Finite-size effects on the structure of grain boundaries

We present a combined experimental and theoretical analysis of the structure of finite-sized Sigma 3 [112] grain boundaries in Au. High-resolution electron microscopy shows lattice translations at the grain boundary, with the magnitude of the translation varying along the finite-sized grain boundaries. The presence of this structural profile is explained using continuum elasticity theory and first-principles calculations as originating from a competition between elastic energy and the energy cost of forming continuous [111] planes across the boundary. This competition leads to a structural transition between offset-free and nontrivial grain boundary structures at a critical grain boundary size, in agreement with the experiments. We also provide a method to estimate the energy barrier of the gamma surface.     

Study of the motion of individual triple junctions in aluminum

The mobility of individual triple junctions in aluminum is studied. Triple junctions with 〈111〉, 〈100〉, and 〈110〉 tilt boundaries are studied. The data obtained show that, at low temperatures, the mobility of the system of grain boundaries with a triple junction is controlled by the mobility of the triple junction (the junction kinetics). At high temperatures, the system mobility is determined by the mobility of the grain boundaries (the boundary kinetics). There is a temperature at which the transition from the junction kinetics to the boundary kinetics occurs; this temperature is determined by the crystallographic parameters of the sample.     

Are the structures of twist grain boundaries in silicon ordered at 0 K?

Contrary to previous simulation results on the existence of amorphous intergranular films at high-angle twist grain boundaries (GBs) in elemental solids such as silicon, recent experimental results imply structural order in some high-angle boundaries. With a novel protocol for simulating twist GBs, which allows the number of atoms at the boundary to vary, we have found new low-energy ordered structures. We give a detailed exposition of the results for the simplest boundary. The validity of our results is confirmed by first-principles calculations.     

Studies on grain boundary effects in spray deposited BICOVOX 0.1 films on platinum-coated stainless steel substrate

Thin film of Bi₂Co_0.1V_0.9O_5.35 is deposited by using spray pyrolysis technique on platinum-coated stainless steel substrate. Impedance measurements done in the frequency range 1 to 10 MHz and in the temperature range 502 to 720 K revealed two relaxation processes with distinguishable time constants. The first corresponds to the grain interior charge transfer while the second could be due to grain boundary. The change in polarization seems to be associated with hopping of charge carriers showing Arrhenius behavior with increase in temperature. The relaxation frequency of grain interior transport for the thin film ranges from 96 kHz to 2.59 MHz. The blocking factor was found to be increasing with increase in temperature at low temperature region from 502 to 640 K. At higher temperature above 640 K, diffusive nature of grain boundaries is inferred with the decrease in blocking factor. The same inference is derived by specific grain boundary conductivity calculations since specific grain boundary conductivity decreases in low temperature region while it increases rapidly at higher temperature. These observations prove the grain boundaries to be blocking in lower temperatures while at higher temperatures above 640 K they turn diffusive. These changes are attributed to structural phase transition, or ordering of vacancies in Bi₂Co_0.1V_0.9O_5.35 films.     

Collective effects in grain boundary migration

In situ high-resolution transmission electron microscopy is used to study grain boundary structure and kinetics in bicrystalline Au films at elevated temperature. We report the first direct evidence for the existence of cooperative atomic motion in grain boundary migration. Certain nanoregions at grain boundaries, typically involving up to several hundred atoms, are found to switch back and forth between neighboring grains. Reversible structural fluctuations at temperatures near 0.5T(m) and above have been discovered in [110] and [001] tilt, as well as in general grain boundaries.     

Atomistic studies of the structure and composition of grain boundaries in Cu3Au and Ni3Al

In this paper we investigate the atomic structure and composition of grain boundaries in Cu₃Au (weakly ordered compound) and Ni₃Al (strongly ordered compound). Computer simulations employing both the molecular statics and Monte Carlo methods were performed and the Finnis-Sinclair type many-body central force potentials used. First, grain boundaries in stoichiometric alloys are studied with the goal to investigate the impact of ordering strength on the grain boundary structure and composition. In Cu₃Au grain boundaries may become compositionally disordered even at room temperature and the compositional disordering is associated with segregation of gold. In contrast, in Ni₃Al grain boundaries remain compositionally ordered up to very high temperatures. Secondly, the structures of grain boundaries and the effect of Ni and Al segregation in non-stoichiometric Ni₃Al are investigated. Nickel segregation leads to compositional disordering at grain boundaries, while aluminum segregation, which is strongly selective, leads to an ordered grain boundary structure with high Al content. The possible relationship between structural and compositional characteristics of grain boundaries and their mechanical properties, in particular the grain boundary brittleness and its alleviation by additional alloying, are then discussed in the light of the results of this study.     

