On-axis Transmission Kikuchi Diffraction Research Articles

Transmission-scanning electron microscopy of interface fracture of ferrite deformation twins

AbstractDeformation twins in the ferrite matrix of an age-hardened duplex stainless steel have been observed using on-axis transmission Kikuchi diffraction (TKD) in a scanning electron microscope. This provided details of the lattice misorientation and dislocation arrangement, including the dislocation-free zone at the twin tip. These observations provide evidence for the link between microcracking of the irregular twin/parent interface and relaxation of the residual strains that arise from twin growth, offering new insights into fracture mechanics in these materials. Graphical abstract

Fatigue-Induced HCP-to-FCC Phase Transformation Resulting in Two FCC-Zr Variants in Pure Zirconium.

This study utilized transmission electron microscopy (TEM) and on-axis transmission Kikuchi diffraction (TKD) to investigate the fatigue-induced HCP-to-FCC phase transformation in industrial pure zirconium under a stress ratio of R = 0.1. The results show that fatigue damages result from phase deformations during cyclic loadings. The fatigue-induced FCC-Zr phases exhibit a B-type orientation relationship with the HCP-Zr matrix. Notedly, due to the different growth directions of Shockley partial dislocations relative to nucleation points, there are two FCC-Zr variants after the HCP-to-FCC phase transformation. The content of these two variants accounts for 65% and 35% of the total FCC-Zr, respectively, appearing as lamellae morphology embedded parallelly within the matrix. The distribution of the two variants includes isolated distribution and adjacent distribution. For the adjacent distribution, a twinning relationship is observed between the two variants. Meanwhile, as an intermediate transition stage of the HCP-to-FCC phase transformation, stacking faults are observed at the boundaries of the FCC-Zr lamellae. These findings offer insights into the microstructural features and formation mechanisms of fatigue-induced HCP-to-FCC phase transformation.

Elastic strain associated with irradiation-induced defects in self-ion irradiated tungsten

Elastic interactions play an important role in controlling irradiation damage evolution, but remain largely unexplored experimentally. Using transmission electron microscopy (TEM) and high-resolution on-axis transmission Kikuchi diffraction (HR-TKD), we correlate the evolution of irradiation-induced damage structures and the associated lattice strains in self-ion irradiated pure tungsten. TEM reveals different dislocation loop structures as a function of sample thickness, suggesting that free surfaces limit the formation of extended defect structures that are found in thicker samples. HR-TKD strain analysis shows the formation of crystallographically-orientated long-range strain fluctuations above 0.01 dpa and a decrease of total elastic energy above 0.1 dpa.

Influence of microscope settings on dislocation imaging in transmission forescattered electron imaging (t-FSEI)

This work analyzes the influence of several microscope settings, namely, sample-forescattered electron detector (FSD) distance, and tilting conditions on the characteristics of the dislocation contrast imaged in transmission forescattered electron imaging (t-FSEI). The dislocation contrast behaviors of characteristic dislocation configurations of two Fe-based alloys, namely an α’-martensitic (body-centered cubic, bcc) Fe33Ni alloy (wt%), and an austenitic (face-centered cubic, fcc) Fe-30Mn-6.5Al-0.3C alloy (wt%) were investigated on thin foil samples by using different on-axis transmission Kikuchi diffraction (TKD) configurations, namely t-FSEI, bright-field (BF) t-FSEI and electron channeling contrast imaging (ECCI). The set-ups use transmission Kikuchi electron patterns to orient the crystal into controlled diffraction conditions. Imaging parameters such as dislocation contrast intensity and information depth are analyzed and compared to those obtained in the ECCI mode under the same microscope conditions. These effects are associated with the attenuation of Bragg scattering by high-angle scattering processes and the electron channeling mechanism, respectively. The experimental analysis sets the microscope settings for optimum dislocation imaging in t-FSEI.

