第一性原理研究Zn偏析对CuΣ5晶界的影响

采用基于密度泛函理论的第一性原理方法研究了Zn偏析Cu晶界的原子构型和电子结构,分析了Zn偏析对Cu晶界力学性能的影响.结果表明,Zn以替换方式偏析到晶界处,Zn—Cu与Cu—Cu的成键方式类似,均为含有共价成分的金属键.Zn偏析导致少量电荷集聚于Zn与近邻Cu之间,有限地增强了晶界的结合.拉伸过程中Zn的d轨道定域性增强,Zn与近邻Cu间的电荷密度下降,削弱了Zn—Cu键,导致晶界断裂发生在Zn—Cu间.     

Sr偏析Al晶界结构的第一性原理计算

采用基于密度泛函理论和局域密度近似的第一性原理赝势方法,计算了纯Al晶界和杂质Sr偏析Al晶界的原子结构和电子结构.结果表明Sr偏析引起了晶界膨胀和晶界处电子密度的大幅度降低,从而导致晶界结合力的减弱.这应为Sr杂质偏析引起的Al晶界脆化的主要根源所在. 关键词： Al晶界 Sr 杂质偏析 第一性原理计算     

An atomistic study of grain boundary segregation and cracking

A Monte Carlo model is applied to study a Σ = 5 (310) fcc tilt boundary structure and impurity segregation to this boundary. The Johnson-type potential functions are used to express atomic interactions for two binary alloys of NiCu and CuSb systems. Through atomic relaxation near absolute zero, the basic structural unit of the Σ = 5 CSL boundary is found to dissociate into two subunits. For the NiCu system, copper segregation to the boundary follows the Gibbsian equilibrium segregation behavior in which the segregation is localized on a few atomic layers, its amount increases with increase in the bulk concentration but decreases with increase in temperature. For the CuSb system, an assumption of the CuCu bond weakening due to the strong, adjacent CuSb bonds is also incorporated to realize the embrittling effect of Sb atoms. The preferential site for the large Sb atoms is the vertex of a pentagonal bipyramid in the Σ = 5 grain boundary. Without bond weakening, the Sb-segregated grain boundary maintains a structure combined with a distorted pentagonal bipyramid and a capped trigonal prism. However, with bond weakening some bonds of the trigonal prisms are disrupted in order to form new pentagonal bipyramids. When a uniaxial strain is applied in the direction perpendicular to the grain boundary plane, the weakened copper bonds become the source of microcracks thus enhancing brittle fracture along the Sb-segregated grain boundary.     

The Electronic Structure of Pristine and Doped (100) Tilt Grain Boundaries in SrTiO3

To understand the electronic properties of doped grain boundaries, we reviewed the atomic scale techniques currently available to study the electronic structure at pristine SrTiO₃ grain boundaries. The knowledge gained from the pristine boundaries is used to interpret experimental and theoretical results from a Mn doped 5 SrTiO₃ grain boundaries. Mn atoms are shown to preferentially substitute at specific Ti sites at the grain boundary core. Furthermore, the formal oxidation state of the Mn atoms at the grain boundary core was found to be reduced compared to the Mn atoms substituting for Ti in the bulk. This change of valence did not, however, significantly affect the atomic structure of the grain boundary, as determined by Z-contrast imaging and electron energy-loss spectroscopy, which revealed similar fine-structure features at both the doped and pristine grain boundary. We conclude, therefore, that composition and atomic structure have different effects on the local electronic structure and should be treated separately in any segregation and electrical conductivity models for grain boundaries.     

ZA27合金晶界处铁、稀土元素的有序化与交互作用

为从理论上揭示铁、稀土元素在锌铝合金晶界处的行为本质，建立了ZA27合金中α相大角度重位点阵晶界模型，利用递归法(Recursion)计算了晶界的电子结构(状态密度、费米能级、结构能).用晶界结构能定义合金的团簇能(有序能)，并计算了偏聚铁及稀土晶界的团簇能.计算结果表明：铁、稀土元素在锌铝合金晶界处团簇能为正值，不能形成团簇，具有有序化倾向，趋于形成稳定的金属间化合物.铁与稀土元素在晶界形成负电中心，降低晶界的费米能级. 关键词： 稀土 晶界 电子结构 有序化     

Grain boundaries of nanocrystalline materials – their widths, compositions, and internal structures

