受控凝固条件下高Nb-TiAl合金的显微组织演化及力学相关性

High Nb-containing TiAl alloys are certainly among the most promising candidates to fulfill the required thermal and mechanical specifications. Although Nb reduces the stacking fault energy, retards diffusion process and modifies the structure of the oxidation layer, Nb addition also inherent to the solubility difference between the liquid and the solid phases, together with non-equilibrium solidification conditions, the microsegregation (i.e. chemical heterogeneities at the scale of the microstructure) cannot be avoided during solidification process. The microsegregation will result in increasing the degree of the inhomogeneity and the cracks tendency, worsen the mechanical properties and the subsequent processability. Therefore, the research project propose a new method named isothermal heat treatment (IHT) during solidification to suppress the formation of microsegregation and control the solidification paths of high Nb-containing TiAl alloys. Firstly, a thorough understanding of the way segregation occurs during solidification is investigated based on clarifying the correlation among the cooling rate, solidification path, phase content and microstructure of high Nb-containing TiAl alloys. Secondly, the microstructure evolution including texture characteristics under controlled solidification process are studied, which is promising to propose the solidification technology to eliminate microsegregation and obtain the uniform and fine fully lamellar microstructure. Thirdly, the correlation between microstructure and mechanical behavior for high Nb-containing TiAl alloys is definitely revealed. The research work is very important and valuable not only for further understanding the phase transformation and the microstructure evolution, but also for developing an advanced solidification technology and further enhance the reliability of the processing route of high Nb-containing TiAl alloys.

高Nb-TiAl是当前TiAl合金的发展方向，但由于beta稳定化元素的添加，凝固过程中液相和固相间溶质浓度差异大，再加上非平衡凝固条件，使其凝固过程中不可避免地发生微观偏析，导致组织不均匀程度和开裂倾向增大，材料力学性能和后续加工性能恶化。本项目从抑制微观偏析形成、控制凝固路径的角度出发，在探讨高Nb-TiAl合金冷却速率-凝固路径-相含量-显微组织相关性的基础上，采用受控凝固方法系统研究凝固过程中恒温处理对微观偏析、组织均匀性、片层微结构的影响，提出控制高Nb-TiAl合金凝固过程、消除微观偏析、获得均匀细小片层铸态组织的工艺方法，并进而揭示显微组织与力学行为的相关性。本项目研究不仅对进一步深入理解高Nb-TiAl合金相变机制、阐明凝固过程中的组织演变规律具有重要意义，而且对发展高Nb-TiAl合金凝固组织调控方法、提高全流程加工的可靠性、促进其应用也具有一定的指导作用。

高Nb合金化是提高TiAl合金服役温度的重要途径，但Nb的加入使得合金在凝固过程中不可避免地产生微观偏析，导致组织不均匀和开裂倾向增大，材料力学性能和后续加工性能恶化。本项目以Ti-45Al-8.5Nb-(W,B,Y)合金为主要研究对象，系统研究了高铌TiAl合金凝固过程中的组织演化行为，揭示了冷却速率、冷却过程恒温处理对显微偏析、组织均匀性和片层结构特征的影响规律，阐明了相的析出机制，提出了β单相区短时保温和略高于Tα+β的保温两步恒温处理消除微观偏析、获得均匀细小且取向随机片层结构的工艺方法。系统研究了高Nb-TiAl合金的开裂行为及其与组织特征的关系，阐明了该类合金低温脆性断裂和高温韧性断裂过程中裂纹萌生、裂纹扩展和孔洞演化规律及其与相组成和微观组织的关系。. 国际合作方面，与法国洛林大学(Université de Lorraine)LEM3 实验室在TiAl合金显微结构表征和相变机制研究方面建立了深入的合作关系，双方联合培养博 士生1名（强凤鸣，在读）。. 研究成果在Acta Materialia、Journal of Alloys and Compounds、Intermetallics、Materials Science and Engineering A、Materials Characterization、Materials Letters等刊物发表SCI学术论文18篇。培养博士研究生 4 名（毕业3名，在读1名），硕士研究生4名（毕业2名，在读2名）。参加国内外学术会议14人次，做邀请报告5人次，口头报告8人次，墙报展出1次。