【作者】 许春香；

【导师】 梁伟；

【作者基本信息】 太原理工大学 ， 材料加工工程， 2010， 博士

【摘要】 经过研究已发展出多种提高金属材料强度的有效途径,但随着材料强度的提高,韧度或塑性急剧下降,从而使材料强度和韧度或塑性相互倒置的关系成为十分突出的问题,形成妨碍学科进步和实际应用的一个巨大障碍和技术瓶颈。如何实现强度和韧度或塑性的同步提高,已成为国内外金属材料领域普遍关注的重大科学问题。在金属材料的研究中,纯净化、细晶化和均质化是改进传统材料和研制新型合金的三大工艺手段。铝镁及其合金组织的细化是现代铝镁加工业中广泛研究的重大课题,晶粒细化是晶界强化的重要机制,可显著提高铝镁材的力学性能和塑性变形能力,是改善铝镁材质量的重要途径。通过加入晶粒细化剂抑制柱状晶和羽状晶的生长,从而生成等轴晶组织,这种处理方法在铝镁加工业生产中被广泛应用。A1-Ti-C中间合金是一种高效的铝及其合金晶粒细化剂,具有Al-Ti-B中间合金晶粒细化剂无法比拟的技术优势,它的出现是铝及其合金晶粒细化技术的一项重大突破,引起世界各国的高度重视,对此开展研究具有重要的理论研究意义和工程应用价值。熔体引爆自蔓延合成法因其具有合成迅速、工艺简单、节省能源等优势成为制备中间合金的一项新技术。特别是在新一代铝镁及其合金晶粒细化剂Al-Ti-C-X中间合金的制备中更显示出独特的优越性,然而熔体引爆自蔓延合成法较好地解决了产物TiC、TiAl3、AlX等化合物含量低、细化相分布不均等问题。尽管自蔓延高温合成技术(SHS)制备各种复合材料、结构陶瓷、功能梯度材料的研究已取得很大进展,但是对熔体引爆自蔓延合成法制备Al-Ti-C-X中间合金晶粒细化剂的反应过程、组织转变及结构演化规律研究甚少,论文在省基金项目的支持下,进行以下几方面的研究。论文利用自蔓延高温合成技术的优势,将其和普通铸造法相结合,将各种粉状组成材料压制成预制块,将其压入到铝熔体中,利用熔体热量将预制块引爆发生高温自蔓延反应来合成Al-Ti-C中间合金晶粒细化剂。通过对预制块中[Ti]-C反应的热力学与动力学进行分析研究并采用普通光学显微镜、扫描电镜、X射线衍射仪、透射电镜等分析手段对Al-Ti-C中间合金晶粒细化剂进行分析研究和表征。研究发现:在本实验条件下,影响Al-Ti-C反应速度的主要因素有体系的温度、铝粉的含量、石墨粉原始粒度和石墨粉颗粒周围反应层的厚度,在高温区内,增加体系的温度,减小过量Al含量,减小石墨粉颗粒的尺寸,减小反应层厚度等,则对Al-Ti-C体系内高温区所发生的[Ti]-C反应是有利的。论文在对Al-Ti-C中间合金晶粒细化剂制备研究的基础上,然后以此合金为基础,通过加入适量稀土Y、La和元素P,采用熔体引爆自蔓延合成法制备出Al-Ti-C-X(X为Y、La、P)复合中间合金晶粒细化剂。而后将这种Al-Ti-C-X复合中间合金晶粒细化剂加入相应的铝镁合金中进行细化变质处理,以期获得细晶化与强韧化的高强度铝镁合金材料。论文研究了Al-Ti-C-X复合中间合金的合成、组织结构特点和遗传效应以及对铝镁合金组织和性能的影响及其作用规律,探讨了Al-Ti-C-X复合中间合金增强铝镁合金的微观本质。论文研究了润湿性对TiC合成的重要性,研究发现:为了改善碳与铝液的润湿性,必须采取措施对石墨的表面进行特殊浸泡预处理,并加入适量的特种活化剂,以改善铝液的界面张力,从而进一步改善碳和铝液之间的润湿性,为了满足不同合金细化变质的需要,也为了进一步改善石墨在铝液中的润湿性,促进TiC的生成,加入适量稀土Y、La和元素P,形成Al-Ti-C-X复合中间合金晶粒细化剂,在该中间合金中除了TiC相外,还有AlX相形成以满足不同合金变质细化的需要;研究还发现,在制备Al-Ti-C-Y中间合金中过程中,由于Y含量的不同,其相组成也不同,当中间合金中含0.5%Y时,是由细针状TiAl3+TiC颗粒+α-Al基体三相组所成;当中间合金中含1.0%Y时,中间合金是由小块状和棒状TiAl3+TiC颗粒+Al3Y+α-Al四相所组成;而当中间合金中含2.0%Y时,中间合金是由长条状TiAl3+Al3Y化合物+ TiC颗粒+α-Al基体四相所组成。在Al-Ti-C合金中加入La时,制备出的Al-Ti-C-La中间合金细化剂主要由TiAl3+TiC+(AlTiLa) +α-Al基体四相所组成;在Al-Ti-C合金中加入P时,制备出的Al-Ti-C-P中间合金细化剂主要由TiAl3+TiC+AlP+α-Al基体四相所组成。为了满足对铝镁合金细化和强化的需要,论文以外加方式将Al-Ti-C-X复合中间合金加入不同的铝镁合金中,由于Al3Y、AlP、TiAl3、TiC颗粒可作为有效的结晶核心质点,使被细化的铝镁合金得到细化,而AlX相既可作为铝镁合金的结晶核心存在,又可对铝镁合金中Al、Mg原子的扩散起到阻碍作用,从而改善结晶组织中离异共晶相的形貌和分布形式,双重作用的结果,使铝镁合金进一步得到细化和强化,使铝镁合金的性能潜力得到进一步充分挖掘。本文经过对AlX、TiC和由AlX、TiC颗粒增强的铝镁合金的制备研究,以及新生细化相结构演化规律研究,探明了细化相在铝镁合金结晶凝固过程中的遗传效应与细化变质作用行为。研究表明:弥散均匀分布在初生相内部的新生细化相可起到细化晶粒强化基体与晶界、降低元素在镁铝合金中的扩散速率、阻碍基体中位错运动和阻止晶界滑移的作用,使晶界相短网和趋于球化,有效细化和强化铝镁合金。更多还原

【Abstract】 After decades of research efforts, several effective routes have been developed to enhance the strength of metallic materials. Correspondingly, the ductility or the plasticity dramatically deceases, thus resulting in the imbalance between the strength and the ductility (plasticity), which is a great obstacle for the advancement of material discipline and also a technical bottleneck for the practical applications. How to improve the strength and ductility (plasticity) simultaneously is a significant scientific issue widely concerned by domestic and international researchers in metallic material field.For metallic materials, purifying, grain-refining and homogenizing are three techniques to improve traditional materials and develop new alloy systems. Grain refinement of aluminum, magnesium and their alloys is an important subject in the modern metal processing industry. Meanwhile, the