High-resolution Electron Backscatter Diffraction Research Articles

Estimation of Dislocation Densities With Nondestructive Scanning Electron Microscope Techniques: Application to Gallium Nitride.

Characterizing threading dislocations (TDs) in gallium nitride (GaN) semiconductors is crucial for ensuring the reliability of semiconductor devices. The current research addresses this issue by combining two techniques using a scanning electron microscope, namely electron channeling contrast imaging (ECCI) and high-resolution electron backscattered diffraction (HR-EBSD). It is a comparative study of these techniques to underscore how they perform in the evaluation of TD densities in GaN epitaxial layers. Experiments reveal that the dislocation line vectors mostly deviate from the growth direction of the film, i.e., ∦ [0001], followed by edge-type dislocations (dislocation lines || [0001]) with insignificant screw character. Furthermore, TDs from the dislocation clusters are characterized as edge- and (edge + mixed)-type TDs. By combining ECCI counting of dislocations and HR-EBSD description of geometrically necessary dislocation density type, it is possible to measure the total TD density and provide the proportion of pure (edge and screw) and mixed TDs. It has also been observed from the analyses of residual elastic strain fields and lattice rotations that it is not possible to identify individual dislocations for the spatial resolution of 50 nm in HR-EBSD. Nevertheless, ECCI and HR-EBSD can be complementarily used to count and characterize the TDs.

Strain-gradient crystal plasticity model with slip-system level GND tracking: Simulation vs experiment for sequential strain path change in AA6016-T4

Crystal plasticity models that track strain gradients and associated geometrically necessary dislocations (GNDs) typically determine the Nye tensor, mimicking the experimental approach. However, estimating GND densities for each slip-system is then intrinsically ambiguous. This study seeks to build upon current state of the art by quantifying GNDs at the slip-system level in the model using the local strain gradients. The model is exercised by replicating experiments undertaken on AA6016, which are performed under multi-step strain paths; both GND and SSD populations are quantified at various stages using both high resolution EBSD (HREBSD) and XRD. A full 3D volume of the material is extracted using ion beam serial sectioning to enable the creation of a high-fidelity model of the material.The combined modeling and experimental campaigns conclude that: 1) Calculation of the GND content at the slip level gives effectively equivalent GND evolution as the tradition Nye tensor method, but provides the significant advantage of knowing unambiguously the contribution on each slip system. 2) The net hardening predicted by the SGCP model is accurate, including prediction of a rapid increase in hardening (and associated dislocation content) following strain path change; and 3) Comparisons of observed and simulated GND populations reveal that buildup in the real sample is dominated by precipitate distribution rather than by grain boundary (GB) networks; such precipitates are not present in the current model, hence this result was not predicted.

Towards measuring absolute residual stress by HR-EBSD with simulated reference patterns

The High-Resolution Electron Backscatter Diffraction (HR-EBSD) technique has drawn attention in recent years, mainly thanks to its ability to assess the elastic strain/stress state with a sub-micron spatial resolution. Each diffraction pattern is correlated with a reference one, and the strain level relative to the reference is retrieved in the procedure. Several studies have attempted to use dynamically simulated diffraction patterns as references to access absolute strain/stress states to circumvent the difficulty of finding a strain-free reference pattern, which is often inaccessible. Numerous factors need to be considered and corrected to register better experimental and simulated diffraction patterns, such as the accurate positioning of the projection center, Kikuchi band (K-band) brightness asymmetry, K-band gray level reversal, optical distortion, or non-uniform electron energy. Here, an integrated digital image correlation method is proposed to register experimental patterns to a master pattern defined on a sphere, where the difficulties with experimental patterns are, for the most part, corrected efficiently. Due to the shallow inspection depth of EBSD, the free-surface property (vanishing stress vector at the observation surface) is also adopted to reduce the degrees of freedom and enhance precision. Through several tests on high-quality experimental patterns, the proposed method proves robust and precise. Both the systematic and random errors of the residual strain results are several 10−4. The method also allows the simultaneous determination of residual stress and the projection center coordinates.

