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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 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]

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]

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 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]

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]