Phase stability, mechanical properties and electronic structure of hexagonal and trigonal Ti5Al2C3: An ab initio study

This paper investigates in detail about the phase stability, mechanical and electronic properties of Ti5Al2C3 in the hexagonal (H) and trigonal (T) structures using the first-principles density functional method. Through a series of calculations, including the total energy as a function of volume, the Gibbs free energy as a function of pressure, the elastic stabilities and electronic structures, we have confirmed that T-type Ti5Al2C3 is more energetically stable structure than H-type. It is found that the two structures is more compressible in the c direction than along the a direction, and the hexagonal structure is easily compressed compared with the trigonal structure. It is interesting to note that the c/a ratio of T-Ti5Al2C3 remains unchanged when the pressure is above 50 GPa. The studies of elastic properties show that T-Ti5Al2C3 has better mechanical properties, while the H-Ti5Al2C3 is more ductile than others Ti–Al–C compounds. The calculated Debye temperature and elastic anisotropic factors indicate that T-Ti5Al2C3 has a higher thermal conductivity and degree of anisotropy. By examining the density of states and Mulliken analysis under various pressures, we also found that the Ti–C bonding in the two structures is stronger than the Ti–Al bonding and indicates that the Ti–C bonding is more resistant to deformation than the Ti–Al bonding. Moreover, the two structures become less stable with the pressure increasing.

► The triagonal (T) structure of Ti5Al2C3 is more stable than its hexagonal (H) one.
► The c/a ratio of the T-Ti5Al2C3 remains constant in the pressures above 50 GPa
► The mechanical characters of T-Ti5Al2C3 are better as compared with H-Ti5Al2C3.
► A higher thermal conductivity can be expected for the T-phase.
► Their chemical bonding features (covalency vs ionicity) have been elucidated.