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Intermetallic titanium aluminide alloys based on the ordered face-centred tetragonal γ (TiAl)-phase represent a good example how fundamental and applied research along with industrial development can lead to a new and innovative class of advanced engineering materials. After almost three decades of intensive R&D activities γ (TiAl)-based alloys have matured from “laboratory curiosities” to novel structural light-weight materials which eventually found their applications in aerospace and automotive industries. Their advantage is mainly seen in low density (3.9–4.2 g/cm 3 ), high specific yield strength and stiffness, good oxidation and ignition resistance, combined with good creep properties up to high temperatures. Particularly at temperatures between 600 °C and 800 °C γ (TiAl)-based alloys are superior to Ti-based alloys in terms of their specific strength. Compared to the heavier Ni-based alloys below 800 °C, their specific yield strength is at least similar. Therefore, the particular constitution and extremely fine microstructure of these alloys are illustrated by several high-resolution transmission electron micrographs. The mechanical properties seem to be largely affected by the evolution of internal stresses and off-stoichiometric deviations of the majority γ (TiAl)-phase. Novel experimental approaches are described that could characterize the relevant deformation mechanisms. The combination of these results with the concepts of continuum mechanics and continuum thermodynamics has allowed developing models to describe thermomechanically controlled processes. A selection of such models is introduced and explained in a comprehensive way. While early modeling attempts were successfully undertaken to elucidate selected aspects of physical metallurgy of Ti-Al alloys, many experimental findings, particularly for modern multi-phase alloys based on γ (TiAl) with rather complex constitution and microstructure are still waiting for explanation. [ABSTRACT FROM AUTHOR]

Abstract: The role of vacancies in work hardening and fatigue of TiAl alloys was investigated by mechanical testing and TEM examination of deformed samples. The evolution of the dislocation glide resistance during straining and after recovery was analysed in terms of thermodynamic glide parameters. Strain path change tests with respect to temperature and recovery experiments were performed in order to characterize the thermal stability of the produced defect structures. The recovery of the deformation induced defect structure was observed by TEM in situ heating experiments. The recovery kinetics is described on the basis of a recently published simulation study of vacancy annihilation. [Copyright &y& Elsevier]

Abstract: The creep behavior of a novel type of TiAl alloy with a composite-like microstructure has been investigated. The constitution and microstructure of the alloy result from decomposition reactions of the high-temperature β/B2 phase. The characteristic constituents are structurally modulated laths that are comprised of several stable and metastable phases. Tensile creep tests have been correlated with characterization by high-resolution transmission electron microscopy. The creep behavior of the material is mainly limited by the early onset of tertiary creep at higher stresses and temperatures. The processes associated with this behavior are several phase transformations towards thermodynamic equilibrium, dynamic recrystallization and the relaxation of constraint stresses that exist between misfitting phases. [Copyright &y& Elsevier]

Abstract: Phase decomposition and ordering reactions in β/B2-phase containing TiAl alloys were utilized to establish a novel, previously unreported, type of in situ composite. The characteristic constituents of this composite are laths with a modulated substructure that are comprised of several stable and metastable phases. The modulation occurs at the nanometer-scale and thus leads to a significant refinement of the material. Microstructural control by conventional thermomechanical processing results in a structurally and chemically very homogeneous material with appreciable mechanical properties. The physical metallurgy of this novel type of alloy has been assessed by high-resolution transmission electron microscopy and mechanical testing. [Copyright &y& Elsevier]

Abstract: The paper describes the heterogeneous nucleation of mechanical twins in a TiAl alloy containing Ti3AlC perovskite precipitates. Examination of the deformation structure by high-resolution electron microscopy has revealed that overlapping planar faults were generated at the perovskite particles. These faults can apparently easily rearrange into embryonic twins and subsequently grow into large twins. The nucleation mechanism was found to be closely associated with the misfit character and stress state of the precipitates. The structural data serves as input for the modelling of the precipitation-induced twinning process. Based on these calculations, the thickness and length of the embryonic twins can be predicted, which agree well with the experimental observations. [Copyright &y& Elsevier]

The paper presents an electron microscope study of diffusion assisted creep processes in a two-phase (α2+γ) titanium aluminide alloy, which had been subjected to long-term creep. The results imply that the high primary creep rate of lamellar TiAl alloys is associated with the relaxation of mismatch structures and coherency stresses present at the interfaces. Long-term creep leads to spheroidization and coarsening of the lamellar morphology, which involve phase transformations and recrystallization. The mechanisms of these morphological changes are closely related to the atomic structure of the α2/γ phase boundaries and probably driven by a non-equilibrium of the phase composition leading to the dissolution of the α2 phase. After long-term creep the formation of precipitates was observed, which has been attributed to the α2→γ transformation, because α2 has a significantly higher solubility for interstitial impurities than the γ phase. [Copyright &y& Elsevier]