grain refinement, a mechanism for grain-boundary strengthening, can elevate the mechanical properties and plastic deformation capabilities and is an effective way to modify the quality of aluminum and magnesium section bars. The addition of grain-refiners can restrict the formation of columnar and twin-columnar grains and promote the equiaxed grains, which is widely utilized in the industrial processes of light alloys. Al-Ti-C master alloy shows excellent grain-refining efficiency on aluminum and its alloy and superior technical predominance over Al-Ti-B master alloy. Its appearance is a great breakthrough in terms of the grain-refining technique for aluminum and its alloy, and great attentions have been paid by countries around the world. Therefore, researching on this subject is of great significance from theoretical and practical perspectives.Self-propagating high-temperature synthesis (SHS)-melting technique has become a new technology for the fabrication of master alloys in that it has the advantages of simple experimental setups and processes, low energy consumption, low cost in production and rapid reaction procedures. The SHS-melting technique shows its superiority during the synthesis of Al-Ti-C-X master alloy since it effectively solve the problems such as low content and inhomogeneous distribution of reaction products including TiC, TiAl3 and AlX compound. Great progress has been made in terms of the fabrication of various composite, structural ceramics and functionally gradient mateials by the SHS technology. However, very limited information is available regarding the reaction processes, microstructural transformation and evolution during the fabrication of Al-Ti-C-X grain-refining master alloy using SHS-melting technique. Supported by Foundations of Shanxi Province, research work is investigated in the following aspects.In this paper, Al-Ti-C grain-refining master alloys have been prepared by combining SHS (self-propagating high-temperature synthesis) technology and conventional casting technology. Various powders were mixed homogeneously and pressed to form the preforms, which were added into molten aluminum to ignite the SHS reaction, resulting in the formation of Al-Ti-C master alloys.The thermodynamics and kinetics of the reaction between [Ti] and the graphite in the perform was analyzed, Constituent phases and compositions in the master alloys were identified by optical microscopy, scanning electron microscopy, X-ray diffraction and transmission electron microscopy.Experimental results show that the main factors influencing the reaction velocity in Al-Ti-C system are the system temperature, aluminum content, original size of graphite particles, and the thickness of reaction layer surrounding the graphite particles. At high-temperature region, higher system temperature, lower content of excess aluminum powder, smaller the particle size of graphite, and thinner the reaction layer surrounding the graphite particles, are positive for the reaction between [Ti] and the graphite of Al-Ti-C system.Based on Al-Ti-C ternary grain-refining master alloy and the SHS-melting technique, Al-Ti-C-X (X=Y, La, P) multiple master alloys have