Highlights: Phase diagram and phase fraction evolution of the Ti-43.5Al-4Nb-1Mo-0.1B-C system  Change in solidification pathway from βto peritectic solidification with carbon addition  Macrotextures of cast/HIP microstructures  Carbon solubility, carbide formation and hardness evolution  Microstructure, phase evolution and creep properties of a Ti-43.5Al-4Nb-1.5Mo-0.1B-0.5C alloy Cover Letter -1 / 17 - ABSTRACTImproving mechanical properties of advanced intermetallic multi-phase γ-TiAl based alloys, such as the Ti-43.5Al-4Nb-1Mo-0.1B alloy (in at.%), termed TNM alloy, is limited by compositional and microstructural adaptations. A common possibility to further improve strength and creep behavior of such β-solidifying TiAl alloys is e.g. alloying with β-stabilizing substitutional solid solution hardening elements Nb, Mo, Ta, W as well as the addition of interstitial hardening elements C and N which are also strong carbide and nitride forming elements. Carbon is known to be a strong α-stabilizer and, therefore, alloying with C is accompanied by a change of phase evolution. The preservation of the solidification pathway via the β-phase, which is needed to obtain grain refinement, minimum segregation and an almost texture-free solidification microstructure, in combination with an enhanced content of C, requires a certain amount of β-stabilizing elements, e.g. Mo. In the present study, the solidification pathway, C-solubility and phase evolution of C-containing TNM variants are investigated. Finally, the creep behavior of a refined TNM alloy with 1.5 at.% Mo and 0.5 at.% C is compared with that exhibiting a nominal Ti-43.5Al-4Nb-1Mo-0.1B alloy composition.

a b s t r a c tA b-solidifying TiAl alloy with a nominal composition of Tie43.5Ale4Nbe1Moe0.1B (in at.%), termed TNMÔ alloy, was produced by a powder metallurgical approach. After hot-isostatic pressing the microstructure is comprised of fine equiaxed g-TiAl, a 2 -Ti 3 Al and b o -TiAl grains. By means of two-step heat-treatments different fine-grained nearly lamellar microstructures were adjusted. The evolution of the microstructure after each individual heat-treatment step was examined by light-optical, scanning and transmission electron microscopy as well as by conventional X-ray and in-situ high-energy X-ray diffraction. The experimentally evaluated phase fractions as a function of temperature were compared with the results of a thermodynamical calculation using a commercial TiAl database. Nano-hardness measurements have been conducted on the three constituting phases a 2 , g and b o after hot-isostatic pressing, whereas the hardness modification during heat-treatment was studied by macro-hardness measurements. A nano-hardness for the b o -phase is reported for the first time.

Intermetallic g-TiAl based alloys are a class of novel, light-weight structural materials with attractive mechanical properties for advanced high-temperature applications. Due to their low density (4 g cm À3 ), their high yield and creep strength up to 800 8C and their good oxidation resistance they have the potential to replace the heavier Ni-based superalloys (8 g cm À3 ) in industrial and in aviation gas turbines as well as in automobile engines. [1] Conventional titanium aluminide alloys consist of tetragonal g-TiAl (L1 0 structure; P 4/m m m) and small small amounts of hexagonal a 2 -Ti 3 Al (D0 19 structure; P 6 3 /m m c). Through special heat treatments various microstructures can be established in these two phase alloys to optimize their mechanical properties. [2] The most restricting factor for a broad industrial implementation of titanium aluminides is their low ductility that also limits their workability. A promising design strategy to overcome the brittleness and to improve the hot workability is to induce the formation of more ductile phases by adding ternary alloying elements. The body-centered cubic (bcc) high-temperature b-Ti(Al) phase (A2 structure; I m 3 m) can act as a ductilizing phase in TiAl alloys because it provides a high number of independent slip systems. In recent years several authors have reported that stabilizing the b phase by alloying elements such as Nb, Mo, Ta, or V, significantly improves the hot workability. [3][4][5] Additionally, novel types of microstructures can be achieved exploiting the ternary solid state transformations. [3,4] In spite of this progress, the exact pathway of phase transformations and thus the evolution of microstructures in b phase containing TiAl alloys are not fully understood up to now. At lower temperatures, the disordered bcc b phase can transform to ordered cubic b o -TiAl phase (B2 structure; P m 3 m). However, calculated and experimental transition temperatures show large discrepancies. [6,7] In high-Nb containing TiAl alloys b and/or b o can decompose to ordered hexagonal v o -Ti 4 Al 3 Nb phase (B8 2 structure; P 6 3 /m m c). [8,9] The formation of an orthorhombic phase (B19 structure; P m m a) is reported in Al-lean and Nb-rich TiAl alloys and is interpreted as a transition structure between the cubic b and/ or b o and the orthorhombic O-Ti 2 AlNb phase (C m c m). [4] The crystallographic data of all phases mentioned above and relevant for this work are listed in Table 1.Ordered phases, such as b o and v o , are often assumed to be detrimental to ductility due to their low crystal symmetry. Otherwise the orthorhombic O phase is known to be relatively ductile and even v o containing TiAl alloys show good plastic formability at 800 8C. [8] Thus, with respect to alloy design and processing, it is of high importance to know which kind of additional phase will be formed and which further phase transformations occur during processing and service.In recent years intermetallic g-TiAl based alloys with additional amounts of the ternary b phase have a...

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Metal-based additive manufacturing (AM) permits layer-by-layer fabrication of near net-shaped metallic components with complex geometries not achievable using the design constraints of traditional manufacturing. Production savings of titanium-based components by AM are estimated up to 50% owing to the current exorbitant loss of material during machining. Nowadays, most of the titanium alloys for AM are based on conventional compositions still tailored to conventional manufacturing not considering the directional thermal gradient that provokes epitaxial growth during AM. This results in severely textured microstructures associated with anisotropic structural properties usually remaining upon post-AM processing. The present investigations reveal a promising solidification and cooling path for α formation not yet exploited, in which α does not inherit the usual crystallographic orientation relationship with the parent β phase. The associated decrease in anisotropy, accompanied by the formation of equiaxed microstructures represents a step forward toward a next generation of titanium alloys for AM.

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 Microstructure and phase evolution of a β-solidifying Ti-43Al-4Nb-1Mo-0.1B alloy with minor C and Si  Deformation study within a temperature range of 1150-1300 °C and a strain rate regime of 0.005-0.5 s-1  Strain-resolved constitutive modeling of the flow behavior up to a true deformation of 0.9  Surface fitting approach of flow curve data via a hyperbolic-sine law  Processing maps in correlation with metallographic post-analysis  Complementary information, i.e. phase fractions and texture, from in situ HEXRD experiments  Critical discussion of experimental difficulties in performing hot-deformation tests-2 / 18-ABSTRACT New high-performance engine concepts for aerospace and automotive application enforce the development of lightweight γ-TiAl based alloys with increased high-temperature capability above 750 °C. Besides an increased creep resistance, the alloy system must exhibit sufficient hot-workability. Therefore, a refined βsolidifying TNM alloy with an alloy composition of Ti-43Al-4Nb-1Mo-0.1B (in at.%) and minor additions of C and Si is investigated by means of uniaxial compressive hot-deformation tests. The occurring mechanisms during hot-working were decoded by ensuing constitutive modeling of the flow curves by a phase field region-specific surface fitting approach via a hyperbolic-sine law as well as by evaluation via processing maps combined with microstructural post-analysis. Furthermore, complementary in situ high energy X-ray diffraction experiments give a deeper insight about the deformation behavior of the alloy, i.e. phase fractions and texture evolution during isothermal and non-isothermal compression.

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