Precession Electron Diffraction Research Articles

Characterization of the crystal structure of In-Ga-Zn-O materials via Precession Electron Diffraction

Characterization of the crystal structure of In-Ga-Zn-O materials via Precession Electron Diffraction

Enhanced nanoscale phase characterisation in modern steels using precession electron diffraction and energy filtering

Enhanced nanoscale phase characterisation in modern steels using precession electron diffraction and energy filtering

Characterization of Precipitates in Petroleum Steels by Using Precession Electron Diffraction Technique

Characterization of Precipitates in Petroleum Steels by Using Precession Electron Diffraction Technique

Scanning precession electron diffraction tilt series for orientation analysis

Scanning precession electron diffraction tilt series for orientation analysis

Grain refinement mechanism in chip and the machined subsurface during high-speed machining of Inconel 718 alloy

Microstructural defects on the machined surface of Inconel 718 alloy can be a significant source of fatigue cracks, potentially leading to failures of the structural components of aerospace engines. This work focused on the grain refinement mechanism induced by the dynamic recrystallization (DRX) behavior in both the chip and machined subsurface during high-speed machining (HSM) of Inconel 718 alloy. Multiscale metallurgical observations were conducted using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and precession electron diffraction (PED) to characterize the crystal structures, including the grain size, grain boundary distribution, dislocation density, and grain orientation distribution. Finite element (FE) simulations were performed to obtain the strain, strain rate, and cutting temperature to analyze the microstructure evolution mechanism under the effect of severe deformation conditions in HSM. The results indicate that grain refinement in the chip primarily occurs in the shear band and secondary deformation zone. The primary cause of the variation in the final grain size is attributed to the distinct thermomechanical loading conditions experienced by the material. The microstructure observed in the machined subsurface reveals a significant gradient distribution, characterized by the presence of an amorphous structure, nanocrystallites, refined grains, and elongated grains along the depth direction. The equiaxed ultrafine nanocrystals observed in the machined near-surface result from both continuous DRX (cDRX) and discontinuous DRX (dDRX) mechanisms. In the deeper zone, the presence of numerous subgrains and a high geometrically necessary dislocation (GND) density reaching up to 1016/m2 within the grains indicate that the refined grains in this zone result from the cDRX mechanism. The variation in DRX mechanisms observed in the machined subsurface are closely related to the gradient distribution of the thermomechanical load generated in HSM.

Large bi-axial tensile strain effect in epitaxial BiFeO3 film grown on single crystal PrScO3

A BiFeO3 film is grown epitaxially on a PrScO3 single crystal substrate which imparts ~ 1.45% of biaxial tensile strain to BiFeO3 resulting from lattice misfit. The biaxial tensile strain effect on BiFeO3 is investigated in terms of crystal structure, Poisson ratio, and ferroelectric domain structure. Lattice resolution scanning transmission electron microscopy, precession electron diffraction, and X-ray diffraction results clearly show that in-plane interplanar distance of BiFeO3 is the same as that of PrScO3 with no sign of misfit dislocations, indicating that the biaxial tensile strain caused by lattice mismatch between BiFeO3 and PrScO3 are stored as elastic energy within BiFeO3 film. Nano-beam electron diffraction patterns compared with structure factor calculation found that the BiFeO3 maintains rhombohedral symmetry, i.e., space group of R3c. The pattern analysis also revealed two crystallographically distinguishable domains. Their relations with ferroelectric domain structures in terms of size and spontaneous polarization orientations within the domains are further understood using four-dimensional scanning transmission electron microscopy technique.

