The effect of length scale on the determination of geometrically necessary dislocations via EBSD continuum dislocation microscopy

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.

A Denoising Autoencoder for Improved Kikuchi Pattern Quality and Indexing in Electron Backscatter Diffraction

The rapid collection and indexing of electron diffraction patterns as produced via electron backscatter diffraction (EBSD) has enabled crystallographic orientation and structural determination, as well as additional property-determining strain and dislocation density information with increasing speed, resolution, and efficiency. Pattern indexing quality is reliant on the noise of the collected electron diffraction patterns, which is often convoluted by sample preparation and data collection parameters. EBSD acquisition is sensitive to many factors and thus can result in low confidence index (CI), poor image quality (IQ), and improper minimization of fit, which can result in noisy datasets and misrepresent the microstructure. In an attempt to enable both higher speed EBSD data collection and enable greater orientation fit accuracy with noisy datasets, an image denoising autoencoder was implemented to improve pattern quality. We show that EBSD data processed through the autoencoder results in a higher CI, IQ, and a more accurate degree of fit. In addition, using denoised datasets in HR-EBSD cross correlative strain analysis can result in reduced phantom strain from erroneous calculations due to the increased indexing accuracy and improved correspondence between collected and simulated patterns.

Character and Distribution of Geometrically Necessary Dislocations in Polycrystalline Tantalum

Geometrically necessary dislocations (GNDs) play a key role in accommodating strain incompatibility between neighboring grains in polycrystalline materials. One critical step toward accurately capturing GNDs in deformation models involves studying the microstructural features that promote GND accumulation and the resulting character of GND fields. This study utilizes high-resolution electron backscatter diffraction to map GND populations in a large polycrystalline sample of pure tantalum, under simple tension. A total of 1,989 grains, 3,518 grain boundaries (GBs), and 3,207 triple junctions (TJs) were examined in a subsurface region of the sample. Correlations between GND density and GB character, and to some extent, TJ character, are investigated. Statistical geometrical relationships between these entities are quantified, and also visualized, using a novel application of two-point statistics. The nature of GNDs across the sample is also visualized and assessed using a recently developed method of mapping the local net Burgers vectors. The different approaches to characterizing GND distribution are compared in terms of how they quantify the size of near boundary gradient zones.

A Modelling Study of the Correlation between the Layer Obtained by Selective Transfer and the Dislocations Movement at the Friction Surfaces Limit

The selective transfer phenomenon (STP) is based on physico-chemical processes occurring in the contact area of a friction pair, with an element from a copper alloy and allows the metallic transfer of particles of micro/nanometric size, forming a thin superficial tribologically performing layer under energy and relative motion conditions. During the formation of the layer, its crystalline network has an excess of defects and this makes the dislocations to come to the surface. The layer thickness is small, porous, and with comparable dimensions to those of the tensions field of the dislocations. This paper presents a review and analysis of the STP based on dislocations movement to establish and know the tensions field influence, the energy (about 0.25 J/m), and the linear tension of dislocations (~2.42 × 10−9 N) at the contact surfaces zone of a friction pair, by which we can ensure a low wear state (~4.16 × 10−5–2.16 × 10−4 g/min), and a reduced friction coefficient (~0.014–0.034). Therefore, the purpose of the paper is to analyze the STP based on the dislocations movement because is proves the existence (presence), importance, and utility of the dislocations, respectively, the dislocations movements during the conditions’ selective transfer, at the limit of the friction surfaces, under the action of a tensions field, whose components are determined analytically by modelling, together with energy and the linear tension. Also, the layer formed through STP has the property of ensuring during the deformation process an agglomeration of dislocations (structural defects) which protects it from destruction, and therefore, a self-regulation of the equilibrium processes, disturbed during the friction process, to maintain the friction and wear of the friction pairs within reduced limits.

Identification of Star Defects in Gallium Nitride with HREBSD and ECCI

This paper characterizes novel “star” defects in GaN films grown with metal–organic vapor phase deposition (MOVPE) on GaN substrates with electron channeling contrast imaging (ECCI) and high-resolution electron backscatter diffraction (HREBSD). These defects are hundreds of microns in size and tend to aggregate threading dislocations at their centers. They are the intersection of six nearly ideal low-angle tilt boundaries composed of $\langle a\rangle$-type pyramidal edge dislocations, each on a unique slip system.

Correlating results from high resolution EBSD with TEM- and ECCI-based dislocation microscopy: Approaching single dislocation sensitivity via noise reduction

High resolution electron backscatter diffraction (HREBSD), an SEM-based diffraction technique, may be used to measure the lattice distortion of a crystalline material and to infer the geometrically necessary dislocation content. Uncertainty in the image correlation process used to compare diffraction patterns leads to an uneven distribution of measurement noise in terms of the lattice distortion, which results in erroneous identification of dislocation type and density. This work presents a method of reducing noise in HREBSD dislocation measurements by removing the effect of the most problematic components of the measured distortion. The method is then validated by comparing with TEM analysis of dislocation pile-ups near a twin boundary in austenitic stainless steel and with ECCI analysis near a nano-indentation on a tantalum oligocrystal. The HREBSD dislocation microscopy technique is able to resolve individual dislocations visible in TEM and ECCI and correctly identify their Burgers vectors.

