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

由α_2Ti_3Al相和γTiAl相组成的片层TiAl合金是一种很有前途的高温结构材料。然而，它们的片层边界不太稳定，在高温变形过程中迁移，导致转变为等轴晶组织(球化)。等轴晶组织的蠕变抗力不如片层组织。在本研究中，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：在较低温度下形成的细片状组织球化更明显，细片状材料的蠕变抗力往往比粗片状材料差。当片层组织通过退火稳定时，边界迁移被延缓，细片状材料表现出细片状组织固有的良好的蠕变抗力。合金稳定化：在TiAl合金中添加W可以减缓蠕变过程中的片层迁移。这使得添加W的材料的抗蠕变性能比不添加W的材料更好。边界性质的控制：层状结构包含α_2/γ、真孪生、假孪生和旋转断层四种边界。它们在高温暴露下的稳定性依次为：α_2/γ&gt；真孪晶&gt；假孪晶&gt；旋转断层。伪孪生和旋转断层边界容易迁移并形成等轴晶结构。α_2相含量较高的低Al合金具有较高的α_2/γ界面密度。随退火温度升高，α_2/γ和真孪晶界密度增加。这些都是获得稳定的片层组织和提高全片层TiAl合金蠕变抗力的方法。

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.