High resolution crystal orientation mapping of ultrathin films in SEM and TEM

Ultrathin metallic films are important functional materials for optical and microelectronic devices. Dedicated characterization with high spatial resolution and sufficient field of view is key to the understanding of the relation between microstructure and optical and electrical properties of such thin films. Here, we have applied on-axis transmission Kikuchi diffraction (TKD) and scanning precession electron diffraction (SPED) to study the microstructure of 10 nm thick polycrystalline gold films. The study compares the results obtained from the same specimen region by the two techniques and provides insights on the limits of each diffraction technique. We compare the physical spatial resolution of on-axis TKD and SPED and discuss challenges due to the larger probe size in scanning electron microscopy (SEM). Moreover, we present an improvement for the physical spatial resolution (PSR) of on-axis TKD through acquisition in immersion mode. We show how this method extends the capabilities of SEM-based microstructure characterization of ultrathin films and achieve PSR comparable to semi-automated SPED. • State of the art on-axis TKD SEM system comparison with SPED TEM system. • High resolution orientation mapping of ultrathin nano-crystalline metal films. • Improving TKD physical spatial resolution maps by immersion mode.

Facilitating the occurrence of dynamic recrystallization in plain extra low-carbon steel by warm asymmetric rolling

The occurrence of dynamic recrystallization in warm rolling of plain extra-low carbon steel was examined by applying 75% thickness reduction in a single pass. The investigated temperature range was ambient to 700 °C. Results show apparently dynamic recrystallization remains absent in symmetric rolling below 600 °C. Nevertheless, dynamic recrystallization not only just occurs but accelerates as the condition of warm rolling changed from symmetric to asymmetric condition. The analysis of grain internal disorientations and geometrically necessary dislocation densities by High-angular Resolution on-axis Transmission Kikuchi Diffraction (HR-TKD) allowed to identify the type of dynamic recrystallization and its evolution through the warm temperature range. New grains formed by either continuous or discontinuous dynamic recrystallization are preferentially orientated according a typical shear texture up to 600 °C in the case of asymmetric rolling. Above 600 °C, dynamically recrystallized grains produce restrengthening in the rolling texture as well as in the overall texture.

Boost reactivity of tri-iodide reduction electrode by highly faceted octahedral PtNi nanocrystals

PtNi octahedra were utilized as the counter electrodes (CEs) of N719-dye based DSSCs and well characterized via high-resolution transmission electron microscopy, powder x-ray pattern scan, and on-axis transmission Kikuchi Diffraction technique, rendering the crystallographic orientation of individual particle. The photovoltaic results show that the power conversion efficiency (PCE) of devices with different CEs is in the following sequence: Pt thin film < Pt7Ni3 octahedra < Pt5Ni5 octahedra < Pt4Ni6 octahedra. The device of Pt4Ni6 octahedra reveal the PCE over 10%. The electrochemical measurements show the tri-iodide reduction activity is in a sequence of Pt4Ni6 octahedra > Pt5Ni5 octahedra > Pt7Ni3 octahedra > Pt thin film. This study provides fundamental understanding of the exposed surface of PtNi octahedra, for which the coverage of {111} facets plays a key role in the tri-iodide reduction and thereby leads to the increase of device PCE with significant decrease of the Pt component in CEs.

Crystallographic Orientation Analysis of Nanocrystalline Tungsten Thin Film Using TEM Precession Electron Diffraction and SEM Transmission Kikuchi Diffraction

Two advanced, automated crystal orientation mapping techniques suited for nanocrystalline materials—precession electron diffraction (PED) in transmission electron microscopy (TEM) and on-axis transmission Kikuchi diffraction (TKD) in scanning electron microscopy (SEM)—are evaluated by comparing the orientation maps obtained from the identical location on a 30 nm-thick nanocrystalline tungsten (W) thin film. A side-by-side comparison of the orientation maps directly showed that the large-scale orientation features are almost identical. However, there are differences in the fine details, which arise from the fundamentally different nature of the spot pattern and Kikuchi line pattern in terms of the excitation volume and the angular resolution. While TEM-PED is more reliable to characterize grains oriented along low-index zone axes, the high angular resolution of SEM-TKD allows the detection of small misorientation between grains and thus yields better quantification and statistical analysis of grain orientation. Given that both TEM-PED and SEM-TKD orientation mapping techniques are complementary tools for nanocrystalline materials, one can be favorably selected depending on the requirements of the analysis, as they have competitive performance in terms of angular resolution and texture quantification.

