Influence of grain boundaries misorientation angle on intergranular corrosion in 2024-T3 aluminium

The special attention has been paid to the influence of misorientation angle of a random grain boundary (GB) on susceptibility to intergranular attack. The detailed observations of the microstructure of the intergranular corrosion (IGC) in 2024-T3 aluminium alloy (AA2024-T3) subjected to galvanic corrosion tests in two different solutions containing chloride ions (0.1 M and 0.5 M NaCl) were carried out using Scanning Electron Microscopy (SEM). The Electron Backscattered Diffraction (EBSD) technique was used to determine the grain boundary character distribution (GBCD) in the initial sample and a GBCD of corroded grain boundaries on a sample subjected to the corrosion test. The results are discussed in terms of the influence of the misorientation angle on the susceptibility of the grain boundaries to corrosion.     

The relationship between the fracture toughness and grain boundary characteristics in hot-dip galvanized zinc coatings

This publication presents an experimental study on the relation between the grain boundary (GB) characteristics and the intergranular cracking resistance in a hot dip zinc coating. The cracking was studied using in situ tensile tests in a scanning electron microscope on small tensile samples of a hot dip galvanized steel sheet. In situ testing offered a series of advantages like monitoring the kinematical evolution of cracking without unloading, or making micrographs and OIM imaging on the same area of the tensile sample. The grain boundaries were classified into random and special boundaries (respectively Low angle boundaries and Coincidence site lattice—CSL boundaries). These special boundaries which account for 3.5% of the whole boundaries clearly show better cracking resistance than the random boundaries. The only special boundaries which present cracking failure are in an orientation with their normal direction close to the tensile direction, i.e. submitted to a maximum effective stress. The grain boundaries characteristics are obtained from EBSD individual orientation measurements.     

Ion conduction and redistribution at grain boundaries in oxide systems

The review provides a comprehensive overview on the major findings regarding ion redistribution at interfaces in oxide systems and its effects on the electrical transport properties. As far as interfaces are concerned, grain boundaries and hence polycrystalline materials are to the fore. Heterocontacts and hence composites are only considered if relevant for the general understanding. Selected examples refer to oxide ceramics but also to composites. As far as the fundamental properties are concerned, major emphasis is laid on the impact on ion conductivity. Purely electronic effects (e.g. interfaces in semiconductors, boundaries in superconductors, or the formation of two-dimensional electron gases at the interface between two insulators) as well as phenomena related with solid/fluid interfaces are not addressed.     

An electron beam induced current study of grain boundaries in zinc selenide

An electron beam induced current investigation of crystalline samples of zinc selenide in the scanning electron microscope has proved particularly useful in revealing the existence of grain boundaries. Changes in the EBIC contrast which are observed when the bias is altered, or at places where twin bands intersect the grain boundaries, are explained in terms of a model which associates a potential energy barrier, similar to two Schottky barriers back to back, with a grain boundary.     

Forming contacts and grain boundaries between MgO nanoparticles

The present article is concerned with how nanoparticles join: it considers MgO nano-cubes as a model, well-defined system. The development of grain boundaries (GBs) between cube particles has been re-examined using MgO smoke. In addition to the face-to-face contact which leads to the well-known low-∑ twist GBs, interactions are also found which initially involve point-to-face contact, edge-to-face contact, or contacts along the cube edges. It is proposed here that the point contact lead to a line contact through the requirement to balance charges, and rotation about such a line of contact leads to formation of the interface, i.e., the grain boundary. The atoms along the edges have lower coordination than the atoms in the bulk, which may contribute to the edge–edge and edge-face boundary formation. The inherently small size of nanoparticles makes transmission electron microscopy (TEM) an invaluable technique for characterizing the contacts between them without modifying them in any way. The present study uses TEM to characterize the types of boundaries formed, discusses the boundary structures, and considers how the particle morphology may determine the formation of low-∑ GBs. Julia Deneen Nowak and C. Barry Carter were previously at Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, MN, 55455, USA.     

Segregation of reactive elements at oxide grain boundaries in FeCrAlRE alloys

Abstract

Extensive previous research has established that the oxidation of FeCrAl alloys at temperatures ≥1000°C results in the formation of α-Al₂O₃ oxide scales, and that minor alloy constituents (particularly Reactive Elements (RE) such as Y, Hf, Zr, etc.) can change the oxide growth mechanism. A knowledge of the segregation behaviour of these REs is thus central to our understanding of the oxidation behaviour of these, technologically important, range of alloys.

