Arantxa Vilalta-Clemente

Using transmission Kikuchi diffraction to study intergranular stress corrosion cracking in type 316 stainless steels

Transmission Kikuchi diffraction (TKD), also known as transmission-electron backscatter diffraction (t-EBSD) is a novel method for orientation mapping of electron transparent transmission electron microscopy specimen in the scanning electron microscope and has been utilized for stress corrosion cracking characterization of type 316 stainless steels. The main advantage of TKD is a significantly higher spatial resolution compared to the conventional EBSD due to the smaller interaction volume of the incident beam with the specimen. Two 316 stainless steel specimen, tested for stress corrosion cracking in hydrogenated and oxygenated pressurized water reactor chemistry, were characterized via TKD. The results include inverse pole figure (IPFZ) maps, image quality maps and misorientation maps, all acquired in very short time (<60 min) and with remarkable spatial resolution (up to 5 nm step size possible). They have been used in order to determine the location of the open crack with respect to the grain boundary, deformation bands, twinning and slip. Furthermore, TKD has been used to measure the grain boundary misorientation and establish a gauge for quantifying plastic deformation at the crack tip and other regions in the surrounding matrix. Both grain boundary migration and slip transfer have been detected as well.

Measurement of probability distributions for internal stresses in dislocated crystals

Here, we analyse residual stress distributions obtained from various crystal systems using high resolution electron backscatter diffraction (EBSD) measurements. Histograms showing stress probability distributions exhibit tails extending to very high stress levels. We demonstrate that these extreme stress values are consistent with the functional form that should be expected for dislocated crystals. Analysis initially developed by Groma and co-workers for X-ray line profile analysis and based on the so-called “restricted second moment of the probability distribution” can be used to estimate the total dislocation density. The generality of the results are illustrated by application to three quite different systems, namely, face centred cubic Cu deformed in uniaxial tension, a body centred cubic steel deformed to larger strain by cold rolling, and hexagonal InAlN layers grown on misfitting sapphire and silicon carbide substrates.

Diffraction effects and inelastic electron transport in angle-resolved microscopic imaging applications

We analyse the signal formation process for scanning electron microscopic imaging applications on crystalline specimens. In accordance with previous investigations, we find nontrivial effects of incident beam diffraction on the backscattered electron distribution in energy and momentum. Specifically, incident beam diffraction causes angular changes of the backscattered electron distribution which we identify as the dominant mechanism underlying pseudocolour orientation imaging using multiple, angle‐resolving detectors. Consequently, diffraction effects of the incident beam and their impact on the subsequent coherent and incoherent electron transport need to be taken into account for an in‐depth theoretical modelling of the energy‐ and momentum distribution of electrons backscattered from crystalline sample regions. Our findings have implications for the level of theoretical detail that can be necessary for the interpretation of complex imaging modalities such as electron channelling contrast imaging (ECCI) of defects in crystals. If the solid angle of detection is limited to specific regions of the backscattered electron momentum distribution, the image contrast that is observed in ECCI and similar applications can be strongly affected by incident beam diffraction and topographic effects from the sample surface. As an application, we demonstrate characteristic changes in the resulting images if different properties of the backscattered electron distribution are used for the analysis of a GaN thin film sample containing dislocations.

Quantitative imaging of anti-phase domains by polarity sensitive orientation mapping using electron backscatter diffraction

Advanced structural characterisation techniques which are rapid to use, non-destructive and structurally definitive on the nanoscale are in demand, especially for a detailed understanding of extended-defects and their influence on the properties of materials. We have applied the electron backscatter diffraction (EBSD) technique in a scanning electron microscope to non-destructively characterise and quantify antiphase domains (APDs) in GaP thin films grown on different (001) Si substrates with different offcuts. We were able to image and quantify APDs by relating the asymmetrical intensity distributions observed in the EBSD patterns acquired experimentally and comparing the same with the dynamical electron diffraction simulations. Additionally mean angular error maps were also plotted using automated cross-correlation based approaches to image APDs. Samples grown on substrates with a 4° offcut from the [110] do not show any APDs, whereas samples grown on the exactly oriented substrates contain APDs. The procedures described in our work can be adopted for characterising a wide range of other material systems possessing non-centrosymmetric point groups.

