No abstract

No abstract

No abstract

No abstract

Intermetallic titanium aluminides offer an attractive combination of low density and good oxidation and ignition resistance with unique mechanical properties. These involve high strength and elastic stiffness with excellent high temperature retention. Thus, they are one of the few classes of emerging materials that have the potential to be used in demanding high‐temperature structural applications whenever specific strength and stiffness are of major concern. However, in order to effectively replace the heavier nickel‐base superalloys currently in use, titanium aluminides must combine a wide range of mechanical property capabilities. Advanced alloy designs are tailored for strength, toughness, creep resistance, and environmental stability. These concerns are addressed in the present paper through global commentary on the physical metallurgy and associated processing technologies of γ‐TiAl‐base alloys. Particular emphasis is paid on recent developments of TiAl alloys with enhanced high‐temperature capability.

Phase decomposition and ordering reactions in β/B2‐phase containing TiAl alloys were utilized to establish a novel, previously unreported, type of laminate microstructure. The characteristic constituent of this microstructure are laths with a nanometer‐scale substructure that are comprised of several stable and metastable phases. Microstructural control can be achieved by conventional thermomechanical processing and leads to a structurally and chemically very homogeneous material with excellent mechanical properties. The physical metallurgy of this novel type of alloy has been assessed by transmission electron microscope investigations and mechanical testing.

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...

For high-temperature applications, creep strength is of major concern, in addition to oxidation and corrosion resistance, and determines the application range of titanium aluminide alloys in competition with other structural materials. Thus, this work was aimed at identifying mechanisms of creep deformation and microstructural degradation and at developing alloying concepts with respect to an enhanced high-temperature capability. The analysis shows that dislocation climb controls deformation in the range of the intended operation temperatures. Further, complex processes of phase transformations, recrystallization, and microstructural coarsening were observed, which contribute to microstructural degradation and limit component life in long-term service. By alloying with high contents of Nb, both room-and high-temperature strength properties can be improved as Nb increases the activation energy of diffusion and increases the propensity for twinning at ambient temperature. For alloys with enhanced hightemperature capability, microalloying with carbon is also of particular use, because carbide precipitates effectively hinder dislocation motion and are thought to increase microstructural stability.