Fe-N系统的第一性原理模拟研究

发布时间：2018-04-29 22:26

本文选题：DFT + Fe-N系统　；参考：《上海交通大学》2015年博士论文

【摘要】：N是钢中的常见添加元素,可以提高钢的各种性能,如强度、硬度,耐腐蚀性等。本论文通过基于密度泛函理论的第一性原理方法(DFT),并结合过渡态和热力学理论,系统的讨论了Fe-N系统中关于点缺陷和面缺陷的四个问题:N扩散进入BCC Fe的原子机制;FCC铁中N和点缺陷的相互作用;N和C原子在BCC铁中晶界的吸附,扩散机制;BCC Fe中对称晶界的迁移机制以及点缺陷对晶界迁移的影响。得到的结论如下:(1)N在BCC Fe表面的最适宜吸附位置和N的覆盖率有关。一旦进入基体,N倾向于占据八面体间隙位置,而不是四面体间隙。然而,四面体间隙在扩散中扮演着过渡态的角色,是扩散过程的鞍点,决定了扩散的能垒。和C在BCC Fe中的行为相比,两者有很多类似之处,但也有区别。除了在吸附能,扩散能垒等物理量上量的区别之外,N和C从BCC Fe(100)面扩散进入亚表面的原子机制完全不同,分析表明这是由于N和亚表层Fe原子之间的排斥作用造成。(2)在FCC Fe中,自旋极化(磁性)会减小N的溶解焓和空位的形成能;N-N之间存在排斥相互作用,且作用会被自旋极化减弱;N和空位(v)之间有很强的相互吸引作用,以至于一个空位可以吸附多个N,形成点缺陷团簇(NVC)。NVC的分布依赖于N的浓度和温度。计算发现随着N浓度和温度的改变,主要占据数的NVC是vN2,vN3,vN4和vN5。N的浓度会影响空位浓度,增大N的浓度可增大空位的浓度达8个数量级,但提高N浓度最终会使空位浓度饱和,这和Kirchheilm的热力学理论预测一致。(3)C和N会对晶界(GB)结构和结合能产生不同的影响。C原子会将构成BCC FeΣ5100(210)GB的两个晶粒拉近,而N原子会将它们推开,但分析结合能大小却发现C原子会弱化Σ5晶界,而N会强化Σ5晶界。C和N原子都会将构成Σ3100(112)GB的两个晶粒推开,且使其脆化。间隙原子与晶界的结合能可以分解为化学能和机械能两部分。间隙原子和Σ5晶界Fe原子之间形成的化学键的化学能是决定其结合能的关键,而间隙原子偏聚在Σ3晶界上则会引起很大的机械能。Σ5晶界的间隙比较大,可以容纳多个间隙原子,但应用第一性原理热力学方法,发现在本文所考虑的间隙原子浓度下,Σ5晶界只可能含有一个C或者N原子,否则会形成化合物。大角晶界可以看作由胞结构构成,间隙原子在晶界胞之间的跳跃实现了间隙原子在晶界中的扩散。计算发现C和N在Σ5晶界中的扩散机制不同。和体扩散过程相比,Σ5晶界会加速N而减弱C的扩散。C和N原子在Σ3晶界中的扩散机制基本相同,且非常类似于体扩散机制。和体扩散对比,Σ3晶界严重阻碍了C和N的扩散。(4)BCC FeΣ5100(210)和Σ5100(310)对称晶界的晶界能相差不大,但理想迁移能垒却相差很大。Σ5(310)相对来说更加稳定。间隙原子会大大提高晶界的迁移能垒,而空位对晶界迁移的影响则相对复杂。空位的参与会大大减小Σ5(310)的迁移能垒,但计算没有发现空位会减小Σ5(210)的迁移能垒。文中提出了描述晶界迁移的形核机制:晶界位错环模型(GBDL)。通过GBDL模型,计算的得到的迁移能垒可以用于预测晶界迁移趋势。
[Abstract]:N is a common addition element in steel, which can improve the properties of steel, such as strength, hardness, and corrosion resistance. In this paper, the four problems of point defects and surface defects in Fe-N systems are discussed systematically through the first principle method based on density functional theory (DFT), and the transition state and thermodynamics theory. The N diffusion enters BCC Fe. Atomic mechanism; the interaction of N and point defects in FCC iron; the adsorption of N and C atoms in the grain boundary of BCC iron, the diffusion mechanism, the migration mechanism of the symmetric grain boundary in BCC Fe and the effect of point defects on the migration of grain boundary. The conclusions are as follows: (1) the optimum adsorption position of N on the BCC Fe surface is related to the coverage of N. Once the matrix is entered, N tends to occupy However, the tetrahedral gap is not a tetrahedral gap. However, the tetrahedral gap plays a role in the transition state in diffusion, is the saddle point of the diffusion process, and determines the energy barrier of the diffusion. Compared with the behavior of C in BCC Fe, there are many similarities but also differences. Apart from the difference in the amount of physical quantities such as adsorption energy, diffusion energy barrier and so on In addition, the atomic mechanism of N and C diffusion from BCC Fe (100) surface to subsurface is completely different. Analysis shows that it is caused by the rejection between N and the subsurface Fe atom. (2) in FCC Fe, the spin polarization (magnetic) will reduce the dissolution enthalpy of N and the formation energy of the vacancy; there is a repulsion interaction between N-N and the effect will be weakened by spin polarization; N There is a strong mutual attraction between the vacancy (V), so that one vacancy can adsorb multiple N, and the distribution of the point defect cluster (NVC).NVC depends