β转变温度附近Ti-B4C体系的原位反应行为和产物特征的调控机制

Because of the enhanced diffusion of Ti near β transition temperature (βt), reactive sintering of Ti-B4C powder system is expected to be accomplished under a much lower temperature compared with presented studies. Low temperature sintering can not only avoid the coarsening of both reinforcements and Ti matrix, but also meet the requirements of energy conservation and pollutant reduction. However, in order to control microstructure characters, such as the morphology, size and amount of reinforcements, and thus adjust mechanical properties of (TiCp+TiBw)/Ti composites, the accelerated diffusion mechanisms and in-situ reaction behavior of Ti-B4C powder system by solid-state sintering near βt of Ti matrix must be further clarified..In the presented proposal, alloying element Al or Mo will be introduced into pure Ti matrix to adjust βt, and then Ti-B4C diffusion-reaction couples will be prepared by solid-state sintering near βt. The characters of reaction products and thickness of diffusion layer will be investigated by characterizing the diffusion-reaction couples. Diffusion behavior and microstructural evolution during in-situ reaction process will be studied by both water-quenching and in-situ observations under high temperature. Subsequently, relationships between interfacial diffusion of Ti-B4C and β transition of Ti matrix will be discussed from diffusion theory, nucleation and growth model of reaction products will be established from the viewpoints of thermodynamics and kinetics, and then reaction behavior of Ti-B4C powder system and the characters of its products will be clarified. Finally, to validate the above theoretic results, (TiCp+TiBw)/Ti composites will be prepared from Ti-B4C powder system by solid-state reactive sintering, and their microstructures and mechanical properties will be also investigated. Results of this proposal are expected to provide an important theoretical guidance for further study and preparation of (TiCp+TiBw)/Ti composites with excellent properties.

基于钛基体在β转变温度附近的加速扩散现象，有望实现Ti-B4C体系的低温反应烧结，对细化组织和节能减排具有积极作用。若能进一步揭示加速扩散机制及合金元素的作用规律，阐明原位反应行为和产物特征(形态和尺度等)的影响机制，则可为(TiCp+TiBw)/Ti复合材料的组织设计与力学性能调控提供重要的理论指导。本项目拟首先向纯钛中添加合金元素Al或Mo调节β转变温度，并在相应β转变温度附近制备Ti-B4C扩散反应偶，结合组织表征确定产物特征和扩散层的厚度，利用静态和原位手段研究反应过程的扩散行为与组织演变规律。随后，结合扩散理论探讨Ti-B4C界面扩散与β转变的相关性，建立产物形核和长大的热-动力学模型，从而阐明Ti-B4C体系反应行为和产物特征的调控机制。最后，通过制备复合材料并考察其组织与力学性能，验证理论成果的可靠性，为高性能(TiCp+TiBw)/Ti复合材料的后续研究与制备提供理论基础。

钛及其合金由于具有高的比强度和比刚度、良好的耐腐蚀能力和抗氧化性能，被广泛用于航空航天领域，但其低的强度和耐磨性成为限制其应用的主要障碍，而通过Ti-B4C原位反应引入TiC颗粒和TiB晶须有望解决这一问题。为了降低Ti-B4C体系的反应温度，实现节能减排和避免组织过度长大，本项目提出了利用钛基体α/β相变温度附近自钛基体中固溶原子的反常扩散现象，实现Ti-B4C复合材料体系低温烧结的目的。执行过程中通过向纯Ti粉末中添加TiO2粉末并进行粉末烧结制备了Ti(O)固溶体，成功实现了对转变温度的调节，随后将其覆盖B4C块体进行放电等离子烧结制备了Ti/B4C扩散反应偶，通过测量反应界面处TiB2层的厚度表征B和C原子在钛基体转变温度附近不同温度下的扩散能力，发现虽然温度越高扩散层厚度越大，但在转变温度点时扩散层的厚度最大，甚至高于处于基体β相区的温度，表明转变温度确实存在B和C原子在钛基体中的反常加速扩散。为了进一步研究这种反常扩散对所制备的(TiCp+TiBw)/Ti复合材料的影响，以Ti-0.25wt%O合金为基体并在其转变温度附近进行烧结，发现在转变温度下反应完全进行且反应产物特别是TiB晶须的尺寸最大，但由于粗大的TiBw与Ti基体界面的开裂导致复合材料的弯曲强度低于其他温度下制备的复合材料。采用等温水淬法和高温激光共聚焦原位研究了Ti-B4C体系的原位反应行为，并结合热力学和动力学计算了Ti侧和B4C内部不同化学反应间的竞争关系，确定了原位反应的过程：B4C在高温下分解为B和C原子并向Ti侧扩散，生成TiB晶须和TiC颗粒；随后Ti原子通过B和C原子扩散后留下的空位进入B4C侧，生成TiB2颗粒和C单质；最后当TiB2长成致密层后阻碍C和B原子的扩散，导致反应终止。制备了不同增强相含量的(TiCp+TiBw)/Ti复合材料，计算了各因素对强度的贡献，发现TiCp/TiBw比例的提高导致复合材料强度的降低和延伸率的提高，当其摩尔比>1:1时，由于团聚TiCp与基体界面在弹性阶段的开裂出现反常的明显屈服现象，同时还提出了TiCp和TiBw的选择性断裂判据。研究结果可为基于Ti-B4C体系的高性能复合材料设计与制备提供指导。