多元多尺寸增强体对钛合金强塑形变形的作用机理

It is of vital importance to research and develop a kind of titanium alloys with excellent microstructure stability as well as mechanical properties, which can provide theories and techniques references to satisfy the demands in aviation and space fields. Based on the research results of our group on the in situ synthesized titanium matrix composites, equal channel angular pressing (ECAP) is put forward to obtain matrix structure with fine grain. The microstructure of titanium matrix composites is optimized, the mechanical properties are improved. The titanium matrix composites reinforced with multi-phase and multi-scale reinforcements are fabricated by in situ technique. Then they are processed by hot-forging and ECAP. Based on the theory of ECAP severe plastic process in obtaining the ultrafine grain microstructure, the key fundamental problem researches will be carried out on the microstructure transformation and mechanical properties of titanium matrix composites reinforced with TiB and La2O3. During the processing of ECAP of titanium matrix composites, both ultrafine grain and the required reinforcement with excellent morphology, distribution and size can be obtained, which can improve the performance of titanium matrix composites largely. The systematically studies focus on the reinforcements' grain refinement mechanism, microstructure evolution characteristics of both reinforcements and matrix, as well as the thermal stability of deformed microstructure. In addition, the effect of the reinforcements with different dimension and distribution on ultra-fine grain microstructure deformation mechanism is studied and the relationship between the microstructure and mechanical properties is also carried in this project. It will provide a viable idea in preparing the ultrafine grain titanium matrix composites at low cost. Meanwhile, it will improve the understanding on the theory of ECAP severe plastic deformation and develop the utilization of severe plastic deformation on the metal matrix composites. It also will offer certain theoretical basis on design and preparation of multiphase metal matrix composites.

研制和开发具有良好综合性能的钛合金对满足航天航空领域的迫切需求具有重要的科学意义和实用价值。本项目基于课题组多年原位自生钛基复合材料的研究，希冀利用等径弯曲通道变形（ECAP）获得具有超细晶的基体组织，进一步改善钛基复合材料组织特征，进而提高钛基复合材料的综合性能。利用原位自生工艺制备TiB和La2O3混杂增强的钛基复合材料，经热锻和多道次弯角挤压制备材料。系统研究多元多尺寸原位自生增强体对ECAP变形的影响规律及其对钛基复合材料组织演变的作用机制，优化原位自生钛基复合材料弯角挤压的制备工艺。阐明多元增强体对晶粒细化及超细晶组织稳定性的影响机制，建立超细晶钛基复合材料的微结构和性能之间的内在关系，为简捷、低成本制备超细晶钛基复合材料提供新的方法。也有利于促进金属材料大塑性变形基础理论的了解，拓展大塑性变形在金属基复合材料中的应用，为多相复合强化金属材料的设计、制备提供指导。

本研究为制备出具有优异性能的超细晶钛基复合材料，成功将等径弯曲通道剧烈塑性变形（ECAP）技术分别应用于(TiB+TiC)/Ti-6Al-4V和(TiB+La2O3)/Ti-6Al-4V多元多尺度混杂增强钛基复合材料，制备获得了具有超细晶结构的钛基复合材料。结果表明经高温剧烈塑性变形，TiB晶须和TiC颗粒等多尺度增强体细化，分布更加均匀，热处理前后增强体形态稳定，与基体之间界面清晰没有界面反应，结合良好。挤压温度对复合材料组织影响显著，经800°C和900°C时，形成大量具有清晰晶界的新生超细晶粒（100-500nm），基体片层平均厚度减小到不足1μm，细化效果尤为明显。TiB晶须的平均长径比随着ECAP温度的升高而减小，在径向和轴向都出现了断裂和开裂；但其平均长径比仍大于临界长径比2.7，依然能够起到有效承载作用。结果也表明挤压道次和温度的影响作用明显，其首道次晶粒细化机制为位错的滑移和孪生交互作用；2~3道次为动态回复机制，平均晶粒尺寸急剧减小，形成典型变形亚晶组织；4道次为晶界转动机制，增强体更加细小而分布均匀，晶界角度逐渐增大，最终形成200nm大小的等轴状具有大角度晶界的超细晶显微组织。而温度较低时位错堆积和缠结占细化主导作用，ECAP温度的升高促进其动态再结晶和新晶粒的形核，最终生成大量新生超细晶粒。