Global DIC approach guided by a cross-correlation based initial guess for HR-EBSD and on-axis HR-TKD

Digital image correlation (DIC) techniques usually performed on deforming speckle patterns are applied here on electron diffraction patterns (EDP) in order to map disorientations with high angular resolution, as well as geometrically necessary dislocation densities and elastic strains. The proposed approach relies on a DIC analysis which is conducted independently from the microscope calibration. It registers EDP as a whole through a single and large region of interest whose relative deformation with respect to a reference EDP is described by a first-order homography. Subpixel registration is performed iteratively in the spatial domain using an inverse-compositional Gauss-Newton (IC-GN) algorithm which integrates the correction of the optical distortions. Its robustness against large orientation changes as well as its computational efficiency are improved by mean of an automated and path-independent initial guess which fairly captures the effects of large rotations on the EDP. Using successive Fourier-Mellin and Fourier transforms based cross-correlation techniques, the initial guess measures the in-plane rotation and translation between the reference and the target EDP, respectively. The performances of the technique are illustrated in markedly plastically deformed steels using two electron diffraction techniques in the scanning electron microscope. A 15% deformed interstitial free steel is investigated by means of electron backscattered diffraction (EBSD) while a quenched and tempered oxide dispersed strengthened steel subjected to martensitic transformation is characterized using a recently developed on-axis Transmission Kikuchi Diffraction (TKD) configuration.

Spatial Resolutions of On-Axis and Off-Axis Transmission Kikuchi Diffraction Methods

Spatial resolution is one of the key factors in orientation microscopy, as it determines the accuracy of grain size investigation and phase identification. We determined the spatial resolutions of on-axis and off-axis transmission Kikuchi diffraction (TKD) methods by calculating correlation coefficients using only the effective parts of on-axis and off-axis transmission Kikuchi patterns. During the calculation, we used average filtering to evaluate the spatial resolution more accurately. The spatial resolutions of both on-axis and off-axis TKD methods were determined in the same scanning electron microscope at different accelerating voltages and specimen thicknesses. The spatial resolution of the on-axis TKD was higher than that of the off-axis TKD at the same parameters. Furthermore, with an increase in accelerating voltage or a decrease in specimen thickness, the spatial resolutions of the two configurations could be significantly improved, from tens of nanometers to below 10 nm. At a voltage of 30 kV and sample thickness of 74 nm, both on-axis and off-axis TKD methods exhibited the highest resolutions of 6.2 and 9.7 nm, respectively.

Improving the imaging capability of an on-axis transmission Kikuchi detector

Transmission Kikuchi Diffraction (TKD) in the scanning electron microscope has been developing at a fast pace since its introduction less than a decade ago. The recently presented on-axis detector configuration, with its optimized geometry, has significantly increased the signal yield and facilitated the acquisition of STEM images in bright field (BF) and dark field (DF) mode, in addition to the automated orientation mapping of nanocrystalline electron transparent samples. However, the physical position of the integrated imaging system, located outside the detector screen, requires its movement in order to combine high resolution STEM images with high resolution orientation measurements. The difference between the two positions makes it impossible to acquire optimal signals simultaneously, leading to challenges when investigating site-specific nanocrystalline microstructures. To eliminate this drawback, a new imaging capability was added at the centre of the on-axis TKD detector, thus enabling acquisition of optimal quality BF images and orientation maps without detector movement. The advantages brought about by this new configuration are presented and the associated limitations are discussed.

