【作者】 范群成；

【导师】 金志浩； 柴惠芬；

【作者基本信息】 西安交通大学 ， 材料科学与工程， 2000， 博士

【摘要】 自蔓延高温合成(SHS)以其独特的优点成为制备化合物及复合材料的新技术。TiC 是高熔点和高硬度的优秀高温耐磨材料;TiC-Fe 由于较高韧性、耐磨性和可热处理性而被广泛应用;NiAl 是优良的低密度、高强度、高温材料,有望替代Ti-基和Ni-基超合金;而NiAl-Cu可改善NiAl 的韧性,并保持其形状记忆效应。因此,研究这些材料的SHS 机理既有重要的科学意义也有重要的工程应用价值。相转变和显微组织转变是开启SHS 机理之门的两把钥匙,本研究主要通过显微组织转变的扫描电镜观察揭示SHS 机理。采用燃烧波淬熄法并对其进行了全面改进,更完整更清晰地记录了显微组织转变过程,为深刻认识SHS 机理提供了确凿而详实的实验证据,得到了下列重要结果: 建立了TiC 燃烧合成的壳-核机制及其模型。C 原子向Ti 粉粒中反应扩散形成了TiC 壳层,被TiC 壳层包裹的Ti 核熔化后,C 原子扩散过TiC 壳层向熔融的Ti 核中溶解,并析出TiC 晶粒。以前提出的TiC 燃烧合成的渗碳机制和溶解-析出机制被有机地统一在壳-核机制中,而且,壳-核机制可解释燃烧合成的TiC 呈轻微烧结状粉末的实验现象。提出了TiC-Fe 燃烧合成的三元反应扩散/ 溶解-析出机制及其模型。在使用较粗Ti 和Fe 粉条件下,Ti 与C 的燃烧反应分别发生在Ti 和Fe 粉粒中,且前者先于后者。Ti 粉粒中的反应通过Fe、特别是C 向Ti 粉粒中的固态三元反应扩散进行;由于Fe 粉粒的熔点随C和Ti 向其中扩散而迅速向三元共晶温度降低,Fe 粉粒中的反应在液态下通过C 和Ti 向Fe液中溶解以及TiC 粒子析出而进行。还提出了TiC-Fe 燃烧合成的双溶解-析出机制及其模型。在使用较细Ti 粉和较粗Fe 粉情况下,虽然Fe 粉粒中的燃烧反应未受影响,但较细Ti 粉粒中的反应变为溶解-析出机制。即,较细Ti 粉粒在反应开始前熔化,C 和Fe 向Ti 液中溶解,并析出TiC 粒子。较细Ti 粉粒在反应前的熔化归因于粉粒的小尺寸效应引起的受尺寸控制的熔点以及C 和Fe 向较细Ti粉粒中快速扩散引起的受成分控制的熔点的显著降低。澄清了Fe 添加物在TiC-Fe 燃烧合成中的作用。Fe 添加物不仅仅起稀释剂和粘结剂的作用,更重要的是,还起了反应源和降低激活能的作用。它不仅通过C 和Ti 向Fe 液中的溶解为Ti 与C 的燃烧反应作了必要的准备,并为TiC 粒子提供了另一个析出源;C 和Fe 向Ti粉粒中的同时扩散,无论引起由TiC 粒子和富Ti 固溶体组成的三元反应扩散层,还是导致反应按溶解-析出机制进行,都改变了无Fe 添加物时C 原子向Ti 粉粒心部迁移的路径,从而降低了激活能。这揭示了加入Fe 粉降低反应点燃温度的原因,也清楚地说明了Fe 粉变为粘结剂的过程。研究了反应物颗粒尺寸对TiC-Fe 燃烧合成特性的影响,并用所提机制和模型进行了满更多还原

