Kenji Higashida

High-angle triple-axis specimen holder for three-dimensional diffraction contrast imaging in transmission electron microscopy.

Electron tomography requires a wide angular range of specimen-tilt for a reliable three-dimensional (3D) reconstruction. Although specimen holders are commercially available for tomography, they have several limitations, including tilting capability in only one or two axes at most, e.g. tilt-rotate. For amorphous specimens, the image contrast depends on mass and thickness only and the single-tilt holder is adequate for most tomographic image acquisitions. On the other hand, for crystalline materials where image contrast is strongly dependent on diffraction conditions, current commercially available tomography holders are inadequate, because they lack tilt capability in all three orthogonal axes needed to maintain a constant diffraction condition over the whole tilt range. We have developed a high-angle triple-axis (HATA) tomography specimen holder capable of high-angle tilting for the primary horizontal axis with tilting capability in the other (orthogonal) horizontal and vertical axes. This allows the user to trim the specimen tilt to obtain the desired diffraction condition over the whole tilt range of the tomography series. To demonstrate its capabilities, we have used this triple-axis tomography holder with a dual-axis tilt series (the specimen was rotated by 90° ex-situ between series) to obtain tomographic reconstructions of dislocation arrangements in plastically deformed austenitic steel foils.

Dislocation tomography made easy: a reconstruction from ADF STEM images obtained using automated image shift correction

After previous work producing a successful 3D tomographic reconstruction of dislocations in GaN from conventional weak-beam dark-field (WBDF) images, we have reconstructed a cascade of dislocations in deformed and annealed silicon to a comparable standard using the more experimentally straightforward technique of STEM annular dark-field imaging (STEM ADF). In this mode, image contrast was much more consistent over the specimen tilt range than in conventional weak-beam dark-field imaging. Automatic acquisition software could thus restore the correct dislocation array to the field of view at each tilt angle, though manual focusing was still required. Reconstruction was carried out by sequential iterative reconstruction technique using FEIs Inspect3D software. Dislocations were distributed non-uniformly along cascades, with sparse areas between denser clumps in which individual dislocations of in-plane image width 24 nm could be distinguished in images and reconstruction. Denser areas showed more complicated stacking-fault contrast, hampering tomographic reconstruction. The general three-dimensional form of the denser areas was reproduced well, showing the dislocation array to be planar and not parallel to the foil surfaces.

Dislocation emission from a crack tip in MgO thin crystals

Abstract Dislocation emission from the tip of a crack in MgO thin crystals has been studied based on the geometry of crack tip dislocations observed using high voltage electron microscopy (HVEM). The effect of crack tip shielding due to the dislocations was also calculated using three-dimensional (3D) Bueckner–Rice weight function theory for the crack–dislocation interaction. Dislocation image analyses showed that dislocations ahead of the crack tip observed in the present study were almost right-handed (R-H) screw dislocations lying on the (0 1 1) plane, while in the crack wake many of the dislocations were left-handed (L-H) on the (0 1 1) plane. Formation of such dislocation configuration can be understood by the emission of dislocation loops from sources at a crack tip, where crack jogs, crack kinks and intersections of crack planes with free surfaces may act as significant sources for dislocation emission. 3D stress analysis exhibits that the largest component of crack tip stress intensities induced is mode I shielding type, and that mode II component is not negligible in the present case, suggesting the induction of the crack tip shielding for the mixed stress modes of I and II through the dislocation emission from the sources indicated above.

AFM and SEM observation on mechanism of fatigue crack growth in an Fe-Si single crystal

Fatigue crack growth tests are carried out on sheets of an Fe-3.2% Si single crystal with a crystallographic orientation appropriate for striation formation. The behaviour of slip near a crack tip during the loading and unloading parts of a fatigue cycle is observed using an Atomic Force Microscope and a Scanning Electron Microscope. The fracture surfaces are also analysed with an AFM and an SEM. The mechanism of fatigue crack growth is discussed based on the observations, and a fundamental kinematic model for fatigue crack growth is proposed. The model gives a reasonable explanation for both the crack growth and striation formation.

