PDF《博士学位论文公示材料》

―1― 博士学位论文公示材料 学生姓名 孟凡顺 学号

1410066 二级学科 材料物理与化学 导师姓名 祁阳 论文题目 纯净及偏析铜晶界性质的第一性原理研究 论文研究方向 第一性原理研究晶界偏析 论文关键词 偏析;

断裂;

第一性原理;

铜晶界 论文摘要(中文) 晶界是材料中普遍存在的面缺陷,杂质或合金原子在晶界处的偏析可以降低或增强界面结合强度, 进而影响材料的力学性能.

得益于当今计算能力的飞速发展,基于密度泛函理论的第一性原理计算方 法成为材料科学发展中的强大工具.常用的理论研究晶界偏析的方法有两类:拉伸试验和 Rice-Wang 热力学模型.在应用这些方法研究对称倾侧晶界体系时,需对特定细节进行谨慎处理.另外,非磁性 原子偏析到磁性材料晶界处的性质已有报道,而磁性原子偏析到非磁性材料晶界的性质尚不明了.原 子偏析浓度、不同拉伸方案对晶界结合强度研究的影响等仍有需要进一步澄清的问题.本文利用第一 性原理方法,以纯净 Cu 晶界及 Fe 和Ag 偏析的 Cu 晶界为载体对上述问题进行了系统研究. 第一,在针对 Cu ∑5(310)[001]晶界的第一性原理拉伸试验中,虽然其能量的释放为平滑模式,但 具有镜面对称的高应变晶界模型却存在亚稳态.亚稳态的出现是由晶界模型在拉伸方向上的镜面对称 性引起的.Cu ∑5 晶界在低应变区倾向于对称倾侧结构,而在高应变区则倾向于非对称倾侧结构,其分 水岭在应变 6%-7%之间.通过对晶界模型施加剪切形变、晶粒相对滑移、特定原子施加扰动等一系列 搜索计算对亚稳态进行了确认.亚稳态的存在会延后晶界的断裂并高估晶界的力学参数.提出了估算 第一性原理拉伸试验误差的方案,强调了对高应变对称倾侧晶界模型中特定原子施加人为扰动的意义. 第二,理论上常规选择晶界断裂路径的方法为测试不同断裂路径获得相应断裂能,依据断裂能的 高低选择最优断裂方式.以Fe 偏析的 Cu 晶界为例,论述了一个特殊情况:晶界断口发生的表面重构 将导致断裂能降低,误导断裂路径的选择.此时应对以断裂能为判据的方法做出原子结构、电荷分布 等方面的分析,以确保断裂路径选择的合理性. 第三,利用第一性原理方法研究了 Fe 偏析对 Cu ∑5(310)[001]晶界结合强度的影响及其磁性等.考 虑了晶界核附近的多个可能偏析位置.Fe 在能量最优偏析位置的偏析能为仅-0.081 eV/atom,显示弱的 偏析趋势.比较纯净 Cu 晶界与 Fe 偏析 Cu 晶界的断裂能(2.24,2.62 eV/m2)可知,Fe 的偏析对晶界结 合起到强化作用.根据 Rice-Wang 热力学模型获得的强化势为-0.382 eV/atom,也体现了强化作用.经 过对强化势的分解得知,在Fe 对晶界强化作用中,由偏析引起的电荷重新分布造成的化学分量起决定 作用.偏析的 Fe 原子利用其自旋向上的电子与周围的 Cu 原子作用.电荷的转移主要发生在 Fe 原子的 d 轨道内,自旋向下电子向自旋向上电子轨道转移.由于配位环境及占用体积的不同,与纯净 Fe 晶体 相比,占据晶界位置的 Fe 原子的磁矩得到了强化,其磁矩甚至高于 Fe 晶体.Fe 周围的 Cu 原子也被 Fe 原子轻微极化. 第四,利用基于密度泛函的第一性原理能量计算方法,研究了不同浓度 Ag 偏析 Cu 晶界的性质. 根据 Rice-Wang 热力学模型分析可知, 单个 Ag 原子偏析到晶界处的不同位置对晶界结合强度的影响不 同,且仅当 Ag 原子偏析到特定位置时才对晶界起轻微的强化作用,其强化势仅为-0.098 eV/atom,其 余位置或更高浓度的 Ag 原子偏析到晶界处均对晶界起到脆化作用.随着偏析浓度的升高,因Ag 原子 与Cu 的尺寸上的差别,导致了晶界处的结构膨胀,从而引起该区域的电荷密度低于晶界模型的其他区 域,故偏析晶界的断裂沿着 Ag 原子间电荷密度较低的路径实现.偏析晶界的断裂能随着偏析浓度的升 高而降低, 向纯净 Ag 晶界的断裂能靠近. 上述两个现象均表明引起晶界脆化的物理根源为键替换机制. 采用现有的三种拉伸方案对单个 Ag 原子偏析的 Cu 晶界进行了第一性原理拉伸试验,除晶粒间插入真 空层的方案外,其余两种方案获得了相似的应变-能量曲线,且均呈现出能量的尖峰释放模式.经谨慎 验证,高应变下具有镜面对称性的偏析晶界是亚稳态结构.经过施加人为扰动,与插入真空层的方案 相结合,获得的理论拉伸强度仅比纯净 Cu 晶界高少许,印证了 Rice-Wang 的预测结果. ―2― 论文摘要(英文) Grain boundaries (GBs) are important planar defects that are ubiquitous in all polycrystalline materials. The segregation of impurities or alloying elements at GBs can enhance or weaken the cohesion of the interface and influence the mechanical properties of materials further. Benefiting from the ever-growing computing power, State-of- the-art first-principles calculations with the help of the density functional theory are powerful tool in the domain of the materials science. Tensile test and Rice-Wang thermodynamic model based on the first-principles calculations are the two most used methods in the theoretical study of the GBs segregation, and both of them show limitations when they are employed to study on the system of symmetric tilt GBs. Additionally, the properties of the GBs of magnetic materials with non-magnetic atoms segregation has been reported, however, the properties of the GBs of non-magnetic materials with magnetic atoms sgregation are still unclear. More works need to be done in the studies of the GB cohesion when the segregants density and tensile scheme are changed. In this thesis, systematical first-principles calculations are performed for clean Cu GB as well as Fe and Ag segregated Cu GBs to declear the issues mentioned above. Firstly, when the first-principles computational tensile tests (FPCTTs) are performed for the clean Cu ∑5(310)[001] GB, the metastable states are discovered in the atomic structure of GBs with high tensile strains and mirror symmetry even if the energy release follows the smooth mode. These metastable states are introduced by the mirror symmetry of the GB structure. The atomic configurations of GB models with low strain prefer symmetric structures, while those with high strains prefer the asymmetric structures. The the watershed for the present clean Cu GB is 6-7%. These metastable states are confirmed by a series of searching calculations, including shear deformation, relative sliding between grains and disturbance exerted on a given atom. The