Characterization of Elastic Strain Field and Geometrically Necessary Dislocation Distribution in Stress Corrosion Cracking of 316 Stainless Steels by Transmission Kikuchi Diffraction

Stainless steel alloys such as SUS 316 are widely used in nuclear power plants because of their excellent performance in high-temperature and corrosive environments. In this work, a stress corrosion crack from a sample tested under simulated primary water from a pressurized water reactor has been characterized.

High-Resolution Electron Backscatter Diffraction in III-Nitride Semiconductors

The large and increasing interest in III-nitrides semiconductors lies in the wide range of useful applications that can be achieved, from high electron mobility transistors (HEMTs) to light emitting LEDs and lasers. However, the III-nitride materials are usually epitaxially grown on foreign substrates, which lead to the formation of a large number of dislocations and significant strain variations in the epitaxial layers that seriously affect the performance of devices based upon them.

Applications of multivariate statistical methods and simulation libraries to analysis of electron backscatter diffraction and transmission Kikuchi diffraction datasets

Multivariate statistical methods are widely used throughout the sciences, including microscopy, however, their utilisation for analysis of electron backscatter diffraction (EBSD) data has not been adequately explored. The basic aim of most EBSD analysis is to segment the spatial domain to reveal and quantify the microstructure, and links this to knowledge of the crystallography (e.g. crystal phase, orientation) within each segmented region. Two analysis strategies have been explored; principal component analysis (PCA) and k-means clustering. The intensity at individual (binned) pixels on the detector were used as the variables defining the multidimensional space in which each pattern in the map generates a single discrete point. PCA analysis alone did not work well but rotating factors to the VARIMAX solution did. K-means clustering also successfully segmented the data but was computational more expensive. The characteristic patterns produced by either VARIMAX or k-means clustering enhance weak patterns, remove pattern overlap, and allow subtle effects from polarity to be distinguished. Combining multivariate statistical analysis (MSA) approaches with template matching to simulation libraries can significantly reduce computational demand as the number of patterns to be matched is drastically reduced. Both template matching and MSA approaches may augment existing analysis methods but will not replace them in the majority of applications.

Understanding Corrosion and Hydrogen Pickup of Zirconium Fuel Cladding Alloys: The Role of Oxide Microstructure, Porosity, Suboxides, and Second-Phase Particles

We have used a range of advanced microscopy techniques to study the microstructure, the nanoscale chemistry and the porosity in a range of zirconium alloys at different stages of oxidation. Samples from both autoclave and in-reactor conditions were available to compare, including ZIRLO, Zr-1.0Nb and Zr-2.5Nb samples with different heat-treatments. (Scanning) Transmission Electron Microscopy ((S)TEM), Transmission Kikuchi Diffraction (TKD) and automated crystal orientation mapping with TEM 2,3 were used to study the grain structure and phase distribution. Significant differences in grain morphology were observed between samples oxidised in the autoclave and in-reactor samples, with shorter, less well-aligned monoclinic grains and more tetragonal grains seen in the neutron irradiated samples. A combination of Energy Dispersion X-ray (EDX) mapping in STEM and Atom Probe Tomography (APT) analysis of SPPs can reveal the main and the minor element distributions respectively. Neutron irradiation seems to have little effect on promoting fast oxidation or dissolution of β-Nb precipitates, but encourages dissolution of Fe from Laves phase precipitates. Electron Energy Loss Spectroscopy (EELS) analysis of the oxidation state of Nb in β-Nb SPPs in the oxide reveal the fully oxidised Nb state in the SPPs deep into the oxide, but Nb in the crystalline SPPs near the metaloxide interface. EELS analysis and automated crystal orientation mapping with TEM have also revealed Widmanstatten-type suboxide layers in some samples with the hexagonal ZrO structure predicted by ab initio modelling. The combined thickness of the ZrO suboxide and oxygen-saturated layers at the metal-oxide interface correlates well to the estimated instantaneous oxidation rate, suggesting that the presence of this oxygen rich zone is part of the protective oxide that is rate limiting in the key in the transport processes involved in oxidation. Porosity in the oxide has a major influence on the overall rate of oxidation, and there is much more porosity in the rapidly oxidising annealed Zr-1.0Nb alloy than found in either the recrystallised alloy or the similar alloy exposed to neutron irradiation.