on the concentration and temperature of N. It is found that with the change of N concentration and temperature, the NVC of the main occupying number is vN2, the concentration of vN3, vN4 and vN5.N will affect the concentration of the vacancy and increase the concentration of the N. The concentration of large vacancies reaches 8 orders of magnitude, but increasing the concentration of N will eventually saturate the concentration of the vacancy, which is consistent with the thermodynamic theory of Kirchheilm. (3) C and N will have different effects on the structure and binding energy of the grain boundary (GB) and the.C atoms will close the two grains that constitute the BCC Fe sigma 5100 (210) GB, and the N atoms will push them open, but analyze the combination of them. It is found that the size of the C atom will weaken the sigma 5 grain boundary, and N will strengthen the.C and N atoms of the sigma 5 grain boundary, which will push the two grains of sigma 3100 (112) GB open and make them brittle. The binding energy of the gap atoms and the grain boundary can be decomposed into two parts of chemical energy and mechanical energy. The chemical energy of the chemical bonds formed between the gap atom and the Fe atom of the sigma 5 grain boundary is determined. The key to the binding energy is determined, and the segregation of the gap atoms on the sigma 3 grain boundary will cause a large mechanical energy. The gap between the sigma 5 grain boundary is larger and can accommodate a number of gap atoms. However, the first principle thermodynamics method is used to find that under the concentration of interstitial atoms considered in this article, the sigma 5 grain boundary may contain only one C or N atom, otherwise it will form a form. The large corner grain boundary can be regarded as the structure of the cell structure. The gap atom's diffusion between the grain boundary cells is realized. The diffusion mechanism of C and N in the sigma 5 grain boundary is different. Compared with the bulk diffusion process, the sigma 5 grain boundary will accelerate the N and weaken the diffusion mechanism of the C's diffusion.C and the N atom in the sigma 3 grain boundary. Basically the same, and very similar to the body diffusion mechanism. Compared with the body diffusion, the sigma 3 grain boundary seriously hinders the diffusion of C and N. (4) the grain boundary of the BCC Fe sigma 5100 (210) and the sigma 5100 (310) symmetry grain boundary is not quite different, but the ideal migration energy barrier is very different. The sigma 5 (310) is relatively stable. The gap atoms will greatly increase the migration energy barrier at grain boundary. The effect of vacancy on the migration of grain boundary is relatively complex. The participation of vacancy will greatly reduce the migration energy barrier of the sigma 5 (310), but the calculation does not find that the vacancy will reduce the transfer energy barrier of the sigma 5 (210). The nucleation mechanism describing the grain boundary migration is proposed: the grain boundary dislocation loop model (GBDL). The obtained migration energy barrier calculated by the GBDL model can be used for the preview. The trend of grain boundary migration.

【学位授予单位】：上海交通大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG141

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