On the depth resolution of transmission Kikuchi diffraction (TKD) analysis

In this paper, we have analyzed the depth resolution that can be achieved by on-axis transmission Kikuchi diffraction (TKD) using a Zr–Nb alloy. The results indicate that the signals contributing to detectable Kikuchi bands originate from a depth of approximately the mean free path of thermal diffuse scattering (λTDS) from the bottom surface of a thin foil sample. This existing surface sensitivity can thus lead to the observation of different grain structures when opposite sides of a nano-crystalline foil are facing the incident electron beam. These results also provide a guideline for the ideal sample thickness for TKD analysis of ≤ 6λTDS, or 21 times the elastic scattering mean free path (λMFP) for samples of high crystal symmetry. For samples of lower symmetry, a smaller thickness ≤ 3λTDS, or ≤ 10λMFP is suggested.

A New and Unexpected Spatial Relationship Between Interaction Volume and Diffraction Pattern in Electron Microscopy in Transmission.

The finding of this study is that the interaction volume in electron microscopy in transmission is well ordered laterally, with a remarkable and unexpected consequence being that lateral subsections of the interaction volume produce subsections of the Kikuchi diffraction pattern. It makes the microstructure of samples directly visible in Kikuchi patterns. This is first illustrated with polycrystalline Ti-10Al-25Nb with an on-axis transmission Kikuchi diffraction set-up in a scanning electron microscope. It is then shown via a Monte Carlo simulation and a large-angle convergent-beam electron diffraction experiment that this phenomenon finds its origin in the nature of the differential elastic and quasi-elastic cross sections. This phenomenon is then quantified by a careful image analysis of Kikuchi patterns recorded across a vertical interface in a silicon sample specifically designed and fabricated. A Monte Carlo simulation reproducing all the geometric parameters is conducted. Experiments and simulations match very well qualitatively, but with a slight quantitativity gap. The specificity of the thermal diffuse scattering cross-section, not available in the simulation, is thought to be responsible for this gap. Beside Kikuchi diffraction, the case of diffraction spots and diffuse background present in the pattern is also discussed.

Complementarity of On-Axis Transmission Kikuchi Diffraction and Forward Scatter Diffraction Imaging in SEM

Complementarity of On-Axis Transmission Kikuchi Diffraction and Forward Scatter Diffraction Imaging in SEM

Crystallography of Growth Blocks in Spheroidal Graphite

A better understanding of spheroidal graphite growth is expected in a near future thanks to widespread use of transmission electron microscopy. However, common transmission electron microscopy is quite time consuming and new indexing techniques are being developed, among them is transmission Kikuchi diffraction in a scanning electron microscope, a recent technique derived from electron backscatter diffraction. In the present work, on-axis transmission Kikuchi diffraction in scanning electron microscope, completed by transmission electron microscopy, was used with the objective of producing new observations on the microstructure of spheroidal graphite. This study shows that disorientations between blocks and sectors in spheroidal graphite are quite large in the early growth stage, which may be indicative of a competition process selecting the best orientations for achieving radial growth along thecdirection of graphite.

A systematic comparison of on-axis and off-axis transmission Kikuchi diffraction

The capabilities of the novel on-axis transmission Kikuchi diffraction (TKD) technique were explored in a systematic comparison with conventional off-axis TKD. The effect of experimental parameters on the appearance of on-axis and off-axis Kikuchi patterns was measured and discussed. In contrast to off-axis TKD, on-axis TKD is more sensitive to changes in beam current and beam energy and less sensitive to changes in working distance and detector distance. Moreover, on-axis TKD has a distinct advantage over off-axis TKD due to enhanced pattern intensity, which allows reduction of the beam current or an increase in the acquisition rate. The physical and effective spatial resolution were measured with detector-typical parameters. Even though the spatial resolution of both configurations did not differ significantly under test conditions, on-axis TKD enables measurement over large areas with the determined resolution, whereas off-axis TKD is more sensitive to beam drift. Band detection by the Hough-transform led to indexing of, on average, one additional Kikuchi band when measuring with on-axis TKD compared to off-axis TKD and operated more stable on on-axis patterns.

