In search of permanent improvement and optimization of existing energy conversion systems, like aero engines or turbocharger wheels, titanium aluminides (TiAl) are an interesting alternative to the established Ni base superalloys. Because of their good high temperature strength, oxidation and corrosion resistance in combination with their low density, titanium aluminides are attractive for rotatory loaded components. Since these intermetallic alloys are used in a high temperature environment, their creep behavior is of major importance. The creep properties of TiAl alloys depend strongly on the microstructure. The best creep resistance is achieved by a fully lamellar microstructure with alternating α2- and γ-laths. Additionally, alloying elements are added to further improve the creep properties.

The alloying elements Nb, Ta and Zr improve the creep properties of fully lamellar γ/α2 titanium aluminides significantly. The superior creep strength and increased microstructural stability of the ternary alloys were ascribed to the solid solution hardening effect, the diffusivity, the partitioning behavior and the influence on the lattice parameters of the alloying elements Nb, Ta and Zr. However, no systematic investigation was carried out to determine the diffusivity and solid solution hardening of Nb, Ta and Zr in titanium aluminides. Since high temperature deformation mainly occurs in the γ-phase of γ/α2 titanium aluminides, the diffusivity and the solid solution hardening effect of these three elements in the γ-Ti-54Al-5X alloys is studied by analyzing the concentration gradients of the alloying elements and the resulting hardness across the interdiffusion zone of diffusion couples by energy dispersive X-ray diffraction and nanoindentation. Furthermore, strain rate jump tests on single γ-phase Ti-54Al-5X alloys were conducted at room temperature 750°C and 900°C to further correlate the diffusivity and solid solution hardening of Nb, Ta & Zr to the macroscopic mechanical properties.

It was found, that Zr has the largest solid solution hardening coefficient but also the highest interdiffusion coefficient. Ta has the lowest interdiffusion coefficient and a similar solid solution hardening coefficient as Nb. The addition of 5 at. % Nb or Ta lead to an increased strength compared to a binary γ-Ti54-Al alloy at all investigated temperatures. The Zr-containing γ-TiAl alloy reveals the highest strength at 750 °C and 900 °C, which is discussed to be due to the strong solid solution hardening effect of Zr. However, in comparison to the other alloys Ti-54Al-5Zr shows quite brittle behavior up to 900 °C. The lower diffusivity of Ta compared to Nb leads to a higher strength of the Ta modified alloy at 900 °C and low strain rates.