β型γ-TiAl合金β→α相变行为及其与片层结构相关性研究

β-solidifying TiAl alloys regarded as the most promising candidates to fulfill the required thermal and mechanical specifications have attracted significant attentions, and their mechanical properties is affected by the lamellar structures. The microstructural characteristics of lamellar structures are determined by the α phase which originate from the β→α solid transformation. Therefore, it is of great importance to investigate the β→α transformation behavior and its relationship with the lamellar structures. In this research project，an in-situ observation by high-temperature laser-scanning confocal microscopy (HTLSCM) combined with controlled heat treatment-quenching method will be utilized to investigate the β→α transformation. Firstly, the nucleation and growth behavior of α phase is studied by the in-situ observation under different conditions, i.e., cooling rates and boron content, and according to the data recorded by HTLSCM, the CCT curves can be established. Secondly, α phase can be retained by the quenching experiments and the microstructural characteristics of α phase, the orientation relationships among boride, β phase and α phase will be investigated. Based on the above experimental results, the nucleation and growth mechanism of α phase, the variational selection effect and the kinetics of the β→α transformation will be analyzed and discussed. Thirdly, the correlation between α phase and lamellar structures is definitely revealed according to the low temperature aging results. The research work is very important and valuable not only for further understanding the microstructure evolution, but also supply a theoretical basis for optimizing the lamellar structures and mechanical properties of β-solidifying TiAl alloys.

β→α固态相变作为β型γ-TiAl合金的重要相变之一，决定了α相的结构特征，从而引起（α2+γ）片层结构的变化。但该相变发生于高温阶段、合金后续固态相变复杂、相变序列对成分敏感，给β型γ-TiAl合金β→α相变行为的研究造成了困难，使得α相及片层结构的调控缺乏深入的理论指导。本项目采用高温激光共聚焦显微镜与可控热处理-淬火相结合的方法对β→α相变进行原位研究，研究冷却速率、硼含量对α相形核生长方式的影响，分析硼化物与β相以及α相之间的位相关系，阐明不同条件下α相的形核生长机制及变体选择行为，揭示β→α相变动力学机制，建立连续冷却转变曲线；在精确控制α相微观结构的基础上，探索具有不同初始组织特征的α相与片层结构的相关性，建立片层结构调控的新方法。本研究不仅对于深入理解β型γ-TiAl合金α相演化规律具有理论意义，而且对实现该类合金的组织-性能调控、促进其应用也具有重要的工程价值。

本项目采用高温激光共聚焦显微镜与可控热处理-淬火相结合的方法对β型γ-TiAl合金β→α相变行为进行了原位研究；在此基础上，研究了后续退火过程中γ相的析出规律。主要结论如下：(1) 分析了Ti-45Al-8Nb-(0.5, 1)B、Ti-40Al-8Nb和Ti-40Al-10V合金在β单相区淬火后的显微组织。随着硼含量的增加，Ti-45Al-8NB-xB合金中的等轴状的非共格α相含量增加，板条状的共格α相含量降低；Nb元素的β稳定能力强于V元素，Ti-40Al-8Nb合金淬火后得到了一种新的全B2相亚稳组织，Ti-40Al-10V合金淬火后获得了(B2+α2+γ)显微组织，其中α相存在12种变体，母相β遵循Burgers位向关系，γ相存在24种变体，与母相遵循Kurdjumov-Sachs位向关系。(2) 采用高温激光共聚焦显微镜对Ti-45Al-8Nb-0.5B合金的 β→α相变进行了原位观察，发现随着冷速的降低，合金中非共格α相含量增加；建立了β→α相变的CCT曲线；晶格错配度计算结果表明由硼化物上形核的非共格α相的形核能量较低，并利用共格α相和非共格α相的竞争生长机制进行了解释。(3) 在800℃和1000℃对Ti-45Al-8Nb-0.5B、Ti-40Al-8Nb和Ti-40Al-10V合金的淬火组织进行了低温时效。Ti-45Al-8Nb-0.5B合金800℃时效5小时后获得了均匀细小且取向随机的近片层结构；由于亚稳B2相极大的过冷度及多重相变作用，Ti-40Al-8Nb在800℃和1000℃回火后分别得到了亚微米尺度的(ω0+γ+B2)及晶粒尺寸为2μm的(α2+γ+B2)细晶组织；Ti-40Al-10V合金在1000℃回火时，α2相发生了分解，获得了亚微米尺寸的(B2+γ)组织。