Nanocrystalline materials contain many atoms at and near grain boundaries. Sufficient numbers of Mössbauer probe atoms can be situated in grain boundary environments to make a clear contribution to the measured Mössbauer spectrum. Three types of measurements on nanocrystalline materials are reported here, all using Mössbauer spectrometry in conjunction with X-ray diffractometry, transmission electron microscopy, or small angle neutron scattering. By measuring the fraction of atoms contributing to the grain boundary component in a Mössbauer spectrum, and by knowing the grain size of the material, it is possible to deduce the average width of grain boundaries in metallic alloys. It is found that these widths are approximately 0.5 nm for fcc alloys and slightly larger than 1.0 nm for bcc alloys. Chemical segregation to grain boundaries can be measured by Mössbauer spectrometry, especially in conjunction with small angle neutron scattering. Such measurements on Fe-Cu and Fe₃Si-Nb were used to study how nanocrystalline materials could be stabilized against grain growth by the segregation of Cu and Nb to grain boundaries. The segregation of Cu to grain boundaries did not stabilize the Fe-Cu alloys against grain growth, since the grain boundaries were found to widen and accept more Cu atoms during annealing. The Nb additions to Fe₃Si did suppress grain growth, perhaps because of the low mobility of Nb atoms, but also perhaps because Nb atoms altered the chemical ordering in the alloy. The internal structure of grain boundaries in nanocrystalline materials prepared by high-energy ball milling is found to be unstable against internal relaxations at low temperatures. The Mössbauer spectra of the nanocrystalline samples showed changes in the hyperfine fields attributable to movements of grain boundary atoms. In conjunction with SANS measurements, the changes in grain boundary structure induced by cryogenic exposure and annealing at low temperature were found to be somewhat different. Both were consistent with a sharper density gradient between the crystalline region and the grain boundary region.     

Nucleation and growth of helium bubble at(110) twist grain boundaries in tungsten studied by molecular dynamics

Migration of He atoms and growth of He bubbles in high angle twist grain boundaries(HAGBs) in tungsten(W) are investigated by atomic simulation method. The energy and free volume(FV) of grain boundary(GB) are affected by the density and structure of dislocation patterns in GB. The migration energy of the He atom between the neighboring trapping sites depends on free volume along the migration path at grain boundary. The region of grain boundary around the He bubble forms an ordered crystal structure when He bubble grows at certain grain boundaries. The He atoms aggregate on the grain boundary plane to form a plate-shape configuration. Furthermore, high grain boundary energy(GBE) results in a large volume of He bubble. Thus, the nucleation and growth of He bubbles in twist grain boundaries depend on the energy of grain boundary, the dislocation patterns and the free volume related migration path on the grain boundary plane.     

La、Al在Fe晶界上协同作用的第一性原理研究

采用基于密度泛函理论的第一性原理,研究了稀土La和Al在bcc-Fe中Fe晶界处的协同作用影响.计算了La和Al在晶界、表面、晶内的形成能,建立La、Al共存于晶界模型,通过电荷密度、布居分布手段分析了La、Al原子对晶界的影响.结果表明:La、Al原子易偏聚在晶界处,且La-Al的原子间距与体系结合能成正比关系. La的掺杂改善了晶界处的电荷分布情况,促进了Al原子与周围Fe原子间的相互作用,态密度曲线的计算结果显示,La原子的加入能够使La、Fe及Al原子间的键的结合力更强,从而提高了界面结合强度.     

微米气室铯原子自旋噪声谱

利用自旋噪声谱技术研究了无缓冲气体133Cs原子气室的自旋动力学和展宽机制.在宏观原子气室中,自旋弛豫速率失谐频率谱的线型为高斯分布;在空间局域较强的微米气室中,自旋弛豫速率失谐频率谱的线型为洛伦兹分布.实验测量得到的自旋弛豫速率失谐频率谱的展宽约4 GHz,明显大于宏观原子气室中约度强烈依赖于激光相对于原子共振跃迁的频率失谐;在微米气室中,由于较强的均匀展宽,总噪声的失谐频率谱中心处出现明显的凹陷.通过建立简化的物理模型来计算微米气室的展宽机制,在实验与理论中解释了原子的均匀展宽特性.     

