The Structure And Mechanical Properties Of Cementite In Nb-heavy Rail Steel: First Principle Study

There are rich niobium resources in Baotou. The microalloying effect of niobium on the application of heavy rail steel has become hot issues that scholars paid so much attention. The heavy rail steel in China is pearlite-type steel with high carbon content. Large quantity of cementite can guarantee high strength, hardness and wear resistant in heavy rail steel. Pearlite-type steel have appeared rail surface cracks due to cementite. The structure and mechanical properties of cementite in Nb-heavy rail steel have not been studied at the micro-scale. This dissertation studies the crystal structure and mechanical properties of cementite(Fe3C) based on density function theory by using first principle. The discussions focus on crystal structure, stability, mechanical properties and anisotropy for Fe3C,(Fe11Nb8d)C4and (Fe11Nb4c)C4. Finally, the differences between electronic structure and mechanical properties of ferrite and cementite are systematically studied. The anisotropic behaviors along different crystal planes for α-Fe are discussed. The main conclusions are as follows:(1) Our calculations indicate that the cohesive energy of (Fe11Nb8d)C4and (Fe11Nb4c)C4is larger than Fe3C,which illustrate niobium element can exist in cementite. The analysis of electronic structure shows that the structure of cementite is dominated by Fe-Fe metal bonding, the impact of Fe-C covalent bonding is not big. Moreover, Niobium atoms in cementite that cause distortions of lattice, which promote that the carbon atoms are close to Fe atoms and hence strengthen Fe-C bonding.(2) Our calculations have predicted that the Vickers hardness of Fe3C is11.21GPa. When Niobium atoms replace iron atoms in cementite to form an alloy cementite, the value of the bulk modulus and the Young’s modulus is increased. Our results indicate a very large shear anisotropy on the {100} plane of cementite.(Fe11Nb8d)C4can improve the anisotropy of cementite. we calculate the value of the Debye temperature is480K,486K and501K,which indicate that the linear relationship between the Debye temperature and the shear modulus.(3) The electronic structure and mechanical properties of ferrite (α-Fe) and cementite(Fe3C) have been studies. Our results indicate the differences of mechanical properties between the two structures are small, which indicate mechanical properties are dominated by Fe-Fe metal bonding, the impact of Fe-C covalent bonding is not big for materials and is mainly determined by the crystal structure.(4) The anisotropic behaviors along different crystal planes for α-Fe are studied. Our results indicate the differences of the anisotropy are because of the crystal planes with different crystal orientations. By establishing the relationships between elastic anisotropic coefficients and Young’s modulus, we have predicted the anisotropic degree of different crystal planes: the crystal planes of (112) is best, the crystal planes of (011) is inferior, the crystal planes of (001) is worst.