發(fā)布時間：2018-04-29 22:26

  本文選題：DFT + Fe-N系統(tǒng)��； 參考：《上海交通大學(xué)》2015年博士論文

【摘要】：N是鋼中的常見添加元素,可以提高鋼的各種性能,如強度、硬度,耐腐蝕性等。本論文通過基于密度泛函理論的第一性原理方法(DFT),并結(jié)合過渡態(tài)和熱力學(xué)理論,系統(tǒng)的討論了Fe-N系統(tǒng)中關(guān)于點缺陷和面缺陷的四個問題:N擴散進入BCC Fe的原子機制;FCC鐵中N和點缺陷的相互作用;N和C原子在BCC鐵中晶界的吸附,擴散機制;BCC Fe中對稱晶界的遷移機制以及點缺陷對晶界遷移的影響。得到的結(jié)論如下:(1)N在BCC Fe表面的最適宜吸附位置和N的覆蓋率有關(guān)。一旦進入基體,N傾向于占據(jù)八面體間隙位置,而不是四面體間隙。然而,四面體間隙在擴散中扮演著過渡態(tài)的角色,是擴散過程的鞍點,決定了擴散的能壘。和C在BCC Fe中的行為相比,兩者有很多類似之處,但也有區(qū)別。除了在吸附能,擴散能壘等物理量上量的區(qū)別之外,N和C從BCC Fe(100)面擴散進入亞表面的原子機制完全不同,分析表明這是由于N和亞表層Fe原子之間的排斥作用造成。(2)在FCC Fe中,自旋極化(磁性)會減小N的溶解焓和空位的形成能;N-N之間存在排斥相互作用,且作用會被自旋極化減弱;N和空位(v)之間有很強的相互吸引作用,以至于一個空位可以吸附多個N,形成點缺陷團簇(NVC)。NVC的分布依賴于N的濃度和溫度。計算發(fā)現(xiàn)隨著N濃度和溫度的改變,主要占據(jù)數(shù)的NVC是vN2,vN3,vN4和vN5。N的濃度會影響空位濃度,增大N的濃度可增大空位的濃度達8個數(shù)量級,但提高N濃度最終會使空位濃度飽和,這和Kirchheilm的熱力學(xué)理論預(yù)測一致。(3)C和N會對晶界(GB)結(jié)構(gòu)和結(jié)合能產(chǎn)生不同的影響。C原子會將構(gòu)成BCC FeΣ5100(210)GB的兩個晶粒拉近,而N原子會將它們推開,但分析結(jié)合能大小卻發(fā)現(xiàn)C原子會弱化Σ5晶界,而N會強化Σ5晶界。C和N原子都會將構(gòu)成Σ3100(112)GB的兩個晶粒推開,且使其脆化。間隙原子與晶界的結(jié)合能可以分解為化學(xué)能和機械能兩部分。間隙原子和Σ5晶界Fe原子之間形成的化學(xué)鍵的化學(xué)能是決定其結(jié)合能的關(guān)鍵,而間隙原子偏聚在Σ3晶界上則會引起很大的機械能。Σ5晶界的間隙比較大,可以容納多個間隙原子,但應(yīng)用第一性原理熱力學(xué)方法,發(fā)現(xiàn)在本文所考慮的間隙原子濃度下,Σ5晶界只可能含有一個C或者N原子,否則會形成化合物。大角晶界可以看作由胞結(jié)構(gòu)構(gòu)成,間隙原子在晶界胞之間的跳躍實現(xiàn)了間隙原子在晶界中的擴散。計算發(fā)現(xiàn)C和N在Σ5晶界中的擴散機制不同。和體擴散過程相比,Σ5晶界會加速N而減弱C的擴散。C和N原子在Σ3晶界中的擴散機制基本相同,且非常類似于體擴散機制。和體擴散對比,Σ3晶界嚴重阻礙了C和N的擴散。(4)BCC FeΣ5100(210)和Σ5100(310)對稱晶界的晶界能相差不大,但理想遷移能壘卻相差很大。Σ5(310)相對來說更加穩(wěn)定。間隙原子會大大提高晶界的遷移能壘,而空位對晶界遷移的影響則相對復(fù)雜。空位的參與會大大減小Σ5(310)的遷移能壘,但計算沒有發(fā)現(xiàn)空位會減小Σ5(210)的遷移能壘。文中提出了描述晶界遷移的形核機制:晶界位錯環(huán)模型(GBDL)。通過GBDL模型,計算的得到的遷移能壘可以用于預(yù)測晶界遷移趨勢。
[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.

【學(xué)位授予單位】：上海交通大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2015
【分類號】：TG141

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