基于动力学多元扩散结开发的近β钛合金等温ω相变研究

Near β Ti alloys are finding widespread application particularly in aerospace as large structural forging components thanks to their advantages of deeper hardenability, super high strength, and adjustable microstructure and mechanical properties, thus becoming an important development direction of high strength Ti alloys. The presence of the ω phase was found to provide the preferential nucleation sites that triggers the formation of the secondary nano-size α phase and subsequently improve the strength of near β Ti alloys. However, the coupling influence of elements on isothermal ω phase transformation in engineering near β Ti alloys is still not well understood largely due to the complexity arising from multicomponent alloying elements. Therefore, the developing of microstructure controlling technique and the designing of new alloys are seriously limited. In this project, the coupling influence of elements on diffusion pathway and isothermal ω phase transformation in engineering near β Ti alloys is systematically investigated based on the development of kinetic Diffusion Multiple (DM) technique and the application of "high-spatial-resolution" microanalysis methods. In addition, the DM will ultimately enable kinetic information to be extracted and composition-kinetics libraries to be generated in a combinatorial fashion. Based on the combinatorial kinetic libraries, a quantitative phase field modeling for isothermal ω phase transformation will be developed. And then the coupling effect of composition field, elastic stress field and interfacial energy on the morphology, size and distribution of the ω particles is studied by using the modeling. Besides, the fundamental understanding of the interrelation and trend that coordinate the kinetics of isothermal ω phase transformation over the arrays of alloy compositions and processing parameters can be gained. Our project has significant potential to develop a novel infrastructure for microstructure controlling technique of near β Ti alloy.

近β钛合金因其具有高淬透性、超高强度以及宽泛的组织性能可调控性，成为高强钛合金的重要发展方向，在航空航天大型结构锻件及高强紧固件上应用广泛。而ω相辅助次生纳米α相形核是近β钛合金重要的强化机制，但是，对于多组元近β钛合金，由于多元素对等温ω相变的耦合作用机制尚不明确，严重制约了微观组织控制技术的发展以及新型合金的开发。本项目通过发展动力学多元扩散结技术，结合"高空间分辨率"测试手段，系统研究合金元素在β基体中的扩散路径及其耦合作用对等温ω相变机制的影响，并建立复合动力学数据库。基于该数据平台，发展描述近β钛合金中等温ω相变过程的定量相场模型，研究成分场、弹性力场以及界面能的耦合作用对ω相形貌演化、尺寸及分布规律的影响，揭示合金成分与热处理工艺对等温ω相变动力学的影响规律，为发展近β钛合金的组织控制技术奠定基础。

以Ti5553、Ti1023、TC18、Ti7333等钛合金为代表的近β型钛合金是高强钛合金的重要发展方向，但是较高的成分敏感性与工艺敏感性导致该类合金的成分设计与组织控制较为困难。本项目提出采用动力学多元扩散结的方法研究近β钛合金中的ω相变与α相变，为成分优化、工艺设计以及组织控制提供依据。首先，根据近β钛合金的成分特征设计并制备了四元与五元扩散结，通过高温固溶处理成功获得了较宽的成分梯度界面，结合低温时效工艺，使得等温ω相与纳米尺寸α相在扩散界面连续析出，为研究合金元素对相变的影响奠定了基础；随后，将EPMA局域成分检测技术、EBSD分析技术与纳米压痕技术相结合，定量研究了成分-组织-力学性能关系，并且结合DICTRA分析软件，建立了Ti-Al-Cr-Mo-V-Fe系近β钛合金扩散移动性数据库；其次，将TEM分析技术与FIB技术相结合，研究了Al、Cr、Mo、V等合金成分对ω相特征的影响规律，获得了发生ω相变的成分范围；最后，建立了描述近β钛合金中ω相变的相场模型，基于此分析了工艺参数对ω相变动力学及形貌的影响，此外，初步探讨了多元多相定量相场模型的建立方法，为发展工程用钛合金定量组织模拟技术奠定了基础。本项目的研究结果可直接用于近β钛合金的成分设计与热处理工艺优化，发挥ω相辅助纳米α相形核的作用，为进一步发掘钛合金强度的提升潜力做出贡献。此外，本项目涉及到复杂的扩散连接技术与组织控制技术，意外的为近β钛合金的扩散连接与组织控制提供了新的思路，基于此，申请与TC18钛合金扩散连接技术相关的发明专利两项，已授权1项。项目研究过程中共发表学术论文6篇（其中SCI收录5篇），培养研究生4名（已毕业3名），利用本项目思路孵化了重点研发计划、自然科学基金以及航空科学基金等省部级以上项目3项。