Precession Electron Diffraction Research Articles

Tensile Properties of a Non-Equiatomic Ni–Co–V Medium Entropy Alloy at Cryogenic Temperature

The development of strong and ductile alloys for application in cryogenic temperatures has long been sought after. In this work, we have developed a face-centered cubic Ni10Co56.5V33.5 multi-principal element alloy (MPEA) that exhibits a balanced combination of high strength and good ductility at 77 K, based on the considerations of large local lattice distortion (LLD) and low stacking fault energy. The small-grained Ni10Co56.5V33.5 MPEA exhibits a yield strength of 1400 MPa and an ultimate tensile strength of 1890 MPa, while preserving a good ductility of 23%. Moreover, precession electron diffraction and transmission electron microscopy revealed multiple deformation mechanisms, including wavy dislocations, atypically severely twisted dislocation bands, hierarchical stacking faults, and deformation twins, which are implicated in the alloy’s outstanding mechanical performance. These insights offer a strategic guide for the design of strong and ductile alloys, particularly for utilization in extreme environments.

From microstructural changes to electrochemical behavior: decoding the corrosion mechanism in Zr-Sn-Nb-Fe-Cr-Mo-O-V alloys

The oxide formed on a Zr-0.4Sn-0.7Nb-0.3Fe-0.1Cr-0.15Mo-0.12O-0.05V, wt% alloy, after exposure for 300 d in 360°C/20 MPa (water with 0 ppb, 300 ppb, and 1000 ppb), was characterized by transmission electron microscope (TEM), precession electron diffraction (PED) and electrochemical impedance spectroscopy (EIS). Results showed that DO accelerated the corrosion of the alloy, and this effect was enhanced with the increase of the DO concentrations. A mechanism of the correlation between electrochemical behavior evolution and microstructural changes of oxide film was proposed, which provided theoretical guidance for the selection of fuel cladding materials for small nuclear reactors.

Determination of Na+ Cation Locations in Nanozeolite ECR-1 Using a 3D ED Method

Until now, the comprehensive structural analysis of single crystals of zeolite ECR-1, an aluminosilicate with the EON topology, has been hindered owing to the submicron dimensions of the obtained crystals. Additionally, this zeolite, which is characterized by a topology comprising alternating periodic building units of MAZ and MOR layers, exhibits stacking faults that impede accurate refinement through the Rietveld method. In this report, we present, for the first time, the structure of ECR-1 elucidated by studying a nanocrystal with a significantly reduced number of stacking faults. The sample used was synthesized hydrothermally using trioxane as the organic structure-directing agent. The structure determination was conducted using precession electron diffraction (PED) at 103 K. Partial dehydration occurred owing to the high vacuum conditions in the TEM sample chamber. From the dynamical refinement (Robs = 0.097), 8.16 Na+ compensating cations were localized on six distinct crystallographic sites, along with approximately four water molecules per unit cell. Furthermore, a canonical Monte Carlo computational study was conducted to compare the experimental cationic distribution and location of water molecules with the simulation.

Study of the epitaxy between ɛ-Fe2O3, hematite and spinel in brown-glazed Chinese ceramics using electron diffraction mapping techniques

Black-to-brown Chinese ceramics have sparked the interest of material scientists since the widespread discovery of complex dendritic structures containing mainly the rare ɛ-Fe2O3 polymorph in their glazes. This study proposes to tackle this issue using two electron diffraction mapping techniques, transmission Kikuchi diffraction and automated crystal orientation mapping–precession electron diffraction, which have not been fully exploited despite their relevance for studying these structures at the scales of interest. It first demonstrates the efficiency of applying these techniques to characterize the structures at the nanoscale and discusses some limitations that may be encountered, in particular due to the specificity of heritage samples. It then focuses on the crystalline orientations obtained with these techniques, which highlight epitaxial relationships among the crystalline planes (001) of the ɛ-Fe2O3 phase, (0001) of the hematite and (111) of the spinel. Finally, it discusses the growth of such complex multi-phase structures by proposing two possible mechanisms, one based on the transformation of ɛ-Fe2O3 into hematite and spinel, and the other based on the growth of hematite and spinel on already crystallized ɛ-Fe2O3 dendrites. It also compares the dendritic structures of the two modern samples with those found in an ancient sample of Jian ware.

Dynamical refinement with multipolar electron scattering factors.

