合金连续凝固组织演变过程模型,并结合实验开展了模拟研究,揭示了微量表面活性元素对偏晶合金连续凝固组织的影响机理。结果表明,表面活性元素富集于基体液相与弥散相液滴界面处,降低偏晶合金液-液相变过程中两液相间的界面能,从而提高弥散相液滴的形核率,降低弥散相液滴的Marangoni速率,促进偏晶合金形成弥散型凝固组织。提出了在氟盐反应法中采用纳米碳管(CNT)作为碳源制备Al-Ti-C中间合金的新思路,制备出了 TiC颗粒弥散度高的Al-Ti-C中间合金,分析了 CNT作为碳源时Al-Ti-C中间合金凝固组织的形成过程,结果表明,Al-Ti-C中间合金制备过程中,TiC颗粒是通过固态C与溶质Ti反应生成,而不是通过溶质C与溶质Ti反应生成;CNT尺寸小,比表面积大,能够增加铝熔体与CNT间的接触面积,同时,CNT结构中存在大量的缺陷和空位,使CNT具有较高的化学活性,这些均有利于促进CNT与溶质Ti之间的反应,促进TiC颗粒高度弥散分布于基体中的Al-Ti-C中间合金的形成。以Al-Pb(Bi)偏晶合金为对象,实验研究了微量化合物TiC对偏晶合金凝固组织的影响。发现微量化合物TiC可以显著细化Al-Pb(Bi)偏晶合金凝固组织;随着TiC颗粒添加量的逐渐增加,弥散相液滴的平均尺寸呈现先保持不变、增大、减小再保持不变的趋势;弥散相体积分数越大,微量化合物TiC对弥散相粒子的细化效果就越好,表明TiC颗粒可以作为Al-Pb(Bi)偏晶合金液-液相变过程中弥散相液滴的有效异质形核质点,能大幅度提高弥散相液滴形核率,促进弥散型偏晶合金凝固组织的形成。建立了 TiC颗粒在偏晶合金熔体中的动力学行为模型及微量化合物TiC作用下偏晶合金凝固组织演变过程模型,模拟计算与实验研究相结合,分析了 TiC颗粒在合金熔体中的动力学行为及其对偏晶合金凝固组织的影响,阐明了微量化合物TiC对偏晶合金凝固组织形成过程的影响机理。结果表明,TiC颗粒自加入偏晶合金熔体后会发生溶解、粗化及冷却时沉淀析出过程,这一过程对偏晶合金凝固组织影响重大,TiC颗粒对弥散相液滴的细化效果主要取决于液-液相变开始温度时熔体中存在的TiC颗粒的数量密度。当TiC颗粒添加量较小,冷却至液-液相变开始温度时熔体中存在的TiC颗粒的数量密度不足时,微量化合物的添加会粗化弥散相液滴;只有当TiC颗粒添加量较大,冷却至液-液相变开始温度时熔体中存在的TiC颗粒的数量密度足量时,微量化合物的添加才会细化弥散相液滴,促使弥散型偏晶合金凝固组织的形成。
[Abstract]:The partial crystal alloy is very wide, and its main characteristics are that a single, homogeneous, monotectic alloy melt will take place in the cooling process to a liquid-liquid phase to form two immiscible liquid phases. If a small amount of the composite material dispersed in the matrix is dispersed in the form of small particles, many of the partial crystal alloys have excellent properties and have a wide application prospect in the industry. However, the process of the liquid-liquid phase transformation of the partial-crystal alloy is very complicated, and the influence factors are various. Under the conventional solidification condition, the solidification structure with serious phase segregation and even two-phase stratification can be easily formed. This limits the industrial development and application of such alloys. It is of great significance to carry out the solidification process of the partial crystal alloy and the research of its structure control. The micro-alloying has an important influence on the solidification process and the microstructure of the metal and the alloy, but so far, the research on the solidification behavior of the partial-crystal alloy under the condition of micro-alloying is rarely reported. The effect of microalloying on the solidification structure of the partial-crystal alloy and the feasibility of controlling the solidification process and the microstructure of the partial-crystal alloy were investigated by microalloying. The main research work and results are as follows: The effect of trace elements on the solidification structure of the metacrystal alloy is studied. The effect of trace element Bi on the solidification structure of Al-Pb (Bi) alloy is investigated. When the trace elements Bi and Sn are found, the solidification structure of the Al-Pb (Bi) partial crystal alloy can be obviously refined; with the increase of the addition of the trace elements, the average size of the dispersed phase liquid drops is kept constant; the larger the volume fraction of the dispersion phase, The better the fine effect of the trace surface active element on the dispersion phase particles in the partial crystal alloy, the better the trace element Bi can be used as the surface active element of the Al-Pb alloy, and the trace element Sn can be used as the surface active element of the Al-Pb (Bi) alloy. The evolution process model of the partial-crystal alloy continuous solidification structure under the action of micro-surface-active elements is established, and the simulation research is carried out in combination with the experiment, and the influence mechanism of the trace surface-active element on the continuous solidification structure of the partial-crystal alloy is disclosed. The results show that the surface-active element is enriched in the interface of the liquid phase of the matrix and the dispersed phase drop, and the interfacial energy between the two liquid phases in the liquid-liquid