Electron Backscatter Diffraction Patterns Research Articles

Accurate and fast localization of EBSD pattern centers for screen moving technology

The authors of this study develop an accurate and fast method for the localization of the pattern centers (PCs) in the electron backscatter diffraction (EBSD) technique by using the model of deformation of screen moving technology. The proposed algorithm is divided into two steps: (a) Approximation: We use collinear feature points to obtain the initial value of the coordinates of the PC and the zoom factor. (b) Subdivision: We then construct a deformation function containing the three parameters to be solved, select a large region for global registration, use the inverse compositional Gauss–Newton (ICGN) to optimize the objective function, and obtain the results of iteration of the PC and the zoom factor. The proposed algorithm was applied to simulated patterns, and yielded an accuracy of measurement of the PCs that was better than 4.6×10−6 of their resolution while taking only 0.2 s for computations. Moreover, the proposed algorithm has a large radius of convergence that makes it robust to the initial estimate. We also discuss the influence of factors of mechanical instability on its results of calibration during the insertion of the detector, and show that errors in measurements caused by the tilt motion of the camera are related only to the tilt angle of its motion and the detector distance, and are unrelated to the distance moved by it.

Indexing high-noise electron backscatter diffraction patterns using convolutional neural network and transfer learning

The crystal orientation indexing method based on Hough transform has many “zero solution” points when indexing the crystal orientation of materials, especially those with large strains. We propose an efficient and migratable indexing method for fast and autonomous crystal orientations indexing of experimental electron backscatter diffraction (EBSD) patterns of various crystal structures by transfer learning. To better evaluate the performance of the model, the performance of the model was assessed both quantitatively based on the numerical metrics and qualitatively through the Inverse Pole Figure (IPF). The results of indexing high-noise nickel EBSD patterns show that the CNN-based indexing method could not only obtain an acceptable indexing accuracy (a mean disorientation error of 1.77° for retraining and 2.31° for transfer learning), but also reasonably index the “zero solution” points remained in the Hough transform-based indexing method. And the results of indexing other materials by transfer learning show that the CNN has strong knowledge migration capability. In addition, the interpretability of the CNN was investigated by visualizing feature maps of each convolutional layer, and the sensitivity analysis of the CNN was realized by testing masked images.

Imaging Threading Dislocations and Surface Steps in Nitride Thin Films Using Electron Backscatter Diffraction.

Extended defects, like threading dislocations, are detrimental to the performance of optoelectronic devices. In the scanning electron microscope, dislocations are traditionally imaged using diodes to monitor changes in backscattered electron intensity as the electron beam is scanned over the sample, with the sample positioned so the electron beam is at, or close to the Bragg angle for a crystal plane/planes. Here, we use a pixelated detector instead of single diodes, specifically an electron backscatter diffraction (EBSD) detector. We present postprocessing techniques to extract images of dislocations and surface steps, for a nitride thin film, from measurements of backscattered electron intensities and intensity distributions in unprocessed EBSD patterns. In virtual diode (VD) imaging, the backscattered electron intensity is monitored for a selected segment of the unprocessed EBSD patterns. In center of mass (COM) imaging, the position of the center of the backscattered electron intensity distribution is monitored. Additionally, both methods can be combined (VDCOM). Using both VD and VDCOM, images of only threading dislocations, or dislocations and surface steps can be produced, with VDCOM images exhibiting better signal-to-noise. The applicability of VDCOM imaging is demonstrated across a range of nitride semiconductor thin films, with varying surface step and dislocation densities.

Parametric simulation of electron backscatter diffraction patterns through generative models

Recently, discriminative machine learning models have been widely used to predict various attributes from Electron Backscatter Diffraction (EBSD) patterns. However, there has never been any generative model developed for EBSD pattern simulation. On one hand, the training of generative models is much harder than that of discriminative ones; On the other hand, numerous variables affecting EBSD pattern formation make the input space high-dimensional and its relationship with the distribution of backscattered electrons complicated. In this study, we propose a framework (EBSD-CVAE/GAN) with great flexibility and scalability to realize parametric simulation of EBSD patterns. Compared with the frequently used forward model, EBSD-CVAE/GAN can take variables more than just orientation and generate corresponding EBSD patterns in a single run. The accuracy and quality of generated patterns are systematically evaluated. The model does not only summarize a distribution of backscattered electrons at a higher level, but also mitigates data scarcity in this field.

