The structural, mechanical and thermodynamic properties of Ti-B compounds under the influence of temperature and pressure: First-principles study

Abstract

The crystal structure and elastic properties of Ti-B compounds were systematically studied by first-principles methods at 0 K. The lattice parameters obtained by generalized gradient approximation (GGA) at 0 GPa match well with those previously reported by other authors and the values of V/V0 under pressure are arranged in the following order: Ti2B < TiB < Ti3B4 <TiB2. The Ti-B compounds are mechanically stable under pressure and their independent elastic constants increase with pressure. Polycrystalline elastic moduli determined from the elastic constants have a similar trend with the applied pressure. From these results, it is derived that the volume change resistance, shear deformation resistance, stiffness, elastic anisotropy and plasticity of the polycrystalline Ti-B compounds increase with pressure. The volume change resistance, shear deformation resistance and stiffness of Ti–B compounds improve with pressure. Ti2B tends to be ductile with pressure, while others exhibit brittle behavior where the brittleness of materials is ranked in the following descending order: TiB2 > Ti3B4 > TiB. Besides, the elastic moduli predicted for TiB2 are larger than others compounds of Ti–B, which indicate that TiB2 has the larger hardness than others. The effects of pressure and temperature on the Debye temperature, heat capacity and bulk modulus of the polycrystalline Ti-B compounds are predicted by using a quasi-harmonic Debye model of the phonon density of states to include the vibrational contribution at the Gibbs energy of the compound. The applied pressure could improve the Debye temperature ΘD and the bulk modulus B of Ti–B compounds at the given temperatures, and the heat capacities (Cv and Cp) decrease slowly with pressure at a constant temperature.