Dislocations, TiAl

Similarly, in studies of lamellar interfaces the calculations using the central-force potentials predict correctly the order of energies for different interfaces but their ratios cannot be determined since the energy of the ordered twin is unphysically low, similarly as that of the SISF. Notwithstcinding, the situation is more complex in the case of interfaces. It has been demonstrated that the atomic structure of an ordered twin with APB type displacement is not predicted correctly in the framework of central-forces and that it is the formation of strong Ti-Ti covalent bonds across the interface which dominates the structure. This character of bonding in TiAl is likely to be even more important in more complex interfaces and it cannot be excluded that it affects directly dislocation cores. 

Figure 9.37. Schematic diagrams showing (a) a perfect edge dislocation in a structure consisting of alternating layers A and B, and (b) the climb dissociation of such a dislocation into two partial dislocations. As shown, the dissociation is due to the preferential precipitation of vacancies at A layers, or of interstitials at B layers. Burgers circuits are shown for the perfect dislocation (a) and the two par-tials (b). In BaTiOs, the A and B layers could correspond to BaO and Ti02 layers...

ATOMIC STRUCTURE AND PROPERTIES OF DISLOCATIONS AND INTERFACES IN TWO-PHASE TiAl COMPOUNDS... 

Fig. 2. Schematic pictures of the core structure of the 1/2(110] screw dislocation in TiAl. (a) Planar core spread in the (111) plane, (b) Non-planar core spread in both (111) and (111) planes.

Hence, a further advancement in studies of dislocations and interfaces in TiAl requires introduction of non-central forces into atomistic calculations. This character of bonding is, of course, included in ab-initio electronic structure calculations but such calculations are still not feasible on the scale needed when investigating dislocations and interfaces. Empirical attempts to include directionality of bonding have been made (Panova and Farkas 1995) but only employment of queintum mechemics based potentials can reveal the effects of non-central forces unambiguously. Such potentials have been emerging recently but have not yet been employed extensively in studies of lattice defects (Aoki and Pettifor, 1994 Memh et al., 1995 Pettlfor et al., 1995). Incorporation of... 

Mryasov ON, Gomostyrev YN, Freeman AJ. Generalized stacking-fault energetics and dislocation properties Compact versus spread unit-dislocation structures in TiAl and CuAu. Phys. Rev. B... 

T. Kawabata and O. Izumi, Dislocation Structures in TiAl Single Crystals Deformed at 77K, Scr. Metall., Vol 21,1987, p 433-434... 

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