Observation of grain boundaries

The observation of grain boundaries using the optical microscope, the electron microscope and the field-ion microscope has considerably aided our understanding of grain boundary structure. The study of etch pits in the optical microscope has proved that low angle boundaries are composed of dislocations. A large number of grain boundary observations have been made by transmission electron microscopy. Here, various classes of boundary structural features have been characterized in grain boundaries; they can be classified into three categories: (1) features which are essentially independent of the true structure of the boundary (thickness fringes, moiré fringes); (2) accidental features which result from a perturbation of the boundary by phenomena originating in the adjacent grains (dislocations, ledges); (3) features of the true structure of the grain boundary (features of the equilibrium structure-features of the perturbed structure). It must be appreciated that certain features of the structure are not resolved because of their poor contrast in the electron microscope image. The field-ion microscope has been used for a relatively short time in studies of grain boundaries. Nevertheless it has yielded very interesting results. Unfortunately, at the present time, it seems that the positions of the atoms may not be determined sufficiently accurately in this instrument to allow a precise determination of the structure. Only in a limited number of cases has it been possible to account for the observed features on the basis of theoretical considerations.     

Imaging and thickness measurement of amorphous intergranular films using TEM

Fresnel fringe analysis is shown to be unreliable for grain boundaries in yttrium-doped alumina: the determined thicknesses do not agree well with those measured from high resolution transmission electron microscopy (HRTEM), the asymmetry between under- and overfocus is very large, and Fresnel fringes are sometimes shown at boundaries which contain no amorphous film. An alternative approach to the analysis of HRTEM images of grain boundary films is demonstrated: Fourier filtering is used to remove the lattice fringes from the image thereby significantly enhancing the visibility of the intergranular films. The apparent film thickness shows a discrepancy between measurements from the original HRTEM image and the filtered image. It was shown that fringe delocalisation and diffuseness of the amorphous/crystalline interfaces will lead to a significant underestimate of the thickness in unprocessed HRTEM images. In contrast to this, the average thickness can be much more accurately measured from the Fourier-filtered image, provided the boundary is oriented accurately edge-on.     

Analysis of grain boundary dislocations by large angle convergent beam electron diffraction

Large angle convergent beam electron diffraction (LACBED) is used to analyse secondary dislocations in sigma3 and sigma9 grain boundaries in silicon. By selecting reflections from crystal planes common to the adjoining grains, LACBED images are insensitive to the boundaries except where dislocations are present. The dislocation images are closely similar to those for dislocations in single crystals and can be analysed by standard Cherns-Preston rules. It is shown that, for both boundaries, sufficient common reflections can be selected for a complete analysis, and that dislocations can be analysed assuming integer values of g x b, implying that the Burgers vectors are Displacement Shift Complete (DSC) lattice vectors. For both sigma3 and sigma9 boundaries, DSC dislocations are identified which are specific to these boundaries. The experimental conditions for the analysis of grain boundaries are explained, and the extension of the method to other coincidence boundaries is discussed.     

Automated image processing for grain boundary analysis

The image processing used in the automated analysis of grain boundaries and triple junctions in scanning electron microscopy images is described. The required image processing includes the location of grain boundaries and triple junctions, calculation of the dihedral angles at triple junctions, and selection of electron backscatter probe points (to obtain grain orientation data).     

Extraction and parametrization of grain boundary networks in glacier ice,using a dedicated method of automatic image analysis

Microstructure analysis of polar ice cores is vital to understand the processes controlling the flow of polar ice on the microscale. This paper presents an automatic image processing framework for extraction and parametrization of grain boundary networks from images of the NEEM deep ice core. As cross‐section images are acquired using controlled surface sublimation, grain boundaries and air inclusions appear dark, whereas the inside of grains appears grey. The initial segmentation step of the software is to separate possible boundaries of grains and air inclusions from background. A Machine learning approach is utilized to gain automatic, reliable classification, which is required for processing large data sets along deep ice cores. The second step is to compose the perimeter of section profiles of grains by planar sections of the grain surface between triple points. Ultimately, grain areas, grain boundaries and triple junctions of the later are diversely parametrized. High resolution is achieved, so that small grain sizes and local curvatures of grain boundaries can systematically be investigated.     