Mapping Anti-phase Domains by Polarity Sensitive Orientation Imaging Using Electron Backscatter Diffraction

Various material properties such as piezoelectricity, spontaneous polarisation, and plasticity are directly dependent on the crystal structure, and any form of deviation from their perfect crystal lattice could significantly alter their fundamental behaviour. Producing defect free materials is a challenging task especially in the case of heteroepitaxial thin film growth. Irrespective of the substrates, the growth plane, or the growth conditions employed, extended defects such as dislocations, stacking faults and grain boundaries are generally observed in the as–grown layers. In addition to these commonly observed extended defects; inversion domains (IDs), antiphase domains (APDs) and antiphase boundaries (APBs) have also been identified in several materials; examples include, layered perovskite structured materials, semiconductors, metallic superlattices and shape memory alloys. Often extended defects are electrically active and are problematic for electronic and optoelectronic devices. This is why structural characterisation techniques which are simultaneously rapid to use, non-destructive and structurally definitive on the nanoscale become a prerequisite.

Analysis of Dislocation Densities using High Resolution Electron Backscatter Diffraction

Cross-correlation analysis of electron backscatter diffraction (EBSD) patterns allows measurement of elastic strain and lattice rotation variations at a sensitivity of ~±10 [1]. Cross-correlation is used to measure shifts between sub-regions of test and reference patterns and simple geometry allows the elastic strains and lattice rotations to be calculated from the measured dispersion of pattern shifts. In deformed metals the lattice rotations are often significantly larger than the elastic strains and in these situations a pattern ‘remapping’ approach has proved necessary to avoid artefacts in the strain fields [2]. One area of application for EBSD has been the determination of dislocation densities. The most explored route for quantifying the dislocation density has been to use the relationship, described by Nye [3] between the geometrically necessary dislocation (GND) density and the lattice curvature. In Nye’s analysis the excess density of dislocations in the crystal is directly related to the gradient of the lattice rotation field that induced. Unfortunately solution of the reverse problem of finding dislocation densities from the measured rotation gradients often does not have a unique solution. This situation is exacerbated by the fact that only 6 of the nine possible rotation gradient terms can be established from EBSD on a single section. Despite these issues, a lower bound estimate of the total dislocation density can be made (though the densities of particular dislocation types are ambiguous). Figure 1 shows an example map of GND density distribution within a Cu polycrystal deformed to 10% tensile strain. Of course the GND density is only a fraction of the total dislocation density because any dipoles or multipoles between the measurement points cause no measureable rotation gradient. This leads to differences in the GND density as the step size is varied. Figure 1 shows the GND density recovered in the same region but with the rotation gradients calculated using three different length scales. This is shown in a more quantitative way in figure 2 which shows how the average GND density recorded in this map reduces as the effective step size is made progressively larger [4]. To estimate the total dislocation density a new approach has been recently proposed [5]. This borrows from peak profile analysis used in X-ray and neutron diffraction assessment of dislocation density. From the EBSD data the variations of stress from the mean value in each grain can be calculated and used to form a plot showing the probability of obtaining a given stress level. In all dislocated crystal we have analyzed the probability has the form of a central Gaussian-like part but has tails showing higher probabilities at the high stress levels with the probability following ܲ(ߪ) = ܣ ߪ ⁄ . The form of these tails is consistent with the high stresses being generated by the localized stresses close to isolated dislocation cores, and the magnitude of the proportionality constant A can be used to determine the dislocation density. As the tails correspond to low probability, where experimental data tends to be somewhat noisy, rather than fitting the data directly we follow Groma’s method [5] developed for analysis of X-ray diffraction peak intensities of using the restricted second moment V2 of the probability ଶܸ(ߪ) = න ܲ(ߪ) ߪ ଶ dߪ ାఙ