Effect of element Re on the grain boundary cohesion of α-Fe

The effect of Re segregation on the α-Fe ∑5 [001] （010） grain boundary （GB） is investigated by using a software called DMol and discrete variational method （DVM）. Based on the Rice Wang model, the calculated segregation energy and defect formation energy show that Re is a strong cohesive enhancer. We also calculated the interatomic energy （IE） and bond order （BO） of several atomic pairs to investigate the mechanism of the cohesive effect of Re microscopically and locally. The results show that IEs of atomic pairs formed by those atoms which cross the plane of GB are strengthened due to the segregation of Re, while the BOs of the corresponding pairs are slightly decreased. This discrepancy demonstrates that IE which contains the Hamiltoniaa of interaction between atoms is a good quantity to describe the bonding strength. The analysis suggests that the electronic effect between atomic pair which comes directly from Hamiltonian is the key factor, The charge density is also presented, and the result indicates that the bonding strength between the Fe atoms on the GB is enhanced due to the segregation of Re, which is consistent with the analysis of IE.     

Benefits of oxygen in CuInSe2 and CuGaSe2 containing Se-rich grain boundaries

Using density functional theory calculation, we show that oxygen (O) exhibits an interesting effect in CuInSe₂ and CuGaSe₂. The Se atoms with dangling bonds in a Se-rich Σ3 (114) grain boundary (GB) create deep gap states due to strong interaction between Se atoms. However, when such a Se atom is substituted by an O atom, the deep gap states can be shifted into valence band, making the site no longer a harmful non-radiative recombination center. We find that O atoms prefer energetically to substitute these Se atoms and induce significant lattice relaxation due to their smaller atomic size and stronger electronegativity, which effectively reduces the anion–anion interaction. Consequently, the deep gap states are shifted to lower energy regions close or even below the top of the valence band.     

Possible effects of oxygen in Te-rich Σ3 (112) grain boundaries in CdTe

Using density functional theory calculation, we show that oxygen (O) exhibits an interesting effect in CdTe. The Te atoms with dangling bonds in a Te-rich rich Σ3 (112) grain boundary (GB) create deep gap states due to strong interaction between Te atoms. However, when such a Te atom is substituted by an O atom, the deep gap states can be shifted toward the valence band, making the site no longer a harmful non-radiative recombination center. We find that O atoms prefer energetically substituting these Te atoms and induce significant lattice relaxation due to their smaller atomic size and stronger electronegativity, which effectively reduces the anion–anion interaction. Consequently, the deep gap states are shifted to lower energy regions close to or even below the top of the valence band.     

Al晶界的第一性原理拉伸试验

基于密度泛函理论和局域密度近似的第一性原理方法，进行了Al晶界的第一性原理拉伸试验.得到Al晶界的理论拉伸强度为9.5 GPa，对应的应变为16%.根据价电荷密度、键长和原子构型随应变的变化，我们证实断裂发生在晶界面，其特征是所有界面键的断裂.同时还发现在周围原子键的数目减少的情况下，界面重构的Al-Al原子键具有共价键的性质.因此Al晶界依然保持着较高的界面强度. 关键词： Al晶界 第一性原理拉伸试验 理论拉伸强度     

Atomic and electronic structures of a grain boundary in iron with impurity segregation

We present a short review on our current investigations of the atomic and electronic structures of a grain boundary in iron. Atomic structures of grain boundaries were simulated and the local electronic densities of states were calculated in the simulated structure. When phosphorus impurity atoms segregated at the grain boundaries in iron, trigonal prismatic FeP clusters were formed. Segregated boron atoms tended to stay at the central site of polyhedra constructed by host atoms in the grain boundaries. The non-bonding states of the iron atom at the grain boundary disappear by forming a strong bonding orbital with the orbital of the segregated impurity atom. This bonding orbital is formed in a Fe3d host band in the case of a boron impurity. On the other hand, the bonding orbital is formed at lower energies for the phosphorus impurity and is less-mixed with the Fe3d host band. Non-bonding states are formed around the Fe₉P clusters. These can give a qualitative explanation for the embrittlement of the impurity segregated grain boundary. Finally, we can explain from the viewpoint of the electronic structure why the interstitial impurity is the only cohesive enhancer.     

