Gamma-based TiAl alloy has been known to be one of the most promising candidates for high-temperature structural applications. However, two typical microstructures of the TiAl alloy, such as the fully lamellar microstructure and the duplex microstructure, have shown a so-called inverse relationship between room-temperature ductility and fracture toughness/creep resistance. The fully lamellar TiAl have excellent creep resistance and fracture toughness, even though poor room temperature ductility. In contrast, the duplex TiAl alloys have considerable room temperature ductility but low levels of creep properties. Therefore, it is necessary to develop a TiAl alloy system with balanced mechanical properties.
This thesis deals with the microstructural control of the fully lamellar and the duplex microstructure of carbon and nitrogen doped alloys Chapter 3 and their tensile properties in Chapter 4 and their creep properties in Chapter 5 and their fracture toughness in Chapter 6.
In chapter 3, Effects of nitrogen addition on the microstructure and the mechanical properties of two-phase TiAl intermeetallic compounds were investigated. 1.0at.% nitrogen addition leads to remarkable grain refinement in the fully lamellar microstructure but had little effect on the duplex microstructure. These facts are caused by ＄Ti_2AlN＄ precipitates which observed in only 1.0at.% nitrogen-doped alloys. The carbon and nitrogen atoms may delayed the diffusion in the lattice. Therefore, heat treatment time of the duplex alloys were required over 24 hours.
In Chapter 4, We examined tensile properties of the alloys of Chapter 3 at room temperature and elavated temperature. Though 0.3at.% nitrogen-doped alloy had much lower elongation than the mother alloy, 1.0at.% nitrogen addition had little effect on room temperature elongation and increased room temperature yield strength by two times in the fully lamellar microstructure. High temperature yield strength of the nitrogen-doped alloys increased due to solute hardening and precipitation hardening of ＄Ti_3AlN＄.
In Chapter 5, we investigated the creep properties of carbon and nitrogen doped alloys seclected in Chapter 3 and 4. Nitrogen addition led to remarkable improvement of creep resistance in the duplex microstructure as well as in the fully lamellar microstructure. In particular, the primary creep deformation of the 1.0at.% nitrogen-doped alloy with the duplex microstructure decreased definitely to a similar or superior level of the fully lamellar alloy. We consider that precipitate hardening of ＄Ti_3AlN＄ precipitates and solute hardening of nitrogen atoms may be responsible for such remarkable creep resistance of the nitrogen-doped alloys.
At last, in Chapter 6, we evaluated the fracture toughness of fully lamellar nitrogen-doped alloys. Effect of nitrogen addition on the fracture toughness of the lamellay alloys is similar to the case of the elongation of the lamellar alloy at the room temperature. Though the lamellar alloy which added with 0.3at.% nitrogen shows the lower ＄K_Q＄ value than the mother alloy, ＄K_Q＄ value of 1.0at.% nitrogen alloy was similar to that value of the mother alloy. These may be caused by ＄Ti_2AlN＄ precipitates.
From these results, we reach a conclusion that the carbon and nitrogen doped alloys had balanced-mechanical properties, in particular creep resistance. Therefore, I have expected to progrees further study as structural intermetallic compounds for high temperature.