【Abstract】 Because of its unique advantages, the self-propagating high-temperature synthesis (SHS) has become a novel technology of producing compounds and composite materials. TiC is an excellent wear-and heat-resisting material with high hardness and melting point; TiC-Fe is extensively used due to its higher toughness, wear-resisting, and heat-treatability; NiAl is a promising potential low density, high strength, and high temperature material which may take the place of conventional Ti-and Ni-base superalloys; and NiAl-Cu can improve the toughness but maintain the shape-memory effect of NiAl. Therefore, studying the SHS mechanism of these materials is of either scientific significance or engineering applied importance. The phase transition and the microstructural evolution are two keys to the SHS mechanism, and the scanning electron microscope (SEM) observation of the microstructural evolution was used for revealing the SHS mechanism in the present work. A combustion front quenching technique was adopted and developed overall so that more details of the microstructural evolution could be recorded more accurately, giving conclusive and detailed experimental evidences for a deep understanding of the SHS mechanism. The following important results were obtained: A shell-core mechanism and correspondent model of the SHS of TiC were established. A TiC shell was formed by reaction diffusion of C atoms into the Ti particle, and after melting of the Ti core coated with the TiC shell, C atoms dissolved into the molten Ti core by means of diffusion through the TiC shell, and then the TiC grains precipitated. The previously proposed carburization mechanism and dissolution-precipitation mechanism for the SHS of TiC were organically united into the shell-core mechanism, and an experimental phenomenon, the combustion-synthesized TiC was present in the form of slight binding powders, could be explained with this shell-core model. A ternary-reaction-diffusion / dissolution-precipitation mechanism and correspondent model of the SHS of TiC-Fe were proposed. Under a condition of using the coarser Ti and Fe powders, the combustion reaction between Ti and C occurred, respectively, in the Ti and Fe particles, and the former occurred earlier than the latter. The reaction in the Ti particle took place in the solid state by a ternary-reaction-diffusion of Fe, especially C into the Ti particle; because the melting point of the Fe particle rapidly decreased toward the ternary eutectic temperature with a diffusion of C and Ti into the Fe particle, the reaction in the Fe particle took place in the liquid state by a dissolution of C and Ti into the molten Fe liquid as well as a precipitation of TiC particles. A dual-dissolution-precipitation mechanism and correspondent model of the SHS of TiC-Fe were also suggested. In the case of using the finer Ti and the coarser Fe powders, although the combustion reaction mechanism in the Fe particle was not affected, the reaction in the finer Ti particle was changed to a dissolution-precipitation mechanism. In other words, the finer Ti particle melted before the reaction started occurring, C and Fe dissolved into the Ti liquid, and TiC particles precipitated. The melting of the finer Ti particle prior to the reaction was attributed to a significant decrease in both the size-controlled melting point due to a small-size effect of a particle and the composition-controlled melting point due to a rapid diffusion of C and Fe into the finer Ti particle. The role of the Fe addition in the SHS of the TiC-Fe was cleared up. The Fe addition not only served as a diluent and binder, what is more, it played the role of source of the reaction and the role of decreasing the activation energy of the reaction. It not only made the necessary preparations for the combustion reaction of Ti+C by the dissolution of C and Ti into the Fe liquid, but also it provided another source for the precipitation of the TiC particles; and the simultaneous diffusion of C and Fe into the Ti particles, leading to whether a formation of a ternary reaction diffusion layer composed of TiC particles and rich-Ti solid solution or a reaction by the dissolution-precipitation mechanism, changed the movement route of C atoms into the central region of the Ti particle in the absence of the Fe addition, so the activation energy was decreased. These revealed the cause why the ignition temperature of the reaction decreased with an addition of the Fe powder and indicated clearly the process in which the Fe powder became the binder. The effects of the reactant particle size on the characteristics of the SHS of TiC-Fe were investigated and explained satisfyingly with the established mechanisms and models. By comparing the characteristics of the SHS of four kinds of Ti-C-Fe mixtures with a same composition but not same size of Ti and Fe powders, it was found that the finer Ti powder led to the more complete reactions and hence the higher combustion temperatures; the finer Ti powder resulted in the higher reaction velocities, but in the case of using the coarser Ti powder, the finer Fe powder greatly decreased the reaction velocity; and the finer Ti powder made the TiC particles with a greater size, the products with a higher density and the layer-shaped pores parallel to the combustion wavefront. These effects were explained satisfyingly with the proposed mechanisms and models, and hence the availability of the mechanisms and models were confirmed. A dissolution-precipitation mechanism and correspondent model of the SHS of NiAl were established. After melting of the Al particle, the Ni particle dissolved into the Al liquid solution, and the NiAl grains precipitated. This was significantly different from that for a thermal explosion combustion synthesis of NiAl, since all the pre-combustion reactions prior to the melting of Al particle and all the possibly intermediate steps in formation of NiAl which occurred during the thermal explosion combustion synthesis were restrained by a rapid increasing of temperature of the reactants during the SHS. A previously doubtful point, i.e. the reaction velocity was affected by the particle size of both Al and Ni despite the melting of the Al particle before the reaction, was explained reasonably. A dissolution-precipitation-substitution mechanism and correspondent model of the SHS of NiAl-Cu were proposed. It was found that the combustion reaction was initiated by melting of the Al particle, and that during the reaction the Cu particles acted as an intermediate, changing the process of the SHS of NiAl. After melting of the Al particle, the Cu particle more rapidly dissolvedinto the Al liquid because of the lower melting point of Cu than Ni, and a CuAl2 phase precipitated. The CuAl2 phase then transformed into a CuAl phase with melting of the Cu particle. The Cu atoms in the CuAl phase were then substituted by the Ni atoms due to the greater affinity after melting of the Ni particle, finally forming β-NiAl particles containing a small amount of Cu and a Cu binder containing a small amount of Al and Ni.更多还原