A simple 2D composite image analysis technique for the crystal growth study of L‐ascorbic acid

This work was destined for 2D crystal growth studies of L‐ascorbic acid using the composite image analysis technique. Growth experiments on the L‐ascorbic acid crystals were carried out by standard (optical) microscopy, laser diffraction analysis, and composite image analysis. For image analysis, the growth of L‐ascorbic acid crystals was captured as digital 2D RGB images, which were then processed to composite images. After processing, the crystal boundaries emerged as white lines against the black (cancelled) background. The crystal boundaries were well differentiated by peaks in the intensity graphs generated for the composite images. The lengths of crystal boundaries measured from the intensity graphs of composite images were in good agreement (correlation coefficient “r” = 0.99) with the lengths measured by standard microscopy. On the contrary, the lengths measured by laser diffraction were poorly correlated with both techniques. Therefore, the composite image analysis can replace the standard microscopy technique for the crystal growth studies of L‐ascorbic acid.     

Grain‐oriented segmentation of images of porous structures using ray casting and curvature energy minimization

We segment an image of a porous structure by successively identifying individual grains, using a process that requires no manual initialization. Adaptive thresholding is used to extract an incomplete edge map from the image. Then, seed points are created on a rectangular grid. Rays are cast from each point to identify the local grain. The grain with the best shape is selected by energy minimization, and the grain is used to update the edge map. This is repeated until all the grains have been recognized. Tests on scanning electron microscope images of titanium oxide and aluminium oxide show that their process achieves better results than five other contour detection techniques.     

Application of image processing to STEM tomography of low-contrast materials

In this study, the effect of various image-processing techniques on the visibility of tomographic reconstructions is investigated for a low-contrast material system of non-uniform thickness containing complex features such as grain boundaries and nanoparticles. Starting with a tilt series of high-angle annular dark-field (HAADF) images from an area of Dy-doped YBa₂Cu₃O_7−x-coated superconductor obtained using a scanning transmission electron microscope, various image-processing techniques were applied. These can be classified as edge detection, contrast-enhancing methods for non-uniform thickness and image sharpening. Although the processing methods violate the projection criterion for tomographic reconstruction, they were found, at least in this case, to enhance contrast and define the correct shape and size of structural features with minimal artifacts. Enhancing the visibility of structural features in this way allows the spatial distribution of the nanoparticles, their size, number density and location relative to each other and grain boundaries to be determined, which are essential to understand the flux-pinning characteristics of these materials.     

Identification of intergranular phases in ceramic nanocomposites

Analytical FEG-TEM was used for nanostructural and nanochemical characterization of Al₂O₃–TiN (composite I) and Si₃N₄–TiN (composite II) ceramic composite systems. The presence of vitreous intergranular phases in pockets at multiple grain junctions and in thin films (≈ 0.8 nm thick) at grain boundaries was revealed by high resolution and Fresnel fringe imaging techniques. The existence of a Ti-rich thin intergranular film at alumina grain boundaries was revealed by EDS line-scanning across internal interfaces at the 1.5 nm lateral resolution level. Extracting interface specific information at subnanometre levels by means of quantitative spatial difference EELS allowed an identification of intergranular phases. Ti sub-oxide existed in thin films at Al₂O₃ and TiN grain boundaries, whereas a mixed Al–Ti–O–N glassy phase was observed in pockets at triple grain junctions in composite I. In composite II, residual siliceous oxide and oxynitride glass phases were identified in thin films at Si₃N₄ grain boundaries and multiple grain junctions, respectively. These observations indicated that the chemistry of the intergranular phase in thin grain boundary films is notably different from that in larger pockets at multiple grain junctions.     

