The thermodynamic, electronic and elastic properties of the early-transition-metal diborides with AlB2-type structure: A density functional theory study

• The thermodynamic characters of TMB2 have been firstly studied using the QHA method.
• WB2 and TaB2 are good candidates for the structural application at high temperature.
• Most of the early-transition-metal diborides cannot be easily machined.
• The correlations between elastic constants and VECs of TMB2 have been discussed.

The thermodynamic, electronic and elastic properties of a class of early-transition-metal diborides (TMB2, TM = Sc, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W) with AlB2-type structure have been investigated using the quasi-harmonic Debye model and the ab initio calculation based on the density functional theory, respectively. According to the characters of temperature dependent bulk modulus and coefficient of thermal expansion, the TMB2 compounds can be divided into three groups. The results also indicate that 4d- and 5d-TMB2 compounds are good high-temperature structural materials. The five independent stiffness coefficients, bulk and shear moduli of the diborides are obtained and well agreement with the available experimental and theoretical data. The correlations between elastic properties and electronic structure are discussed in detail. Due to the high values of hardness, the VIB-transition-metal diborides with relatively high B/G and B/C44 ratios are still difficult to machine with usual methods.