压力下新型高铌TiAl合金的相关系及其演变机制研究

The TiAl alloys containing high content of Nb (8-10at.%) have excellent high temperature properties compared with conventional TiAl alloys, and are being the focus of intensive research in TiAl intermetallics. HIP process is particularly important in order to close pores for cast high Nb content TiAl alloys. However, the modest pressures used in HIP process are large enough to influence the proportion of phases and thus the properties of HIPped samples, and its effect on microstructure is usually neglected. It's necessary to identify the detailed influence of pressure in HIP process on microstructure and phase transformation in order to use its advantages to optimize microstructures. The aim of this project is to investigate the microstructural and phase proportion changes as the temperature and pressure varies based on our earlier work, get the thermodynamic and kinetic properties for phase transformations. The effect of alloying element and the as-cast primary microstructure are considered. The diffusion, interface reaction behaviour in high Nb containing TiAl alloys under different pressures and the interphase structures are also to be investigated, then the phase transformation dynamic mechanisms will be made clear. Finally, the relationship between pressure and temperature, alloying content, phase stability will be disclosed. The research results are useful to the alloying design for high Nb containing TiAl alloys and the HIP process control to optimize the microstructure and properties as well as the development of solid state phase transformation theories for TiAl alloys.

高铌含量(8-10at.%)的TiAl合金具有比普通TiAl合金更好的高温性能，为当前TiAl金属间化合物研究领域的重点。热等静压是发展铸造高铌TiAl合金必不可少的工艺，其压力改变了合金常压下的相平衡关系，影响了其力学性能。但是以往的研究忽略了热等静压对材料微观结构的影响。为了充分发挥它的优势，揭示压力对组织及相演变的影响规律是必须要解决的科学问题。本项目拟在前期研究工作的基础上，通过研究常压和热等静压下不同元素合金化和冷却速度的高铌TiAl合金凝固组织随着温度和压力的变化，获得压力下相转变的热力学和动力学规律；并且通过研究不同压力下合金元素的扩散行为及相界面特征，揭示相和组织演变的动力学机制和微观机理；最终揭示压力、温度-成分-相和组织结构相互影响规律。项目的研究结果将为优化高铌TiAl合金成分，调控其组织和力学性能及制定铸件生产工艺提供理论依据，而且可以丰富TiAl合金固态相变理论。

高铌含量(8-10at.%)的TiAl合金具有比普通TiAl合金更好的高温性能，为当前TiAl金属间化合物研究领域的重点。铸态合金一般经历的热等静压不仅能消除气孔、缩松等缺陷，而且有望改变合金的相结构。本项目研究了常压和热等静压下不同元素合金化的高铌TiAl合金的组织和相结构随着压力的变化，发现与同温度相同热处理时间的普通热处理相比热等静压促进了β/B2相转变为α2相和γ相，1280℃/200MPa×4h热等静压后合金中的β相已基本消除，但同时带来了层片组织粗化的负面效应。热等静压造成了γ相的晶格畸变增大。对不同温度不同压力下的高Nb-TiAl合金进行了研究，获得不同温度不同压力下相转变的热力学和动力学规律，较高的温度较高的压力有利于β/B2相分解，缩短了分解的时间。研究了不同压力下α-γ扩散偶两侧合金元素的扩散行为及相界面特征，发现热等静压降低了界面反应厚度，抑制了Al和Ti元素的界面扩散。在两种热处理状态下添加的合金元素均能降低界面两侧Al和Ti 元素的扩散距离；最终揭示了压力、温度-成分-相和组织结构相互影响规律。项目的研究结果为优化高铌TiAl合金成分，调控其组织和力学性能及制定铸件生产工艺提供了理论依据。并根据合金化设计原则，优化出一种综合性能优良的铸造高Nb-TiAl合金，Ti45Al8Nb0.2W0.25Cr0.2B0.02Y(at.%)，并优化了其热加工工艺。