β单相凝固TiAl基合金定向凝固及层片取向同步调控

TiAl-based alloys are potentially used as high-temperature structural materials with a high specific strength. The mechanical properties of TiAl-based alloys are extremely anisotropic with respect to the lamellar orientation of the microstructures. A balance combination of room-temperature ductility and strength can be achieved when the lamellar orientation are aligned parallel to the tensile stress direction. Two methods named seeding and alloying are used to align the lamellar orientation by directional solidification for peritectic TiAl-based alloys. β-solidifying γ-TiAl based alloy is a novel TiAl based alloy with excellent mechanical properties, and its microstructures are composed of B2/γ phases and (α2+γ) lamellar structures. However, how to obtain aligned lamellar structures in β-solidifying γ-TiAl based alloys is also lack of in-depth understanding and exploration. Therefore, the aim of this project is to investigate phase selection and lamellar orientation control in β-solidifying γ-TiAl based alloys by using directional solidification technique. The project selects typical β-solidifying γ-TiAl based alloys with excellent mechanical properties as the research object, which intend to study the microstructure adjusting, lamellar orientation control, segregation of elements and the influence of alloying elements on lamellar orientation by directional system solidification experiments and theoretical analysis. The laws between solidification parameters, lamellar orientation and mechanical properties will be established in this project because of the deficiency of clear understanding of the regularity in the microstructure related to the complex solidification path.It can provide theoretical and experimental evidence for control the microstructures and lamellar orientation precisely and implement large scale production of TiAl castings. The results of project research are expected to be of great significance on the preparation of microstructure aligned β-solidifying γ-TiAl based alloys with more excellent mechanical properties by using directional solidification.

TiAl基合金的力学性能强烈依赖于其片层取向，对于传统的包晶两相(α+β)凝固TiAl基合金，通过定向凝固或取向热加工技术可以控制片层取向从而提高其力学性能。对于新型β单相凝固TiAl基合金，近年来发现定向凝固后其室温和高温性能均得到显著提高，而利用定向凝固技术可以同时调控β单相凝固TiAl基合金的凝固组织和晶体取向，对该型合金具有非常重要的意义。因此，本项目选择最具代表性的β单相凝固TNM合金和新型β单相凝固高强TiAl合金为对象，通过系统的定向凝固实验和理论研究，探索β单相凝固TiAl基合金液固相变和固固相变相关性、微观组织和片层取向变化规律、探索定向凝固β单相凝固TiAl合金的相组成、相体积分数和层片取向协同调控机制，严格控制层片取向的同时使其凝固组织直接达到/接近最终服役组织状态，实现合金力学性能的提高和制备流程的简化，为β单相TiAl基合金的研究和应用提供理论和实验依据。

TiAl基合金是一种极具应用前景的中高温结构材料，对于新型β单相凝固TiAl基合金，通过定向凝固技术控制其微观组织可提高其综合力学性能。本项目选取β单相凝固TiAl合金为研究对象，采用Bridgman定向凝固技术与高温激光共聚焦技术进行系统性的研究，探究了β单相凝固TiAl合金定向凝固与层片控制的调控规律。.为改善β单相凝固TiAl基合金Ti–44Al–5Nb–3Cr–1.5Zr的力学性能，制备了Ti–44Al–5Nb–3Cr–1.5Zr–xMo–yB合金分别研究Mo含量和B含量对其性能的影响。研究表明Mo可以提高合金中B2相体积分数并改善合金的高温压缩性能，B可以显著细化组织并提高合金的室温压缩性能。.对Ti-44Al-4Nb-1Mo-0.1B合金进行了热稳定处理实验，为定向凝固初始固液界面准备探究经验。结果显示，其铸态组织是以(α2+γ)片层团为主体，网络状B2相、细小的γ相及硼化物分布于片层团间的近片层组织。热稳定时间越长、TNM合金固/液界面越平整，界面前沿温度场和溶质场趋向均匀，但为减少坩埚对熔体的污染，热稳定处理时间不宜过长，30~60 min的热稳定时间即可提供平整的定向凝固启动界面。 热稳定处理影响着TNM合金固/液界面附近固液两相中Al元素的分布，进而影响硼化物的分布。.采用不同抽拉速率对Ti-44Al-5Nb-1.5Cr-1.5Zr-1Mo-0.1B合金进行定向凝固实验，研究抽拉速率对合金组织与显微硬度的影响。研究结果表明，β相为该合金定向凝固的领先相。随着抽拉速率的增加，固液界面有胞状生长转变为枝状生长。片层间距λs与抽拉速率V服从λs=3.39V -0.31的规律。凝固过程中β稳定元素Nb、Cr在初生β相中富集，导致凝固偏析;而且，随着生长速率的增加，偏析程度增加，在高生长速率下，片层团中出现了B2相。由于组织细化作用，维氏硬度Hv与抽拉速率V符合Hv=328.69V 0.072。.采用高温激光共聚焦技术对β单相凝固的Ti-40Al合金的在不同冷却速率下的β/α转变进行了研究，结果表明在2K/min的冷却速率下，相变的形核方式为界面失稳形核和激发形核并且α相以等轴生长和魏氏组织两种形式生长。而在108K/min的冷却速率下，α相只以界面失稳形核和魏氏组织的形式形核生长。高冷却速率会显著提高魏氏组织的生长速度。

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