Electron Channeling Contrast Imaging Techniques Research Articles

Microstructure refinement of cold-sprayed copper investigated by electron channeling contrast imaging.

The electron channeling contrast imaging technique was used to investigate the microstructure of copper coatings fabricated by cold gas dynamic spray. The high velocity impact characteristics for cold spray led to the formation of many substructures, such as high density dislocation walls, dislocation cells, deformation twins, and ultrafine equiaxed subgrains/grains. A schematic model is proposed to explain structure refinement of Cu during cold spray, where an emphasis is placed on the role of dislocation configurations and twinning.

Coupling of Electron Channeling with EBSD: Toward the Quantitative Characterization of Deformation Structures in the SEM

The coupling of electron channeling contrast imaging (ECCI) with electron backscatter diffraction (EBSD) provides an efficient and fast approach to perform ECCI of crystal defects, such as dislocations, cells, and stacking faults, under controlled diffraction conditions with enhanced contrast. From a technical point of view, the ECCI technique complements two of the main electron microscopy techniques, namely, EBSD and conventional diffraction-based transmission electron microscopy. In this review, we provide several application examples of the EBSD-based ECCI approach on microstructure characterization, namely, characterization of single dislocations, measurement of dislocation densities, and characterization of dislocation substructures in deformed bulk materials. We make use of a two-beam Bloch wave approach to interpret the channeling contrast associated with crystal defects. The approach captures the main features observed in the experimental contrast associated with stacking faults and dislocations.

Twinning during recrystallization cooling in α-Zr alloy

Recently, twinning induced by intergranular thermal residual stresses (ITRSs) has been first reported in a Zr alloy after β→α furnace cooling. To further explore this kind of twinning behavior, microstructures and textures of the Zr alloy during α annealing between 650 and 750°C after β quenching were investigated in the present work, by use of X-ray diffraction, electron backscatter diffraction and electron channeling contrast imaging techniques. Results indicate that the ITRS-induced twins could also occur during cooling after α-recrystallization annealing in the Zr alloy, closely related to both the grain size and the texture. When the recrystallization texture is relatively random, high tensile ITRSs up to ~200MPa along c-axis could be accumulated. Tensile twins can only be induced by the tensile ITRS in the nearly texture-free Zr specimen with an average grain size larger than ~20μm.

Scanning and transmission electron microscopy investigations of defect arrangements in a two-phase γ-TiAl alloy

Different methods of scanning and transmission electron microscopy (SEM, TEM) were applied on a γ-TiAl alloy TNB-V5 after a thermo-mechanical fatigue test. Electron channelling contrast imaging (ECCI) and electron backscattered diffraction were carried out on bulk specimen. In addition, ECCI and scanning transmission electron microscopy in the SEM were carried out on a TEM foil in the electron opaque and the electron transparent region, respectively. The investigations were completed by transmission electron microscopy in the form of standard bright field imaging as well as by taking corresponding diffraction patterns. The results demonstrate in an impressive way that the ECCI technique applied in scanning electron microscopy can successfully supplement or in some cases replace imaging of dislocation arrangements in TEM.

OS1306 破壊様式が異なるCu-10%Zn合金の破面近傍の微視的組織

The purpose of the present study is to show possibility of a new scheme which can clarify fracture process by means of microstructure observation near fracture surface. In the present study, strip specimens of Cu-10%Zn alloy were fractured under monotonic tensile deformation, fatigue, and corrosive conditions. Microstructure near fracture surface was investigated by electron channeling contrast imaging (ECCI) technique and by electron backscatter diffraction (EBSD) analysis. For the specimen fractured under tensile deformation, the microstructure imaged by the ECCI technique consisted of blurred patches, which would be caused by heavily distorted grains. In the ECC image of the fatigued specimen, dislocation wall structure - which was self-organized by cyclic plastic deformation - was observed. No specific structure was detected for the stress-corrosion cracking. Through the EBSD analysis, distortion of a grain near fracture surface was investigated quantitatively. For the tensile deformation test, the maximum deviation angle from average grain orientation was amounted to about 10°, while those for the fatigue and the stress-corrosion cracking were less than 2°. It is hence suggest that fracture mode could be identified by the ECCI and EBSD analyses of the microstructures developed near fracture surface.

OS1610 ECCI法による疲労き裂近傍の転位構造観察(OS16-2 微視的な視点からの変形と強度,OS-16 ナノ・マイクロの視点からの変形と破壊の力学)

Dislocation structures formed around a fatigue crack in copper were observed with electron channelling contrast imaging (ECCI) technique. The ECCI technique enables us to observe dislocations lying near surface using a scanning electron microscope (SEM). For a cyclically-deformed copper, self-organized dislocation structures such as vein structure were successfully observed with the ECCI technique. To investigate relationship between dislocation structure around a fatigue crack and stress concentration, a fatigue crack growth test was conducted on a center-cracked tensile (CCT) copper specimen. The ECCI observation along the fracture surface revealed that cell structures were generated at the close vicinity of the fracture surface and the vein structure existed around outer edge of the cell structure. The formation of the cell structure was affected by the degree of stress concentration: measured size of the cell-structured area l_c was a power function of stress intensity factor range ΔK_I. To estimate distribution of ΔK_I value along a fracture surface, a lot of ECCI observations of the subsurface microstructure were performed on a smooth-shape copper specimen which was fractured under push-pull fatigue. The distribution of ΔK_I value, which was calculated from the above l_c-ΔK_I relation, was consistent with crack growth direction that was predicted from striation.

