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This paper aims to systematically investigate the stability, electronic structures and mechanical properties of Mo2FeB2 and Mo2NiB2 ternary borides by the first-principles calculations. The models of Mo2FeB2, orthorhombic Mo2NiB2 (O-Mo2NiB2) and tetragonal Mo2NiB2 (T-Mo2NiB2) have been established and calculated. All three crystal lattices are thermodynamically and mechanically stable. O-Mo2NiB2 shows the highest bulk modulus (B), shear modulus (G) and Young’s modulus (E) as 303.1, 183.1 and 457.3 GPa, respectively. Furthermore, O-Mo2NiB2 is supposed to show much higher hardness of 20.4 GPa compared with those of Mo2FeB2 (17.3 GPa) and T-Mo2NiB2 (14.7 GPa). However, T-Mo2NiB2 shows the largest B/G ratio as 1.923, indicating its considerable ductility. According to the analysis of electronic structures and Mulliken population, it can be found that B-B, Mo-B and Fe/Ni-B covalent bonds play a predominant role in the borides’ mechanical properties. In O-Mo2NiB2, Ni-B bonds are obviously stronger, accounting for its higher elastic moduli and hardness. Benefiting from the strongest covalent bonding character, O-Mo2NiB2 shows the highest Debye temperature and melting point. In addition, O-Mo2NiB2 is expected to show the least anisotropy considering the anisotropic factors and 3D contour of Young’s modulus. On the whole, O-Mo2NiB2 is the best candidate boride for the applications in high-temperature circumstance in spite of its intrinsic brittleness. Keywords: Ternary borides, Mechanical properties, Chemical bonding character, Anisotropy, First-principles calculations