Grain Structure of Ni3Mn Alloy Ordered in Annealing

The grain boundary ensembles in Ni₃Mn alloy with L1₂ superstructure and different manganese content were investigated. It was found that the process of atomic ordering is accompanied by the migration of grain boundaries. The presence of oxygen atoms in solid solution and oxide particles at the grain bound- aries hinders the formation of the long-range atomic order in the crystalline lattice and migration of grain boundaries.     

Dynamics of moving interacting atoms in a laser radiation field and optical size resonances

The forces acting on interacting moving atoms exposed to resonant laser radiation are calculated. It is shown that the forces acting on the atoms include the radiation pressure forces as well as the external and internal bias forces. The dependences of the forces on the atomic spacing, polarization, and laser radiation frequency are given. It is found that the internal bias force associated with the interaction of atomic dipoles via the reemitted field may play an important role in the dynamics of dense atomic ensembles in a light field. It is shown that optical size resonances appear in the system of interacting atoms at frequencies differing substantially from transition frequencies in the spectrum of atoms. It is noted that optical size resonances as well as the Doppler frequency shift in the spectrum of interacting atoms play a significant role in the processes of laser-radiation-controlled motion of the atoms.     

Interdiffusion in two-layer Pd/Ag films. IV. Diffusion-induced Σ13 boundary migration

Fast growth of grains with homogeneous composition and 13 orientation was observed in Pd/Ag single-crystal thin films during annealing at 400°C. It was found that nucleuses of these orientation are contained in an initial structure. The grain growth obeys the mechanism of diffusion-induced grain boundary migration and the migration velocity was >-10^-7 m/s. The assumption was made that an elementary act of fast grain boundary migration is a transition of an atomic group determined by a Coinsidence Site Lattice, and the reconstruction occurs as correlated displacements of n atoms resulting in the transition of m atoms from one grain to another. A free activation energy of such a process and velocity of the special grain boundary were estimated in the frames of the model.     

稀土La在$lt;em$gt;α$lt;/em$gt;-Fe中占位倾向及对晶界影响的第一性原理研究

本文采用重合位置点阵理论构建了 α-Fe的Σ3[110]（112）对称倾转晶界模型,通过基于密度泛函理论的平面波超软赝势方法研究了稀土La元素在 α-Fe中的占位倾向. 结果表明,La在 α-Fe晶界的杂质形成能最低,因而La原子倾向于占据晶界区;掺杂La前后的 α-Fe晶界电子结构计算结果显示,La占位于 α-Fe晶界会使体系中的电荷发生重新分配,将提供更多电子用于晶界区成键,使得Fe原子得到更多的电子,这将导致掺杂区原子间结合有离子化趋势,从而使La与晶界区相邻Fe原子之间的相互作用加强,也使晶界原子与晶界两侧Fe原子的键合加强,从能量角度解释了材料宏观力学性能变化的原因;计算同时发现,La加入后,也使晶界上的原子成键区态密度左移,降低了体系的总能量,使晶界结构更为稳定. 关键词： La α-Fe')" href="#">α-Fe 晶界 第一性原理     

Grain boundary sliding in pure and segregated bicrystals: a molecular dynamics and first principles study

Sliding behaviors of Σ9(221) grain boundary bicrystals have been investigated in pure metals (Al, Ag, Au, Cu, Pt and Co) and in segregated metals (Cu segregated by Al, Ag, Au, Pt and Co) by molecular dynamics simulations and first-principles calculations. The grain boundary energy, the atomic size and the electronegativity of the segregated elements were not critical for the occurrence of grain boundary sliding. On the other hand, the sliding rate increased as the minimum charge density decreased at the bond critical point. This was the case for both pure grain boundary models and segregated grain boundary models. Therefore, it seems that the sliding rate depends on atomic movement at sites with minimum charge density, irrespective of the elements involved and of the presence of segregated atoms.     

Effect of atomic displacement on the parameters of the grain boundary ensemble in nickel-based alloys with <Emphasis Type="Italic">L</Emphasis>1<Subscript>2</Subscript> superstructure

The effect complex stacking fault energy and mean-square atomic displacement have on the parameters of the grain boundary ensemble in nickel-based alloys with L1₂ superstructure is established experimentally. The higher the complex stacking fault energy, the lower the average number of special grain boundaries per parent grain. The lower the mean-square atomic displacement, the smaller the proportion of the Σ3 twins in the special grain boundary spectrum.