Kikuchi Patterns Research Articles

Orientation mapping with Kikuchi patterns generated from a focused STEM probe and indexing with commercially available EDAX software

Relating a crystal's microscopic structure—such as orientation and size—to a material's macroscopic properties is of great importance in materials science. Although most crystal orientation microscopy is performed in the scanning electron microscope (SEM), transmission electron microscopy (TEM)-based methods have a number of benefits, including higher spatial resolution. Current TEM orientation methods have either specific hardware requirements or use software that has limited scope, utility, or availability. In this article, a technique is described for orientation mapping using Kikuchi diffraction patterns generated from a focused STEM probe. One key advantage is that indexing and analysis of the patterns and maps occurs in the robust OIM Analysis software, currently widely used for electron backscatter diffraction (EBSD) and transmission Kikuchi diffraction (TKD) analysis. It was found that with minimal to no image processing and by changing only a few software parameters, reliable indexing of Kikuchi patterns is possible. Three samples, a deformed β-Titanium (Ti), a medium carbon heat-treated steel, and BaCe0.8Y0.2O3-δ were tested to determine the effectiveness of the approach. In all three measurements the algorithms effectively and reliably determined the phases and the crystal orientations of the features measured. For the two orientation maps produced, less than 5% of the patterns were misindexed including boundary areas where overlapping patterns existed. An angular resolution of 0.15° was achieved while features <25 nm were able to be spatially resolved.

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.

Transmission EBSD (t-EBSD) to determine grain and grain boundary properties on nanostructured superconductor samples

The knowledge of the properties of the grains and their grain boundaries is essential for the further optimization of sample performance of various high-Tc samples (YBa2Cu3Oy (YBCO), Bi-based cuprates, MgB2, iron-based materials). In these materials either the grains are of nanometer dimensions or the various additions of secondary phase(s) to improve the flux pinning are required to be of nanometer dimensions, so a high achievable resolution and better imaging properties are important to obtain reasonably high image quality of the recorded Kikuchi patterns to enable automated orientation mapping using the EBSD technique. Therefore, the newly developed transmission-EBSD (t-EBSD) technique is the method of choice to analyze the grain and the grain boundary properties in such nanostructured superconducting materials. Several results obtained on MgB2, melt-textured, infiltration-growth processed YBCO and electrospun Bi2Sr2CaCu2O8 nanowires are presented.

Automated Reconstruction of Spherical Kikuchi Maps.

An automated approach to fully reconstruct spherical Kikuchi maps from experimentally collected electron backscatter diffraction patterns and overlay each pattern onto its corresponding position on a simulated Kikuchi sphere is presented in this study. This work demonstrates the feasibility of warping any Kikuchi pattern onto its corresponding location of a simulated Kikuchi sphere and reconstructing a spherical Kikuchi map of a known phase based on any set of experimental patterns. This method consists of the following steps after pattern collection: (1) pattern selection based on multiple threshold values; (2) extraction of multiple scan parameters and phase information; (3) generation of a kinematically simulated Kikuchi sphere as the "skeleton" of the spherical Kikuchi map; and (4) overlaying the inverse gnomonic projection of multiple selected patterns after appropriate pattern center calibration and refinement. The proposed method is the first automated approach to reconstructing spherical Kikuchi maps from experimental Kikuchi patterns. It potentially enables more accurate orientation calculation, new pattern center refinement methods, improved dictionary-based pattern matching, and phase identification in the future.

Controlled piezotronic properties on recoverable energy storage density in rare-earth ions doped epitaxial PZT thin films