A novel fast fabrication of crystal-oriented hydroxyapatite ceramics via gel-casting in high magnetic field

Hydroxyapatite ceramics with oriented crystal structures are successfully fabricated via a novel and fast gel-casting process in high magnetic field for the first time. Experimental conditions as direction and duration of gel-casting in magnetic field were altered for selective crystal orientation of a,b-/c-plane of hydroxyapatites. The effect of slurry preparation process as ball-milling and freeze-drying on orientation degree and grain growth behavior of hydroxyapatite particles is elucidated with detailed analysis of the size distribution of particles, viscosity of slurry, microstructure and crystal structure of sintered bodies with assistance of SEM, XRD and advanced mapping observation of EBSD patterns. In addition, the influence of sintering temperature on the crystal phase transformation of the oriented outermost surface is also clarified through Raman spectroscopy in depth. It was found that the freeze-drying processed slurry shows potential in enhancing the crystal orientation degree than the ball-milling case, during where a suppressed grain growth phenomenon and inhibition of crystal phase transformation on highly oriented hydroxyapatite was also observed.

Damage assessment for steel structures subjected to cyclic pre-strain Part2: SEM in-situ observations clarifying the effect of stress state on damage evolution during plastic deformation

The authors conducted fracture toughness test results after the cyclic plastic strain and compared them with virgin steel in the previous report (Part 1 [1]). In this previous report, it was revealed that prestraining under high triaxiality stress state made more degradation of toughness. In this study, in order to deeply investigate the effect of triaxiality on material damage, tensile tests in a scanning electron microscope (SEM) were performed on two types of specimens with different triaxiality histories. First, material damage was evaluated by in-situ observation of the evolution of material damage under tensile loading using the EBSD (Electron Back Scattered Diffraction Pattern) method. As a result, it was confirmed that material damage tended to be more significant in the specimen with a higher triaxial field at the same equivalent plastic strain level and that material damage propagated more rapidly. The results of the slip evaluation based on the slip model for crystals in steel materials suggest that the ease of damage development associated with plastic deformation in a high triaxial field is due to the magnitude of movement of a particular slip plane.

Heterogeneous nucleation and orientation relationships of icosahedral phase with TiB2 inoculants

This study confirms that crystalline TiB2 particles can act as a potent site for the heterogeneous nucleation of the metastable icosahedral quasicrystalline phase (IQC) formed in cast Al-Mn-Si-Cu-Mg alloys. The results show that a primary metastable IQC phase nucleates on facets of TiB2 particles. Electron backscatter diffraction patterns and selected-area electron diffraction patterns confirmed that at least five orientation relationships (ORs) exist between the stable TiB2 and the metastable IQC phase. The IQC adopts different ORs with the crystallineTiB2 particles to form low energy interfaces by matching close-packed planes at the interfaces. In the IQC phase, the close-packed planes are the 5-fold and 2-fold. Microstructural analyses show that inoculation has a noticeable influence on the size and distribution of the primary IQC phase, which in turn has a noticeable influence on the compressive properties of the investigated alloys. Due to their exceptional mechanical properties, these alloys could become a prime candidate for lightweight applications, especially in the automotive and aerospace industries.

Mapping of lattice distortion in martensitic steel—Comparison of different evaluation methods of EBSD patterns

To visualize the varying tetragonal distortions in high carbon martensitic steels by EBSD, two different approaches have been applied on backscattered Kikuchi diffraction (BKD) patterns. A band-edge refinement technique called Refined Accuracy (RA) (Oxford Instruments) is compared with a technique called Pattern Matching (PM), which optimizes the fit to a simulated BKD signal. RA distinguishes between hypothetical phases of different fixed c/a, while PM determines a best fitting continuous c/a by projective transformation of a master pattern. Both techniques require stored BKD patterns.The sensitivity of the c/a-determination was tested by investigating the microstructure of a ferritic steel with an expected c/a=1. The influence of the Kikuchi pattern noise on c/a was compared for a single or 40 averaged frames per measuring point, and turned out to be not significant. The application of RA and PM on the martensitic microstructure delivered qualitatively similar maps of c/a. The comparison of RA and PM shows that RA is suitably fast and precise during mapping the martensite c/a ratio in analyses of high carbon martensite, especially for fast initial surveys. As RA leads quantitatively to higher noise in c/a, the PM analysis can be used for higher precision results.