phase of the partial-crystal alloy is reduced, so that the nucleation rate of the dispersed phase liquid drops is improved, the Marangoni rate of the dispersed phase liquid drops is reduced, And the dispersion type solidification structure is formed by promoting the partial crystal alloy. A new method for preparing Al-Ti-C intermediate alloy by using carbon nanotubes (CNT) as a carbon source in the fluorine salt reaction method is proposed, and the Al-Ti-C intermediate alloy with high TiC particle dispersion is prepared, and the forming process of the solidification structure of the Al-Ti-C intermediate alloy when the CNT is used as a carbon source is analyzed, the results show that, in the preparation process of the Al-Ti-C intermediate alloy, the TiC particles are generated by the reaction of the solid state C and the solute Ti, not by the reaction of the solute C and the solute Ti; the CNT size is small, the specific surface area is large, the contact area between the aluminum melt and the CNT can be increased, and meanwhile, The CNT structure has a large number of defects and vacancies, so that the CNTs have higher chemical activity, which are beneficial to promoting the reaction between the CNTs and the solute Ti, and promoting the formation of the Al-Ti-C intermediate alloy in the matrix. The effect of the micro-compound TiC on the solidification structure of the Al-Pb (Bi) alloy is studied in this paper. It is found that the micro-compound TiC can refine the solidification structure of the Al-Pb (Bi) alloy, with the increasing of the amount of the TiC particles, the average size of the dispersed phase droplets will remain unchanged, increase, and reduce the tendency to remain unchanged; the larger the volume fraction of the dispersion phase, The better the fine effect of the micro-compound TiC on the dispersion phase particles, the better the TiC particles can be used as the effective heterogeneous nucleation points of the dispersed phase droplets in the liquid-liquid phase transformation of the Al-Pb (Bi) partial crystal alloy liquid-liquid phase transformation, and can greatly improve the dispersion phase liquid drop shape nuclear rate and promote the formation of the dispersion type partial crystal alloy solidification structure. The dynamic behavior model of TiC particles in the melt of the partial-crystal alloy and the evolution process model of the solidification structure of the partial-crystal alloy under the action of the micro-compound TiC are established, and the simulation calculation is combined with the experimental research. The dynamic behavior of TiC particles in the alloy melt and its influence on the solidification structure of the partial-crystal alloy are analyzed, and the influence mechanism of the micro-compound TiC on the formation process of the solidification structure of the partial-crystal alloy is illustrated. The results show that TiC particles can be dissolved, coarsened and cooled in the process of precipitation and precipitation, which has a significant influence on the solidification structure of the partial-crystal alloy. The effect of TiC particles on the dispersion phase droplets depends mainly on the amount and density of TiC particles present in the melt at the beginning of the liquid-liquid phase change. when the amount of the TiC particles in the melt is less than the amount of the TiC particles present in the melt when the amount of the TiC particles is small, the dispersion phase liquid drops are coarsened by the addition of the trace compound; only when the amount of the TiC particles is large, When the quantity and density of the TiC particles present in the melt at the beginning of cooling to the liquid-liquid phase change are sufficient, the addition of the micro-compound can refine the dispersion phase liquid droplets and promote the formation of the diffusion-type partial crystal alloy solidification structure.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG146.21

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