Structure of interfaces in the lamellar TiAl: effects of directional bonding and segregation

In this paper we first analyze the effect of covalent bonding upon the structure of stacking fault type planar defects on (111) planes and γ/γ interfaces in the lamellar TiAl. A remarkable finding of ab initio LDA based calculations is that the anti-phase domain boundary (APB) is not a metastable fault in TiAl. The main reason why this has not been found in studies employing central-force potentials is that the covalent bonding naturally leads to a rather high APB energy and, consequently, to an instability. A combination of atomistic calculations and HREM observations revealed that the covalent character of bonding, in particular the d-bond between Ti atoms, also remarkably affects the local atomic structure of the ordered twin with APB. The propensity for preservation of these bonds induces an asymmetry in the structure of this interface. A similar effect has also been found in the complex stacking fault (CSF). Understanding of this phenomenon is crucial when interpreting HREM observations. Furthermore, the Monte Carlo calculations presented in this paper suggest that in Ti-rich alloys a significant segregation takes place to some γ/γ interfaces leading to the formation of a very narrow region of the DO19-Ti3Al in their cores.