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合金是高温应用结构材料的有希望的候选者。然而，它们的层状边界不太稳定，并且在高温变形过程中发生迁移，导致转变为等轴晶结构（球化）。等轴晶组织的抗蠕变性比层状组织差。在这项研究中，研究了如何稳定层状结构并保持其精细的层状间距。研究结果如下： 1．热处理稳定化：在较低温度下形成的细层状结构中，球化发生得更显着，并且细层状材料通常表现出比粗层状材料更差的抗蠕变性。当层状结构通过退火稳定后，边界迁移被延迟，细层状材料表现出细层状结构固有的良好的抗蠕变性。2．合金化稳定化：在 TiAl 合金中添加 W 可以减缓蠕变变形过程中的层状迁移。这导致添加W的材料的抗蠕变性比不添加W的材料更好。3。边界特征控制：层状结构包含四种类型的边界：α_2/γ、真孪晶、伪孪晶和旋转断层边界。它们在高温暴露期间的稳定性按以下顺序降低：α_2/γ>真孪晶>伪孪晶>旋转故障。假孪晶和旋转断层边界很容易迁移并形成等轴晶粒结构。含有高比例α_2相的低Al合金具有较高密度的α_2/γ边界。 α_2/γ和真孪晶界的密度随着退火温度的升高而增加。这些是形成稳定的层状结构并提高全层状 TiAl 合金的抗蠕变性的方法。

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.