Quantitative Analysis of Strengthening Mechanism of Titanium Aluminide by Means of Microstructural Stabilization

由α_2Ti_3Al和γTiAl相组成的层状TiAl合金是一种很有前途的高温结构材料。但是，它们的片层边界并不稳定，在高温变形过程中发生迁移，导致向等轴晶组织转变（球化）。等轴晶粒组织的抗蠕变性比层状组织差。本文研究了如何稳定片层结构并保持其细小的片层间距。主要研究结果如下：1.热处理稳定化：球化在较低温度下形成的细层状结构中发生得更显著，并且细层状材料通常显示出比粗层状材料差的抗蠕变性。当片层结构通过退火稳定时，边界迁移被阻滞，并且细片层材料显示出细片层结构固有的良好抗蠕变性。2.合金化稳定化：在TiAl合金中添加W减缓了蠕变变形过程中的层状迁移。这导致添加W的材料的抗蠕变性优于不添加W的材料。边界特征控制：片层结构中有α_2/γ、真孪晶、假孪晶和旋转断层四种边界。它们在高温下的稳定性顺序为：α_2/γ >真孪晶>假孪晶>旋转断层。假孪晶和旋转断层边界容易迁移，并形成等轴晶粒结构。α_2相含量高的低Al合金具有较高的α_2/γ晶界密度。随着退火温度的升高，α_2/γ晶界和真孪晶晶界密度增加。这些都是制备稳定的层片组织和提高全层片TiAl合金抗蠕变性能的途径。

Lamellar TiAl alloys consisting of α_2Ti_3Al and γTiAl phases are promising candidates of structural materials for high temperature applications. However, their lamellar boundaries are not so stable, and migrate during high temperature deformation, resulting in the transformation to equiaxed grain structure (spheroidization). The equiaxed grain structure is inferior in creep resistance to the lamellar structure. In this research it was studied how to stabilize the lamellar structure and to keep its fine lamellar spacing. The results are summarized as follow:1. Stabilization by heat treatment: The spheroidization takes place more significantly in a fine lamellar structure formed at lower temperature, and the fine lamellar material often shows poorer creep resistance than a coarse lamellar material. When the lamellar structures are stabilized by annealing, the boundary migration is retarded and the fine lamellar material shows the good creep resistance inherent to the fine lamellar structure.2. Stabilization by alloying: Addition of W to TiAl alloys slows down the lamellar migration during creep deformation. This result in the creep resistance of the W added material better than the one without W.3. Control of boundary character: The lamellar structure contains four types of boundaries: α_2/γ, true twin, pseudo twin and rotational fault boundaries. Their stability during high temperature exposure decreases in the following order: α_2/γ > true twin > pseudo twin > rotational fault. The pseudo twin and rotational fault boundaries readily migrate and form an equiaxed grain structure. A low Al alloy containing a high fraction of α_2 phase has a higher density of α_2/γ boundaries. The densities of α_2/γ and true twin boundaries increase with rising annealing temperature. These are ways to make a stable lamellar structure and improve creep resistance of fully lamellar TiAl alloys.