Hot-working of intermetallic γ titanium aluminides is often hampered by their liability to shear localization and subsequent crack formation, thereby preventing a refinement of the microstructure by dynamic recrystallization. The mechanisms leading to shear localization and cracking were microstructurally analyzed using scanning (SEM) and transmission (TEM) electron microscopic techniques.

Hot-working of γ Titanium Aluminides

The development of structural materials which can replace the currently used nickel-based superalloys in aircraft engines is an area of current research. The material developed should have a high specific strength up to at least the service temperature of 700 °C resulting in reduced centrifugal forces and an increase in engine efficiency. Intermetallic titanium aluminides are promising in this respect because of their low density and high structural stability which stems from the crystal structure. Of particular interest are the so-called high niobium containing TiAl alloys within the approximate composition range Ti-(41-47)at. %Al-(5-8)at. %Nb which contain the intermetallic phases γ(TiAl) and α₂(Ti₃Al) [1].

The microstructures of the alloys are chemically and structurally very inhomogeneous after casting due to segregation processes mainly consisting of lamellar colonies composed of γ(TiAl) and α₂(Ti₃Al) platelets and a small number of γ grains at the colony boundaries. Such inhomogeneities are very harmful to the alloys, which are intrinsically brittle in accordance with their structural stability, because critical values of constraint stress can develop after very small deformation strains and lead to premature failure. They must however be deformable to a certain degree when used for highly demanding components as aircraft engine blades to ensure safe service and also for manufacturing reasons. The chemical and structural inhomogeneities must therefore be reduced significantly.

The inhomogeneities can most effectively be reduced by hot-working. During hot-working the material is severely deformed at high temperatures thereby triggering recrystallization processes ideally leading to completely transformed and homogeneous microstructures.

Hot-working of γ titanium aluminides is, however, hampered by the very same features which provide for the structural stability. Accordingly, the mobility of the dislocations which mainly contribute to deformation is low. Also the mobility of the lattice atoms and the grain boundaries is low thereby impeding recrystallization. Moreover the material tends to deform only locally in narrowly confined regions, the so-called shear zones [2]. This results in large volumes of the material remaining nearly undeformed so that the desired material consolidation is not achieved. In this context the mechanisms leading to shear localization and cracking will briefly be discussed.

Formation of Shear Bands and Cracks

Figure 1 shows a shear band in the longitudinal section of a sample which underwent cyclic torsional deformation combined with compression. Under these deformation conditions extremely high accumulated deformation strains can be achieved so that the underlying hot-working mechanisms can be easily studied [2]. The shear band (arrow 1) is very narrow and cracks are formed within the band. Initial stages of shear band formation are manifested as kink bands (arrow 2). They develop when the lamellae bend beyond a critical value. The lamellae then become separated by dynamic recrystallization. The regions where the lamellae separate are preferred sites for the formation of very fine grains providing a high density of additional grain boundaries that can act as further sites for recrystallization. The deformation of such grains may consequently lead to intensive recrystallization resulting in a thickening of the buckled interfaces at the expense of the adjacent lamellae so that a shear band is eventually formed. Such concentrated deformation leads to extremely high strains within the shear bands. In the course of deformation voids develop at places where the transfer of deformation is difficult. Growth and coalescence of the voids, as seen in front of the crack tip (arrow 3 in the insert) lead to a complete separation of the material during later stages of deformation [2].

Deformation Processes within the Shear Band

The shear bands predominantly consist of γ grains which are intensively deformed by twinning (fig. 2). 1/6<112    ]{111} mechanical twins carry the deformation in the γ-phase beside ½<110] ordinary dislocations, <011], and ½<112] superdislocations [3,4]. Figure 3 shows a twin in atomic resolution. Twins become wider by the successive glide of twin dislocations along the twin boundaries. The intensive twinning was attributed to the Nb additions which reduce the stacking fault energies thereby promoting the dissociation of the superdislocations into twin dislocations [5].

However twins may also impede deformation by acting as glide obstacles. The ordinary dislocations in figure 4 are partly immobilized at the twin boundaries. This is indicated by the straight dislocation segments, which are aligned parallel to the twin boundaries (arrows 1 and 2). The resulting constraint stresses may support the nucleation of new grains at the twin boundaries. This is suggested from the α₂ grain shown in figure 5, which was formed within the γ matrix at the intersection zone of two twins (arrow), where very high stresses can be expected. Accordingly, in addition to grain boundaries, twin boundaries can also act as sites for recrystallization thereby further stabilizing the continued deformation along the shear bands in that the recrystallized grains are readily deformable in accordance with their low defect densities.

As a consequence of these findings a hot-working technique must be used which produces comparatively small shear stresses and high hydrostatic stresses to reduce the liability to shear localization and crack formation.

Acknowledgements
The author thanks S. Eggert, U. Lorenz, M. Oehring, J.D.H. Paul, and F. Pyczak for helpful discussions and support.

References
[1] Appel F. et al.: Intermetallics 8, 1283-1312 (2000)
[2] Froebel U. and Appel F.: Met. Mat. Trans. 38A, 1817-32 (2007)
[3] Blackburn M.J.: The Science, Technology, and Applications of Titanium, ed. by Jaffee R. and Promisel N.E., Pergamon Press, 1970, 633-43
[4] Lipsitt H.A. et al.: Met. Trans. 6A, 1991-96 (1975)
[5] Yamaguchi M. and Umakoshi Y., Prog. Mater. Sci. 34, 1-148 (1990)
[6] Lee J. K. and Yoo M. H., Metall. Trans. 21A, 2521-30 (1990)
[7] Paul J. D. H. et al.: Acta Mater. 46, 1075-85 (1998)

Author
Dr. Ulrich Fröbel (corresponding author for Email request)
Helmholtz-Zentrum Geesthacht
Institute of Materials Research
Geesthacht, Germany
http://www.hzg.de