Dynamical refinement is a well established method for refining crystal structures against 3D electron diffraction (ED) data and its benefits have been discussed in the literature [Palatinus, Petříček & Corrêa, (2015). Acta Cryst. A71, 235-244; Palatinus, Corrêa et al. (2015). Acta Cryst. B71, 740-751]. However, until now, dynamical refinements have only been conducted using the independent atom model (IAM). Recent research has shown that a more accurate description can be achieved by applying the transferable aspherical atom model (TAAM), but this has been limited only to kinematical refinements [Gruza et al. (2020). Acta Cryst. A76, 92-109; Jha et al. (2021). J. Appl. Cryst. 54, 1234-1243]. In this study, we combine dynamical refinement with TAAM for the crystal structure of 1-methyluracil, using data from precession ED. Our results show that this approach improves the residual Fourier electrostatic potential and refinement figures of merit. Furthermore, it leads to systematic changes in the atomic displacement parameters of all atoms and the positions of hydrogen atoms. We found that the refinement results are sensitive to the parameters used in the TAAM modelling process. Though our results show that TAAM offers superior performance compared with IAM in all cases, they also show that TAAM parameters obtained by periodic DFT calculations on the refined structure are superior to the TAAM parameters from the UBDB/MATTS database. It appears that multipolar parameters transferred from the database may not be sufficiently accurate to provide a satisfactory description of all details of the electrostatic potential probed by the 3D ED experiment.

NbC thin films via reactive DC sputtering in a CH4/Ar atmosphere

We have demonstrated the growth of NbC-Amorphous C films that manifest a well-known nanocomposite structure (Klages and Memming, 1990; Nedfors et al., 2011; Sala et al., 2021). The films were deposited using reactive DC sputtering in a CH4/Ar atmosphere using an insulating substrate near room temperature. A metallic Nb sputtering target was sputtered in an argon/CH4 atmosphere with a pressure range of 5.9 to 8.5 mTorr as the CH4 flow was also varied. Crystalline NbC films were deposited at room temperature at the lowest CH4 flow rate. The films exhibited ohmic electrical response at room temperature. The DC electrical resistivity scaled almost 4 orders of magnitude from 4.39 × 10−4 Ω-cm to 1.08 × 10−1 Ω-cm through changes in CH4 flow rates and the resultant C content of the films. The large variation in resistivity cannot be attributed to the changes in sample composition, e.g. volumetric mixing. It is postulated from XRD and TEM results that the large change in resistivity is due to the decreasing NbC crystallite size with increasing CH4. This leads to additional three-dimensional nanoscale intragranular interfaces and results in increased resistivity. X-ray reflectivity measurements were conducted, and the modelled film density was found to match the expected density for the film compositions as measured using X-ray photoelectron spectroscopy. Elastic recoil detection analysis (ERDA) was used to quantify H content of the films. The H content increased from 5.5 to 22 atomic percent with corresponding increase in CH4 flow. Transmission electron microscopy utilizing precession electron diffraction (PED) was conducted and showed a strong dependence of crystallinity on CH4 flow. A multilayer layer sample with varying CH4 flowrates was examined with TEM and showed that films were amorphous in nature for flow rates above 10 SCCM CH4; in agreement with XRD results.

Diffraction-Based Multiscale Residual Strain Measurements.

Modern analytical tools, from microfocus X-ray diffraction (XRD) to electron microscopy-based microtexture measurements, offer exciting possibilities of diffraction-based multiscale residual strain measurements. The different techniques differ in scale and resolution, but may also yield significantly different strain values. This study, for example, clearly established that high-resolution electron backscattered diffraction (HR-EBSD) and high-resolution transmission Kikuchi diffraction (HR-TKD) [sensitive to changes in interplanar angle (Δθθ)], provide quantitatively higher residual strains than micro-Laue XRD and transmission electron microscope (TEM) based precession electron diffraction (PED) [sensitive to changes in interplanar spacing (Δdd)]. Even after correcting key known factors affecting the accuracy of HR-EBSD strain measurements, a scaling factor of ∼1.57 (between HR-EBSD and micro-Laue) emerged. We have then conducted "virtual" experiments by systematically deforming an ideal lattice by either changing an interplanar angle (α) or a lattice parameter (a). The patterns were kinematically and dynamically simulated, and corresponding strains were measured by HR-EBSD. These strains showed consistently higher values for lattice(s) distorted by α, than those altered by a. The differences in strain measurements were further emphasized by mapping identical location with HR-TKD and TEM-PED. These measurements exhibited different spatial resolution, but when scaled (with ∼1.57) provided similar lattice distortions numerically.

Structural and magnetic properties of Ni cluster embedded (111)NiO layers grown on (0001)GaN films

(111) NiO epitaxial layers embedded with crystallographically oriented Ni-clusters are grown on c-GaN/Sapphire templates using the pulsed laser deposition technique. Structural and magnetic properties of the films are examined by a variety of techniques including high-resolution x-ray diffraction, precession-electron diffraction, and superconducting quantum interference device magnetometry. It has been found that Ni-clusters are formed in the NiO matrix only at sufficiently low oxygen pressures during the growth. The study further reveals that the orientation, shape, size, density, and magnetic properties of these clusters depend strongly on the growth temperature (TG). Although most of the Ni-clusters are found to be obtained crystallographically aligned with the NiO matrix with Ni(111) ∥NiO(111), clusters with other orientations also exist, especially in samples grown at lower temperatures. The average size and density of the clusters increase with TG. The proportion of the Ni(111) ∥ NiO(111)-oriented clusters also improves as TG is increased. All cluster embedded films show ferromagnetic behavior even at room temperature. Easy-axis is found to be oriented in the layer plane in samples grown at relatively lower temperatures. However, it turns perpendicular to the layer plane for samples grown at sufficiently high temperatures. This reversal of easy-axis has been attributed to the size dependent competition between the shape and magnetoelastic and the surface anisotropies of the clusters. This composite material thus has great potential to serve as a spin-injector and spin-storage medium in GaN-based spintronics of the future.

