First - Principles Calculation And Molecular Dynamics Simulation Of Boron Carbon In Steel

Boron and carbon are very important nonmetallic elements in steels. The addition of carbon, even very small amount, can significantly influence the performance of the steels. In the present work, first principles calculations and molecular dynamics simulations are employed to investigate the physical fundamentals of the behavior of boron and carbon in the steel. Our studies include four aspects as follows. The first is the hard phases formed in the steel. The second is the occupation of boron and carbon in BCC and FCC iron lattice. The third is the interactions among the boron, carbon and vacancy in different iron lattices. The fourth is the migration and diffusion of boron and carbon in iron lattice.There are many transition metal carbides and borides in the steel. In the third chapter of this thesis, we mainly investigate the mechanical and thermodynamic properties of M02XB2 and MoX2B4 (X=Fe, Co, Ni) compounds in the steel. We find that all the ternary borides are thermodynamic and mechanical stability. Among the six ternary bordies, Mo2FeB2 has the biggest shear modulus(G) and Young’s modulus(E). According to the Poisson’s ratio, Mo2NiB2 and MoNi2B4 are ductile, other four ternary bordies are brittle. All of the six ternary bordies have high hardness except for MoNi2B4. The analyze of the electronic structure shows that these ternary bordies are chemically bonding behavior of metal and covalent bonds. Enthalpy shows an approximately linear function of the temperature>300 K. The entropy of these compounds are zero at 0 K, and increase rapidly with increasing temperature when temperature is below 450 K. The Gibbs free energy of MoCo2B4 is the lowest in Mo2XB2 and MoX2B4 (X=Fe, Co, Ni) compounds, which indicating the strongest formation ability.A boron atom prefers to occupy a substitution site in both BCC and FCC Fe. However, the octahedral interstitial site is the most favorable site for a carbon atom in the iron lattice. We can see that weak covalent bonds are formed between boron (carbon) atom and its neighbor Fe atoms. Two foreign interstitial atoms (B or C) repel each other. As the distance between two interstitial atoms increases, the repulsion tends to decrease.The equilibrium separation values of C-C, B-B and C-B are about 5.31 A and 6.42 A in BCC and FCC Fe lattice, respectively. A boron (or carbon) atom and a vacancy attract each other. A vacancy can serve as a tapping center and accommodate multiple interstitial atoms to form various clusters.The optimal migration paths and the minimum diffusion energy barrier of B and C in both Bcc and Fcc iron have also been investigated. Boron hops directly from its stable interstitial site to another adjacent interstitial site. The migration energy barrier are 0.69 eV and 1.02 eV. Carbon will move to the unstable interstitial site first and then hop to another adjacent stable interstitial site. The migration energy barrier are 0.87 eV and 0.98 eV. At last, the diffusion of B and C atoms in BCC and FCC Fe are simulated by molecular dynamics methods. The migrations of the B and C atom on different octahedral interstitial sites are vividly observed. We also obtained the diffusion activation energy and diffusion constant, our results are in good agreement with other available calculation and experimental results.