existence of the metastable states can usually delay the occuring fracture and overestimate the mechanical parameters. The method for evaluation of the errors in FPCTTs is presented, and the importance of the artificial corrections is emphasized. Secondly, common sense would suggest that the favorable fracture path of GBs can be obtained by testing different fracture paths and the one with the lowest fracture enenrgy is preferred. However, the reconstruction of the created surface of GB might lead to a lower fracture energy which might produce misleading results in the determination of fracture path. The system of Fe segregated Cu GB was chosen to state this phenomenon. The atomic structure and charge density should be considered to analyze the rationality of the selection of the GB fracture path once the reconstruction of the created surface happened. Thirdly, first-principles electronic calculations are performed to investigate the segregation effect and magnetism of Fe in Cu ∑5(310)[001] GB. Several segregation sites along the GB were considered. The segregation energy for the most energetically favorable site is -0.081 eV/atom which shows a weak tendency of iron segregates to GB. Iron plays as a cohesive enhancer to Cu GB by comparing the fracture energy between clean and Fe segregated Cu GB (2.24, 2.62 J/m3 ), and the strengthening potency based on the Rice-Wang model, -0.382eV/atom, is also in line with that predication. After decomposing the strengthening potency we can obtain that the chemical contribution (i.e. charge redistribution) plays a determinable role in the effect of iron on Cu GB cohesion. Irons bond with neighboring Cu atoms through its majority spin electrons. Charge transfer is dominant by the d orbit of iron that the minority spin electrons transfer to majority spin states. As a result, an enhancement of the local magnetic moments of iron in GB (2.916 μB ) or bulk (2.668 μB) or surface (2.978 μB) are presented compared to the FCC iron crystal (2.187 μB). The magnetic moment of iron at different sites dependents on the different occupied atomic volume. The surrounding Cu atoms are slightly polarized by the doped iron atom. Finally, the properties of Cu ∑5(310)[001] GB with dense segregated Ag atoms are studied by the first-principles calculations. A single silver atom plays the different roles in the GB cohesion when it located at ―3― different sites. It has weak cohesive effect on Cu GB cohesion when it occupies the specific site with the strengthening potency of -0.098 eV/atom. Silver plays as an embrittler when more than one silver atom segregated at GB sites. With the increasing of the concentration of the segregated silver atom around GB core, the structure of the GB core exhibits a significant expansion due to the size difference between Ag and Cu atoms. Consequently, the charge density at the GB core is lower than at other locations of the model, and the GB fracture is taken place through the segregated silver atoms. The GB fracture energy is decreased with the increasing of the segregated Ag concentration and it approaches to that of the pure Ag ∑5(310)[001] GB. The facts mentioned above declare that the bond replacement mechanism should responsible for the embrittling effect of the Ag segregated Cu GB. The FPCTTs are performed for the single Ag atom segregated Cu GB under the three existed tensile schemes. Besides the tensile scheme that inserting an increasing thickness vacuum between grains, the rest two schemes produce the similar strain-energy file which is the sharp release mode. The GB atomic structures upon the peak are verified to be metastable states. Based on the error evaluation method, the range of the theoretical tensile strength of the Ag segregated Cu GB is tiny higher than that of clean Cu GB, which agrees well with the Rice-Wang predication. 论文主要创新点 1. 发现了第一性原理拉伸试验中高应变的对称倾侧 Cu 晶界存在亚稳态,这是由晶界在拉伸方向上的 镜面对称性造成的.该状态的存在导致晶界拉伸断裂被延后、力学参数被高估;