First-Principles Study on the Phase Stability and Mechanical Properties of Boron Carbides in Boron-Bearing High-Speed Steel

The phase stability and mechanical properties of boron carbides in Fe-Cr-B boron-bearing high-speed steel were determined via the first-principles calculation method of the density functional theory. The types of boron carbides in this steel were determined through equilibrium thermodynamic phase diagrams and X-ray diffraction analysis of the material. In addition, the lattice constant, binding energy, electronic density of states, mechanical properties, and hardness of boron carbides were calculated with the VASP software. The results showed that VC, WC, Fe2B, Mo2B, W2B, and W2FeB2 were stable in Fe-Cr-B boron-bearing high-speed steel. Among these compounds, VC exhibited the highest thermodynamic stability (enthalpy of formation: -44.16 kJ mol(-1)). The boron carbides all exhibited metallic characteristics. Furthermore, the corresponding electronic density of states was contributed mainly by the P orbit of B and C atoms and the d orbit of Fe, Mo, V, and W. Except for the ratio of WC, the B-H/G(H) ratios of all phases were >1.75, indicating that these phases were all ductile. Poisson's ratios of similar to 0.25 were obtained, consistent with the ionic-covalent nature of the compounds. When the B measurement ratio increased, the hardness of the phases with rich B surpassed the hardness of the phases with rich C. The highest Debye temperature (906.65 K) was obtained for VC, which had the strongest covalent bond, as evidenced by the width of its pseudogap.