Property enhancement of TiAl alloys by element additions

Intermetallic alloys based on titanium and aluminum, so-called TiAl based alloys, exhibit attractive properties, such as high melting point and high specific elastic modulus as well as good creep and oxidation resistance up to 700C. Combined with a low density, which is only half of that of conventionally used nickel based superalloys, this profile makes them key materials for several high temperature applications. The use of lightweight structural materials could help to enhance the efficiency of advanced environmental-friendly aero-engines, because air traffic alone holds a share of approximately 3.5% of the human-induced climate change. The use of these innovative structural materials will contribute in the reduction of CO2 emission. Furthermore, the fuel consumption could be minimized significantly, which helps to save resources. However, for the next generation of jet- engines TiAl alloys with an application temperature of up to 850C are required. In order to understand the physical hardening processes that occur in the temperature range of 800C to 900C, the Austrian-French project team has developed a unique research strategy, which is based on the investigation of well-selected model alloys using a number of complementary structural and mechanical characterization methods. The goal of the submitted proposal is to gain a fundamental understanding of the role of substitutional and interstitial alloying elements on the constituting phases and physical processes activated during deformation and isothermal ageing, which form the microstructure, thus controlling the mechanical properties at high temperatures. The material will be produced by a powder metallurgical approach, employing spark plasma sintering of pre-alloyed gas atomized powders. The constituting phases of the microstructure will be characterized from macroscopic scale down to atomic scale, using conventional as well as most advanced microscopic and analytical techniques, such as electron microscopy and atom probe tomography. Concurrently, the activation parameters of the deformation mechanisms will be measured through instrumented mechanical tests. In sum, the proposed fundamental project will gain a better understanding of the high temperature mechanisms in advanced multi-phase structural material systems, which will provide the basis of future knowledge-driven alloy design. The obtained results, although of fundamental character, can then be used to design even more advanced aero-engines, which support both industry and environment.

This project targeted the influence of substitutional and interstitial elements, i.e. Mo, W, C, and Si, on the properties of intermetallic -TiAl based alloys, especially the physical hardening processes occurring in the temperature range of 800 to 900 C as well as the thermodynamics. The project was formulated as an international collaboration between the Chair of Physical Metallurgy, Montanuniversität Leoben, in Austria and the Center for Materials Elaboration and Structural Studies in France. The -TiAl based alloys containing Mo, W, C, and Si possessed improved strength at room and elevated temperatures as well as an increased creep resistance compared to the binary reference alloy. In particular, the W-containing and W+C-containing alloys were found to be the most promising candidates for an improvement of the high-temperature properties. In order to understand the interaction of these elements with dislocations and, thus, the influence on the hardening mechanisms, the chemical environment in the vicinity of deformation-bearing dislocations was studied. After characterization by transmission electron microscopy, atom probe tomography was combined with a newly developed site-specific preparation procedure to study the elemental distribution around such dislocations at the atomic scale. In the case of the alloy containing W, no segregations at dislocations were observed, after neither room temperature nor creep deformation. Consequently, the enhancement of creep strength by the addition of this element is attributed to a reduction of the overall diffusion activity of the phase and solid solution hardening by the W atoms. In the alloy containing W and C, local enrichments of both elements were observed at dislocations after creep deformation. This shows that the presence of C enhances the segregation tendencies of W and increases the creep resistance due to additional pinning of dislocations. The developed characterization workflow for the elemental environment of dislocations can also be applied to other alloying elements in -TiAl based alloys as well as to many other material classes in the future. Concerning the thermodynamics of -TiAl based alloys, Mo and W were found to significantly alter the microstructure evolution during solidification and subsequent heat treatments by stabilization of the /o phase and evoking phase transformations not present in the binary Ti-Al system. Silicon was found to act as a -stabilizing element, increase solid-solid transformation temperatures, and form Ti5Si3 precipitates. In-situ high-energy X-ray diffraction combined with a newly developed experimental setup was used to study the powder densification by spark plasma sintering concerning the occurring equilibrium and non-equilibrium phase transformations for the first time in a time-resolved manner. The obtained thermodynamic data for the individual alloying elements will be implemented in databases, thus, improving the accuracy of thermodynamic calculations and aiding in future alloy development programs.