EBSD patterns simulating multilayer twins based on dynamical theory of electron diffraction.

Electron backscatter diffraction (EBSD) can be employed to determine crystal structures but has not been used alone to identify defects at the atom scale due to the lack of understanding of the EBSD patterns generated by various structure defects. In the present work, the EBSD patterns of FCC-Fe with 9-layer, 6-layer and 3-layer twin structures are simulated, respectively, using the revised real space (RRS) method and compared with the counterpart of perfect crystals. Our results show that when the electron beam is incident along a direction parallel to the twin plane, the pattern appears symmetrical with respect to the corresponding Kikuchi band of the twin plane, and the diffraction details within the Kikuchi band also exhibit symmetry with respect to the middle line of the Kikuchi band. Moreover, the overall clarity of the patterns decreases, and the pattern becomes more blurred with increasing the distance from the Kikuchi band corresponding to the twin plane. By contrast, the incident electron beam along the direction perpendicular to the twin plane results in diffraction superposition of the matrix region and the shear region, which shows twofold rotational symmetry with respect to the Kikuchi pole corresponding to the normal to the twin plane. In addition, some extra Kikuchi bands appear in the EBSD patterns due to the long-period structures of the multilayer twins. As the number of multilayer twins decreases, the number of extra Kikuchi bands decreases and the area of the blurring pattern increases. The correlation between twin structures and EBSD patterns provides theoretical insights for identifying twin structures by the EBSD technique.

Atomic-scale observation and prediction of gallium ion implantation induced amorphization on natural diamond surface

Gallium ion implantation is an important solution to modify the physical properties of diamonds, in which the critical condition of gallium ion implantation-induced amorphization of diamond has a crucial influence on the modification effect. Therefore, this work attempts to establish an analytical prediction model of the critical implantation dose for diamond amorphization considering the acceleration voltage, implantation angle and crystal orientation. The prediction accuracy of the model is validated with the electron backscatter diffraction pattern technology. The results show that the model has a satisfactory prediction accuracy when the acceleration voltage is higher. Moreover, the atomic-scale observation of gallium ion implantation-induced amorphization of natural diamonds is realized on a spherical aberration projection electron microscope. The results presented in this work are significant to modify the natural diamond tool for service life improvement with focused ion beams in the future as well as to guide doping diamonds to manufacture n-type semiconductor diamonds.

EBSD patterns simulation of dislocation structures based on electron diffraction dynamic theory

Electron backscatter diffraction (EBSD) technology is a powerful tool for materials characterization including crystal orientation mapping, phase identification, and strain analysis. However, it is still challenging for using EBSD to identify crystallographic defects due to the insufficient understanding of the diffraction patterns of different defect structures. In the present work, EBSD patterns of FCC-Fe with 1/2 < 110 > edge dislocation dipole and 1/6 < 11̅2 > screw dislocation quadrupole structures are simulated by the revised real space (RRS) method. Our results showed that the presence of dislocations deteriorates the overall quality of the diffraction pattern and have different effects on different Kikuchi bands and Kikuchi poles. The edges of the Kikuchi band corresponding to the edge dislocation glide plane are sharp and the diffraction details within the band are clear. The sharpness of the edges of the Kikuchi band corresponding to the crystal plane normal to the dislocation Burgers vector is reduced, but the intra-band diffraction details are clear. Other Kikuchi bands show obvious anisotropic blurring. The diffraction details of the Kikuchi pole corresponding to the screw dislocation Burgers vector are clear, the edges of the Kikuchi bands across this pole are sharp, and the diffraction details within the bands are clear in the segments close to this pole and blurred in the segments far away from it. Other Kikuchi bands and Kikuchi poles are blurred. Our results indicate that the EBSD pattern can be simulated based on the electron diffraction dynamic theory and the correlation between dislocation structure and EBSD pattern is revealed, which provides theoretical guidance for the resolution of dislocation structures by the EBSD technique.