The present study describes the influence of rare-earth (RE = La, Eu, Dy and Ho) ions on recoverable energy storage density in PLD grown epitaxial Pb(ZrxTi1−x)O3 (PZT) thin films on SRO/LSAT (0 0 1) hetrostructures. Special attention has been paid to remove the pyrochlore phase, which is a prominent unwanted phase in ferroelectric specimens with rare earth dopants. The evidence of single crystallinity of the hetrostructures was confirmed from x-ray diffraction. The presence of single zone points in the Kikuchi patterns in electron back scattering diffraction reveals the epitaxial nature of rare earth doped PZT thin films. In addition, an x-ray photoelectron spectroscopy experiment was carried out to determine the oxidation states after doping the rare-earth ions in the PZT sample. The enhancement of spontaneous polarization in the donor (La, Eu) doped PZT films compared to the pure one is attributed to easy orientation and mobility of domain walls. It is found that the piezoelectric coefficient (d33) is more (130 pm V−1) for La doped PZT films (La:PZT), compared to the other rare earth doped PZT thin films. In our results, we found asymmetric polarization versus electric field hysteresis loop and energy storage efficiency () values of 30% were recorded in the case of pure PZT film and the value decreased to 28% with Ho doping, however, a high value of 46% could be realized in La:PZT.

EBSD Characterization of Specific Microstructures in RE-BCO Superconductors

Using the electron backscatter diffraction (EBSD) technique, we analyze in this contribution the microstructure of YBa2Cu3O x (YBCO) thick films with embedded a -axis oriented grains. The YBCO films were prepared by liquid phase epitaxy in air using NdGaO3 substrates. Using this technique, a / c -grain boundaries (GBs) with a well-defined facet of the YBCO film characterized by a single crystalline nature are obtained. This type of GB is very important for the basic physics of GBs in high- T c superconductor materials, and will contribute to a better understanding of GBs also in bulk materials. The application of EBSD to the analysis of a / c -GBs in YBCO requires a high Kikuchi pattern quality (i.e., image quality) to enable a proper distinction of the orientation, as the a -axis corresponds to about one-third of the c -axis, which can cause a misinterpretation of the resulting data. Having solved this problem, the EBSD-based determination of the GB misorientation angles directly proof the specific grain orientation achieved in the film growth.

Analysis of the microstructure of bulk MgB2 using TEM, EBSD and t-EBSD.

EBSD analysis can provide information about grain orientation, texture and grain boundary misorientation of bulk superconducting MgB2 samples intended for supermagnet applications. However, as the grain size of the MgB2 bulks is preferably in the 100-200 nm range, the common EBSD technique operating in reflection mode works only properly on highly dense samples. In order to achieve reasonably good Kikuchi pattern quality on all types of MgB2 samples, we apply here the newly developed transmission EBSD (t-EBSD) technique to spark-plasma sintered MgB2 samples. This method requires the preparation of TEM slices by means of focused ion-beam milling, which are then analysed within the SEM, operating with a custom-built sample holder. To obtain multiphase scans, we identified the Kikuchi pattern of the MgB4 phase which appears at higher reaction temperatures and may act as additional flux pinning sites. We present here for the first time EBSD mappings of multiple phases, which include MgB2 , MgB4 and MgO. LAY DESCRIPTION: The electron backscatter diffraction (EBSD) technique operating in the scanning electron microscope provides information on the crystallographic orientation the material by recording Kikuchi patterns. In polycrystalline samples, it becomes possible to analyse the orientations of the grains to each other. The metallic superconductor with the currently highest superconducting transition temperature, MgB2 with a Tc of 38.5 K, can be used in applications in polycrystalline form. One such application of interest are trapped field magnets or supermagnets, where the superconductor cooled in an applied magnetic field can trap the magnetic field as vortices at numerous flux pinning sites in the sample. When the external magnetic field is removed, the sample will stay magnetised as long as it is kept cool, and importantly, the trapped magnetic fields can be much higher as for any permanent magnet. However, the small size of the MgB2 grains in the 100-200 nanometre range requires a different approach when using the EBSD technique on such samples. The recently developed EBSD technique working in transmission mode (t-EBSD) helps considerably to image such materials. In this approach, a tiny TEM slice has to be milled out from the original sample by using focused ion beam milling. To understand the properties of the flux pinning in the spark-plasma sintered MgB2 sample, we had to identify the Kikuchi pattern of MgB4 , which is another, non-superconducting phase appearing at higher reaction temperatures required to compact the material. Using this information, we could perform EBSD scans using three different phases, MgB2 , MgB4 and MgO. The EBSD mappings enable to see where the secondary phase particles are located in the sample, and to judge if the particles could work as flux pinning sites.