Improvement in the resolution of three-dimensional data sets collected using optical serial sectioning

In this paper an approach for improving the quality of 3-D microscopic images obtained through optical serial sectioning is described and implemented. A serially sectioned image is composed of a sequence of 2-D images obtained by incrementing the focusing plane of the microscope through the specimen of interest; ideally, the image obtained at each focusing plane should be in focus, and should contain information lying only within that plane. In practice, however, the images obtained contain redundant information from neighbouring focusing planes and are blurred by a three-dimensional low-pass distortion. These degradations are a consequence of the limited aperture of any optical system; using principles of geometric optics and allowing for the passage of light through the specimen, we are able to demonstrate that the microscope distortion can be described as a linear system, if the absorption of the specimen is assumed to be linear and non-diffractive. The transfer function of the microscope is found to zero a biconic region of 3-D spatial frequencies orientated along the optical axis; a closed-form expression is derived for the low-pass transfer function of the microscope outside the region of missing frequencies. The planar resolution of the serial sections can be greatly improved by convolving the image obtained with the inverse of the low-pass distortion function, although the missing cone of frequencies is not recoverable. The reconstruction technique is demonstrated using both simulated images, to demonstrate more clearly the effects of the distortion and the accuracy of the subsequent reconstruction, and actual experiments with a pollen grain and a stained preparation of human cerebellum tissue.     

Masked illumination scheme for a galvanometer scanning high-speed confocal fluorescence microscope

High-speed beam scanning and data acquisition in a laser scanning confocal microscope system are normally implemented with a resonant galvanometer scanner and a frame grabber. However, the nonlinear scanning speed of a resonant galvanometer can generate nonuniform photobleaching in a fluorescence sample as well as image distortion near the edges of a galvanometer scanned fluorescence image. Besides, incompatibility of signal format between a frame grabber and a point detector can lead to digitization error during data acquisition. In this article, we introduce a masked illumination scheme which can effectively decrease drawbacks in fluorescence images taken by a laser scanning confocal microscope with a resonant galvanometer and a frame grabber. We have demonstrated that the difference of photobleaching between the center and the edge of a fluorescence image can be reduced from 26 to 5% in our confocal laser scanning microscope with a square illumination mask. Another advantage of our masked illumination scheme is that the zero level or the lowest input level of an analog signal in a frame grabber can be accurately set by the dark area of a mask in our masked illumination scheme. We have experimentally demonstrated the advantages of our masked illumination method in detail.     

An autonomous phase-boundary detection technique for colloidal hard sphere suspension experiments

Colloidal suspensions of monodisperse spheres are used as physical models of thermodynamic phase transitions and as precursors to photonic band gap materials. Current techniques for identifying the phase boundaries involve manually identifying the phase transitions, which is very tedious and time-consuming. In addition, current image analysis techniques are not able to distinguish between densely packed phases within conventional microscope images, which are mainly characterized by degrees of randomness or order with similar grayscale value properties. We have developed an intelligent machine vision technique that automatically identifies colloidal phase boundaries. The technique utilizes intelligent image processing algorithms that accurately identify and track phase changes vertically or horizontally for a sequence of colloidal hard sphere suspension images. This technique is readily adaptable to any imaging application wherein regions of interest are distinguished from the background by differing patterns of motion over time.     

A light efficient optically sectioning microscope

We describe a simple method by which optically sectioned images may be obtained. The system geometry is similar to that of a tandem scanning microscope but a one-dimensional grid pattern is used rather than an array of pinholes. This produces a composite image consisting of an optically sectioned image superimposed on a conventional image. A blank sector on the disc is used to provide a wide-field image. Image subtraction yields the optically sectioned image in real time.     

Direct 3D mask modeling for nonplanar workpieces in microabrasive jet machining

Recently, a new microengraving technology, microabrasive jet machining, has been studied as a machining technology for highly brittle materials. The technology implements the machining by using an abrasive jet and it uses mask structures to achieve microscale geometrical accuracy. The mask structure selectively blocks the abrasive jet at the portions of the surface that are not to be machined. Modeling and fabrication of the mask structure are thus key processes in microabrasive jet machining. Microstereolithography is believed to be a better means of mask fabrication for general planar and nonplanar workpieces. However, it is not easy to model a precise 3D mask structure from a given pattern image. Because of inconsistencies between the computer-aided design (CAD) model and the actual workpiece, mask structures modeled from workpiece CAD models often fall off. We therefore propose an automated modeling algorithm for the corresponding 3D nonplanar mask structure by using measured geometry directly. The algorithm takes the workpiece geometry as section images acquired from computer tomography and generates the CAD mask model directly from the section and mask images. Application software was developed to verify the algorithm and was tested by verification and actual cases.     