Dislocation cells in additively manufactured metallic alloys characterized by electron backscatter diffraction pattern sharpness

Dislocation cells in additively manufactured metallic alloys characterized by electron backscatter diffraction pattern sharpness

Use of electron backscatter diffraction patterns to determine the crystal lattice. Part 3. Pseudosymmetry.

A pseudosymmetric description of the crystal lattice derived from a single wide-angle Kikuchi pattern can have several causes. The small size (<15%) of the sector covered by an electron backscatter diffraction pattern, the limited precision of the projection centre position and the Kikuchi band definition are crucial. Inherent pseudosymmetries of the crystal lattice and/or structure also pose a challenge in the analysis of Kikuchi patterns. To eliminate experimental errors as much as possible, simulated Kikuchi patterns of 350 phases have been analysed using the software CALM [Nolze et al. (2021). J. Appl. Cryst. 54, 1012-1022] in order to estimate the frequency of and reasons for pseudosymmetric crystal lattice descriptions. Misinterpretations occur in particular when the atomic scattering factors of non-equivalent positions are too similar and reciprocal-lattice points are systematically missing. As an example, a pseudosymmetry prediction depending on the elements involved is discussed for binary AB compounds with B1 and B2 structure types. However, since this is impossible for more complicated phases, this approach cannot be directly applied to compounds of arbitrary composition and structure.

Use of electron backscatter diffraction patterns to determine the crystal lattice. Part 1. Where is the Bragg angle?

The derivation of a crystal structure and its phase-specific parameters from a single wide-angle backscattered Kikuchi diffraction pattern requires reliable extraction of the Bragg angles. By means of the first derivative of the lattice profile, an attempt is made to determine fully automatically and reproducibly the band widths in simulated Kikuchi patterns. Even under such ideal conditions (projection centre, wavelength and lattice plane traces are perfectly known), this leads to a lattice parameter distribution whose mean shows a linear offset that correlates with the mean atomic number Z of the pattern-forming phase. The consideration of as many Kikuchi bands as possible reduces the errors that typically occur if only a single band is analysed. On the other hand, the width of the resulting distribution is such that higher image resolution of diffraction patterns, employing longer wavelengths to produce wider bands or the use of higher interference orders is less advantageous than commonly assumed.

Use of electron backscatter diffraction patterns to determine the crystal lattice. Part 2. Offset corrections.

A band width determination using the first derivative of the band profile systematically underestimates the true Bragg angle. Corrections are proposed to compensate for the resulting offset Δa/a of the mean lattice parameters derived from as many Kikuchi band widths as possible. For dynamically simulated Kikuchi patterns, Δa/a can reach up to 8% for phases with a high mean atomic number Z, whereas for much more common low-Z materials the offset decreases linearly. A predicted offset Δa/a = f(Z) is therefore proposed, which also includes the unit-cell volume and thus takes into account the packing density of the scatterers in the material. Since Z is not always available for unknown phases, its substitution by Z max, i.e. the atomic number of the heaviest element in the compound, is still acceptable for an approximate correction. For simulated Kikuchi patterns the offset-corrected lattice parameter deviation is Δa/a < 1.5%. The lattice parameter ratios, and the angles α, β and γ between the basis vectors, are not affected at all.

Evolution of Martensite Tetragonality in High-Carbon Steels Revealed by In Situ High-Energy X-Ray Diffraction

The martensitic transformation was studied by in situ and ex situ experiments in two high-carbon, 0.54 and 0.74 wt pct C, steels applying three different cooling rates, 15 °C/s, 5 °C/s, and 0.5 °C/s, in the temperature range around Ms, to improve the understanding of the evolution of martensite tetragonality c/a and phase fraction formed during the transformation. The combination of in situ high-energy X-ray diffraction during controlled cooling and spatially resolved tetragonality c/a determination by electron backscatter diffraction pattern matching was used to study the transformation behavior. The cooling rate and the different Ms for the steels had a clear impact on the martensitic transformation with a decrease in average tetragonality due to stronger autotempering for a decreasing cooling rate and higher Ms. A slower cooling rate also resulted in a lower fraction of martensite at room temperature, but with an increase in fraction of autotempered martensite. Additionally, a heterogeneous distribution of martensite tetragonality was observed for all cooling rates.