Scanning precession electron diffraction data analysis approaches for phase mapping of precipitates in aluminium alloys

Mapping the spatial distribution of crystal phases with nm-scale spatial resolution is an important characterisation task in studies of multi-phase materials. One popular approach is to use scanning precession electron diffraction which enables semi-automatic phase mapping at the nanoscale by collecting a single precession electron diffraction pattern at every probe position over regions spanning up to a few micrometers. For a successful phase mapping each diffraction pattern must be correctly identified. In this work four different approaches for phase mapping of embedded precipitates in an Al-Cu-Li alloy are compared on a sample containing three distinct crystal phases. These approaches are based on: non-negative matrix factorisation, vector matching, template matching and artificial neural networks. To evaluate the success of each approach a ground truth phase map was manually created from virtual images based on characteristic phase morphologies and compared with the deduced phase maps. The percentage accuracy of all methods when compared to the ground truth was satisfactory, with all approaches obtaining scores above 98%. The optimal method depends on the specific task at hand. Non-negative matrix factorisation is suitable with limited prior data knowledge but performs best with few unique diffraction patterns and requires substantial post-processing. It has the advantage of reducing the dimensionality of the dataset and handles weak diffracted intensities well given that they occur repeatedly. The current vector matching implementation is fast, simple, based only on the Bragg spot geometry and requires few parameters. It does however demand that each Bragg spot is accurately detected in each pattern and the current implementation is limited to zone axis patterns. Template matching handles a large range of orientations, including off-axis patterns. However, achieving successful and reliable results often require thorough data pre-processing and do require adequate diffraction simulations. For artificial neural networks a substantial setup effort is demanded but once trained it excels for routine tasks, offering fast predictions. The implemented codes and the data used are available open-source. These resources and the detailed assessment of the methods will allow others to make informed decisions when selecting a data analysis approach for 4D-STEM phase mapping tasks on other material systems.

Improving orientation mapping by enhancing the diffraction signal using Auto-CLAHE in precession electron diffraction data

Precession electron diffraction (PED) is a powerful technique for revealing the crystallographic orientation of samples at the nanoscale. However, the quality of orientation indexing is strongly influenced by the quality of diffraction patterns. In this study, we have developed a novel algorithm called Auto-CLAHE (automatic contrast-limited adaptive histogram equalization), which automatically enhances low-intensity diffraction pattern signals using contrast-limited adaptive histogram equalization (CLAHE). The degree of enhancement is dynamically adjusted based on the overall intensity of the diffraction pattern, with greater enhancement applied to patterns with fewer spots (i.e., away from zone axes) and little or no enhancement applied to patterns with many spots (i.e., at a zone axis). By improving the visibility of low-intensity diffraction spots, Auto-CLAHE significantly improves the template matching between experimentally acquired and simulated diffraction patterns, leading to orientation maps with dramatically higher quality and lower noise. We anticipate that Auto-CLAHE provides an efficient and practical solution for preprocessing PED data, enabling higher-quality crystal orientation mapping to be routinely obtained.

Mapping strain across Co80Ta7B13 / Co62Ta6B32 glassy interfaces

While multilayered thin film metallic glasses offer promising mechanical properties due to their interfaces, the correlation between chemical composition and and strain at the interfaces has not been investigated so far. Hence, atomic distance and composition variations across glassy Co80Ta7B13 / Co62Ta6B32 interfaces are revealed by correlative transmission electron microscopy, nanobeam precession electron diffraction (NBPED) mapping and atom probe tomography (APT). A composition dependent mean atomic distance is identified for the individual layers. At the interfaces, a chemically graded region of 4 nm thickness is observed with APT, while the mean atomic distance gradient - investigated by NBPED mapping - extends over 9 nm and cannot solely be explained by chemical composition changes. Instead, the excess thickness of the atomic distance gradient compared to the chemical gradient can be rationalized by the presence of strain at the interface.

Oxidation behaviour of Ni-base superalloy 617 in simulated Advanced Ultra Supercritical (AUSC) steam

Alloy 617 was exposed in a simulated Advanced Ultra Supercritical (AUSC) steam oxidation test loop at 650–710 ℃, 31 MPa for 600 h. Detailed characterization of the oxide scales was carried out using Transmission Electron Microscopy coupled with Precession Electron Diffraction. The alloy 617 showed insignificant weight gains with super-parabolic kinetics. The oxide scale formed on the alloy consisted of a thin inner Cr2O3 layer and outer islands of (Mn,Ni,Fe)Cr2O4 spinel with negligible internal oxidation zone comprising Al2O3. The mechanistic aspect of AUSC steam oxidation is discussed.

