Ti-Ti bonding in γ-TiAl and F.C.C. Ti

The flow and fracture of high-temperature intermetallic alloys is strongly influenced by the propagation and cross-slip of ordinary and super dislocations. One factor associated with the poor ductility of TiAl is an apparently large Peierls stress, or lattice friction stress for the glide of ordinary (½〈110〉) dislocations. This Peierls relief is intimately related to the underlying crystal and electronic structure of TiAl, and specific bonding states have been identified by some investigators as contributing to the pinning of dislocations along certain line directions. In this study we quantify the relative strength of these bonds using several complementary electronic structure methods. Aspects of the electronic structure, equilibrium lattice constants and bond energies of γ-TiAl are compared with hypothetical f.c.c. Ti as obtained by the full-potential linearized augmented-plane-wave (FLAPW) method, the DMol molecular cluster method and the linear muffin-tin orbital (LMTO) Green function (GF) method. Pair energies, bond occupations of Ti-Ti and Ti-Al bonds in L1₀ TiAl and f.c.c. Ti are calculated using the LMTOGF method. Comparing the d_xy bonding states for these two crystal structure, we find the in-plane directional d-d bonding between Ti atoms on the (001) plane are strengthened in TiAl relative to f.c.c. Ti. These observations suggest that the alternating (001) planes of Al and Ti atoms presented in TiAl enhance these bonding states. Conversely, transition-metal ternary additions substituted on the Al sites in TiAl are expected to weaken these bonding states. Using the FLAPW method, we examine the changes in charge density in the Ti(001) planes when Mn atoms are substituted on Al sites in TiAl. Finally, LMTOGF methods are used to study the effect of Mn substitutional point defects on the Ti(001) d_xy bonding states.