First-principles Study of the Mechanical Properties of TiAl-Based Alloys

TiAl-based alloys have been suggested as advanced high-temperature structural materials replacing the Ni-based superalloys, due to their low density, high modulus, good oxidation resistance and creep resistance. However, their poor ductility and toughness at low temperature limits their applications in industry. Generally, controlling the microstructure and alloying elements are the most useful methods to improve the mechnical properties of TiAl-based alloys. The TiAl-based alloys have the lamellar structure consisting of the main γ-TiAl phase with L10 structure and the minor α2-Ti3Al phase with D019 structure. In the present thesis, first I focus on the microstructure of the two-phase interface (γ/α2). According to the available experimental works, the orientation relationship for the interface of the two phases is (111)γ//(0001)α2 and 〈1-10〉γ//〈11-20〉α2 . Thus, I select the (111)γ and (0001)α2 orientations to build up a coherent two-phase interface model. In order to model an ideal interface, the convergence of surface energy of the (111)γ and (0001)α2 slabs is discussed. Comparing the interface energies of different configurations and rotations of the two-phase interface, the most stable γ/α2 interface structure is obtained, which is in agreement with the experimental results. The multilayer cleavage energy (Ecl) and generalized stacking fault energies (GSFEs) of the γ/α2 interface structure are calculated. The results of the energy barriers for the [11-2] dislocation show that the γ/α2 interface has relatively low energy barriers, indicating that the two-phase interface can improve the ductility of the TiAl-based alloys. Second I carry out a systematical investigation of the deformation modes and the relationship between the selection of deformation modes of pristine γ-TiAl alloys. I study the three major deformations in γ-TiAl alloys at atomic level, which are ordinary dislocation (OD), superlattice dislocation (SD), and twin (TW), with the Burgers vectors 1/2〈110], 〈101], and 1/6〈11-2] respectively. Taking the symmetry of the L10 structure and the external stress orientation into account, a plastic deformation model is constructed. Effective energy barrier is introduced to analyse the competition of the different deformations. Combining the previous experimental data and the calculated effective energy barriers, I explain the breakdown of the Schmid’s law in γ-TiAl by considering the planar activation of the TW and OD. Based on this model, the atomic-level mechanisms behind so-called channeled flow mechanism is revealed for the first time. In my third project, I investigate the Nb alloying effect on the plasticity of γ-TiAl alloys. At the beginning, the site occupation of Nb with different concentration in γ-TiAl is confirmed by calculating the formation energy and the local lattice relaxation. Results show that Nb subsititutes the Ti sublattice at low alloying concentration, which is in agreement with the previous experimental and theoretical results, while in high-Nb alloys, Nb prefers the Al sites. When Nb occupies the Al-sublattice, it decreases the stacking fault energy of γ-TiAl. In particular, we focus on γsisf , γusisf , γcsf , γucsf , γtw, γutw, γapb, and γuapb, which represent the stable and unstable superlattice intrinsic stacking fault (SISF), complex stacking fault (CSF), twinning (TW), antiphase boundary (APB) of the γ phase, respectively. The energy barriers of all deformation paths decrease