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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]

Deformation twinning is an important mode of plastic deformation in γ(TiAl)-based titanium aluminide alloys. The paper describes conventional and high-resolution electron microscope investigations of twinning arrangements in γ based titanium aluminide alloys. The major areas of study are: (i) twin nucleation and propagation, (ii) effects of solutes and precipitates on the kinematics and dynamics of twin propagation, (iii) association of mechanical twinning and fracture. The implications of the results will be discussed with respect lo alloy design strategies towards improved mechanical properties. [ABSTRACT FROM AUTHOR]

Titanium aluminide alloys containing relatively large Nb additions and subjected to precipitation hardening exhibit attractive thermo-physical properties, which extend the service range of conventional TiAl alloys. In order to use these materials at their full potential, structural and chemical consolidation is required. To this end, the feasibility of wrought processing of high Nb containing alloys was systematically investigated. The major areas of the study involve: (i) high-temperature deformation mechanisms, (ii) primary ingot break-down, (iii) secondary processing, (iv) texture evolution, and (v) mechanical properties. [Copyright &y& Elsevier]

Mechanical twinning has long been recognized as an important deformation mechanism in many intermetallics including γ(TiAl) based alloys. The generation of a twin can be triggered by dislocations at grain boundaries. The reduction of the stored strain energy in relation to a configuration without any twins can be shown by applying an established transformation criterion, considering twinning as a transformation shearing process. In addition, the application of a thermodynamical extremal principle allows one to predict a distinct twin nucleus in good agreement with experimental observations. The concept is rather general and can be adapted to any initial eigenstress field produced by dislocations present at misfitting interfaces. [Copyright &y& Elsevier]

Weak-beam transmission electron microscope (TEM) analyses of 1/2〈112〉 superdislocations were performed in order to determine the stacking fault energies in Ti–45Al, Ti–49Al, Ti–45Al–10Nb and Ti–49Al–10Nb alloys (at.%) at 900 °C. In Al-lean binary TiAl alloys, the SISF energies decrease significantly with decreasing Al concentration in the γ phase, from 97 mJ m−2 for 49.6% Al to 64 mJ m−2 for 48% Al. The ternary alloys exhibit a relatively low SISF energy of about 68 mJ m−2, which is practically independent of the Al concentration. [Copyright &y& Elsevier]

The deformation behaviour of two-phase titanium aluminides was investigated in the intermediate temperature interval 450–750 K. The observed deformation characteristics include discontinuous yielding and negative strain rate sensitivity, which are indicative of the operation of the Portevin–LeChatelier effect. The pinning processes occurring at the dislocations and associated with these phenomena were studied by static strain ageing experiments. A wide range of alloy compositions and microstructures was investigated in order to identify the relevant defect species. Accordingly, dislocation pinning is characterized by a relatively small activation energy of Qa=0.7eV being not consistent with a conventional diffusion process. Furthermore, the strain ageing phenomena are most pronounced in Ti-rich alloys. This gives rise to the conclusion that antisite defects are involved in the pinning process. [ABSTRACT FROM AUTHOR]

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]

The yield strengths of Ti–45Al–xNb and Ti–49Al–xNb (x=0, 10) with a nearly-equiaxed gamma microstructure were measured at 900 °C. Nb addition was found to largely increase the yield strengths but the variation of Al content has little influence on the yield strength of binary TiAl alloys. To understand the strengthening effect of Nb, the stacking fault energies in these alloys were measured using weak-beam transmission electron microscope (TEM) techniques. The SISF energy decreases significantly with the decreasing of Al content of γ phase in binary alloys but is practically independent of the Al content in ternary alloys. The implications of the dissociation behavior on the mechanical properties of the material will be discussed. [Copyright &y& Elsevier]

Examines the mechanical alloying preparation of TiAl powders with grain size of 15 nanometer by hot isotonic pressing. Exhibition of the material globular microstructure; Observation of Hall Petch type dependence on grain size of microhardness measurements; Decrease ductility at room temperature in compression test result.