A method for crystallographic mapping of an alpha-beta titanium alloy with nanometre resolution using scanning precession electron diffraction and open-source software libraries.

An approach for the crystallographic mapping of two-phase alloys on the nanoscale using a combination of scanned precession electron diffraction and open-source python libraries is introduced in this paper. This method is demonstrated using the example of a two-phaseα/β titanium alloy. The data were recorded using a direct electron detector to collect the patterns, and recently developed algorithms to perform automated indexing and analyse the crystallography from the results. Very high-quality mapping is achieved at a 3nm step size. The results show the expected Burgers orientation relationships between the α laths and β matrix, as well as the expected misorientations betweenα laths. A minor issue was found that one area was affected by 180° ambiguities in indexing occur due to this area being aligned too close to a zone axis of the α with twofold projection symmetry (not present in 3D) in the zero-order Laue Zone, and this should be avoided in data acquisition in the future. Nevertheless, this study demonstrates a good workflow for the analysis of nanocrystalline two- or multi-phase materials, which will be of widespread use in analysing two-phase titanium and other systems and how they evolve as a function of thermomechanical treatments.

Quantitative Scanning Transmission Electron Microscopy–High‐Angle‐Annular Dark‐Field Study of the Structure of Pseudo‐2D Sb2Te3 Films Grown by (Quasi) Van der Waals Epitaxy

Scanning transmission electron microscopy (STEM) techniques are used to improve the understanding of out‐of‐plane oriented Sb2Te3 thin films deposited by sputtering on SiO2 and Si substrates. Nanobeam precession electron diffraction, energy‐dispersive X‐ray spectroscopy, and high‐angle‐annular dark‐field imaging show that the presence of 1–2 atomic planes of Te on top of the substrate is a crucial factor for successful growth of such films, which can be achieved by optimizing cosputtering of Te and Sb2Te3 targets. The formation of an actual van der Waals (vdW) gap between the substrate and the first Sb2Te3 quintuple layer allows for vdW epitaxy. This gap is larger than those separating Te planes in the pseudo‐2D Sb2Te3 structure. HAADF image analysis provides detailed information on the atomic arrangement such as interplanar distances, vdW gaps, and Debye–Waller coefficients, all these with a few pm precision. For the anisotropic atomic displacements, a new methodology is introduced based on the statistical analysis of atomic column positions that provides information on the low‐frequency phonon modes. Ab initio calculations are used to support our results. Overall, this study provides quantitative STEM tools particularly well suited for nonperiodic pseudo‐2D materials, such as Sb2Te3/GeTe superlattices.

High precision orientation mapping from 4D-STEM precession electron diffraction data through quantitative analysis of diffracted intensities

The association of scanning transmission electron microscopy (STEM) and detection of a diffraction pattern at each probe position (so-called 4D-STEM) represents one of the most promising approaches to analyze structural properties of materials with nanometric resolution and low irradiation levels. This is widely used for texture analysis of materials using automated crystal orientation mapping (ACOM). Herein, we perform orientation mapping in InP nanowires exploiting precession electron diffraction (PED) patterns acquired by an axial CMOS camera. Crystal orientation is determined at each probe position by the quantitative analysis of diffracted intensities minimizing a residue comparing experiments and simulations in analogy to x-ray structural refinement. Our simulations are based on the two-beam dynamical diffraction approximation and yield a high angular precision (∼0.03°), much lower than the traditional ACOM based on pattern matching algorithms (∼1°). We anticipate that simultaneous exploration of both spot positions and high precision crystal misorientation will allow the exploration of the whole potentiality provided by PED-based 4D-STEM for the characterization of deformation fields in nanomaterials.

New insights into microstructure refinement in crack initiation region of very-high-cycle fatigue for SLM Ti-6Al-4V via precession electron diffraction

The formation mechanism of crack initiation region of selective laser melted (SLM) Ti-6Al-4V in very-high-cycle fatigue (VHCF) was investigated by using the novel approach of transmission electron microscopy (TEM) with precession electron diffraction (PED). It is the first time to reveal the microstructure details of nanograin layer in the crack banks of both main crack and sub-crack in SLM Ti-6Al-4V via TEM with PED, which is the result of the crack closure and contacting between fracture surfaces. This revelation provides new insights into the microstructural evolution in the crack initiation region for SLM titanium alloys and other metallic materials, which is helpful for further understanding the crack initiation mechanism in VHCF.

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.