Observation of stacking faults in a scanning electron microscope by electron channelling contrast imaging

Abstract The electron channelling contrast imaging technique was applied in a scanning electron microscope equipped with a field emission gun and high beam current mode for imaging stacking faults, in particular in two different variants of cast TRIP steel after tensile and cyclic deformation. Besides well developed deformation bands, individual stacking faults with low dissipation width were observed in the metastable austenitic matrix.

Epitaxial SiC Growth Morphology and Extended Defects Investigated by Electron Backscatter Diffraction and Electron Channeling Contrast Imaging

Electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) were employed to investigate epitaxial SiC growth on 4H-SiC mesa structures. SiC polytypes were identified by indexing Kikuchi maps recorded from various points on the mesa surfaces. Orientation contrast was observed between different polytype surfaces using ECCI by forescattered electron detection. Extended defects in 3C-SiC were imaged directly by ECCI. Additionally, the ECCI technique was utilized to correlate dislocations with atomic step morphologies for various mesa surfaces. Evidence of vertical growth enhancement in the form of additional faceting was attributed to the presence of threading screw dislocations at mesa surfaces. Atomic steps were observed very near the edges of some mesa surfaces free of dislocations.

Characterizing dislocation structure evolution during cyclic deformation using electron channelling contrast imaging

This paper briefly reviews development of the electron channelling contrast imaging (ECCI) technique, which uses diffraction contrast to reveal dislocation structures underlying the surface of bulk samples imaged in the scanning electron microscope. In particular, the application of ECCI to the study of evolution of dislocation structures during cyclic deformation of single crystal copper samples will be described. Quantitative measurements of the vein width, separation and volume fraction within the matrix phase were made and these parameters showed rapid changes during the initial cyclic hardening of the sample, with smaller variations continuing after stress saturation. The volume fraction of persistent slip bands (PSBs) increased continuously with cycling beyond stress saturation. Measurements of PSB widths and spacings indicated that new PSBs tend to form near existing PSBs. The ECCI technique revealed PSBs of finite length with tapered ends. Stress concentrations occurring ahead of the tapered PSBs may be the cause of preferential nucleation of new PSBs close to existing ones.

549 疲労により自己組織化したメゾスケール転位構造の SEM による観察

Dislocation structures of fatigued copper and stainless steel crystals were observed with a scanning electron microscope (SEM) optimized for electron channeling contrast imaging (ECCI). With a help of the ECCI technique, one can detect dislocations underlying near surface of a bulk crystal. After fatigue tests, self-organized dislocation patterns such as persistent slip bands (PSBs) were successfully observed in both the copper and stainless steels. In addition, the ECCI technique revealed evolution of dislocation cell structure around a crack tip at which the observation with transmission electron microscopy (TEM) has been difficult.

Advances in SEM-based diffraction studies of defects and strains in semiconductors

Two scanning electron microscope (SEM)-based diffraction techniques, i.e. electron channelling contrast imaging (ECCI) and electron back-scatter diffraction (EBSD) are used to study lattice defect and local elastic strain distributions in Si1-xGe(x) epilayers grown on Si substrates patterned with mesas. The ECCI technique allows the misfit dislocations to be imaged in bulk samples. The misfit dislocations caused plastic relaxation of the strain in planar regions between mesas and in the wider mesas. In the narrower mesas the removal of lateral constraint at the mesa side faces had reduced the stress sufficiently to suppress the propagation of dislocations parallel to the closely spaced side faces. The measurements of small changes in the positions of two zone axes in EBSD patterns caused by variations in the local strain field were used to determine the strains and rotations making up the generalized plane strain tensor describing the deformation in the long mesa structures. The strain sensitivity of the method was determined to be approximately +/- 2 x 10(-4). Distributions of strains and rotations across mesas of several dimensions are reported and differ significantly between mesa for which the mesas width to epilayer thickness is high and low.

Study of dislocation structures near fatigue cracks using electron channelling contrast imaging technique (ECCI).

The fatigue of copper single crystals, orientated for single slip, has been studied using electron channelling contrast imaging in a scanning electron microscope. With the incident beam set at the Bragg condition, changes in the backscattered electron intensity occur as the beam is scanned over dislocations that cause a local tilting of the diffraction planes. This technique allows the evolution of dislocation structures over large areas to be followed through different stages of the fatigue life. Furthermore, it enables direct imaging of dislocation configurations at crack tips. The technique is compared with transmission electron microscopy and electron backscatter diffraction in its application to fatigue studies.