HVEM observation of crack tip dislocations in silicon crystals

Abstract Dislocation configurations near the tip of a crack in Si crystals has been investigated by using a high voltage electron microscope (HVEM). A ( 1 10 ) crack was introduced into a (001) silicon wafer by using indentation method at room temperature, and the specimen was annealed at 823 K to emit dislocations from the tip of the crack under the presence of residual stress due to the indentation. An array of dislocations was seen not only in front of the crack tip but also in the crack wake. The dislocations were emitted on the ( 1 1 1 ) plane which is oblique to the ( 1 10 ) crack. The effect of crack tip shielding due to the dislocations is analyzed to be mainly mode I. Dense dislocation region is also found near the tip, suggesting the occurrence of dislocation multiplication around the crack tip.

HVEM observations of dislocation structures near a crack tip in MgO crystals

Abstract Dislocation structures near the tip of a crack in MgO thin plate crystals have been investigated using a high voltage electron microscope (HVEM). In (001) thin plate crystals, a (010) crack is most commonly observed and an array of dislocations is seen along the [100] direction in front of the crack tip. The dislocations in the array are those of (011[ 0 1 ¯ 1 ] or ( 0 1 ¯ 1 )[011] slip systems and their Burgers vectors have large screw components. Dislocation image analyses have been applied to determine the signs of their Burgers vectors, and the screw dislocations on the (011) plane are found to be left handed and those on the ( 0 1 ¯ 1 ) plane are right handed. The effect of crack tip dislocations on fracture behavior is qualitatively examined and clarified to be a mode I shielding type so as to accommodate the tensile stress concentration near the crack tip.

Crack tip dislocations in silicon characterized by high-voltage electron microscopy

Abstract The nature of crack tip dislocations and their multiplication processes in silicon crystals have been examined by using high-voltage electron microscopy. Cracks were introduced by the Vickers indentation method at room temperature, and the specimen indented was annealed at high temperatures to induce dislocation generation around the crack tip under the presence of residual stress due to the indentation. In the specimens annealed, fine slip bands with the step heights of around 1 nm were formed along the {111} slip planes near the crack tip. The crack tip dislocations observed were characterized by matching their images to those simulated, and it was found that two different slip systems were activated even in the early stage of dislocation emission. With the increase in the number of crack tip dislocations, more complicated dislocation configurations such as dislocation tangles were formed around the crack tip, showing the beginning of multiplication of crack tip dislocations which causes effective crack tip shielding.

Structural and Resistivity Changes in YBa2Cu3Oy Ceramics by Heat-Treatment in Air

Effect of heat-tretment in YBa2Cu3Oy ceramics was investigated using the methods of electrical resistivity, TG, DTA and X-ray diffraction. In the heating process, a mass increase and a resistivity decrease are observed in the sample in the temperature range 630-780 K. Heat-treatment in air at temperatures above 780 K causes the marked increase of resistivity as well as the decrease of oxygen content. The YBa2Cu3Oy compound is decomposed gradually above 1200 K and completely at around 1290 K. The degradation and recovery of structural and transport properties by heat-treatment in air are also reported.

Inhomogeneous Deformation Observed Using High-Precision Markers Drawn by Electron Beam Lithography in a Magnesium Alloy with LPSO Phase

Inhomogeneous deformation in a magnesium alloy with long-period stacking order (LPSO) phase has been investigated using high-precision markers drawn by electron beam lithography. Mg alloys containing Zn and rare earth elements such as Y have a characteristic microstructure including the LPSO phase and the usual hcp matrix phase. The mechanical performance of this alloy is remarkably enhanced by warm-extrusion. The microstructure developed by such extrusion consists of elongated grains with fine-lamellae of LPSO phase and fine-grained matrix of hcp phase. In order to clarify the details of inhomogeneous deformation which should relate with the superior mechanical properties in this alloy, high-precision marking method using electron beam lithography has been employed. By using the method, local displacement due to tensile deformation in the Mg alloy was directly measured.

Transmission electron microscopy of bulk specimens over 10 µm in thickness

We succeeded the observation of microstructures in bulk-sized specimens of over 10µm in thickness by employing a technique that combines transmission electron microscopy (TEM) with energy-filtered imaging based on electron energy-loss spectroscopy (EELS). This method is unique in that it incorporates the inelastically scattered electrons into the imaging process. Using this technique, bright and sharp images of dislocations in crystalline silicon specimens as thick as 10µm were obtained. A calibration curve to determine foil thickness of such a thick specimen was also derived. This method simply extends the observable thickness range in TEM. If combined with tilt series of observation over a significant range of angle, it will disclose three dimensional nanostructures in a µm-order block of a specimen, promoting our understanding of the controlling mechanisms behind various bulky material properties.