受控冷却下β型γ-TiAl合金B2相演变行为及热稳定性研究

In the field of aerospace, β-solidifying TiAl alloy is one of the most potential lightweight high-temperature structural materials, and the β/B2 phase in alloy exhibits excellent deformability. However, the ordered B2 phase would deteriorate the room-temperature ductility and reduce the performance stability after decomposing at evaluated temperature. Therefore, the close control of B2 phase is required to improve the mechanical properties of β-solidifying TiAl alloy. In the past, the research on removing the B2 phase is limited to hot isostatic pressing or heat treatment, which neglects the significant influence of cooling process on the formation of B2 phase. Also, the role of original microstructure on the thermal stability of B2 phase has not been well understood, which is a severe constraint to the development of β-solidifying TiAl alloy. For this reason, based on the control of cooling rate and isothermal stage after solidification (controlled cooling), the current subject will clarify the evolution behavior of B2 phase in β-solidifying TiAl alloy and establish the adjusting diagram of B2 phase under cooling control, which can be accomplished by the means of microstructure analysis and thermodynamics/ kinetics calculation. Further, the rule and mechanism of B2 evolution will be revealed combining with the influence of original microstructure. Also, the interrelation between the original microstructure and the thermal stability will be clarified. Therefore, the results of the subject can provide an important guide for the engineering of β-solidifying TiAl alloy.

在航空航天领域，β型γ-TiAl合金是目前最具发展潜力的一种轻质高温结构材料。合金中β/B2相具有良好的热变形能力，但有序B2相会恶化合金的室温塑性，经高温分解后会降低合金力学性能的稳定性。因此，需要对B2相进行精确调控以改善材料性能。但以往研究局限于热等静压/热处理来消除B2相，而忽略了初始冷却过程对B2相形成的重要影响，并缺乏初始组织状态对B2相热稳定性影响的认识，由此严重制约了β型γ-TiAl合金的发展。对此，本项目基于β型γ-TiAl合金凝固后冷速和恒温过程的控制（受控冷却），通过初始组织特征和热/动力学分析，阐明B2相的演变行为，进而建立受控冷却下B2相特征的调控选择图。在此基础上，考虑初始组织状态的影响，分析并揭示B2相在高温时效过程中分解、转变的规律及机制，明确初始组织和B2相热稳性之间的内在关联。因此，本项目的研究结果对推动β型γ-TiAl合金工程化具有重要指导意义。

本项目选用Ti-45Al-8Nb-(W, B, Y)和Ti-44Al-4Nb-2Cr-0.1B两种典型β型TiAl合金成分，基于控温加热-凝固-冷却技术，系统深研究了β型TiAl合金从熔体到室温阶段、在不同冷却参数下的组织演化和相变过程，并重点考察分析了β/B2相在冷却和高温时效过程中的形成、分解、转变规律和机制。结果表明，糊状区的控温凝固可以显著地影响后续固态相变，β单相区恒温有利于成分均匀化并伴有晶粒长大；随冷速增加，从母相β相析出的α相生成滞后、细化、具有Burgers关系、从等轴发展成细条状。随后，根据组织演化和相变情况建立了高Nb-TiAl合金的CCT和TTT曲线。进一步，发现分析了不同组织状态下β/B2相在高温时效过程中的粗化、退化现象和规律，并通过STEM揭示了β/B2、ω、γ相之间相互转变的位相关系和结构转变机制。最后，探索了β型TiAl合金在高温拉伸、蠕变和高压三种载荷作用下β/B2相的热稳定规律和行为。综上，本项目完成了全部的计划书内容，阐明了β型TiAl合金中B2相在受控冷却过程中的形成规律及机制，并揭示了初始组织状态和B2相热稳定性的内在关联。