Surface patterning for multi-scale strain analysis of in-situ SEM mechanical experiments

Scanning electron microscopy (SEM) enables microstructure characterization at multi-scale, having potential to quantify strain distribution across multiple fields of view (FOVs) and exploring coordinated relationships among the deformations of multi-scale microstructures. Optimizing distribution of multi-scale speckles is crucial for achieving high precision and efficiency in digital image correlation (DIC) computations. In this work, a multi-scale strain measurement method is proposed for in-situ SEM mechanical experiments, which relies on an uncomplicated multi-scale speckle preparation method. Multi-scale speckle preparation can be achieved by the combination of spin-coating and magnetron sputtering. By using micro- and macro-speckle, multi-scale strain calculations can be achieved during the in-situ SEM mechanical loading. The InSn macro-speckle is composed of controllable-sized InSn nanoscale speckles, ensuring that the accuracy of DIC measurements in the microscopic regions are not compromised by the coverage of macro-speckles in the testing region. In regions covered exclusively by SiO2 nano-speckles, in-situ SEM mechanical loading allows for the acquisition of high-resolution DIC (HR-DIC) and electron backscatter diffraction (EBSD) data from the same region and under the same deformation state. Experiments are conducted to illustrate the functionality and utility of the multi-scale speckles. Such experimental analyses offer an opportunity to further explore multi-scale strain heterogeneity in some advanced structural materials and provide reliable experimental verification for material mechanics simulations.

Fracture behaviors and crack propagation anisotropy in multi-ions modified bismuth titanate ferroelectrics

Bismuth titanate (BIT) is widely considered a promising lead-free piezoelectric material for advanced high-temperature applications. Despite significant advances in high-performance BIT ferroelectrics, there remains little understanding of mechanical properties including fracture failure and crack propagation behaviors, hindering the optimization of stability and reliability for next-generation high-temperature ferroelectrics. In this work, we investigated mechanical fracture and crack propagation behaviors of a type of high-performance bismuth titanate-based ceramic (Bi3.96Ce0.04Ti3–2xWxNbxO12, BCTWN) with different doping contents and electric poling states. High-resolution transmission electron microscopy (HRTEM) and electron backscatter diffraction (EBSD) results reveal that the BCTWN ceramics possess a single ferroelectric phase and exhibit layered crystal structures characterized by periodic fringe features. In addition, the striped domain structures are observed on the surface of plate-like grains. Mechanical measurements indicate that fracture behaviors of BCTWN ceramics are significantly affected by doping contents. The ceramics with the largest grain size present inferior microstructure with more defects, endowing the lowest bending strength and fracture toughness. For the samples with different poling states, strong anisotropy in crack propagation and fracture toughness is observed, attributed to the different domain switching mechanisms driven by local incompatible stress fields, which is experimentally validated by the intriguing evolutions of striped domain structures around crack tips. This work advances our understanding of the fracture and crack propagation mechanisms of BIT-based ceramics and provides insight into tailoring the mechanical behaviors through doping and poling strategies.

Microstructure and mechanical properties of additively manufactured Ti–6Al–4V alloy based on large area, high-resolution EBSD mapping

The laser direct energy deposition (L-DED) is well-suited for metal manufacturing, offering advantages of excellent performance, high precision, intricate structures and material flexibility, etc. In this study, the microstructure and mechanical property of Ti–6Al–4V alloy were investigated using L-DED under various printing parameters. A large-area, high-resolution Electron Backscatter Diffraction (EBSD) mapping technique, enabling to characterize the coarse columnar β-grains with individual crystals up to a centimeter along the building direction, and the sub-grain acicular α′ phase down to 50 nm in the same image, was utilized to analyze the crystalline phases and their lattice orientations in additively manufactured Ti–6Al–4V parts. The microstructure evolution, involving the high temperature β grain structures, β+α phase transformation, as well as acicular α′ martensitic phases, were studied during the periodically attenuated thermal cycles of L-DED. The formation of the “fish-scale” morphology results primarily from the radically different α phase in size between the interior and edges of the “fish-scale” morphology. Additionally, the effect of processing parameters, i.e., scanning speed and laser power, on microstructures and mechanical properties is discussed in detail.

