两级层状结构Ti-(TiBw/Ti)复合材料的设计制备与断裂机理研究

In order to solve the bottleneck problem of low toughness and ductility of metal matrix composites, the proposal aims to fabricate laminated titanium matrix composites with two-scale hierarchical structures by low energy ball milling and hot pressing based on the previous work. The two-scale hierarchical structures contain laminated (Ti-TiBw/Ti) and network (TiBw/Ti) structures, so as to achieve “nacre-like multi-scale hierarchical structure” reinforcing and toughening design and optimization. Then hot rolling is employed to decrease layer thickness and control microstructure, and further improve reinforcing and toughening effect. The two-scale hierarchical structure, interfacial characteristics and microstructures evolution during hot pressing and rolling process will be investigated systematically. By tensile, bending and impact test, effects of structural and rolling parameters on mechanical properties of laminated titanium matrix composites with two-scale hierarchical structures are investigated, and the relationship of fabrication parameters, structure parameters and mechanical properties will be clarified. Using in situ tension test and fracture morphology analysis to reveal crack initiation, propagation paths and fracture mechanism of a series of laminated titanium matrix composites with two-scale hierarchical structures, and to clarify the toughening mechanism of two-scale hierarchical structure, interfacial characteristics and microstructures during the deformation and fracture process. Through designing, controlling and optimizing of laminated Ti-(TiBw/Ti) composites with two-scale hierarchical structures, this work will provide experimental as well as theoretical fundamentals on strengthening and toughening of metal matrix composite.

针对金属基复合材料韧塑性低的瓶颈问题，本项目在前期工作的基础上，首先采用低能球磨和真空热压烧结方法制备含有一级层状(Ti-TiBw/Ti)和二级网状(TiBw/Ti)结构的两级层状钛基复合材料，以实现“仿生贝壳多级结构”强韧化设计和调控的目的，然后采用热轧制变形降低层厚和调控微观组织，进一步达到增强和增韧的目的。系统研究制备和热轧过程中两级结构、界面特征和显微组织的演变规律，利用拉伸、弯曲、冲击实验，研究结构和热轧参数对两级层状结构钛基复合材料力学性能的影响规律，建立工艺参数—组织结构—力学性能的关系。利用SEM原位拉伸观察和断口分析，揭示系列两级层状结构钛基复合材料裂纹萌生、扩展路径和断裂机理，并阐明两级结构、界面特征和显微组织在变形和断裂过程中的增韧机理。通过对两级层状Ti-(TiBw/Ti)复合材料结构和组织的设计、调控和优化，为实现金属基复合材料强韧化提供设计思路和理论基础。

传统固溶强化、弥散强化、位错强化以及均匀构型的增强体在提高金属材料强度的同时，总以牺牲塑韧性作为代价。本项目根据贝壳材料多级结构仿生增韧理念，采用低能球磨和真空热压烧结方法制备一种含有一级层状Ti-TiBw/Ti结构和二级网状(TiBw/Ti)结构的两级层状钛基复合材料，以实现“仿生贝壳多级结构”强韧化设计和调控的目的。然后采用热轧制变形降低层厚和调控微观组织，进一步达到增强和增韧的目的。研究表明：采用热压/扩散温度为1200oC，压强为25MPa，真空度为10-2Pa时，可获得综合力学性能优异的层状钛基复合材料，基体组织呈现梯度晶粒结构。系统研究了层厚比、层厚、增强体体积分数对层状Ti-TiBw/Ti复合材料显微组织和力学性能的影响规律，当层厚比为1:1，增强体体积分数为8%，层厚为300微米时，强度为624MPa，断后延伸率为23%。两级层状钛基复合材料在承载和断裂过程中，呈现出多重颈缩，脱层断裂和多处隧道裂纹现象。随烧结温度的升高，层间界面结合强度会逐渐升高，导致横向拉伸性能逐渐升高。由于1200oC烧结后，层状钛基复合材料具有较高的层间结合强度，同时残余热应力适中，因此获得最高的纵向拉伸强度和断裂延伸率，而1300oC烧结后的层状钛基复合材料，由于残余热应力过大，导致纵向拉伸性能最低。1100oC烧结后的层状钛基复合材料，由于具有较弱的层间界面结合强度，在弯曲过程中极易发生脱层断裂，这样吸收了大量的裂纹扩展能，从而获得最高的弯曲韧性。两级结构钛基复合材料的冲击韧性为单一网状结构的5倍，呈现出优异的强韧化效果。随着热轧温度的降低，TiB晶须断裂越来越严重，且层状钛基复合材料出现裂纹的趋势越来越严重，抗拉强度随时效温度升高而降低，断后延伸率则随之升高。该项目可为金属基复合材料强韧化提供设计思路、理论基础和实验支撑。