| Iron is one of the most abundant elements on earth, but the pure iron can not be directly used in engineering for its low strength, low hardness and low scalability. Adding small amounts elements will have a huge impact on mechanical, chemical and physical properties of iron. So a variety of alloy steels are widely used. Currently, TiC and TiN have characteristic of high hardness, low friction coefficient, and good chemical stability, etc. So, the effect of Ti, C and N in bulk Fe have been extensively researched. But its microscopic strengthening mechanisms and microcosmic migration principle are unclear. So, the studies of microscopic mechanism from its atomic and molecule level are studied particularly important. The results are summarized as follows:First, the electronic structures and bond characters of bulk α-Fe with Ti, C, N additions were studied using the first-principle pseudopotential plane-wave method. The total energy, cohesive energy and mechanical property were calculated, the mulliken population, overlap population, density of states and charge density were also analyzed, which could give a microscopic reason why the mechanical property was improved after the infiltration of Ti, C, N in bulk Fe. The calculated results show that as the alloying elements Ti(0-12.5at%), C(0-11.11at%), N(0-11.11at%) content increasing, the cohesive energy of alloy increases slowly and the structures keep stable. The addition of Ti, C, N in the alloys enhanced reciprocal hybridization in Fermi energy level, the binding ability of Ti, C, N, Fe becomes stronger. The pseudo-gap near the Fermi energy level means coexistence of covalent and metallic bonds in alloy. With the content of alloying elements increasing, the covalent bonding between C, N and Ti, Fe becomes stronger, part of C and N atoms will binding with Ti atoms and form TiC, TiN particle, the dispersion strengthening will be effected.Second, in this paper, the energy barrier for C and N atoms migration in pure Fe and bulk Fe with Ti atom have been studied using the LST/QST method of CASTEP module, when the Fei6(2x2x2) and Fe24(2x2x3) super cell models were used as bulk Fe, respectively. The results show that when the N and C atoms occupy the same position, the energy barriers for N atom migration in bulk Fe are less than C atom. When the distance of C atom and Ti atom or N atom and Ti atom is shorter, the energy barriers for migration is greater; on the contrary, When the distance of C atom and Ti atom or N atom and Ti atom is greater, the energy barriers for migration is less. When the content of Ti is low in bulk Fe, the energy barriers for C, N atoms migration in bulk Fe are decreased; Otherwise, when the content of Ti is large, the energy barriers for C, N atoms migration in bulk Fe are increased. Third, the migration of the Fe(100) planes with Ti, C and N also have been systematically studied through optimizing the crystal structures of the Fe(100) planes with Ti, C and N in this paper. The strengthening mechanisms of Fe(100) planes with Ti, C and N were analyzed from the electronic structures. The structure of Ti, C or N alone occupied first player is best stability. The structure of two Ti and C simultaneously occupied the first player is best stability. The structure of Ti and N simultaneously occupied the first player is best stability.Last, the interface models of Fe/TiC and Fe/TiN which respectively contents four layers of Fe and TiC(N) were also builded in this paper. The total energy and cohesive energy of the interface models were calculated, the density of states, overlap population and charge density were also analyzed, which can theoretically explained the adhesion, electronic structure, and bond characters of Fe/TiC and Fe/TiN interfaces. The calculated results show that Fe atoms form strong covalent bond with the C(N) atoms and form metallic bond with Ti atoms in the interfaces. So the strong interface adhesion of Fe/TiC and Fe/TiN interfaces are contributed by the strong covalent bond of Fe-C and Fe-N. |
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