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

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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.