Fatigue mechanisms at 450°C of a highly twined (>70%) and HIP-densified IN718 superalloy additively manufactured by laser beam powder bed fusion

Fatigue resistance at elevated temperatures is crucial for certifying aerospace structures additively manufactured by the laser beam powder bed fusion (PBF-LB) method with IN718 superalloy. This study employed a multi-step, supersolvus heat treatment process with hot isostatic pressing (HIP), called RHSA, to minimize pores and brittle phases. Stress intensity factor (△K) calculations using data from X-ray computed tomography and shape factors referencing finite element analysis (FEA) studies confirmed the suppression of △K below the threshold of conventional IN718 (∼5 MPa√m), shifting fatigue behavior to grain-structure-dominated. Despite a very high twin boundary (TB) fraction (>70%), fatigue tests at 450°C and R = 0.1 demonstrated low scatter. Slip trace analysis and high-resolution electron backscatter diffraction (EBSD) revealed that TB-induced strain concentration became prominent only at high △K, causing cracking at 45⁰ to the loading direction. The randomly oriented TBs with higher angles (60⁰) compared to high-angle grain boundaries (HAGBs) (30–40⁰) likely enhanced slip resistance and provided a net strengthening effect, which can explain the lower-than-average TB% along fracture paths. These insights suggest that a high TB fraction is not detrimental if fatigue stress is not excessive, alleviating concerns about annealing twins during defect minimization in AM IN718, allowing novel processes to improve fatigue resistance in PBF-LB IN718.

Damage-tolerant oxides by imprint of an ultra-high dislocation density

Dislocations in ductile ceramics offer the potential for robust mechanical performance while unlocking versatile functional properties. Previous studies have been limited by small volumes with dislocations and/or low dislocation densities in ceramics. Here, we use Brinell ball scratching to create crack-free, large plastic zones, offering a simple and effective method for dislocation engineering at room temperature. Using MgO, we tailor high dislocation densities up to ∼1015 m−2. We characterise the plastic zones by chemical etching, electron channelling contrast imaging, and scanning transmission electron microscopy, and further demonstrate that crack initiation and propagation in the plastic zones with high-density dislocations can be completely suppressed. The residual stresses in the plastic zones were analysed using high-resolution electron backscatter diffraction. With the residual stress being subsequently relieved via thermal annealing while retaining the high-density dislocations, we observe the cracks are no longer completely suppressed, but the pure toughening effect of the dislocations remains evident.

Imaging and Segmenting Grains and Subgrains Using Backscattered Electron Techniques.

We present two new methods of processing data from backscattered electron signals in a scanning electron microscope to image grains and subgrains. The first combines data from multiple backscattered electron images acquired at different specimen geometries to (1) better reveal grain boundaries in recrystallized microstructures and (2) distinguish between recrystallized and unrecrystallized regions in partially recrystallized microstructures. The second utilizes spherical harmonic transform indexing of electron backscatter diffraction patterns to produce high angular resolution orientation data that enable the characterization of subgrains. Subgrains are produced during high-temperature plastic deformation and have boundary misorientation angles ranging from a few degrees down to a few hundredths of a degree. We also present an algorithm to automatically segment grains from combined backscattered electron image data or grains and subgrains from high angular resolution electron backscatter diffraction data. Together, these new techniques enable rapid measurements of individual grains and subgrains from large populations.

Mechanism of Fatigue-Life Extension Due to Dynamic Strain Aging in Low-Carbon Steel at High Temperature.