Precession electron diffraction tomography for structural analysis of nanoparticles: a study of minimal size and associated dynamical effects

Precession electron diffraction tomography for structural analysis of nanoparticles: a study of minimal size and associated dynamical effects

Low Energy Twist Defects in Pentatwinned Silver Nanowires

Transmission electron microscopy investigation of networks of pentatwinned silver nanowires has identified the presence of nanoscale defects within individual wires, showing narrow regions of banded contrast normal to the nanowire axis (bamboo faults). Structural analysis using machine learning decomposition of scanning precession electron diffraction data identifies these bamboo faults as a pair of twist boundaries normal to the wire axis creating low energy coincident site lattices between the faulted region and the parent nanowire. This leads to a conservation of the relative misorientation of the twinned structure within the faulted region, leading to an apparent local rotation of the nanowire. Their presence after spraying and nonthermal processing suggests that they may form during nanowire synthesis. However, examination of the networks before and after cyclic straining finds an increase in the density of the defects, indicating that they may also form as a result of mechanical deformation.

Scanning precession electron diffraction study of carbide precipitation sequence in low alloy martensitic Cr-Mo-V tool steel

Precipitation of carbides after tempering of a medium carbon low alloyed Cr-Mo-V tool steel at 600°C was studied with scanning precession electron diffraction (SPED) in a transmission electron microscope (TEM). The precipitation sequence was evaluated by mapping the carbide distribution on carbon extraction replicas prepared from samples tempered for different durations of up to 24 h. The SPED results were supplemented by equilibrium calculations and energy dispersive x-ray spectroscopy (EDX) measurements in a TEM. It was found that ε-Fe2C precipitates within martensite laths during quenching via auto-tempering. During reheating to tempering temperature ε-Fe2C was dissolved and replaced by cementite, θ-M3C, which predominately form on martensite boundaries. It was further found that the small carbides in the early stage of tempering have predominantly a cubic MC structure, even if the V/Mo-ratio of the studied steel was only 0.12, and it is known from literature that V and Mo form cubic MC or hexagonal M2C carbides, respectively. Later during tempering more stable carbides, such as M7C3 and M23C6, also form, and it was concluded that the M7C3 form both by separate nucleation and precipitation on the cementite/matrix interface. The latter phenomenon was seen as particles with a core of cementite and a shell of M7C3 after 24 h of tempering.

Elastic strain field measurements in the TEM for metallurgical applications

In the last twenty years, several transmission electron microscopy techniques have been developed to measure nanoscale elastic strain fields, but with very little applications to metals and alloys. Here, we show that scanning precession electron diffraction can be used to measure elastic strain fields induced by nanometric coherent particles in an aluminium alloy. The agreement with theory is excellent, despite small discrepancies very near the particle. Strains are examined in both cartesian and cylindrical coordinate systems. Examination of the radial strains around the particle reveals that the deformation mode in the matrix is close to plane-strain. This results from having prepared samples as thin foils, which is required for observation in the microscope. Plane strain implies that only a partial information about the strain field in the bulk material is available, but it ensures that the measurements are not disturbed by variations of strain through the foil thickness. Strategies for ensuring plane strain in future observations are discussed at the end of the paper.

Locating Hydrogen Positions for COF-300 by Cryo-3D Electron Diffraction.

Covalent organic frameworks (COFs) have wide-ranging applications, and their host-guest interactions play an essential role in the achievement of COF functions. To investigate these host-guest interactions, it is necessary to locate all atoms, especially hydrogen atoms. However, it is difficult to determine the hydrogen atomic positions in COFs because of the complexities in synthesizing high-quality large single crystals. Three-dimensional electron diffraction (3D ED) has unique advantages for the structural determination of nanocrystals and identification of light atoms. In this study, it was demonstrated for the first time that the hydrogen atoms of a COF, not only on the framework but also on the guest molecule, can be located by 3D ED using continuous precession electron diffraction tomography (cPEDT) under cryogenic conditions. The host-guest interactions were clarified with the location of the hydrogen atoms. These findings provide novel insights into the investigation of COFs.

