Unexpected relationship between interlayer distances and surface/cleavage energies in γ-TiAl: density functional study

Density functional calculations were performed to study the γ-TiAl (001), (100), (110) and (111) surfaces. The (100) surface is the most stable under Ti-rich conditions, while the Al-termination (110) surface becomes the most stable with the increase of Al chemical potential. We calculate that in γ-TiAl intermetallic compound the larger the interlayer distance, the larger the surface energy and cleavage energy. This is different from the situation in a pure metal. This phenomenon can be explained by the analysis of the bonding characteristics in γ-TiAl. In particular there are both metallic and covalent bonds in γ-TiAl, and the strongest covalent bonds mainly focus on the center of three Ti–Al–Ti atoms. It is the covalent bonds that affect greatly the cleavage energy, the surface energy and the surface stability.