The aim of this study was to gain an insight into the development of near-surface stress, which has been proposed to play a role in environmental embrittlement, as a function of depth after exposure. Investigations were performed, using both energy and angle-dispersive X-ray diffraction, to determine the residual stress as a function of depth for flat specimens made from two γ-based TiAl alloys in as-polished and exposed (700 °C/1 h/air) conditions. It has been found that large compressive stresses are present at the surface of as-polished specimens due to sub-surface deformation that remains despite careful grinding/polishing. After exposure the compressive residual stresses at depths below 0.6 μm are significantly reduced, probably due to the annealing out of deformation induced defects, but remain compressive in nature. However, within 0.6 μm from the outer surface, tensile stresses were present and reached a maximum of around +350 MPa at 0.15 μm depth. The extent to which the development of superficial tensile residual stress and reduced deeper compressive stress contribute to embrittlement remains unclear. [ABSTRACT FROM AUTHOR]

Abstract: The equilibrium site fraction of vacancies increases with temperature and, thus, annealing and rapid quenching may lead to states with a significant vacancy supersaturation. Excess vacancies can then gradually annihilate at available sinks represented by jogs at dislocations, by grain boundaries or free surfaces. Significant supersaturation by vacancies may also lead to the nucleation and growth of Frank loops acting as additional sinks. Three models corresponding to three different annihilation mechanisms are developed in this paper. They refer to annihilation of excess vacancies at jogs at dislocation with a constant density, at homogeneously distributed Frank loops with a constant density and at grain boundaries. The simulations based on the models are performed for individual annihilation mechanisms under isothermal and non-isothermal conditions as well as for simultaneous annihilation of vacancies at Frank loops and dislocation jogs and grain boundaries using different cooling conditions. [Copyright &y& Elsevier]

Abstract: Shot peening is assessed as a potential method for improving the fatigue strength of titanium aluminide alloys based on γ(TiAl). Metallographic characterization was performed with regard to surface roughness, microhardness, residual stress profiles, and structural changes occurring in the subsurface region. The fatigue performance that can be achieved by shot peening mainly depends on the compressive stresses left in the surface region, but is also governed by the fine details of the microstructure. In view of the anticipated high-temperature application of TiAl alloys, particular emphasis is placed on the thermal stability of the surface hardening developed by shot peening. [Copyright &y& Elsevier]

Mechanical or deformation twinning is first explained as a shearing mechanism and compared to crystallographic slip. Then twinning in several metals is discussed. A twin can be represented as a thin layer, bound by an upper and lower plane. The average width of a disk-shaped twin is either given by the average diameter of a microregion (grain) or is calculated in the case of twin nuclei. An energy balance is outlined in detail for the untwinned and twinned status of a representative volume element. The elastic strain energy due to the twinning shear eigenstrain is studied in detail both analytically and numerically. An energy criterion for the stability of equilibrium allows to formulate a twinning condition which yields a minimum thickness in the case of a deformation twin. In the case of a twin nucleus Onsager''s principle of maximum dissipation rate is engaged as a further criterion to find the dimensions of the twin nucleus. Comparisons with experimentally observed twins in TiAl intermetallics are reported finally. Further consequences of the study are listed, such as the estimation of the critical resolved shear stress for twinning. [Copyright &y& Elsevier]

Abstract: The quality of wrought discs made from large Ti-45Al-5Nb-0.2B-0.2C (at. %) ingots has been assessed at various stages of processing. Such components suffer from different types of defects that can usually be traced back to structural and chemical inhomogeneities that are present in the starting ingot. In conventional manufacturing routes the imparted mechanical strain is often insufficient for complete dynamic recrystallisation, particularly for large parts. In an attempt to overcome these problems, a new multi-stage method has been developed, which has the advantages that: i) the strain is significantly higher than that imparted using conventional routes, ii) the flow direction of the material is inverted between the first and second processing stages in the sense that the length of the ingot is first increased via extrusion, and then the resulting extruded bar is shortened via axial forging in the subsequent processing stages, and iii) the geometrical constraints associated with the production of large components from ingots can be overcome. The reliability of the method has been assessed by metallographic and mechanical characterisation. The challenges that have to be addressed for the new technique to be employed for the manufacture of large components are discussed. [Copyright &y& Elsevier]

Abstract: The influence of the Al content and the addition of further alloying elements on the cast microstructure of γ(TiAl)+α2(Ti3Al) alloys has been examined. The results show that particularly fine and homogeneous microstructures without strong segregation can be obtained for certain alloy compositions solidifying through the β phase. This behavior can be attributed to the avoidance of peritectic solidification and to the alloying influence on the kinetics of the β⇒α transformation following solidification. The experimental findings were used to propose a design concept for γ-TiAl+α2-Ti3Al alloys. This concept aims at the production of high-quality castings as well as at ingot material for wrought processing routes because the chemically homogeneous and fine-grained microstructures would be a good precondition for improved workability. [Copyright &y& Elsevier]