Formation and crystallographic orientation study of quasicrystal, 2/1 and 1/1 approximants in Cd–Mg–Y system using electron backscatter diffraction (EBSD)

ABSTRACT A partial isothermal ternary phase diagram of the Cd–Mg–Y system including icosahedral quasicrystal (i-QC) and 1/1 approximant (APP) was obtained at 673 K. The stability of the i-QC and APPs in terms of composition and temperature was studied, as a result of which the 2/1 APP phase was discovered at 773 K. Moreover, the i-QC phase was found to be stable at lower Y concentrations of ∼12–13 at.%, as compared to the 1/1 APP, which showed ∼15–16 at.% Y. Single i-QC, 2/1 and 1/1 APP grains were also synthesized applying centrifuging system. Electron backscatter diffraction (EBSD) characterization of the obtained i-QC and APPs revealed almost the same Kikuchi patterns for the i-QC and 2/1 APP, indicating high resemblance of their structures. However, the Kikuchi pattern acquired from the 1/1 APP was distinguishable from the one obtained from the i-QC as it showed split bands. The occurrence of the split bands was associated with significant deviation from a perfect icosahedral symmetry. This was also confirmed by analysing calculated electron diffraction and Kikuchi patterns along pseudo-five-fold axes of the 2/1 and 1/1 APPs. Finally, orientation relationships between the i-QC, 1/1 APP and α-Mg were investigated by analysing acquired EBSD Kikuchi patterns from the respective domains. Accordingly, three perpendicular two-fold axes of the i-QC and {100} axes of the 1/1 APP occur along three mutually orthogonal {0001}, {10-10} and {11-20} axes of the α-Mg.

Constraints on the effective electron energy spectrum in backscatter Kikuchi diffraction

Electron Backscatter Diffraction (EBSD) is a technique to obtain microcrystallographic information from materials by collecting large-angle Kikuchi patterns in the scanning electron microscope (SEM). An important fundamental question concerns the scattering-angle dependent electron energy distribution which is relevant for the formation of the Kikuchi diffraction patterns. Here we review the existing experimental data and explore the effective energy spectrum that is operative in the generation of backscatter Kikuchi patterns from silicon. We use a full pattern comparison of experimental data with dynamical electron diffraction simulations. Our energy-dependent cross-correlation based pattern matching approach establishes improved constraints on the effective Kikuchi pattern energy spectrum which is relevant for high-resolution EBSD pattern simulations and their applications.

Determination of Local Strains in a Neighborhood of Cracks in a Welded Seam of Ni-Cr-Fe According to the Power Fourier Spectrum of Kikuchi Patterns

A discrete two-dimensional Fourier transform and the power Fourier spectrum are used for determination of average strains near cracks in a welded seam of Ni-Cr-Fe alloy. The alignment of Kikuchi images with the help of genetic algorithms and subtraction of white Gaussian noise made it possible to more fully take into account the influence of instrumental factors on the formation of electron backscatter diffraction patterns.

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.

Discrimination of dynamically and post‐dynamically recrystallized grains based on EBSD data: application to Inconel 718

The importance of angular resolution in EBSD analyses is discussed based on an Inconel 718 sample containing several populations of recrystallized grains, with subtle differences in dislocation contents. Classical EBSD analyses (with angular resolution in the range of 0.5-1°) do not allow for distinguishing recrystallized grains grown dynamically or post-dynamically. The angular resolution of EBSD orientation and misorientation data can be significantly improved (down to about 0.1-0.2°) either using more sophisticated Kikuchi pattern indexing methods and/or using the recently proposed LLASS denoising filter (Local Linear Automatic Smoothing Splines). Then the coexistence of both dynamically and post-dynamically recrystallized grains in the sample can be confirmed and quantified. ECCI images unambiguously confirm the conclusions drawn from the analysis of improved angular resolution EBSD data, and furthermore reveal the presence of thermal stress induced dislocations with typical patterns in water quenched Inconel 718 recrystallized grains. LAY DESCRIPTION: EBSD is widely used to study recrystallization phenomena. Conventional EBSD is nevertheless not able to distinguish dynamic recrystallized grains from post-dynamic recrystallized grains which differ by subtitle differences in dislocation contents. In this paper, we show that improving the orientation precision of EBSD data by means of different methods allows distinguishing these two recrystallized grains populations. Analyses and discussion are based on an Inconel 718, a famous Nickel-based superalloy in aeronautic.

