定向凝固TiAl合金非择优柱状晶连续平直生长控制及其生长特征

The columnar crystals obtained in directionally solidified TiAl alloy are the advanced structure to achieve the aeroengine blades, and further the columnar crystals with close-packed hexagonal structure are controlled to grow along the <112 ̅0> direction, which can promote the blades owning the optimal strengthen and plastic. However, the non-preferred growth is thermodynamics instability, which can easily lead to the interruption of the continued growth of the columnar crystal and cause the impurity crystals and the tilting growth defect. Continued and parallel growth controlling of the non-preferred columnar grains has been the advanced technology for the high-quality blade structure. .In this study the self-seeding method is used for the crystal orientation controlling, and the continued growth of the non-preferred columnar crystals is obtained through improving the initial interface quality and the growth stability to eliminate the block of the impurity crystals. Furthermore the precipitation of the impurity crystal and the competitive growth are also investigated, to obtain the kinetics conditions of the non-preferred growth and the technique prototype. Simultaneously the relationship of the growth direction, preferred orientation and the heat flow direction with the effect of the solidification conditions is investigated, and the growth morphology and the growth mechanism of the non-preferred crystal are clarified, and finally the non-preferred growth direction consisted with the heat flow direction is achived. The studies above can supply the theory and the experimental foundation for the continued and parallel growth controlling of the non-preferred columnar crystals, which is expected to achive the optimal mechanical property of the TiAl blades structure.

定向凝固TiAl合金获得柱状晶组织是制备航空发动机叶片的先进手段，进一步控制密排六方结构的柱状晶沿非择优<11-20>取向平直生长，可以使叶片具有最佳的强度和塑性。然而非择优生长存在不稳定性，易导致柱状晶连续生长中断并引起杂晶、斜晶等缺陷。控制非择优柱状晶连续平直生长已成为高性能叶片材料制备的关键技术。.本项目应用自引晶法进行TiAl合金晶体取向控制，通过改善引晶界面质量以及提高生长稳定性，消除杂晶析出的阻碍作用，实现非择优柱状晶的连续生长。并阐明该过程中杂晶析出条件以及晶粒间竞争生长机制，获得非择优“强制”稳定生长的动力学条件和制备技术原型；同时揭示晶体生长方向、择优取向和热流方向之间及其与凝固条件的耦合关系，阐明非择优晶粒生长形态及其生长机制，实现非择优生长与热流方向的一致性。以上研究可为非择优柱状晶连续平直生长控制提供理论和技术指导，有望制备出具有最优力学性能的TiAl合金叶片材料。

定向凝固TiAl合金获得柱状晶组织是制备航空发动机叶片的先进手段，进一步控制密排六方结构的柱状晶沿非择优<11-20>取向平直生长，可以使叶片具有最佳的强度和塑性。然而非择优生长存在不稳定性，易导致柱状晶连续生长中断并引起杂晶、斜晶等缺陷。因此，研究定向凝固非择优柱状晶连续平直生长控制及非择优生长特征对高性能叶片材料制备具有现实意义。.本项目应用自引晶法进行TiAl合金晶体取向控制，通过优化合金成分设计、改善引晶界面质量以及工艺条件提高非择优生长稳定性。首先，确定了合金成分Al、Nb含量对初生β相和初生α相凝固过程以及层片组织取向的影响，高Nb的添加促进了β相凝固过程以及硬脆B2相的析出，Al含量的增加有利于具有完全α相的凝固过程。进一步采用高Al低Nb的Ti-50Al-(4~6)Nb合金设计可以实现初生α相的凝固过程、溶质均匀分布并获得全部α2/γ层片组织。通过自引晶法进行层片取向控制，利用EBSD反极图和多向切割的方法测量领先α相生长取向和层片取向，在合适的生长速度(5μm/s≤V≤15μm/s)区间，柱状晶沿着热流方向具有<101 ̅0>、<224 ̅1>等的晶体取向，非择优晶粒能够稳定遗传并连续生长，保证获得平行的层片组织。同时，非择优α相受界面过冷度、界面各向异性和凝固动力学参数的影响具有复杂生长形态，固液界面处非择优柱状晶沿着热流方向发生倾斜，并随生长速度的增加倾斜角度增大。倾斜枝晶的前侧面表现为高度发达的二次以及三次枝晶，三次枝晶的高度发展使其与一次枝晶间保持相同生长速度，促进倾斜柱状晶连续生长。非择优晶粒的连续生长体现为晶粒间的竞争生长，在低速时凝固界面夹杂容易引起重新形核，最终阻碍非择优晶粒遗传到定向凝固区。在较高的生长速度下，非择优柱状晶发生严重的倾斜生长，倾斜枝晶以及前侧面高的过冷度有利于择优晶粒的析出，择优晶粒将会迅速长大并阻碍非择优晶粒生长，导致最终出现垂直层片组织。上述工作获得了非择优柱状晶连续平直生长控制的技术原型，可为高性能TiAl合金叶片材料的制备提供理论和技术指导。