Depth Resolution Dependence on Sample Thickness and Incident Energy in On-Axis Transmission Kikuchi Diffraction in Scanning Electron Microscope (SEM)

Transmission Kikuchi diffraction is an emerging technique aimed at producing orientation maps of the structure of materials with a nanometric lateral resolution. This study investigates experimentally the depth resolution of the on-axis configuration, via a twinned silicon bi-crystal sample specifically designed and fabricated. The measured depth resolution varies from 30 to 65 nm in the range 10-30 keV, with a close to linear dependence with incident energy and no dependence with the total sample thickness. The depth resolution is explained in terms of two mechanisms acting concomitantly: generation of Kikuchi diffraction all along the thickness of the sample, associated with continuous absorption on the way out. A model based on the electron mean free path is used to account for the dependence with incident energy of the depth resolution. In addition, based on the results in silicon, the use of the mean absorption coefficient is proposed to predict the depth resolution for any atomic number and incident energy.

Characterization of Porous, TiO2 Nanoparticle Films Using On-Axis TKD in SEM -a New Nano-Analysis Tool for a Large-Scale Application

A combined methodical approach is tested with respect to the characterization of the inner structure of porous TiO2 layers as typically used in modern dye sensitized solar cells (DSSC). Their performance is directly linked to the surface area of the pore network. The micrometer thick layer employed was manufactured by screen-printing of a starting TiO2 powder constituted of shape-controlled, bipyramidal anatase nanoparticles on FTO/glass substrates. The analytical methods exploited in our study are Focused Ion Beam (FIB) slicing followed by 3D reconstruction as well as the new approach transmission Kikuchi diffraction (TKD) technology in the scanning electron microscope (SEM). Size and shape distribution of the TiO2 NPs within the layer can be extracted. SEM in transmission mode and atomic force microscopy (AFM) have been used to verify the dimensional data obtained by the new combined methodical approach. Its analytical benefits but also the challenges and limitations are highlighted.

On-axis Transmission Kikuchi Diffraction for Orientation Mapping of Nanocrystalline Materials in the SEM

Journal Article On-axis Transmission Kikuchi Diffraction for Orientation Mapping of Nanocrystalline Materials in the SEM Get access E Brodu, E Brodu Laboratoire d'Etude des Microstructures et de Mécanique des Matériaux (LEM3), UMR CNRS 7239, Université de Lorraine, Metz, FranceLaboratory of Excellence on Design of Alloy Metals for low-MAss Structures (DAMAS), University of Lorraine, Metz, France Search for other works by this author on: Oxford Academic Google Scholar E Bouzy, E Bouzy Laboratoire d'Etude des Microstructures et de Mécanique des Matériaux (LEM3), UMR CNRS 7239, Université de Lorraine, Metz, FranceLaboratory of Excellence on Design of Alloy Metals for low-MAss Structures (DAMAS), University of Lorraine, Metz, France Search for other works by this author on: Oxford Academic Google Scholar J-J Fundenberger J-J Fundenberger Laboratoire d'Etude des Microstructures et de Mécanique des Matériaux (LEM3), UMR CNRS 7239, Université de Lorraine, Metz, FranceLaboratory of Excellence on Design of Alloy Metals for low-MAss Structures (DAMAS), University of Lorraine, Metz, France Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 23, Issue S1, 1 July 2017, Pages 530–531, https://doi.org/10.1017/S1431927617003336 Published: 04 August 2017

On-axis TKD for orientation mapping of nanocrystalline materials in SEM

A new configuration for Transmission Kikuchi Diffraction (TKD) in SEM was recently developed, with a scintillator perpendicular to the electron beam. This configuration, “on-axis” TKD, makes grains and twins below 10nm well visible in orientation maps and with high indexation rate, as demonstrated with an electrodeposited nanocrystalline Ni. This high lateral spatial resolution is achieved by combining high accelerating voltage and low sample thickness to reduce the interaction volume. Among the advantages of the on-axis TKD, orientation mapping of particularly thin samples can be realized thanks to the advantageous axial position of the detector. Indeed, the reduction of the solid angle of the transmitted intensity around the beam axis with decreasing sample thickness is much less of a problem with an axial detector than with an off-axis detector. With an on-axis detector, the only condition for production of an orientation map is that the sample must be thick enough to produce Kikuchi diffraction in addition to spots.