【作者】 严家安；

【导师】 王崇愚；

【作者基本信息】 清华大学 ， 物理学， 2005， 博士

【摘要】 基于密度泛函理论,本文对位错芯区结构和杂质-位错相互作用进行了理论研究。利用第一原理总能计算,首次在 Peierls-Nabarro 框架下研究了量子力学自旋效应和广义梯度近似修正对体心立方结构铁中广义堆垛能以及刃位错芯区结构和性质的影响。计算结果表明,滑移系的广义堆垛能敏感于体系的自旋状态,因而对广义堆垛能的计算需要考虑与磁性相关的自旋效应;对滑移系[1-11](110),仅在包含自旋的情况下可以给出合理的广义堆垛能曲线。与非自旋极化的计算相比,自旋极化的计算给出了较窄的位错芯区、较高的不稳定堆垛能和较大的最大原子回复应力。利用第一原理 DMol 方法,研究了体心立方结构铁中[100](010)刃位错芯区间隙型碳掺杂的择优占位、能量学和掺杂电子结构。计算发现,在位错芯区,电荷密度呈现不均匀分布;芯区 Fe 原子的局域态密度在 Fermi 能级附近出现明显劈裂;C 原子与近邻 Fe 原子成键,且杂化主要来自 C 的 2p 轨道和 Fe 的 3d轨道。能量学计算表明,与压缩区相比,C 原子更倾向于进入位错的扩张区和芯区中心,位错芯区中心对 C 原子表现出捕获效应,因而可形成碳-位错复合体。利用第一原理计算,研究了 B2 结构 FeAl 中轻杂质 B、C、N、O、P 和 S与刃位错的相互作用以及掺杂效应,通过将杂质与位错的相互作用分解为原子尺寸不匹配相关的力学效应和电子结构相关的化学效应,分析了这两种效应对杂质-位错相互作用的影响,并与经典理论进行了对比。基于实验观测结果,研究了 B 在 FeAl [100](010)刃位错芯区的偏聚,讨论了影响 B 偏聚的可能因素。B 的 Cottrell 气团出现在位错压缩区且伴有 Al 减少的实验现象不能归结为简单的 B 替换 Al 或间隙式 B 原子掺杂。计算表明,存在如下几种可能的偏聚机制:B 在位错芯区的扩张区以间隙原子形式存在,与经典的 Cottrell 理论预期相符;同时,B 间隙原子借助于 B 替位能够在压缩区和位错线形成聚集态;B 原子倾向于与空位形成复合体而向位错线偏聚。更多还原

【Abstract】 Based on density functional theory, the dislocation core structure and theimpurity-dislocation interactions are theoretically investigated in thisdissertation.The effects of spin polarization and generalized gradient corrections onthe generalized stacking-fault (GSF) energies and the edge dislocationproperties in bcc Fe are firstly studied by using the first-principles totalenergy calculations within the framework of Peierls-Nabarro model. Ourcalculation shows that the GSF energy is sensitive to the spin state of thesystem and only spin-polarized calculations can give a reasonable GSF energyfor the [1-11](110) slip system. Hence, it is crucial to include thespin-polarization for calculating the GSF energies in bcc Fe. We demonstratethat the spin-polarized calculations give a narrower dislocation core width,higher unstable stacking fault energy, and larger maximum restoring stress, ascompared with the non-spin-polarized calculations.Using the first-principles real-space DMol method, we have investigatedthe site preferences, energetics, and doping effects of interstitial carbon in thebcc Fe [100](010) edge dislocation core. It is found that: (i) aninhomogeneous electron density distribution occurs in the dislocation coreregion;(ii) an evident splitting of the local densities of states near the Fermienergy can be identified for the Fe atoms in the dislocation core;(iii) thehybridization between C and Fe mainly comes from C-2p and Fe-3d orbitals.Energetic calculations show that C prefers to segregate to the expansionregion and the center in the dislocation core. The strongest trapping effectappears at the dislocation core center, indicating the formation ofC-dislocation complex.The interactions of light impurities B, C, N, O, P, and S with [100](010)edge dislocation in B2-ordered FeAl as well as their doping effects are studiedin detail. The impurity-dislocation interaction is attributed to two parts: themechanical effect which is correlated with atomic size misfit, and thechemical effect which is related to the electronic bonding. The underlyingfactors governing the impurity-dislocation interactions are analyzed.The segregation of boron to [100](010) edge dislocation core inB2-ordered FeAl is studied through the first-principles total energycalculations. Several possible microscopic mechanisms are initially proposedaccording to the experimental observations. We demonstrate that the observedboron Cottrell atmosphere accompanied with large Al depletion in thecompressed region of dislocation core can not be ascribed to the singlesubstitution of boron for Al or simple interstitial states. Instead, ourcalculation results suggest that: (i) boron can segregate to the dilated region asinterstitials, which is consistent with the classical Cottrell theory;(ii) theboron interstitials possibly cluster together via the bridges of B substitution(for Al) both along the dislocation line and in the compressed regions, and (iii)boron and vacancy tend to form boron-vacancy complex and segregate to thedislocation line.更多还原