Grain boundary wetting phase transition and analysis of the energy characteristics of grain boundaries in the Sns-(Zn/Sn)l system

Regularities of the interaction of tin grain boundaries (special Σ5 and general Σ17 〈001〉) and a Sn-Zn melt of equilibrium composition were studied. The grain boundary wetting phase transition temperature was determined; for Σ5 and Σ17, it is 216°C. More than 90% of the general grain boundaries were completely wetted by the melt over a range of temperatures, from the eutectic melting temperature to the tin melting temperature. It was shown that the anisotropy of interphase energy at the solid tin-Zn-Sn melt interface is 64 ± 10 mJ m^?2 at 216°C. The energies of the Σ5 and Σ17 grain boundaries in the range of 201–216°C were obtained on the basis of the experimental dependence of the dihedral angle on temperature.     

Grain boundary potential and charge density at grain boundaries of chemically prepared thin silver sulfide films

The grain boundary potential and interface state charge density at the grain boundaries of silver sulfide (Ag₂S) thin films prepared by chemical conversion of cadmium sulfide (CdS) films have been determined from the dc resistance of the material and are found to be sensitive to annealing. A reduction in the grain boundary potential and the grain boundary charge density of the film has been noticed when the source CdS film is annealed at different temperatures prior to chemical conversion. The variation in the grain boundary charge density of the grown Ag₂S film with source annealing temperature has been found to be similar to that of thin cadmium sulfide film, reported earlier. An additional low temperature heat treatment of the sample results in an enhancement in the charge density at the grain boundaries. The change in the silver vacancy and/or oxygen and sulfur content of the films as revealed from the energy dispersive spectra of the films suggests possible role of film composition on the grain boundary charge density.     

Effect of the wetting of grain boundaries on the formation of a solid solution in the Al-Zn system

Phase transitions in the bulk and at grain boundaries in the (Al-20 wt % Zn) alloy have been studied by means of differential scanning calorimetry and transmission electron microscopy. Polycrystals with a high specific area of grain boundaries have been obtained using severe plastic deformation (high-pressure torsion). It has been shown that the Zn-based solid phase completely wets the grain boundaries in aluminum at a temperature of 200°C. The position of the grain boundary solvus line (solubility line), which is above the bulk solvus by 40?C45 K, has been determined.     

Theoretical study of hydrogen absorption near symmetric tilt grain boundaries in Pd and TiFe

The results of calculations of the atomic and electron structure of Pd and TiFe with symmetrical Σ5 tilt grain boundaries obtained using the methods of electron density functional theory are reported. Hydrogen sorption at tilt grain boundaries and corresponding surfaces is considered. It is shown that the hydrogen absorption energy increases in magnitude by ∼0.2 eV at the Pd Σ5(210) grain boundary and by ∼0.5 eV in B2-TiFe with the Σ5(310) grain boundary. The binding energy of hydrogen in palladium, as well as in TiFe, in the most preferred positions at the surface is higher than near grain boundaries. It is found that, as in the case of a defect-free material, the following tendency is observed at a symmetrical tilt grain boundary: the strong bond of the impurity at the grain boundary in the metal or alloy matrix reduces the sorption energy of hydrogen.     

Misorientation Dependence of the Grain Boundary Energy in Magnesia

Geometric and crystallographic data obtained from a well annealed magnesia polycrystal have been used to specify the five macroscopic degrees of freedom for 4665 grain boundaries. The results indicate, that for this sample, the five parameter grain boundary character space is fully occupied. A finite series of symmetrized spherical harmonics has been used to approximate the misorientation dependence of the relative grain boundary energy. Best fit coefficients for this series were determined by assuming that the interfacial tensions at each triple junction are balanced. The grain boundary energy function shows Read-Shockley behavior at small misorientations and a broad minimum near the 3 misorientation. Furthermore, misorientations about the ‹100› axis create boundaries with relative energies that are less than those created by misorientations about the ‹110› or ‹111› axes.     

