NiTi-TiB2复合材料的原位合成及其结构功能特性研究

NiTi shape memory alloys exhibit shape memory effect, superelasticity and high damping due to reversible thermoelastic martensitic transformation, and thus have assumed tremendous importance in the fields of science and engineering. The presence of a TiB2 ceramic second phase within the SMA matrix may lead to a new composite with increased strength, stiffness, hardness and so on, and the martensitic transformation characteristics and functional properties of the composites can be modified by control of the particles. Compared with common ceramic-metal composites, a higher plasticity may be expected for this composite, because the stress-induced martensitic tranformation may relax the internal stress concentration and hence hinder cracking. Many TiNi-ceramic composites have been studied, such as NiTi-TiC, NiTi-TiN and so on, however, little systematically investigation about NiTi-TiB2 composite was reported. Furthermore, most previous porposed TiNi-ceramic composites are synthesised by powder metallurgy of Ni, Ti and ceramic mixture, with relatively low interface strength and low densification, which may cause a degradation of the structural-functional properties. In this project, an in-situ NiTi-TiB2 composite was prepared by arc melting of Ti, Ni and B based on the self-propagation high temperature synthetic reaction between titanium and boron. The metallurgical bonding of the NiTi/TiB2 phase interface would contribute immensely to the mechanical and functional properties of the composite, and make it superior to previous porposed TiNi-ceramic composites. The aim of this project is to prepare an in-situ NiTi-TiB2 composite with superior structural-functional properties, study the coupling effect between NiTi and TiB2, and propose the strengthening mechanism result from interface constraint.

利用超弹性NiTi记忆合金增韧TiB2陶瓷，有望使材料获得超常性能，集高强度、高模量、超弹性、高硬度、高耐磨等力学性能于一身。此外，与TiB2陶瓷耦合会改变NiTi的相变行为，容易产生崭新功能特性。然而，以往报道的记忆合金-陶瓷复合材料多采用外加陶瓷法非原位合成，界面结合相对较差，致密度低，限制了人们对其真实性能的认知。并且，NiTi-TiB2报道很少，缺乏系统研究。本项目根据Ti和B的自蔓延放热反应，采用熔炼法原位合成NiTi-TiB2复合材料。该材料中NiTi与TiB2冶金结合，界面结合强度高，致密，克服了以往外加陶瓷复合材料的不足，有利于全面而真实地反应出材料的性能。本项目旨在制备具有超常力学性能和崭新功能特性的NiTi-TiB2原位复合材料，搞楚界面约束对NiTi的增强作用机制并建立TiB与TiNi之间耦合作用机制。

常规的通过外加陶瓷颗粒方法制备的记忆合金-陶瓷非原位复合材料往往界面结合相对较差，致密度低，严重影响材料的性能发挥。本项目根据Ti和B的自蔓延放热反应，采用熔炼法原位合成NiTi-TiB2复合材料。该材料中NiTi与TiB2冶金结合，界面结合强度高，致密，克服了以往外加陶瓷复合材料的不足，有利于全面而真实地反应出材料的性能，使本项目所获得的材料兼具有高强度、高硬度、塑性好等力学性能。项目设计并制备了多种成分的NiTi-TiB2复合材料，并通过一系列显微组织分析、成分测定和力学测试，探索了成分配比对显微组织和材料性能的影响；研究了NiTi-TiB2记忆合金陶瓷复合材料的增强、增韧机制。结果标明：当B含量高于4%（at.%）时，大量Ti2Ni相会与NiTi、TiB2相共存，并且陶瓷相粗大，容易导致材料性能劣化。而当B含量低于4%（at.%）时，Ti2Ni相含量显著降低，材料性能较为优异；压缩实验结果显示，4%B铸态样品的压缩强度高达2400MPa，断裂应变为33%。拉伸实验结果显示，直径1mm的2%B拔丝样品，拉伸断裂强度可高达1300MPa；NiTi基体通过应力诱发马氏体相变，能起到抑制裂纹扩展作用，具有增韧效果；复合材料的强化机制为弥散颗粒强化机制，通过陶瓷颗粒阻挡位错滑移增强基体，复合材料混合定律并不适用于该材料。但这也导致该材料不适合用于研究记忆合金基体与第二相间的耦合。因此，为研究记忆合金基体与第二相间的载荷传递，以及因耦合而产生的新特性，项目还拓展研究了NiTi-NbTi系列纳米线增强记忆合金复合材料。