The new SuperSTEM microscope at the Daresbury Laboratory offers considerable potential for a detailed study of the segregation to oxide grain boundaries at the atomic level. The microscope has an aberration corrector fitted to the objective lens, allowing the formation of sub-Angstrom probe for simultaneous ultra-high resolution high angle annular dark field (HAADF) imaging and atomic-column electron energy loss spectroscopy (EELS). This paper reports on an initial study of oxide grain boundary segregation in commercial and model FeCrAlRE alloys containing controlled additions of the reactive elements, yttrium, zirconium and hafnium oxidised at 1250°C, in air, for 50 hours. Both yttrium and hafnium are shown to segregate to the grain boundaries while hafnium rich particules form in the outer region of the scale.     

The chemistry of cements formed between zinc oxide and aqueous zinc chloride

A series of cements has been prepared from zinc oxide powder and aqueous zinc chloride, using solutions corresponding to concentrations of 20%, 30%, 40%, 50% and 60% and a ratio of ZnO powder to zinc chloride solution of 1:1. As with cements of the zinc oxide/zinc nitrate system, these ZnO/ZnCl2 cements were found to be weak in compression (not exceeding 10 MPa) with strength rising with increasing concentration of ZnCl2. The pH change as the reaction proceeded was monitored and generally showed a rapid increase, followed by a slight decrease, and a subsequent slower increase. This is assumed to arise because the doubly charged aquo-zinc cation, Zn(H2O)2+n (n=4 or 6) behaves as a weak acid, due to so-called salt hydrolysis: Zn(H2O)2+n+H2OZnOH(H2O)+(n-1)+H3O+ and reacts to form a salt, thus setting up a classic weak acid/salt buffer system. Finally, cements were stored in water for 1 month, and were generally found to increase in mass during the first week, with the greatest increase occurring in the cement made from 20% ZnCl2 solution. All cements lost mass between 1 week and 1 month, showing them to be sparingly soluble at room temperature. © 1998 Chapman & Hall     

氧化锌掺钇透明导电薄膜的制备及光电特性研究

采用射频磁控溅射法,室温下在玻璃衬底上制备出了具有良好附着性、低电阻率和高透过率的新型透明导电薄膜YZO(ZnO掺杂Y2O3简称YZO)。在薄膜厚度为600nm的情况下,研究了薄膜电学特性随溅射功率和溅射气压的变化情况。X射线衍射谱表明YZO薄膜是多晶膜,具有ZnO的六角纤锌矿结构,最佳取向为(002)方向。最佳溅射条件下制备的薄膜电阻率为8.71×10-4Ω.cm,在可见光范围内平均透过率达到92.3%,禁带宽度为3.57eV。     

Carrier transport in polycrystalline transparent conductive oxides: A comparative study of zinc oxide and indium oxide

Highly doped indium-tin oxide films exhibit resistivities ρ as low as  1.2 × 10^− 4 Ω cm, while for ZnO films resistivities in the range of 2 to 4 × 10^− 4 Ω cm are reported. This difference is unexpected, if ionized impurity scattering would be dominant for carrier concentrations above 10²⁰ cm^− 3. By comparing the dependences of the effective Hall mobility on the carrier concentration of ZnO and ITO it is found that grain barriers limit the carrier mobility in ZnO for carrier concentrations as high as 2 × 10²⁰ cm^− 3, independently, if the films were grown on amorphous or single crystalline substrates. Depending on the deposition method, grain barrier trap densities between 10¹² and 3 × 10¹³ cm^− 2 were estimated for ZnO layers. Also, crystallographic defects seem to reduce the mobility for highly doped ZnO films. On the other hand, for ITO films such an influence of the grain barriers was not observed down to carrier concentrations of about 10¹⁸ cm^− 3. Thus the grain barrier trap densities of ZnO and ITO are significantly different, which seems to be connected with the defect chemistry of the two oxides and especially with the piezoelectricity of zinc oxide.     

Relationship between structure,segregation and electrical activity in grain boundaries

Using the contactless microwave phase-shift technique (μ W-PS) and High Resolution Transmission Electron Microscopy (HRTEM), we show that the twist and mixed parts of a < 110 > Σ = 51(θ = 16.10^∘) grain boundary in germanium (Ge) are electrically active. We also show that we can passivate the electrically active grain boundaries by sulfur segregation which has been studied by energy filtering HRTEM. Atomistic simulations show that the most favorable places for this segregation are the high energy sites of grain boundary.     

