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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]

Wrought TiAI products manufactured by conventional hot-working procedures, such as forging and extrusion, suffer from structural and chemical inhornogeneities, which are major concerns for the reliability of components. In an attempt to overcome these problems, TiAI alloys were hot worked by torsional deformation. Using this method, a much higher strain and mechanical work can be imparted, which triggers dynamic recrystallization. The metallurgical potential of this technique will be assessed. Particular emphasis is placed on shear localization processes, which often lead to premature failure of the workpiece. [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]