An enhancement in fatigue life for ferrite-pearlite low-carbon steel (LCS) at high temperature (HT) has been discovered, where it increased from 190,873 cycles at room temperature (RT) to 10,000,000 cycles at 400 °C under the same stress conditions. To understand the mechanism behind this phenomenon, the evolution of microstructure and dislocation density during fatigue tests was comprehensively investigated. High-power X-ray diffraction (XRD) was employed to analyze the evolution of total dislocation density, while Electron Backscatter Diffraction (EBSD) and High-Resolution EBSD (HR-EBSD) were conducted to reveal the evolutions of kernel average misorientation (KAM), geometrically necessary dislocations (GND) and elastic strains. Results indicate that the enhancement was attributed to the dynamic strain aging (DSA) effect above the upper temperature limit, where serration and jerky flow disappeared but hindrance of dislocations persisted. Due to the DSA effect, periods of increase and decrease in the total dislocations were observed during HT fatigue tests, and the fraction of screw dislocations increased continuously, caused by viscous movement of the screw dislocations. Furthermore, the increased fraction of screw dislocations resulted in a lower energy configuration, reducing slip traces on sample surfaces and preventing fatigue-crack initiation.

Automated analysis framework of strain partitioning and deformation mechanisms via multimodal fusion and computer vision

Simultaneously investigating strain partitioning and the underlying deformation mechanisms for both the grain interior and the grain boundary (GB) is essential for understanding the complex plastic deformation of hexagonal close-packed metals. To this end, an automated analysis framework based on high-resolution digital image correlation (HRDIC) and electron backscatter diffraction (EBSD) data fusion and computer vision, integrating nanoscale resolution and a large field of view, is proposed. This framework consists of: (1) HRDIC-EBSD data fusion; (2) Segmenting the strain field into individual grains each with a core and a mantle; (3) Data clustering of the Matrix and slip bands (SBs) for each grain; (4) Full slip system (SS) identification and SS assignment to the SBs. The capabilities of this framework were demonstrated on Mg-10Y during compression. The strain field data, which was segmented into different clusters, including grain mantle, grain core, Matrix, and SBs, was analyzed statistically and quantitatively. The pixel-based slip activity, which considers the SB morphology, was obtained from a statistical perspective. Inter-granular accommodating mechanisms, including GB strain, slip transfer, and GB sliding, were quantitatively analyzed. Overall, this analysis framework, which can be applied to other materials, can automatically and statistically evaluate both nanoscale strain fields and underlying intra- and inter-granular deformation mechanisms grain-by-grain. This work provides valuable experimental insights into plastic deformation and accommodation mechanisms for polycrystals.

Mapping of Microscopic and Local Stress-Strain Curve Information by Combination of Digital Image Correlation and High-Resolution Electron Backscatter Diffraction Methods

Mapping of Microscopic and Local Stress-Strain Curve Information by Combination of Digital Image Correlation and High-Resolution Electron Backscatter Diffraction Methods

Dynamic recrystallisation: A quantitative study on grain boundary characteristics and dependence on temperature and strain rate in an aluminium alloy

Dynamic recrystallisation (DRX) usually occurs during hot forming of metallic materials, significantly impacting the mechanical properties of the final parts. Although DRX has been studied for decades, the types of DRX that occurs in high stacking fault energy (SFE) materials like aluminium alloys are still controversial, and their dependence on both temperature and strain rate is surprisingly missing. To fill these gaps, in the present study, uniaxial compression tests on an aluminium alloy AA6061 were carried out at different temperatures from 250 to 475 °C and strain rates from 0.01 to 1 s-1. The grain orientation and misorientation before and after the hot deformation were quantitively characterised using high-resolution Electron Backscatter Diffraction (EBSD) with a misorientation resolution of 0.05°, enabling high-angle grain boundaries (HAGBs), low-angle grain boundaries (LAGBs) and geometrically necessary dislocations (GNDs) to be accurately quantified and correlated against temperature and strain rate. Results reveal that both continuous dynamic recrystallisation (CDRX) and geometric dynamic recrystallisation (GDRX) occurred concurrently, resulting in the formation of discontinuous HAGBs and fine equiaxed grains, respectively. The misorientation angle distributions of the discontinuous HAGBs exhibit an opposite trend compared to those of the fine grains’ HAGBs. Both temperature and strain rate significantly affect the density of HAGBs formed by DRX, but have little effect on their misorientation angle distributions. This study sheds light on the fundamental understanding of DRX and provides comprehensive experimental data for future modelling of DRX processes in high SFE materials.