Bohmian trajectory-bloch wave approach to dynamical simulation of electron diffraction in crystal

The Bohmian trajectory method is employed to study electron diffraction in crystalline materials. It provides a fresh understanding of the process of electron diffraction, including traveling channels of electrons and formation of diffraction patterns. By combining it with the Bloch wave method, the electron trajectories can be calculated more efficiently than the traditional wave-packet propagation algorithm. Meanwhile, we propose a momentum expectation approach which is a good approximation method with even higher computational efficiency. Both methods result in intuitive and accurate electron trajectories for the simulation of the electron backscatter diffraction (EBSD) pattern. Excellent agreement has been obtained between the simulated trajectory distributions and the experimental EBSD pattern from Mo (001) at 20 kV, where the Kikuchi patterns and higher order Laue zone rings are characterized.

Crystallization and microstructure of a glass seal for rapid laser sealing in the system CaO/Al2O3/SiO2

A glass with the composition 61 CaO·30 Al2O3·9 SiO2 was studied with respect to its crystallization behavior and its suitability as a rapidly crystallizing material for laser sealing. The glass was studied by differential scanning calorimetry; from the profiles recorded, Avrami activation energies and Avrami coefficients were calculated. The latter are in the range between 0.99 and 1.55 which is supposedly attributed to sole surface crystallization. During thermal treatment as well as during laser sealing, Ca12Al14O33, CaAl2O4 and Ca3Al2O6 are formed. These phases were also observed in SEM micrographs as evidenced by electron backscatter diffraction from the attributed Kikuchi patterns. Transmission electron microscopy showed a crystallized CaO- and SiO2-enriched interface which strongly adhered to the Al2O3 ceramic. The porosity of the crystallized seal was in the order of few percent. The studied glass proved suitable as crystallizing seal for rapid laser sealing.

Methods for Conducting Electron Backscattered Diffraction (EBSD) on Polycrystalline Organic Molecular Thin Films.

Electron backscattered diffraction (EBSD) is a technique regularly used to obtain crystallographic information from inorganic samples. When EBSD is acquired simultaneously with emitting diodes data, a sample can be thoroughly characterized both structurally and compositionally. For organic materials, coherent Kikuchi patterns do form when the electron beam interacts with crystalline material. However, such patterns tend to be weak due to the low average atomic number of organic materials. This is compounded by the fact that the patterns fade quickly and disappear completely once a critical electron dose is exceeded, inhibiting successful collection of EBSD maps from them. In this study, a new approach is presented that allows successful collection of EBSD maps from organic materials, here the extreme example of a hydrocarbon organic molecular thin film, and opens new avenues of characterization for crystalline organic materials.

Transmission Kikuchi diffraction from nano-crystalline Ti and TiN thin-films

Nano-crystalline Ti and TiN thin-films fabricated on <100> P-type Si wafers via magnetron sputtering are characterized by transmission Kikuchi Diffraction (TKD) in an FEI Sirion scanning electron microscope (SEM). A simple and cost-effective sample preparation combined with the TKD experiment at working distance of ∼ 3 mm and a tilt of 35° produced indexable Kikuchi patterns. Pole figures generated by automatic and manual indexing revealed <0001>|| normal direction (ND) fiber texture and <111>||ND fiber texture in monolithic Ti and TiN thin-films, respectively. Glancing angle x-ray diffraction results corroborated the texture in the samples. Spatial resolution of ∼ 60 nm is achieved that is close to the limit of resolution in conventional electron backscatter diffraction (EBSD). It is demonstrated that a simple sample preparation method in TKD technique combined with selected area electron diffraction (SAED) in transmission electron microscope (TEM) may reveal fundamental knowledge about different mechanisms occurring during fabrication and processing of the thin-film systems.