Minimizing defects and controlling the morphology of laser welded aluminum alloys using power modulation-based laser beam oscillation

Due to sensitivity to heat, laser welding of aluminum alloys is always accompanied by serious defects such as porosity and cracks, which restricts their wide application for structures of lightweight applications. Beam oscillating welding can potentially eliminate these defects to a large extent but with shortcoming of low heat utilization rate. For better solution, a Full Domain Power Modulation (FDPM) system was promoted to conduct bead-on-plate welding tests of 6061 aluminum alloys using circular oscillation. The impact of three welding modes (traditional Laser Welding, Constant Power (CP), Gradient Power (GP)) and oscillation frequency on the weld morphology, porosity, and microstructure was examined by Optical Microscopy (OM), Electron backscatter diffraction patterns (EBSD) and industrial Computed Tomography (CT). The results demonstrated that at the same frequency, the weld depth-to-width ratio in GP mode exceeded the CP mode. The strong stirring effect promoted the pores to escape and increased the heterogeneous nucleated particles of the molten pool, which yielded the lowest weld porosity and the largest equiaxed crystal ratio in GP mode. The mechanism of porosity removal and grain refinement was determined based on the calculated energy distribution. This study provides a brand new approach for porosity inhibition and microstructure optimization of laser-welded aluminum alloys.

Theoretical Diffraction Pattern Characteristics of Cubic Twin Crystal

Twin crystal is a common lattice arrangement of crystal structure. The commonly used characterization methods for crystal structure include transmission electron diffraction (TED) pattern and electron backscatter diffraction (EBSD) pattern, but there is currently no specific reference standard for twin crystal. In this research, the mathematical relationship between crystal structure and TED and EBSD patterns of twin was calculated. The characteristics of twin electron diffraction spectrum, as well as the calibration of TED and EBSD patterns, were discussed and analyzed towards the examples of face-centered cubic crystal and body-centered cubic crystal. Overall, our results establish a theoretical calculation standard of diffraction spectrum, which provides a reference for further explorations to characterization of twin crystal structures.

Microstructure of Co–Cr Dental Alloys Manufactured by Casting and 3D Selective Laser Melting

The review analyses the microstructure of the commercial Co–Cr–(Mo, W) dental alloys fabricated by 3D digital selective laser melting (SLM), which is the most promising technique among the emerging additive fabrication technologies used for metal products manufacturing in dentistry. In this regard, the main goal is to compare the microstructures of the metal dental products produced by two currently used technologies, namely, conventional casting and SLM. We consider the latest research published from 2013 to 2022. The microstructures are evaluated using optical microscopy (OM), scanning electron microscopy with energy-dispersive x-ray spectroscopy (SEM–EDS), x-ray diffractometry (XRD), electron backscatter diffraction (EBSD) pattern analysis, and atomic force microscopy (AFM). The microstructure analysis allows concluding whether the SLM fabrication process is suitable for dental applications. As shown, the microstructure of the Co–Cr dental alloys depends on both the chemical composition of the samples and the parameters of the manufacturing technique used. Experimental results have proven that, in contrast to the conventional casting, the SLM-fabricated specimens display superior microstructure due to complete local melting and rapid solidification. Additionally, the SLM process minimizes residual flaws and porosity. As a result, SLM allows producing the dense material comprising homogeneous fine-grain microstructure.

Microstructural evolution and precipitate behavior of an AZ61 alloy plate processed with ECAP and electropulsing treatment

Electropulsing treatment processing (EPT)-induced microstructural evolution and precipitation behaviour of the β-Mg17Al12 phase in the AZ61 alloy were investigated by using scanning electron microscopy, X-ray diffraction and electron backscattering diffraction patterns. The results revealed that the EPT method is efficient in achieving a near-spherical Mg17Al12 phase (approximately 300 nm) and uniform ultrafine-grained structure (approximately 900 nm) featuring a weakened texture in AZ61 alloy plates after EPT for 40 μs-3 min. The improved structural characteristics are attributed to the rapid recrystallization and grain refinement induced by EPT at lower temperatures and in a short time. On this basis, kinetics analysis of the spheroidization and the dissolution of the β-Mg17Al12 precipitation phase and grain refinement and growth in the samples after EPT were performed. Furthermore, the mechanism of precipitation phase transformation and grain refinement during EPT was discussed.