Locating Hydrogen Positions for COF‐300 by Cryo‐3D Electron Diffraction

Locating Hydrogen Positions for COF‐300 by Cryo‐3D Electron Diffraction

Influence of Precession Electron Diffraction Parameters and Energy Filtering on Reduced Density Function Analysis of Thin Amorphous Silica Films—Implications for Structural Studies

We investigated the influence of precession angle, energy filtering and sample thickness on the structural parameters of amorphous SiO2 thin films from the electron reduced density functions obtained by applying precession electron diffraction. The results demonstrate that the peak positions in the electron reduced density functions are generally insensitive to the studied experimental conditions, while both precession angle and energy filtering influence peak heights considerably. It is also shown that introducing precession with small angles of up to 2 degrees and energy filtering results in higher coordination numbers that are closer to the expected theoretical values of 4 and 2 for Si and O, respectively, for data obtained from a thicker sample.

Structure determination in a new type of amorphous molecular solids with different nonlinear optical properties: a comparative structural analysis

The microscopic structures of two amorphous molecular solids with extremely nonlinear optical properties have been studied. They consist of organotetrel chalcogenide clusters with the chemical formula [(RSn)4S6]. The basic molecular building blocks are adamantane-like {Sn4S6} cores with organic ligands R attached to the Sn atoms. While the material equipped with R=naphthyl generates frequency doubling upon irradiation with a simple infrared laser diode, the material decorated with R=phenyl responds by emitting brilliant white light. The structural differences were investigated using x-ray scattering and extended x-ray absorption fine structure combined with molecular Reverse Monte Carlo. Transmission electron microscopy and scanning precession electron diffraction were used to examine structural differences from mesoscopic down to microscopic scales. Characteristic differences were found on all scales. While close core-to-core distances between {Sn4S6} cluster cores and molecular distortions are found in the white light emitting material, undistorted molecules and significantly larger core distances characterize the material showing frequency doubling. Here however, results of scanning precession electron diffraction reveal the formation of nanocrystalline structures in the amorphous matrix, which we identify as cause for the suppression of white light emission.

WITHDRAWN: Precession electron diffraction study on the microtexture of the zirconium oxide film in aqueous solutions

The oxide film formed on zirconium alloy (Zr-0.45 Sn-0.38 Nb-0.30 Fe-0.05 Cu-0.015 Si-0.13 O, wt. %) respectively corroded for 300 days in 360 °C/18.6 MPa pure water and lithiated water was characterized using precession electron diffraction (PED). The results revealed that the grain morphology (columnar and equiaxed) and the grain crystallography orientation (angular deviation of {112¯}m-ZrO2 from the oxide growth direction) gradually changed periodically with the distance of metal/oxide (O/M) interface. In lithiated water sample, the proportion of oxide grains deviated from the {112¯}m-ZrO2 texture by 50-60° is higher than that in pure water, resulting in an early corrosion transition. This is attributed to that Li+ ions were unfavorable for the formation of well-oriented and coarse columnar grains. In the pure water, a small amount of hexagonal ZrO (h-ZrO) suboxide was found at the O/M interface where just go trough a high-rate corrosion transition, while in the lithiated water, a continuous h-ZrO suboxide layer was formed at the O/M interface where experiences a low-rate corrosion just before transition. This suggests that the present of suboxide layer greatly correlates to the instantaneous oxidation rate. Electron energy loss spectroscopy (EELS) further found that the presence of h-ZrO affected the oxygen level in the nearby substrate metal.

Correcting for probe wandering by precession path segmentation.

Precession electron diffraction has in the past few decades become a powerful technique for structure solving, strain analysis, and orientation mapping, to name a few. One of the benefits of precessing the electron beam, is increased reciprocal space resolution, albeit at a loss of spatial resolution due to an effect referred to as 'probe wandering'. Here, a new methodology of precession path segmentation is presented to counteract this effect and increase the resolution in reconstructed virtual images from scanning precession electron diffraction data. By utilizing fast pixelated electron detector technology, multiple frames are recorded for each azimuthal rotation of the beam, allowing for the probe wandering to be corrected in post-acquisition processing. Not only is there an apparent increase in the resolution of the reconstructed images, but probe wandering due to instrument misalignment is reduced, potentially easing an already difficult alignment procedure.