First Principles Calculations On The Physical Properties Of Transition-metal Borides

Due to the excellent properties of good heat resistance, good wear resistance, high hardness, high thermal conductivity, high melting point, the transition- metal borides have aroused more and more interests of researchers. we have studied the thermodynamic properties, electronic structure and elastic constants of the early-transition- metal borides crystallized. The detail contents are as follow:(1) We systematically investigated the crystal structure, stability, elastic properties, chemical bonding and electronic properties of the early-transition- metal(Sc, Ti, V, C r, Y, Zr, Nb, Mo, Hf, Ta, and W) monoborides using the ab initio calculations based on the density functional theory. The results indicated that all 11 TMBs crystallized to a Cr B-type structure are thermodynamically and mechanically stable. The elastic constants were calculated using the ?nite strain method. The correlation between the electronic structure and elastic properties was discussed. YB was found to have high machinability(B/C44=1.73) and low hardness(C44 =43 GPa). The weak interaction between the interleaved yttrium planes and weak pd bonding resulted in the good machinability of YB.(2) The thermodynamic of a class of early-transition- metal diborides with Al B2-type structure have been investigated using the quasi- harmonic Debye model. According to the characters of temperature dependent bulk modulus and coefficient of thermal expansion, these TMB2 compounds can be divided into three groups. The results also indicate that 4dand 5d-TMB2 compounds are good high-temperature structural materials.(3) The electronic and elastic properties of a class of early-transition- metal diborides with Al B2-type structure have been investigated using the ab initio calculation based on the density functional theory.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 electro nic 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.