Numerical analysis of anisotropic yield behavior of single-phase titanium aluminide constituents γ-TiAl and α2-Ti3Al by means of finite element models based on crystal plasticity theory

Kurzfassung

Two-phase (γ-TiAl + α2Ti3Al) lamellar TiAl alloy shows anisotropic behavior which resulted from the local anisotropy of the ductile γ-TiAl (FCT) phase and brittle α2Ti3Al (HCP) phase. The yield anisotropy of lamellar TiAl under uniaxial loading has been investigated in many studies to understand the deformation behavior with respect to lamellar orientation. However, the contribution of single-phase anisotropy on the overall uniaxial anisotropic response has not been well studied. This information is needed to understand the role of local phases in the macroscopic deformation behavior in order to establish a good correlation of the microstructure-property, which eventually is a prerequisite for the design of high performance TiAl alloy. Aiming to get an in-depth knowledge of the anisotropy of the TiAl alloy, the local anisotropy of single phases, γ-TiAl and α2Ti3Al, has been numerically investigated. Using a classical slip-based Crystal Plasticity FE model (CPFEM), the deformation of the TiAl phases under different load-orientation has been simulated. Then, the yield surfaces were evaluated to see the anisotropic yield response of the phases. The role of active slip systems on the anisotropic deformation for the ductile γ-TiAl and brittle α2Ti3Al phase has been investigated in detail. The activations of slip systems were studied using parametric analysis by varying CP parameters of different slips. By these analyses, one may find how the loading angles influence the slip activity, and how the local deformation influences the yield surfaces of these phases. Further, the yield surfaces of the single phases were compared with the yield surface of two-phase TiAl. This comparison gives a clear understanding of how the local anisotropy of the single phases influences the anisotropy of the two-phase TiAl systems. The detailed investigation presented in this thesis work illuminates the correlations between the role of CP parameters (hence, the slip activity) due to load angles, the shape (nature) of anisotropic yield of single phases, and the collective anisotropic response of two-phase TiAl.