been successively prepared through adding proper amount of Y, La and P, respectively. The Al-Ti-C-X multiple master alloys were introduced into aluminum, magnesium alloys so as to obtain materials with fine grain size and excellent comprehensive mechanical properties. The synthesis, phase constitutes and hereditary effect of Al-Ti-C-X multiple master alloys were investigated; the evolution and the origin of the microstructure and mechanical properties of aluminum, magnesium alloys after the addition of multiple master alloys were also studied.It is well-known that the wettability between carbon and liquid aluminum is crucial for the synthesis of TiC particles. Experimental results show that pre-immersion treatment of graphite particles and adding special activators, are indispensable to improve the wetting of the graphite with the aluminum melt. Meanwhile, the addition of Y, La or P could further promote the wettability and subsequent synthesis of TiC. In addition, the Al-X phases in the Al-Ti-C-X multiple master alloys show grain refining effect on various aluminum, magnesium alloys.Experimental results indicate that the phase constitutes of Al-Ti-C-Y master alloy differs as the Y content varies. When the Y content is 0.5%, the master alloy consists of needle-like TiAl3, TiC andα-Al matrix; while increasing the Y content to 1.0%, the master alloy comprises rod-like and blocky TiAl3, TiC, Al3Y andα-Al matrix; further increasing to 2.0%, the master alloy is mainly composed of large lath-shaped TiAl3, net-like Al3Y, TiC andα-Al matrix. The addition of La into Al-Ti-C alloy results in Al-Ti-C-La master alloy that comprises TiAl3, TiC, AlTiLa andα-Al matrix, while the addition of P into Al-Ti-C alloy results in Al-Ti-C-P master alloy that comprises TiAl3, TiC, AlP andα-Al matrix.In order to refine and strengthen aluminum, magnesium alloys, the Al-Ti-C-X multiple master alloy is introduced. Since the Al3Y、AlP、TiAl3 or TiC particle can either act as effective nucleating site or restrict the diffusion of solute atomic Al and Mg, the grain size of the matrix is reduced and morphology, distribution of divorced eutectic phase are modified, which is positive to the improvement of mechanical properties of aluminum, magnesium alloys.In this paper, based on the research in terms of the synthesis of AlX, TiC phase in Al-Ti-C-X multiple mater alloy, the development of aluminum, magnesium alloys reinforced by AlX, TiC phase, and microstructural evolution of the newborn refining phases, hereditary effect and grain-refining behavior of the refining phases during the solidification and crystallization process of aluminum, magnesium alloys have been explored.Experimental results show that the newborn refining phases, with dispersive distribution at the inner part of the primary phase, can refine the grain, strengthen the matrix and grain boundaries, lower the diffusion rate of solute element, hinder dislocation movement in the matrix, prevent the grain boundary sliding; reduce the size of the phase at the grain boundary, render its morphology to be spherical; finally, aluminum, magnesium alloys with fine grain size and excellent mechanical properties are achieved.更多还原