Evidence of a novel intermetallic Mg7Ag3 phase in Mg–Ag binary alloy system

A novel intermetallic Mg7Ag3 phase in the Mg–Ag binary alloy system at 573 and 623 K was discovered by the diffusion couple technique combined with the equilibrated alloy method. The composition and crystal structure of the Mg7Ag3 phase were identified via scanning electron microscopy/energy dispersive spectrometry, X-ray diffractometry (XRD), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The refinement of the XRD patterns was performed by Rietveld analysis. The XRD data have shown that Mg7Ag3 crystallizes in a body-centered crystal structure with Immm/orthorhombic symmetry (space group No. 71) and lattice parameters a = 14.2172, b = 14.6184, c = 14.177 Å and α = 90°. The well indexed selected area electron diffraction patterns and Kikuchi pattern obtained from TEM and EBSD confirmed the crystallographic structure obtained by XRD.

Characteristic orientation relationships in nanoscale Al-Al2Cu Eutectic

A fully lamellar Al-Al2Cu eutectic comprising alternate nano-scale α-Al and θ-Al2Cu phases was synthesized through laser surface remelting of a cast Al-32.7 wt% Cu alloy. The orientation relationship between α-Al and θ-Al2Cu phases was investigated from Kikuchi patterns obtained using electron backscattered diffraction (EBSD) and t-EBSD techniques. The characteristic orientation relationship (OR), {211}Al2Cu||{111}Al and 〈120〉Al2Cu||〈110〉Al, was observed independent of interlamellar spacing. This OR has two variants where, besides the two parallel planes, the variant I exhibits (001)Al2Cu||{001}Al and the variant II exhibits (001)Al2Cu||{111}Al. Variant I prevails over variant II in laser-processed α-Al and θ-Al2Cu nano-laminates, and atomistic modeling indicates that this is due to the low energy interfaces associated with variant I, i.e., {211}Al2Cu||{111}Al or (001)Al2Cu||{001}Al. A comprehensive understanding of the crystallographic orientation relationships between two phases may help predict or explain the active slip systems in fine-scale Al-Al2Cu eutectic and other cubic-tetragonal two-phase materials.

Human dental enamel: A natural nanotechnology masterpiece investigated by TEM and t-EBSD

The micro- and nanostructures of human dental enamel were investigated by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). For this purpose, TEM slices were prepared from teeth by focused ion-beam milling. These slices enabled TEM and transmission-EBSD (t-EBSD) investigations to be performed, while standard EBSD on bulk tooth samples resulted only in Kikuchi patterns. On the TEM slices, t-EBSD enabled automated mapping. The TEM images and the EBSD data clearly elucidated the arrangement of the hydroxyapatite crystals on the nanometer scale. Information regarding the crystallographic orientation of the apatite grains enabled the deduction of novel nanotechnological building principles of the enamel structure based on a chain-like arrangement of the crystallites.

Simulation of kinematic Kikuchi diffraction patterns from atomistic structures

One of the limitations of atomistic simulations is that many of the computational tools used to extract structural information from atomic trajectories provide metrics that are not directly compatible with experiments for validation. In this work, to bridge between simulation and experiment, a method is presented to produce simulated Kikuchi diffraction patterns using data from atomistic simulations, without requiring a priori specification of the crystal structure or defect periodicity. The Kikuchi pattern simulation is based on the kinematic theory of diffraction, with Kikuchi line intensities computed via a discrete structure factor calculation. Reciprocal lattice points are mapped to Kikuchi lines using a geometric projection of the reciprocal space data. This method is validated using single crystal atomistic models, and the novelty of this approach is emphasized by simulating kinematic Kikuchi diffraction patterns from an atomistic model containing a nanoscale dislocation loop. Deviations in kinematic Kikuchi line intensities are explained considering the displacement field of the dislocation loop, as is done in diffraction contrast theory.