纯铜[0 1 1]倾侧型非共格∑3晶界结构稳定性分子动力学模拟研究

有研究表明, 非共格∑₃晶界的行为在中低层错能面心立方金属晶界 特征分布演化中发挥着重要作用. 为了掌握不同界面匹配的非共格∑₃晶界的特性, 本文利用分子动力学(MD)模拟方法首先研究了纯铜的[0 1 1]倾侧型 非共格∑₃晶界在700–1100 K温度范围内和常压下的结构稳定性. MD模拟采用原子间相互作用长程经验多体势, 步长为5×10^-15 s. 模拟结果表明: 所研究的五个非共格∑₃晶界, 其结构稳定性存在很大差异, 其一般规律是, 与(1 1 1)/(1 1 1)共格孪晶界之间的夹角(Φ角)越小, 晶界匹配值越大, 则非共格∑₃晶界越稳定; 反之亦然. Φ角最小的 (2 5 5)/(2 1 1)非共格∑₃晶界较稳定, 在退火过程中几乎不发生变化. 随着Φ角的增大, 非共格∑₃晶界不再稳定, 这类晶界会通过Miller指数较高一侧晶体每三层原子面合并为一层原子面 (或Miller指数较低一侧晶体每一层原子面分解为三层原子面)的机理 转变为亚稳的“台阶”状晶界, 台阶面部分地处于精确的能量极低 的{111}/{111}共格孪晶界上; 当提高温度退火时, 这种“台阶”状晶界最终会全部转变成稳定平直的{111}/{111}共格孪晶界. 关键词： 纯铜 ∑₃晶界')" href="#">非共格∑₃晶界 分子动力学模拟     

Short-Circuit Diffusion in Ceramics

This article discusses grain boundary diffusion in ceramics. It gives a brief review of the experimental data available for ionic oxides and the problems of interpretation associated with it. The fundamental differences between grain boundary diffusion in metals and ceramics are noted. Calculations of the segregation of defects and impurities to grain boundaries are discussed together with methods of calculating diffusion coefficients in these boundaries. New results for alumina and chromia are presented. The problem of defining a grain boundary width is discussed with respect to new calculations on nickel oxide.     

Anisotropy of P Grain Boundary Segregation in a Rapidly Solidified Fe-0.6wt%P Alloy

P grain boundary segregation in an Fe-0.6wt%P alloy quenched from the melt was quantified by X-ray Mapping (XRM) in a Scanning Transmission Electron Microscope (STEM). The misorientation across the boundaries was determined by using ACT (Automatic Crystallography for TEM (Transmission Electron Microscopy)) and CBED (Convergent Beam Electron Diffraction). A significant range of the degree of P segregation to individual grain boundaries was found. Combination of chemical and structural studies provides evidence that P segregation to low-angle grain boundaries is reduced.     

An extended read model for grain boundaries in GaAs

The space charging properties of grain boundaries in GaAs are discussed using an extended Read dislocation model. Model calculations for medium to high angle grain boundaries show: (1) the core-site occupational probability for a dislocation at the grain boundary is at least an order of magnitude lower than would be expected on the basis of simple Read theory; (2) due to the strong core-core interaction of the dislocations, the depletion width W decreases with an increase in tilt angle θ: and (3) W is approximately independent of θ in highly doped (N_D ? 10₁₇/cm³) material. Comparisons between model predictions and experimentally determined depletion widths of grain boundaries in GaAs are presented.     

Internal friction in Al bicrystals with ⟨111⟩ tilt and twist grain boundaries

Internal friction measurements were performed on various ?111? tilt and twist grain boundaries in high-purity Al bicrystals. The temperature dependence of the grain boundary internal friction peak was determined, and the activation parameters of grain boundary relaxation were obtained. These parameters were found to change in a wide range depending on boundary geometry. The activation enthalpy of boundary relaxation and the pre-exponential factor of the relaxation time are related according to the compensation effect. The results are discussed in terms of the model of correlated relaxations. Bicrystals with vicinal Σ3 boundaries were observed to behave like single crystals, i.e. an internal friction peak did not appear. This evidences that both coherent and incoherent (60° ?111? tilt) twins possess a high mechanical resistance.     

Chevron defect at the intersection of grain boundaries with free surfaces in Au

We have identified a new defect at the intersection between grain boundaries and surfaces in Au using atomic resolution transmission electron microscopy. At the junction line of 90 degrees <110> tilt grain boundaries of (110)-(001) orientation with the free surface, a small segment of the grain boundary, about 1 nm in length, dissociates into a triangular region with a chevronlike stacking disorder and a distorted hcp structure. The structure and stability of these defects are confirmed by atomistic simulations, and we point out the relationship with the one-dimensional incommensurate structure of the grain boundary.     

First-principles calculations of interfacial and segregation energies in α-Cr2O3

The interfacial energies of three twin boundaries with low-index boundary planes: prismatic (101?0), basal O-terminated (0001), and basal Cr-terminated (0001), and the segregation energies of five doping elements (Ce, Hf, La, Y and Zr) have been calculated as a function of temperature. The static energies at 0 K were obtained through first-principles calculations and the energies at finite temperatures were derived based on the Debye model. The calculation results show that both the interfacial and segregation energies decrease as temperature increases and the segregation energies are found to be proportional to the ionic size mismatch and the interfacial energy. Our combined approaches suggest an efficient and less computationally intensive way to derive grain boundary energetics at finite temperatures.