Phase stability, mechanical properties and electronic structures of TiAl binary compounds by first principles calculations

The stability, mechanical properties and electronic structures of Ti3Al, TiAl, TiAl2 and TiAl3 have been systematically investigated by the first-principles calculation. The four TiAl binary compounds are thermodynamically stable intermetallics depending on their negative formation enthalpy and cohesive energy. The bulk modulus (B), shear modulus (G), Young's modulus (E), Vicker's hardness (HV) and Poisson's ratio (ν) are calculated. With the increment of Al content, the bulk modulus and Poisson's ratio decrease while the shear modulus, Young's modulus and hardness gradually increase. The ratio of B/G decreases with Al increment, implying the weakening of ductility. In addition, the mechanical anisotropy of TiAl compound has been revealed by the analysis of the anisotropy index, three-dimensional surface contours of the Young's modulus and the planar projections on the low index planes (including (100), (010), (100) and (110)). TiAl shows the most anisotropic structure while Ti3Al has the least anisotropy. The density of states (DOS) and electron density distribution map are discussed to analyze the electronic structures and chemical bonding characters. The TiAl binary compounds are composed of both metallic and covalent bonds, and the covalent bonding character can be strengthened with the increase of Al content.