Employing Constrained Nonnegative Matrix Factorization for Microstructure Segmentation.

Materials characterization using electron backscatter diffraction (EBSD) requires indexing the orientation of the measured region from Kikuchi patterns. The quality of Kikuchi patterns can degrade due to pattern overlaps arising from two or more orientations, in the presence of defects or grain boundaries. In this work, we employ constrained nonnegative matrix factorization to segment a microstructure with small grain misorientations, (<1∘), and predict the amount of pattern overlap. First, we implement the method on mixed simulated patterns-that replicates a pattern overlap scenario, and demonstrate the resolution limit of pattern mixing or factorization resolution using a weight metric. Subsequently, we segment a single-crystal dendritic microstructure and compare the results with high-resolution EBSD. By utilizing weight metrics across a low-angle grain boundary, we demonstrate how very small misorientations/low-angle grain boundaries can be resolved at a pixel level. Our approach constitutes a versatile and robust tool, complementing other fast indexing methods for microstructure characterization.

Unraveling factors affecting the reversibility of martensitic phase transformation in FeNiCoAlTi shape memory alloys: Insights from HR-EBSD and acoustic emission analysis

The reversibility of the martensitic phase transformation is crucial for the functionality of shape memory alloys. The easier the martensite is pinned, the faster occurs the degradation of the shape memory effect during cyclic loading. This study investigates various factors influencing the stabilization of martensite during deformation of single-crystalline states of the FeNiCoAlTi system. In order to investigate early pinning mechanisms in more detail a 〈011〉 single crystal with expected lower reversibility of the martensitic phase transformation and, consequently, lower superelasticity was chosen. Complementary in situ methods, such as digital image correlation and acoustic emission, were used to characterize the evolution of the martensitic phase transformation and the role of dislocations during deformation. Thereby, experimental evidence was elaborated pointing at mechanisms that so far have been underrated in terms of functional degradation. Beside interaction of martensitic variants and dislocation activity, detwinning, associated with high acoustic energy, was identified as an additional significant factor contributing to reduced reversibility and superelasticity. In addition, high-resolution electron backscatter diffraction measurements were applied for the first time to identify residual stresses in the austenitic matrix, which significantly contribute to the reverse transformation. Micro-mechanical experiments using pillar compression tests were carried out to study the influence of residual back stresses of the austenitic matrix on the reversibility of the martensitic phase transformation. A reduced reversibility was found in the case of the absence of back stresses. The findings from this study foster the understanding of pinning mechanisms during loading of the FeNiCoAlTi shape memory alloy eventually enabling targeted optimization for enhanced superelastic material behavior.

The twin formation under electric current stressing and its effect on the properties of copper

Copper as one of the major electronic metals will experience electric current stressing during electronics application. The study comprehensively investigated microhardness and electrical resistivity variations of copper under current stressing at current densities 1.4 x 104 and 1.7 × 104 A/cm2 for up to 8 h. The variations in these properties were correlated with dislocation density and twin fraction. The dislocation density and twin fraction were estimated, respectively, with high resolution transmission electron microscope and Electron Backscatter Diffraction. The twin boundary acts as obstacle to dislocation movement as revealed by high resolution transmission electron microscopy. The twin boundary predominates over dislocation in governing the microhardness and electrical resistivity of copper under current stressing.

