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It is commonly agreed that the α2 phase in γ(TiAl)-based alloys is quite difficult to deform plastically especially by order twinning. In the present study, for the first time we report deformation twinning in the α2 phase in a nearly-lamellar two-phase γ-TiAl alloy investigated by electron microscopy. Diffraction pattern and high-resolution images indicate that the twinning plane is ( 20 2 ¯ 1 ) and the twinning direction is [ 10 14 ¯ ] , which is essentially analogous to what has been observed in disordered hexagonal structures. The twinning and anti-twinning mechanisms are interpreted. The twinning is a combined result of superpartial dislocation glide activated by the dissociation of type superdislocation, accompanied by atomic shuffling and short-range diffusion which is facilitated by the anti-structural bridges diffusion mechanism during high-temperature compression. The appearance of deformation twinning in the α2 phase can play an important role for the excellent mechanical properties of high Nb containing TiAl alloys, which is of great importance for the further development of γ-TiAl alloys.

Intermetallics 100, 70 - 76 (2018). doi:10.1016/j.intermet.2018.06.006, The addition of carbon in TiAl alloys can improve the mechanical properties by precipitate hardening through the perovskite Ti$_3$AlC carbide. Usually precipitates coarsen during continuous annealing. However, in the Ti-45Al-5Nb-0.75C alloy a splitting of the perovskite carbides was observed in later stages of annealing. By investigation with transmission electron microscopy the details of this splitting process are revealed after annealing at 900 °C. The results show that the re-orientation of the $γ$ phase regions between sub-particles is associated with the splitting step from carbide needles into small sub-particles. $γ$ domains with a different orientation with respect to the $γ$ matrix nucleate and gradually replace the $γ$ matrix phase in regions between the carbide sub-particles. The progress of the splitting process is locally different in different carbides and also in one individual carbide. By increasing the temperature from 800 to 900 °C the growth of the emerging carbide conglomerates and the splitting process of the carbides are greatly accelerated. It is found that both are diffusion-controlled processes., Published by Elsevier Science, Amsterdam [u.a.]

This work monitors the nucleation, growth and coarsening of carbides in powder metallurgically processed, high Nb containing TiAl alloys. The effects of carbon content, annealing conditions and internal defects on the precipitation and stability of carbides were systematically investigated by high energy X-ray diffraction and transmission electron microscopy. In general, at 800 °C the carbide microstructure can still change significantly up to 1000 h of annealing. It is found that a higher carbon concentration promotes the carbide precipitation process and increases the thermal stability of carbides. Internal interfaces and other crystallographic defects act not only as heterogeneous nucleation sites for perovskite Ti 3 AlC carbides but also as carbon sinks. This retards the carbide nucleation in the interior of γ-TiAl grains. By homogenising the carbon distribution through solution heat treatment the nucleation of carbides in the γ-TiAl matrix is significantly accelerated as an effect of higher matrix carbon content.

Intermetallics 33, 44-53 (2013). doi:10.1016/j.intermet.2012.09.023, Published by Elsevier Science, Amsterdam [u.a.]

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.

The fracture behaviour of two-phase titanium aluminide alloys was characterized by fracture toughness tests performed in a wide temperature range on chevronnotched three point bending bars. Temperature and rate dependent deformation processes were characterized by temperature and strain rate cycling tests. The alloy investigated had compositions and microstructures which are currently being considered for engineering applications. The paper considers the effects of microstructure and crack tip plasticity on the crack growth resistance. The temperature dependence of the fracture toughness was rationalized in terms of micro-processes which determine the glide resistance of the dislocations in the plastic zone of crack tips. The implications of such observations for the engineering application of the materials are addressed briefly.