【作者】 张成军；

【导师】 傅恒志； 徐达鸣；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2008， 博士

【摘要】 以γ-TiAl金属间化合物为基的合金由于具有低密度、高弹性模量、优异的高温强度和抗氧化性,是一种极具潜力并且人们寄予厚望的高温结构材料。特别是由γ相和少量α₂相所形成的全片层组织材料,显示出良好的室温与高温综合性能。由于这种合金的各向异性,当片层取向合适时,使其更适合于航空发动机叶片等只承受一维载荷的场合。如何获得按一定方向排列的片层组织,是多年来许多专家学者所致力研究的课题。采用籽晶法定向凝固来控制片层组织的排列方向是一种比较成功的方法。本文首先根据片层组织力学性能的各向异性的特点建立了片层最大承载方向的数学模型,导出了不同组织状态下片层的最大承载方向。得出了当γ+α₂片层组织中α₂相的折算分数大于30%时,其最大承载方向为平行于片层组织方向的结论。这为指导本文进行γ-TiAl基合金的定向凝固实验研究,以获得平行于生长方向的片层组织提供了理论依据。本文提出了利用冷坩埚制备籽晶的新方法。由于冷坩埚的激冷能力强,可以制备出尺寸较大的籽晶。同时由于定向凝固实验与与籽晶制备使用同一个冷坩埚,这就为原位籽晶定向凝固新技术创造了必要的条件。采用刚玉管作铸型进行了籽晶法定向凝固实验,对定向凝固片层组织的取向进行了控制,制备出了片层组织取向与籽晶原始取向一致并平行于定向凝固方向的试样。经显微组织观察,片层组织能在整个定向凝固区间内保持与定向凝固方向平行生长长度达50mm的片层组织。成功地利用刚玉管籽晶定向凝固法实现了γ-TiAl基合金片层组织取向的控制。在用冷坩埚进行定向凝固中,发现其晶体生长机理与现有的籽晶法定向凝固完全不同。由于初始成分过渡区的存在,有可能使籽晶完全失去作用。为此,本文提出了消除定向凝固初始成分过渡区的方法——成分调整法。经过多次实验研究和成分组织分析,解决了消除初始成分过渡区的技术难点,从而消除了初始成分过渡区。实现了定向凝固从一开始就可以进入稳定生长状态,解决了开始部位的组织、成分与稳定生长组织、成分不一致的难题。结合成分调整技术,采用本文提出的冷坩埚原位籽晶定向凝固法,对Ti-46at%Al-1.5at%Mo-1.2at%Si合金片层组织的取向控制进行了实验研究。将制备的籽晶直接用于定向凝固实验,集籽晶制备与定向凝固片层组织控制于一体。使原来由籽晶制备、籽晶加工和定向凝固三步构成的工艺,合并成一步进行,从而显著简化了整个工艺过程,并省去了传统制备籽晶工艺中的籽晶切取过程,避免了由此带来的污染,这会给以后的实际生产应用带来很大的经济效益。实验结果表明,采用冷坩埚原位籽晶定向凝固法必须进行成分调整以消除初始成分过渡区;经成分调整后,籽晶中的片层组织能保持其取向顺利通过重熔界面进入定向凝固区。定向凝固区内平行于生长方向的片层组织能够保持其取向生长至少20mm长度以上。从而表明了采用冷坩埚原位籽晶定向凝固法来控制γ+α₂片层组织的取向是完全可行的。更多还原

【Abstract】 γ-TiAl based intermetallics alloys are of many advantages such as low density, high young’s modulus, and excellent elevated temperature strength and oxidation resistance. Therefore, they are very promising materials for elevated temperature construction material. Especially those materials with full lamellar structure consisting ofγ-TiAl with a little amount ofα₂ phase indicate fine synthetical mechanical properties both at room temperature and at high temperature. Because of its aeolotropy, lamellar structure is much more appropriate in such applications as aeroengine blades that mainly bear one-dimensional loads. Therefore, many researchers have paid much attention to getting full lamellar structure with designed orientation. Among the significant results, the combination of seeding and directional solidification （DS） is a successful method to get full lamellar structure with a designed orientation.In this paper, a mathematical model was first established to calculate the maximum load bearing direction of lamellar structures with different lamellar orientations. The results suggested that the maximum load bearing direction of aγ+α₂ lamellar structure is parallel with the lamellar orientation when the equivalent volume fraction ofα₂ is more than 30% which is relatively easy to meet for mostγ+α₂-based alloys. This provides a basic theory for lamellar orientation control: the best lamellar orientation is parallel with the growth direction of DS.Experiments using DS technique with seeding in ceramic tube were carried out to control the lamellar orientation. The observation results showed that the lamellar orientation of the obtained DS structure is parallel with the growth direction and keeps that of the seed throughout the DS zone （about 50mm）. The structure control using DS technique with seed in ceramic tube is successful.An initial concentration transient zone （CTZ） may appear in a cold crucible DS process with seeding. The existence of CTZ can invalidate the effect of the seed. A composition adjustment （CA） method was proposed in the present work to eliminate the CTZ. The problems in CA method were solved in this paper. The experimental results showed that CTZ can be eliminated using the CA method. With the CA method, a stable growth may be established immediately at the outset of the DS, so that the lamellar structure may grow from the seed to the directionally solidified zone smoothly. The problem that the structure and composition in the initial part of DS defer from those in the stable growth part was solved successfully.Based on the proposed CA method, experiments using cold crucible DS technique with seeding were carried out to investigate the feasibility to control the lamellar orientation. An in situ seeding DS, in which the seed made in the experiment was used directly in the cold crucible DS, was proposed and employed. This method combined cold crucible DS with seed making, which significantly simplified the traditional technique consisting of seed making, seed cutting, seed orientation positioning and DS process. This may be beneficial to the application in production. The results indicate that the CA method should be used to eliminate the CTZ in cold crucible DS with seed; with the CA method, the lamellar structure in the seed can grow into the DS zone and keep its original orientation. The lamellar structure parallel to the growth direction in the DS zone may grow continuously longer than 20mm. Therefore, the lamellar orientation control using cold crucible DS technique with seeding is shown to be feasible.更多还原