The micromechanics of fracture of zirconium hydrides

Zirconium alloys are susceptible to hydrogen embrittlement and hydride precipitation. The precipitation of hydrides is accompanied by a transformation strain, but the contribution of this strain to the fracture of hydrides is not well-understood. In this study, deformation mechanisms and the micromechanics of fracture of hydrides are investigated by conducting in-situ and interrupted ex-situ tensile experiments on hydrided zirconium specimens. Electron backscatter diffraction (EBSD) is used to measure the macro-EBSD maps that contain the grains located in the gauge section of the specimens’ surfaces. Further, high spatial resolution EBSD is used to determine orientation variations induced by the precipitation of hydrides and by the external mechanical load. The measured grain orientations are mapped into a crystal plasticity finite element (CPFE) model to examine the performance of eight different crack initiation criteria. It is shown that a multiscale approach is essential for studying the fracture of hydrides as the details of hydride morphology, orientation, local grain neighborhood, and hydride-hydride interactions are important in such analysis. It is shown that neglecting the effects of hydride-induced transformation strain leads to inaccuracies in predicting both the location and direction of microcracks within hydrides. Among the examined methods, the combination of resolved shear stress and resolved shear strain on slip systems, i.e., the highest shear energy density, consistently predicts the correct locations of hydride microcracks as well as their propagation direction. Further, it is shown that the significant deformation that takes place within hydrides is the main driving force for the fracture of hydrides.

A mechanistic study of grain boundary behaviour during irradiation-induced growth in zirconium

Grain boundaries play a significant role during deformation and can exhibit both beneficial and adverse effects on deformation behaviour during irradiation. For instance, macroscopic irradiation growth is increased by the presence of a high density of grain boundaries. Grain boundaries can act as sinks for point defects and barriers to dislocation motion. Thus, a mechanistic assessment of the impact of grain boundaries is essential for better understanding irradiation-induced deformation. The interplay between irradiation-induced microstructure and mechanical properties has been an area of research for many decades, but many uncertainties remain. In the present work, the deformation fields adjacent to grain boundaries in pure zirconium are investigated by using high angular resolution electron backscatter diffraction (HR-EBSD) after a period of irradiation growth. It is observed that the extent of grain boundary deformation is associated with the crystallographic orientation difference between adjacent grains. High-angle grain boundaries function as a deformation constraint, whereas a significant grain boundary migration occurs at some low-angle grain boundaries, which likely act as an active diffusion path for the irradiation-induced point defects. In addition, the change in residual strain fields associated with the irradiation damage is quantified with respect to a non-irradiated reference. A net change in the state of residual elastic strain in the order of ∼10−3 has been estimated for ∼8 dpa proton irradiation. Furthermore, the origin of residual elastic strain and lattice misorientation concentration along (0002) traces are discussed.

Grain boundary localized damage in hexagonal titanium

Fully recrystallized hexagonal titanium tubes were processed through industrial cold pilgering. Two different pilgering sequences, generalized as case I and case II, were used. Both pilgering processes involved identical effective pilgering strains, but they had different strain modes. Case I, for example, used a pilgering pass with more (∼2.5 times) deviation from ideal plane strain compression. Both fully pilgered tubes showed similar grain fragmentations, misorientations, and tensile properties. However, their crystallographic textures and residual stresses significantly differed. Further, the inner surfaces of case II tube showed localized intergranular damage. Direct experimental observations, combining interrupted tensile tests with high-resolution electron backscattered diffraction (HR-EBSD), revealed that the damage originated only at select grain boundaries. In particular, dislocation pile-up plus stress localization appeared to be responsible for the grain boundary damage initiation. Microstructurally, this originated from the relative difference in elastic moduli between neighboring grains and the Luster-Morris parameter, representing slip transfer across the grain boundary.

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.