First Principles Study On The Mechanical Properties Of Metal Borides In High Speed Steel

Machinery and tool manufacturing is the main application field of high speed steel. Improving the performance of high speed steel and ensure to be able to apply to harder situation, we can get more benefits. So it has got an unprecedented attention. We can improve the hardenability, mechanical properties, physical properties, chemical properties and of high speed steel by adding trace boron in high speed steel. Borides replace carbides as hard alloy phase, can possess alloys of higher hardness and abrasive resistance, and lower the cost. Recently, high speed steel has attracted more and more attention. However, few people pay close attention to the properties of borides. So we focus on the calculation and analysis of borides. We analyse the stability, mechanical properties and electronic properties. The results can provide theoretical support for designing, selecting metal borides and improving the efficiency of study. We have predicted the stability, mechanical properties and electronic properties of borides by the first principle calculation combined with semi-empirical theory. Although it is hard to do the experiments, we can offer reliable theoretical guidance. The main research contents and results are as follows:We have got the cohesive energy and formation enthalpy of MB (M=V, Nb and Ta), MB2(M=V, Nb, Ta, Cr, Mo and W) and M3B2(M=V, Nb and Ta). The values of the cohesive energy and formation enthalpy are negative which implies the thermodynamically stable structure. We also get the elastic constants of these metal borides and meet stable conditions of the structures. It states the structural stability of these metal borides again. The Young’s modulus (E) for MB, MB2and M3B2is within ranging of391.1-524.5GPa,33.7-563.4GPa and238.5-255.1GPa. The bulk modulus (B) is within ranging of263.0-278.4GPa,84.2-324.7GPa and238.5-255.1GPa. The shear modulus (G) is within ranging of179.7-230.2GPa,138.7-241.1GPa and171.4-203.0GPa. The hardness is within ranging of7.75-16.87GPa,4.3-13.4GPa and9.5-21.63GPa. The compound with high hardness also has large bulk modulus. The Poissons ratio of MB (M=V, Nb and Ta) and M3B2(M=V, Nb and Ta) are within ranging of0.166-0.245and0.18-0.22, they are all under0.26which implies that they have the feature of ceramic. The Poisson’s ratio of MB2(M=V, Nb, Ta, Cr, Mo and W) is range from0.18to0.32which show trend from brittle materials to ductile materials.The Debye temperature of MB (M=V, Nb and Ta) and M3B2(M=V, Nb and Ta) are within ranging of419.3-794.3K and299-526K. The longitudinal velocities are within ranging of6178.0-10045.3m/s and5888-9431m/s. The transverse velocities are within ranging of3638.7-6384.2m/s and3540-5886m/s. We have calculated the universal anisotropic index AU and percent anisotropy AB and AG.We also build a curved surface of a three-dimensional (3D) which can state the elastic anisotropy of crystal structure on crystallographic directions and outline the projections of Young’s modulus at the (001) and (110) crystal planes. So we can analyse the anisotropy of MB2(M=V, Nb, Ta, Cr, Mo and W). We can summarize that the anisotropy of WB2is stronger than others MB2(X=V, Nb, Ta, Cr and Mo) and he mechanical anisotropy of Group VIB (CrB2, MoB2and WB2) is stronger than Group VB (VB2, NbB2and TaB2). We also calculate and analyse the electronic properties of MB2. Through the total density of states (DOS) and partial of states (PDOS) at equilibrium lattice constants and charge density distribution maps of MB2, we can conclude that the bonding behavior of MB2are the combinations of covalent and metallic bonds. We also have the Mulliken population analysis to the overlap population and the charge calculations and also find that the bonding behaviors of these borides are the combinations of covalent and metallic bonds.