After almost three decades of intensive fundamental research and development activities intermetallic titanium aluminides based on the -TiAl phase have found applications in automotive and aircraft engine industries. The advantages of this class of innovative high-temperature materials are their low density as well as their good strength and creep properties up to 750°C. A drawback, however, is their limited ductility at room temperature, which is reflected by a low plastic strain at fracture. This behavior can be attributed to a limited dislocation movement along with microstructural inhomogeneity. Advanced TiAl alloys, such as β-solidifying TNM™ alloys, are complex multi-phase materials which can be processed by ingot or powder metallurgy as well as precision casting methods. Each production process leads to specific microstructures which can be altered and optimized by thermo-mechanical processing and/or subsequent heat-treatments. The background of these heat-treatments is at least twofold, i.e. concurrent increase of ductility at room temperature and creep strength at elevated temperature. In order to achieve this goal the knowledge of the occurring solidification processes and phase transformation sequences is essential. Therefore, thermodynamic calculations were conducted to predict phase fraction diagrams of engineering TiAl alloys. After experimental verification, these phase diagrams provided the base for the development of heat treatments to adjust balanced mechanical properties. To determine the influence of deformation and kinetic aspects, sophisticated ex- and in-situ methods have been employed to investigate the evolution of the microstructure during thermo-mechanical processing and subsequent multi-step heat-treatments. For example, in-situ high-energy X-ray diffraction was conducted to study dynamic recovery and recrystallization processes during hot-deformation tests. Summarizing all results a consistent picture regarding microstructure formation and its impact on mechanical properties in TNM alloys can be given.

A robust processing route at low cost is an essential requirement for high-temperature materials used in automotive engines. Because of their excellent high-temperature properties, their low density, high elastic modulus as well as high specific strength, intermetallic γ-TiAl based alloys are potential candidates for application in advanced automotive turbochargers. So-called 3rd generation alloys, such as TNM™ alloys with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at%), are multi-phase alloys consisting of γ-TiAl, α2-Ti3Al and a low volume fraction of βo-TiAl phase. In this paper a novel hot-processing route, which is a combination of a one-shot hot-forging step and a controlled cooling treatment, leads to mechanical properties required for turbocharger turbine wheels. The observed strength can be attributed to the small lamellar spacing within the α2/γ colonies of the nearly lamellar microstructure. In order to analyze the microstructure and the prevailing phase fractions microscopic examinations and X-ray diffraction measurements were conducted. The mechanical properties were determined by hardness measurements as well as tensile and creep tests. The evolution of the microstructure during the hot-forming process is described and its relation to the obtained mechanical properties.

Microstructure and texture formation were investigated in an intermetallic Ti-43.4Al-4.2Nb-1.1Mo-0.1B (in at.%) alloy after near conventional forging in the (a þ b) phase field region and subsequent isothermal annealing treatments at 1150°C for different holding times. During forging and the following cooling process a fine grained microstructure is formed consisting of lamellar a 2 /g colonies and elongated grains of the b o phase situated at the colony boundaries. The isothermal annealing treatment leads to a significant change of the forged microstructure. After hot-forging, all phases show typical fiber textures as it is expected for uniaxial deformation. In the as-forged state the a 2 (a) phase exhibits a h11-20i fiber, the b o (b) phase has a strong and sharp h001i fiber and the g phase shows a h110i and a weaker h101i fiber. After additional annealing at 1150°C for 8 h, the textures are almost unchanged in contrast to the microstructure. A. Gaitzenauer et al./Microstructure and Texture Evolution in an Intermetallic b-Stabilized g-TiAl Alloy ADVANCED ENGINEERING MATERIALS 2014, 16, No. 4

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