Twinning, 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. 

Fig. 5 Bright-field and dark-field imaging (A) BF image of lamellar y/ 2 titanium aluminide (B) corresponding SAD pattern (see Fig. 4) (C) and (D) DF images of the reflections marked in (B). Each of these reflections is characteristic for one twin variant of tetragonal y-TiAl appearing with high intensity in the corresponding DF image. (View this art in color at www.dekker.com.)...

Fig. 7 High-resolution transmission electron microscopy. HRTEM micrograph of lamellar y/ot2 titanium aluminide. From top to bottom, first twin variant of tetragonal y-TiAl, hexagonal 2-Ti3Al, second twin variant of y-TiAl and again ot2-Ti3Al. Incident beam direction for the tetragonal phases is 1 1 0, for the hexagonal phase 1 1 0. (View this art in color at www.dekker.com.)...

There are two primary types of lamellar interfaces in multiphase TiAl TLAl/TLAl interfaces between two twin-related lamellae with LIq structures, emd TLAl/TigAl interfaces between lamellae with Llo and DO19 structures, respectively. In the former case the interfacial planes are (111) and 10) vectors in adjacent lamellae are parallel. In the latter casejnterfacial plemes are (111) in TiAl emd (0001) in TiaAl eind (l 10) and (ll20) vectors in adjacent lamellae are parallel. 

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