Imaging of thermal and elastic surface properties by scanning electron acoustic microscopy

Scanning electron acoustic microscopy is a new technique for imaging the thermal and elastic properties of surfaces and detecting subsurface flaws. It can be carried out in a modified scanning electron microscope. The effects of electron beam energy and phase angle on scanning electron acoustic images of the thermal and elastic properties of surfaces were studied with an alumina fiber/aluminum matrix composite for fiber directions both transverse and coaxial to the surface. Images produced with 10- and 30-keV electrons at beam modulation frequencies of 80–1200 kHz appeared to be identical, with the exception of a lower signal-to-noise ratio for the lower electron energy. This observation suggests that the energy input from the beam can be considered to occur at the surface for electron energies below 30 keV and frequencies below 1200 kHz. Images recorded at 0° phase angle mapped regions of different thermal and elastic properties. Images recorded at 90° phase angle highlighted the boundaries between such regions. Scanning electron acoustic microscopy can image features of different thermal and elastic properties at greater depth than traditional imaging with backscattered electrons. The practical application of the technique to the study of surfaces is illustrated by the imaging of grain structure and subsurface particles for an extruder barrel.     

A comparison of grain imaging and measurement using horizontal orientation and colour orientation contrast imaging, electron backscatter pattern and optical methods

The problems associated with the definition of a grain, grain size measurement, and the issues associated with making one- and two-dimensional measurements on a three-dimensional structure are discussed. The relatively new scanning electron microscope (SEM)-based techniques of colour orientation contrast imaging (COCI) and automated electron backscatter pattern (EBSP) are explained and examples given. Comparisons with conventional (horizontal) orientation contrast imaging (HOCI) in the SEM are made. A direct comparison is made between conventional metallographic methods and these new techniques on precisely the same region of an interstitial free iron specimen. Both optical imaging and HOCI were found to miss a large number of grain boundaries (7 and 12%, respectively), and to create boundaries (≈ 2%). COCI was found to be reliable, with only 3% of boundaries missed. EBSP was taken to be the standard against which the others were compared, as it unambiguously measured changes in crystallographic orientation. Errors in the number of grain boundaries that are detected have a considerable effect on grain size measurements, e.g. mean linear intercept, and a follow-on effect on the modelling of mechanical properties. New methods for increasing the acquisition speed of orientation maps are discussed, along with examples. The combination of COCI (for grain location) and EBSP (for orientation measurement) is promising, but requires improvements in either imaging or image analysis to be totally reliable.     

Reconstruction of 3D grain boundaries from rock thin sections,using an advanced polarised‐light microscopy method

Grain boundaries play a significant role in materials by initiating reactions and collecting impurities. Here we present FAGO (Fabric Analyser Grain boundary recOnstruction), a first step towards the automatic determination of three‐dimensional (3D) grain boundary geometry using polarised light. The trace of the grain boundaries from 2D rock thin sections are determined primarily from data acquired using an automatic fabric analyser microscope and the FAME software (fabric analyser‐based microstructure evaluation; Peternell and colleagues and Hammes and Peternell). Based on the Fabric Analyser G50's unique arrangement of nine differently oriented light sources the retardation can be determined independently for each light direction and at each pixel in the field of view. FAGO combines these retardation datasets for each individual pixel together with retardation profiles across grain boundaries, to determine the orientations of the boundaries. The grain boundary traces are then broken up into segments of equal orientation, using the profile‐obtained orientation data. Finally, a 3D grain boundary model is reconstructed. The data processing is almost fully automatic using the MATLAB® environment. Only minor manual inputs are required.     

X-ray beam induced current method at the laboratory x-ray source

The x-ray beam induced current method (XBIC) is realized on the laboratory x-ray source using the polycapillary x-ray optics. It is shown that rather good images of grain boundaries in Si can be obtained by this method. The parameters of x-ray beam are estimated by the simulation of Schottky diode image. A good correlation between the experimental and calculated grain boundary XBIC contrast is obtained. The possibilities of laboratory source based XBIC method are estimated.     

基于ImageJ的复摆颚式破碎物料复杂粘连图像的分割

通过CCD采集作业现场复摆颚式破碎机破碎物料图像,以计算机处理平台,进行了颗粒复杂粘连图像的分水岭算法、分割等处理,并得到相应结果;由其数据可确定物料破碎比。