Two-phase titanium aluminide alloys are being considered as lightweight materials to replace nickel-based superalloys in some high-temperature aero-engine applications. However, at an intended service temperature of 700 °C the strength and creep resistance of the titanium aluminides are inferior to those shown by superalloys. Therefore, precipitation-hardening treatments using carbon additions were tested for an improvement of the strength. Ageing of a Ti-48.5 at. % Al-0.4 at. % C alloy for 24 h at 750 °C after a solution treatment for 4 h at 1250 °C, followed by quenching, leads to the formation of perovskite-type precipitates, significantly improving the high-temperature strength. The size distribution and volume fraction of the perovskite precipitates were measured using small-angle neutron scattering (SANS). The results after extended ageing periods show that although the precipitate size distribution changes significantly, the effect on the yield strength is relatively small. [ABSTRACT FROM AUTHOR]

Thermal neutrons were used to investigate the texture of intermetallic Ti[sub 47]A1[sub 49](Cr, Nb, Mn, Si, B)[sub 4] and Ti[sub 45]A1[sub 45]Nb[sub 10] alloys. The aim was to learn about the influence of different extrusion temperatures and annealing treatments. A relatively strong ingot texture was weakened by extrusion, independently of the processing temperature (1250-1380 °C). However, subsequent annealing has no remarkable influence on the texture sharpness. Therefore, texture has only a minor influence on the anisotropy of hotworked materials. A comparison of the two TiA1 alloys shows a pseudo-cubic deformation behavior for Ti[sub 45]A1[sub 45]Nb[sub 10], which results in a 'double-fiber' < 101] < 110] texture. This pseudo-cubic behavior indicates a high potential of Ti[sub 45]A1[sub 45]Nb[sub 10] for plain strain deformation with sufficient ductility at ambient temperature. [ABSTRACT FROM AUTHOR]

Resilience is the ability to deal with shocks and stresses, including the unknown and previously unimaginable, such as the Covid-19 crisis. This paper assesses (i) how different farming systems were exposed to the crisis, (ii) which resilience capacities were revealed and (iii) how resilience was enabled or constrained by the farming systems' social and institutional environment. The 11 farming systems included have been analysed since 2017. This allows a comparison of pre-Covid-19 findings and the Covid-19 crisis. Pre-Covid findings are from the SURE-Farm systematic sustainability and resilience assessment. For Covid-19 a special data collection was carried out during the early stage of lockdowns. Our case studies found limited impact of Covid-19 on the production and delivery of food and other agricultural products. This was due to either little exposure or the agile activation of robustness capacities of the farming systems in combination with an enabling institutional environment. Revealed capacities were mainly based on already existing connectedness among farmers and more broadly in value chains. Across cases, the experience of the crisis triggered reflexivity about the operation of the farming systems. Recurring topics were the need for shorter chains, more fairness towards farmers, and less dependence on migrant workers. However, actors in the farming systems and the enabling environment generally focused on the immediate issues and gave little real consideration to long-term implications and challenges. Hence, adaptive or transformative capacities were much less on display than coping capacities. The comparison with pre-Covid findings mostly showed similarities. If challenges, such as shortage of labour, already loomed before, they persisted during the crisis. Furthermore, the eminent role of resilience attributes was confirmed. In cases with high connectedness and diversity we found that these system characteristics contributed significantly to dealing with the crisis. Also the focus on coping capacities was already visible before the crisis. We are not sure yet whether the focus on short-term robustness just reflects the higher visibility and urgency of shocks compared to slow processes that undermine or threaten important system functions, or whether they betray an imbalance in resilience capacities at the expense of adaptability and transformability. Our analysis indicates that if transformations are required, e.g. to respond to concerns about transnational value chains and future pandemics from zoonosis, the transformative capacity of many farming systems needs to be actively enhanced through an enabling environment. [Display omitted] • Comparison of pre-Covid-19 findings and Covid-19 crisis • All systems displayed adequate agility to activate coping capacities. • The crisis triggered reflexivity about the operation of the farming systems. • Transformative capacities were not observed. • The systematic resilience assessment revealed system vulnerabilities. [ABSTRACT FROM AUTHOR]