New levels of high angular resolution EBSD performance via inverse compositional Gauss-Newton based digital image correlation

Conventional high angular resolution electron backscatter diffraction (HREBSD) uses cross-correlation to track features between diffraction patterns, which are then related to the relative elastic strain and misorientation between the diffracting volumes of material. This paper adapts inverse compositional Gauss Newton (ICGN) digital image correlation (DIC) to be compatible with HREBSD. ICGN-based works by efficiently tracking not just the shift in features, but also the change in their shape. Modeling a shape change as well as a shift results in greater accuracy. This method, ICGN-based HREBSD, is applied to a simulated data set, and its performance is compared to conventional cross-correlation HREBSD, and cross-correlation HREBSD with remapping. ICGN-based HREBSD is shown to have about half the strain error of the best cross-correlation method with a comparable computation time.

Comparison of dislocation characterization by electron channeling contrast imaging and cross-correlation electron backscattered diffraction

In this work, the relative capabilities and limitations of electron channeling contrast imaging (ECCI) and cross-correlation electron backscattered diffraction (CC-EBSD) have been assessed by studying the dislocation distributions resulting from nanoindentation in body centered cubic Ta. Qualitative comparison reveals very similar dislocation distributions between the CC-EBSD mapped GNDs and the ECC imaged dislocations. Approximate dislocation densities determined from ECC images compare well to those determined by CC-EBSD. Nevertheless, close examination reveals subtle differences in the details of the distributions mapped by these two approaches. The details of the dislocation Burgers vectors and line directions determined by ECCI have been compared to those determined using CC-EBSD and reveal good agreement.

Size effects resolve discrepancies in 40 years of work on low-temperature plasticity in olivine

The strength of olivine at low temperatures and high stresses in Earth's lithospheric mantle exerts a critical control on many geodynamic processes, including lithospheric flexure and the formation of plate boundaries. Unfortunately, laboratory-derived values of the strength of olivine at lithospheric conditions are highly variable and significantly disagree with those inferred from geophysical observations. We demonstrate via nanoindentation that the strength of olivine depends on the length scale of deformation, with experiments on smaller volumes of material exhibiting larger yield stresses. This "size effect" resolves discrepancies among previous measurements of olivine strength using other techniques. It also corroborates the most recent flow law for olivine, which proposes a much weaker lithospheric mantle than previously estimated, thus bringing experimental measurements into closer alignment with geophysical constraints. Further implications include an increased difficulty of activating plasticity in cold, fine-grained shear zones and an impact on the evolution of fault surface roughness due to the size-dependent deformation of nanometer- to micrometer-sized asperities.

Influence of Noise-Generating Factors on Cross-Correlation Electron Backscatter Diffraction (EBSD) Measurement of Geometrically Necessary Dislocations (GNDs)

Studies of dislocation density evolution are fundamental to improved understanding in various areas of deformation mechanics. Recent advances in cross-correlation techniques, applied to electron backscatter diffraction (EBSD) data have particularly shed light on geometrically necessary dislocation (GND) behavior. However, the framework is relatively computationally expensive-patterns are typically saved from the EBSD scan and analyzed offline. A better understanding of the impact of EBSD pattern degradation, such as binning, compression, and various forms of noise, is vital to enable optimization of rapid and low-cost GND analysis. This paper tackles the problem by setting up a set of simulated patterns that mimic real patterns corresponding to a known GND field. The patterns are subsequently degraded in terms of resolution and noise, and the GND densities calculated from the degraded patterns using cross-correlation ESBD are compared with the known values. Some confirmation of validity of the computational degradation of patterns by considering real pattern degradation is also undertaken. The results demonstrate that the EBSD technique is not particularly sensitive to lower levels of binning and image compression, but the precision is sensitive to Poisson-type noise. Some insight is also gained concerning effects of mixed patterns at a grain boundary on measured GND content.

Comparison of quartz crystallographic preferred orientations identified with optical fabric analysis, electron backscatter and neutron diffraction techniques

Three techniques are used to measure crystallographic preferred orientations (CPO) in a naturally deformed quartz mylonite: transmitted light cross-polarized microscopy using an automated fabric analyser, electron backscatter diffraction (EBSD) and neutron diffraction. Pole figure densities attributable to crystal-plastic deformation are variably recognizable across the techniques, particularly between fabric analyser and diffraction instruments. Although fabric analyser techniques offer rapid acquisition with minimal sample preparation, difficulties may exist when gathering orientation data parallel with the incident beam. Overall, we have found that EBSD and fabric analyser techniques are best suited for studying CPO distributions at the grain scale, where individual orientations can be linked to their source grain or nearest neighbours. Neutron diffraction serves as the best qualitative and quantitative means of estimating the bulk CPO, due to its three-dimensional data acquisition, greater sample area coverage, and larger sample size. However, a number of sampling methods can be applied to FA and EBSD data to make similar approximations.