Correlation between grain boundary misorientation and M23C6 precipitation behaviors in a wrought Ni-based superalloy

The correlation between the grain boundary misorientation and the precipitation behaviors of intergranular M₂₃C₆ carbides in a wrought Ni–Cr–W superalloy was investigated by using the electron backscattered diffraction (EBSD) technique. It was observed that the grain boundaries with a misorientation angle less than 20°, as well as all coincidence site lattice (CSL) boundaries, are immune to precipitation of the M₂₃C₆ carbides; in contrast, the random high-angle grain boundaries with a misorientation angle of 20°–40° provide preferential precipitation sites of the M₂₃C₆ carbides at the random high-angle grain boundaries with a higher misorientation angle of 55°–60°/[2 2 3] turn to retard precipitation of M₂₃C₆ carbides owing to their nature like the Σ3 grain boundaries and retard the precipitation of M₂₃C₆ carbides. The low-angle and certain random grain boundary segments induced by twins were found to interrupt the precipitation of the M₂₃C₆ carbides along the high-angle grain boundaries.     

Changes in grain boundary misorientation caused by emission of dislocation pairs

A new geometric micromechanism responsible for changes in the misorientation of grain boundaries in the course of (super)plastic deformation is proposed. According to this, the misorientation changes as a result of the motion of grain-boundary disclinations accompanied by the emission of lattice dislocation pairs. Within the framework of the proposed model, the energy characteristics of the motion of grain-boundary disclinations are calculated.     

Electromigration of grain boundaries in aluminum

The electromigration of grain boundaries has been investigated with aluminum wires (99.999% purity, 1 mm in diameter) in the temperature range 340°–540°C under a current stress of 6×10³ A cm^-2. Grain boundary migration in aluminum is found to be reduced or enhanced by the currents stress when the grain boundary migrates in or against the current direction, respectively. The effects of additions of copper, silicon and zirconium to aluminum on the electromigration of the grain boundaries have also been examined. The contribution of electromigration to normal grain boundary migration is found to be increased by the addition of solutes. The results are discussed on the basis of the impurity drag mechanism of grain boundary migration.     

涂敷ZnO纳米线的石英晶体微天平的湿度敏感特性

采用石英晶体微天平传感器电极上旋涂纳米氧化锌薄膜,构造湿度传感敏感元件,实现对湿度检测.介绍了检测系统的组成,纳米氧化锌的制备.通过DAQ数据采集卡和Labview软件实现数据的实时采集.实验结果表明旋涂纳米氧化锌薄膜的传感器频率随测试的相对湿度变化明显,用纳米氧化锌作为敏感元件检测湿度具有很高的灵敏度和重复性.     

The spectrum of atomic excess free volume in grain boundaries

The grain boundary (GB) excess volume is an important structural factor that is strongly correlated with various thermodynamic and kinetic properties of GBs such as GB energy, GB mobility, GB diffusivity, and GB segregation energy, etc. However, the excess volume is usually reported as an average value of the entire GB. Such simplification does not consider the spectral nature of the excess volume in a GB, which cannot be used to describe the atomic mechanisms of some kinetic process, such as GB migration, that involves only a few atoms at a time. Here, we explore the spectrum of atomic excess volume in representative nanocrystalline Ni and Al samples as well as 388 Ni bicrystals based on the Olmsted dataset by using atomistic simulations. It is found that the nanocrystalline Ni and Al models show a skew-normal distribution in the spectrum of both the atomic excess volume and the atomic excess energy in the GBs, which show a weak inverse correlation between them. This is in stark contrast to the widely reported positive correlation between GB energy and excess volume based on the average value. We further show based on the statistical analysis that the correlation between the atomic excess volume and excess energy strongly depends on the GB type and a universal trend between them does not exist. While low ∑ Ni GBs generally shows a strong inverse linear correlation between these two properties, such correlation is weak for high ∑ Ni GBs. Moreover, we find that the spectrum of the excess volume shows characteristics distribution in some special Ni GBs. For example, twist GBs generally show a symmetrical unimodal distribution while most surveyed ∑3 Ni GBs with anti-thermal behavior show an apparent bimodal distribution. Nevertheless, a strong correlation is found between the atomic excess volume and the segregation energy based on the nanocrystalline Al model with Mg impurity, which implies a possible universal trend between the two properties. The current study thus shows that the excess volume provides useful insights in revealing the elemental structure–property correlations in GBs, which may be used as a structural variable in future thermodynamic modeling of GBs.

     

氧化物透明导电膜的研究进展

介绍了透明导电薄膜主要的种类及发展过程,对氧化物透明导电薄膜的不同的制备工艺作了比较,并概括了氧化物透明导电薄膜的应用领域;在对氧化物透明导电薄膜的最新研究进展进行综合评述的同时,提出了透明导电薄膜的研究方向和目前亟待解决的问题.