Imaging Dislocations in Duplex Tial Using Electron Channeling Contrast

Electron channeling contrast imaging (ECCI), performed using a scanning electron microscope, has been used to observe dislocations in bulk TiAl with a duplex microstructure. The ECCI technique is based on the dependence of the back-scattered electron (BSE) yield (of the incident beam orientation) on the crystal lattice and defect orientation near the specimen surface. This allows nearsurface defects to be imaged in bulk specimens [1, 2]. ECCI was carried out in a Camscan 44FE FEG-SEM operated at 25 kV using a beam divergence of ∽8 mrad and a beam current of ∽ 2nA. Specimens were observed at a working distance of approximately 11 mm. To record the BSE signal, the microscope was fitted with an annular (four quadrant) silicon diode detector array attached to the final lens pole-piece, resulting in a solid angle collection of approximately ∽0.6 π str. Images were recorded as 32 frame averages using ∽1.1 frames/sec.

Application of the electron channeling contrast imaging technique to the study of dislocations associated with cracks in bulk specimens

The application of the electron channeling contrast imaging (ECCI) technique to the study of dislocation behavior in the region of crack tips in high-purity stoichiometric single crystal NiAl is presented. Because the ECCI technique, which is based on the dependence of the intensity of backscattered electrons on the incident beam orientation with respect to the crystal lattice and defects near the specimen surface, allows near-surface defects to be imaged in bulk specimens, it has specific advantages over transmission electron microscopy approaches. The technique eliminates the tedious task of thin foil preparation and avoids many of the constraints imposed by the thin foils. Dislocations associated with crack propagation were observed by examining bulk 4-point bend specimens deformed in situ in a field emission gun scanning electron microscope. Dislocations were directly imaged in the crack tip region as well as along the crack paths. Crack deflection and blunting were observed to occur primarily by plastic zone formation in the immediate region of the crack tip.

Characterizing dislocation structures in bulk fatigued copper single crystals using electron channelling contrast imaging (ECCI)

Fatigue of copper single crystals, oriented for single slip, has been studied using electron channelling contrast imaging (ECCI) in a scanning electron microscope. This technique was used to detect and characterize dislocation structures in bulk specimens. With the incident beam set at the Bragg condition, changes in the backscattered electron intensity occur as the beam is scanned over dislocations which cause a local tilting of the diffraction planes. ECCI observations on specimens fatigued up to the saturation plateau (resolved shear stress τ s = 28 MPa) show that the dislocation substructures, principally ladder structures and elongated cells, are identical to those observed using transmission electron microscopy on thin foils. One of the main advantages of the ECCI technique is that one can follow all stages of the formation and evolution of dislocation structures over large areas in the same bulk specimen during fatigue interrupted at different stages of the fatigue life. ECCI should prove a powerful tool for investigating dislocation configurations at crack tips or at extrusions/intrusions or persistent slip band systems.

The magnetic permeability of steel: F. P. Fahy. (Chemical and metallurgical engineering, vol. 19, no. 5, September 1, 1918.)

In conventional transmission electron microscopy, specimens to be observed are placed in between the objective lens pole piece and therefore exposed to a strong magnetic field about 2 T. For a ferromagnetic specimen, magnetization of the specimen causes isotropic and anisotropic defocusing, deflection of the electron beam as well as deformation of the specimen, which all become more severe when the specimen tilted. Therefore electron tomography on a ferromagnetic crystalline specimen is highly challenging because tilt-series data sets must be acquired without changing the excitation condition of a specific diffraction spot. In this study, a scanning transmission electron microscopy (STEM) tomography method without magnetizing a ferromagnetic specimen has been developed for three-dimensional (3D) visualization of dislocations in α-Fe, which is a typical ferromagnetic material. Magnetic-field-free environment down to 0.38 ± 0.07 mT at the specimen position is realized by demagnetizing the objective lens pole piece of a commercial STEM instrument. By using a spherical aberration corrector with the magnetic-field-free environment, an “aberration corrected Low-Mag STEM mode” with no objective lens field reaches a convergence semi angle ∼1 mrad and a spatial resolution ∼5 nm, and shows an adequate performance of imaging dislocations under a two-beam excitation condition for a low-index diffracted beam. The illumination condition for the aberration corrected Low-Mag STEM mode gives no overlap between the direct beam disk (spot) and neighboring diffraction disks. An electron channeling contrast imaging technique, in which an annular detector was located at a doughnut area between the direct beam and the neighboring diffracted beams, was effectively employed with the aberration corrected Low-Mag STEM mode to keep image intensity high enough even at large specimen-tilt angles. The resultant tomographic observation visualized 3D dislocation arrangements and active slip planes in a deformed α-Fe specimen.