The application of light-weight intermetallic materials to address the growing interest and necessity for reduction of CO2 emissions and environmental concerns has led to intensive research into TiAl-based alloy systems. However, the knowledge about phase relations and transformations is still very incomplete. Therefore, the results presented here from systematic thermal analyses of phase transformations in 12 ternary Ti-Al-Nb alloys and one binary Ti-Al measured with 4–5 different heating rates (0.8 to 10 °C/min) give insights in the kinetics of the second-order type reaction of ordered (βTi)o to disordered (βTi) as well as the three first-order type transformations from Ti3Al to (αTi), ωo (Ti4NbAl3) to (βTi)o, and O (Ti2NbAl) to (βTi)o. The sometimes-strong heating rate dependence of the transformation temperatures is found to vary systematically in dependence on the complexity of the transformations. The dependence on heating rate is nonlinear in all cases and can be well described by a model for solid-solid phase transformations reported in the literature, which allows the determination of the equilibrium transformation temperatures.

Precise knowledge of the phase equilibria in the Ti-Al-Nb system between 700 and 900 °C is of crucial importance for the urgently needed improvement of TiAl-based turbine materials already in industrial use to achieve further energy savings. As a result of the occurrence of the two ternary intermetallic phases ωo (“Ti4NbAl3”) and O (“Ti2NbAl”), which form in the solid state just in the range of the application-relevant temperatures, the phase relations are very complex and not well studied. In the present investigation, isothermal sections of the Ti-rich part of the Ti-Al-Nb system at 700, 800, and 900 °C were determined by a systematic study of 15 ternary alloys, one solid-solid diffusion couple, and three liquid-solid diffusion couples. Using scanning electron microscopy, electron probe microanalysis (EPMA), x-ray diffraction (XRD), high-energy XRD (HEXRD), differential thermal analysis (DTA), and transmission electron microscopy (TEM) investigations, type and composition of phases as well as phase transitions were determined. With these results, the phase equilibria were established. A focus of the investigations is on the homogeneity ranges of the two ternary phases ωo and O, which both are stable up to temperatures above 900 °C. Based on the compositions measured for the ωo phase and its crystal structure type, a new formula (Ti,Nb)2Al is suggested. The results also indicate that the phase field of the ωo phase is split into two parts at 900 °C because of the growing phase field of the ordered (βTi,Nb)o phase.

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Intermetallic titanium aluminides based on the ordered γ-TiAl phase fulfil many of the key requirements of lightweight high-temperature applications. In addition to a high melting point, high specific Young's modulus and strength at elevated temperatures, excellent creep properties, and a satisfactory oxidation and burn resistance, especially their low density of roughly 4 g cm À3 makes them a material of choice for challenging structural applications such as found, e.g., in environmentally friendly combustion engine options. [1][2][3][4] Capable of withstanding extreme conditions, γ-TiAl-based alloys have already entered service as low-pressure turbine blades in jet engines, such as the GEnx engine by GE Aviation, [5] the Geared Turbofan (GTF) engine by Pratt and Whitney, [6] as well as the LEAP engine by CFM International, a joint company between Safran Aircraft Engines and GE. [7,8] In these applications, γ-TiAl-based turbine blades substitute some of the blades made of twice as heavy Ni-base superalloys in a temperature range up to 750 °C. The implementation of the advanced, lightweight material, which is also attended by an adaptation of the design, entails a substantial reduction in weight, fuel consumption, and CO 2 and NO x emissions. [9] Apart from the aircraft engine industry, γ-TiAl-based alloys have also found applications in the automotive industry, e.g., as engine valves in sports and racing cars or as turbocharger turbine wheels. [1,3,[10][11][12][13][14] The past few decades have witnessed great efforts to develop intermetallic γ-TiAl-based alloy systems that are suitable for service in these demanding areas while being economically competitive at the same time. To this end, various alloy compositions and processing routes have been studied intensively. [1,[15][16][17][18][19][20][21] Recent progress has been reviewed, e.g., in the works by Appel et al., Clemens et al., and Mayer et al. [1,3,9,22] Due to the complexity of the intermetallic alloy system, which encompasses a multitude of phases and phase transformations, [23] purposeful alloy design requires the use of manifold characterization methods. Diffraction and scattering techniques, in particular, offer access to the atomic structure of the material and provide insights into a variety of microstructural parameters. [24][25][26][27] High-energy X-rays, such as available at today's synchrotron radiation sources, and recent developments in hardware technology nowadays allow to conduct so-called in situ experiments. Experiments of this kind reveal-at a high temporal resolution-the relationship between selected external

TiAl alloys are increasingly used as a lightweight material, for example in aero engines, which also leads to the requirement for suitable repair techniques. Transient liquid phase bonding is a promising method for the closure of cracks (in non-critical or non-highly loaded areas). The brazing solder Ti-24Ni was investigated for brazing the alloy Ti-45Al-5Nb-0.2B-0.2C (in at. %). After brazing, the joint exhibits different microstructures and phase compositions. The transient liquid phase bonding process was investigated in the middle of the joint region in situ to acquire time resolved information of the phases, their development, and thus the brazing process. These investigations were performed using high-energy X-ray diffraction at the “High-Energy Materials Science” beamline HEMS, located at the synchrotron radiation facility PETRA III at DESY in Hamburg, Germany. For this, we used an induction furnace, which is briefly described here. During the analysis of the diffraction data with Rietveld refinement, the amount of liquid was refined with Gaussian peaks and thus could be quantified. Furthermore, while brazing four different phases occurred in the middle of the joint region over time. Additionally, the degree of ordering of the βo phase was determined with two ideal stoichiometric phases (completely ordered and disordered). Altogether, the phase composition changed clearly over the first six hours of the brazing process.