【作者】 薛金祥；

【导师】 刘燕萍； 王宝俊；

【作者基本信息】 太原理工大学 ， 机械工程， 2012， 硕士

【摘要】 铁是地球上含量最多的元素之一,但纯铁因强度、硬度、延展性较差而通常无法直接使用于工程中。然而,在铁中添加微量的合金元素就会对其力学性能和物理、化学性能产生很大的影响。因此各种类的合金钢被广泛的使用。由于TiC和TiN具有高硬度、低摩擦系数和化学稳定性好等优良性能,目前关于Ti,C,N合金元素对Fe基体性能的影响已经被广泛的研究。但Ti,C,N在Fe基中的微观强化机理和微观迁移机理尚不清楚,因此本文从原子、分子层次上研究了Ti,C,N在Fe基中的微观作用机理。首先,本文基于第一性原理赝势平面波方法研究了合金元素Ti,C,N对α-Fe基电子结构及键合性质的影响,计算了含Ti,C,N的Fe基固溶体的总能量、结合能和力学性能,分析了态密度、电荷聚居数、交叠聚居数和电荷密度,从理论上解释了在Fe基中固溶Ti,C,N后其性能改善的原因。结果表明：随着Fe基固溶体中Ti(0-12.5at%),C(0-11.11at%),N(0-11.11at%)含量增加,结合能略有增加；Ti,C,N的固溶使各Fe基固溶体在费米能级处强烈成键,结合能力增强,并且在费米能级附近出现赝能隙,表明固溶体中金属键与共价键共存。随着Ti,C,N含量的增加,C,N分别与Ti,Fe之间的共价键结合强度加强,部分C,N原子会与Ti原子结合形成TiC, TiN颗粒,起到沉积相颗粒强韧化作用。其次,本文通过采用CASTEP模块的LST/QST搜索过渡态方法,分别以Fe16(2×2×2)和Fe24(2×2×3)超胞作为Fe基体,计算了C和N原子在纯Fe和固溶Ti后Fe基中的迁移能垒。结果表明：当N和C原子占位相同时,N原子的迁移能垒小于C原子的迁移能垒。C,N原子与Ti原子间距越小,迁移能垒越大；C,N原子与Ti间距越大,迁移能垒越小。当Ti含量较小时,C,N原子的迁移能垒减小,当Ti含量较大时,C,N原子的迁移能垒增大。再次,本文通过搭建含有Ti,C,N合金元素的Fe(100)表面,并对其进行结构优化,系统研究了Ti,C,N在Fe(100)表面中的迁移情况,并通过分析电子结构阐述了Ti,C,N合金元素对Fe(100)表面的强化机理。结果表明,当Ti,C或者N原子单独向Fe(100)表面迁移时,其存在于表面第一层时结构稳定性最好；当Ti和C同时向Fe(100)表面迁移时,Ti和C同时占据第一层时结构稳定性最好；当Ti和N同时向Fe(100)表面迁移时,Ti和N同时占据第一层时结构稳定性最好。最后,本文搭建了具有4层Fe和4层TiC(N)的界面模型Fe/TiC和Fe/TiN,通过计算界面模型的总能量和结合能,分析态密度、交叠聚居数和电荷密度,从理论研究了Fe/TiC和Fe/TiN界面之间的结合强度、电子结构和键合作用。计算结果表明：在Fe/TiC和Fe/TiN界面中当Fe原子与C或者N原子直接结合时,Fe与C,N原子形成强的共价键；当Fe原子与Ti原子直接结合时,形成金属键,即Fe/TiC和Fe/TiN界面的结合强度主要由界面中的Fe-C和Fe-N共价键贡献。更多还原

【Abstract】 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.更多还原