The research and development of c-TiAl based alloys for aero-engine and automotive components have been the target of several R & D projects since more than 20 years. [1][2][3] Titanium aluminides are considered for future advanced aero-engines due to their potential of significant component weight savings. Although, remarkable progress has been made, today, titanium aluminides have not been applied for aeroengine parts. Both fundamental materials research and design as well as production technologies have achieved an advanced state of maturity. But overall, the limited tensile ductility, poor crack propagation resistance and detrimental effects of defects, damage and long term cycling loads as well as exposure to hot oxidizing atmospheres on the fatigue life are the mayor concerns in the area of aero-engine components reliability and lifetime issues. There are further needs of understanding the source and effect of the different relevant damages and defects on the life-prediction for a particular titanium aluminide alloy and aero engine component. The attempts of scaling up the production of ingot materials, castings and forgings, have not yet met the required targets of reproducibility and affordability. Large-scale production of titanium aluminides ingots and parts requires further alloy and process development to become a reliable technology. Current titanium and nickel alloys exhibit balanced properties and achieve all requirements of the current design practices.Intermetallic c-TiAl based alloys are certainly among the most promising candidates to fulfill the required thermal and mechanical specifications. Especially, TiAl alloys with high Nb-contents showing a baseline composition of Ti-(42-45)Al-(5-10)Nb-(0-0.5)B (all compositions are stated in at%), termed TNB alloys, have attracted much attention because of their high creep strength, good ductility at room temperature, good fatigue properties, and excellent oxidation resistance. [1][2][3][4][5][6][7] Nb reduces the stacking fault energy in c-TiAl, retards diffusion processes and modifies the structure of the oxidation layer. [4,6,8] Cast alloys based on Ti-(42-45)Al, which solidify via the body-centered cubic b-phase, exhibit an isotropic, equiaxed and texture-free microstructure with modest micro-segregation, whereas peritectic alloys (solidification via the hexagonal a-phase) show anisotropic microstructures as well as significant texture and segregation. [9] Alloy design concepts for c-TiAl based alloys showing refined cast microstructures were recently reported by Imayev et al. [10] An alloy design strategy to improve the hot-workability of TiAl alloys is to exploit a combination of thermo-mechanical processing and additional alloying elements to induce the disordered b-phase at elevated temperatures as ductile phase. [11][12][13][14][15][16][17] The disordered b-phase with bcc lattice provides a sufficient number of independent slip systems. Thus, it may improve the deformability at elevated temperature, where, for example, processes such as rollin...

High-energy X-rays between 30 keV and 1 MeV, such as provided by modern synchrotron radiation sources as the ESRF and HASYLAB, bear the advantage of high penetration into most materials. Even heavy element compositions can be accessed in their volume. The range of applications is huge and spreads from nuclear spectroscopy to the characterization of metal extrusion under industrial conditions. This article compiles an overview over the most common instrumental diffraction techniques.Modern two-dimensional detectors are used to obtain rapid overviews in reciprocal space. For example, diffuse scattering investigations benefit from the very flat Ewald sphere as compared to low energies, which allow mapping of several Brillouin zones within one single shot. Diffraction profiles from liquids or amorphous materials can be recorded easily. For materials science purposes, whole sets of Debye-Scherrer rings are registered onto the detector, their diameters and eccentricities or their intensity distribution along the rings relating to anisotropic strain or texture measurements, respectively. At this point we stress the resolution of this technique which has to be carefully taken into account when working on a second generation synchrotron source.Energy-dispersive studies of local residual strain can be studied by a dedicated three-circle diffractometer which allows accurately to adjust the scattering angle from a defined gauge volume.Triple axis diffractometry and reciprocal space mapping is introduced and can be employed for highest resolution purposes on single crystal characterization, even under heavy and dense sample environments. Thus, the perfection of single crystals can be mapped and strain fields and superstructures as introduced by the modulation from ultrasonic waves into crystals or epitaxially grown Si/Ge layers can be investigated in detail. Phase transitions as magnetic ordering can be studied directly or through its coupling to the crystal lattice. Time resolved studies are performed stroboscopically from a sub-nanosecond to a second time scale.The combination of these techniques is a strong issue for the construction and development of future instruments.

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The C distribution within the -TiAl-phase of a Ti-45Al-5Nb-0.5C alloy with near- microstructure has been studied by atom probe tomography. In most areas the C atoms are homogenously distributed, and only a few C enriched features were detected which are presumably Cottrell atmospheres surrounding dislocation cores. The C concentration within the -phase was measured to be approximately 0.25 at.%, which is a factor of ten higher than the solubility limit reported for other TiAl alloys. The reason for this unusually high C solubility is explained by an existing model which relates the number of octahedral sites consisting of six Ti atoms to the solubility limit of interstitials. The large amount of C in solid-solution can explain the results of a recent study which showed that the C-containing alloy had an approximately 30% increase in yield strength when compared with a C-free sheet containing the same Ti, Al and Nb concentration.

Solid-state phase transformations and phase transition temperatures in Ti-45 at.% Al and Ti-45 at.% Al-(5, 7.5, 10) at.% Nb alloys were analyzed experimentally and compared to thermodynamic calculations. Results from scanning electron microscopy, high-energy and conventional X-ray diffraction as well as differential scanning calorimetry were used for the characterization of the prevailing phases and phase transformations. For the prediction of phase stabilities and phase transition temperatures, thermodynamic calculations using the CALPHAD method were conducted. In order to achieve better agreement between calculated and experimental results, a commercially available database was modified using our own results from thermo-physical measurements and annealing treatments.