High-resolution Electron Backscatter Diffraction Research Articles

Local Stress Measurements in Polycrystalline Metallic Tensile Specimens Using High Resolution EBSD.

Local Stress Measurements in Polycrystalline Metallic Tensile Specimens Using High Resolution EBSD.

Acquisition of microscopic and local stress-strain curves by combination of HR-EBSD and DIC methods

A method for obtaining microscopic and localized stress-strain curves is proposed by combining strain measurements using the digital image correlation (DIC) method with stress measurements using high-resolution electron backscattered diffraction (HR-EBSD). A dataset containing 11 steps of stress–strain information in a single subset was created by applying georeferencing to DIC strain maps and HR-EBSD stress maps measured in a deformation-interrupted microstructure. This analysis allowed us to obtain local stress-strain curves and strain energy maps of polycrystalline nickel alloys. A comparison of the stress-strain curves obtained from any subset in the map revealed that these curves show extremely diverse shapes depending on the location in the microstructure.

An iterative method for reference pattern selection in high-resolution electron backscatter diffraction (HR-EBSD).

For high (angular) resolution electron backscatter diffraction (HR-EBSD), the selection of a reference diffraction pattern (EBSP0) significantly affects the precision of the calculated strain and rotation maps. This effect was demonstrated in plastically deformed body-centred cubic and face-centred cubic ductile metals (ferrite and austenite grains in duplex stainless steel) and brittle single-crystal silicon, which showed that the effect is not only limited to measurement magnitude but also spatial distribution. An empirical relationship was then identified between the cross-correlation parameter and angular error, which was used in an iterative algorithm to identify the optimal reference pattern that maximises the precision of HR-EBSD.

Effect of Heat Treatment on Microstructural Evolution in Additively Manufactured 316L Stainless Steel

316L stainless steel samples were prepared by selective laser melting (SLM) and annealed at 1000 °C for durations of between 1 and 6 h to investigate both the kinetics of microstructural evolution during heat treatment and the effect of annealing on mechanical properties. The as-printed materials contain a high density of oxide particles and dislocations, forming a dislocation cell substructure that shows high thermal stability during heat treatment. Moreover, coarsened oxide particles act as pinning barriers for moving dislocations and grain boundaries, thus extending the recovery and recrystallization process. The process of recrystallization can be effectively tracked by measuring the density of the low-angle misorientation boundaries associated with the oxide particles and dislocations, as characterized by high-resolution EBSD. The evolution of mechanical properties during annealing shows a strong relationship with the observed microstructural changes, suggesting possible optimization of strength and ductility of SLM-prepared metal samples by use of appropriate heat treatments.

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.

Crystal orientation and deformation state analysis from Kikuchi patterns via pattern reconstruction aided deep Siamese network

Fast and accurate analysis of orientations and deformation states from Kikuchi patterns is vital to understand the crystalline materials’ micro- and macro-mechanical behavior. Here, a pattern reconstruction-aided Siamese network is developed to extract the crystal orientations and deformation states from Kikuchi patterns. We demonstrate the pattern reconstruction technique is essential for accurate predictions at large-area EBSD scans since the remapping operation enables the Siamese network to consider changes in experimental setup parameters dynamically. Moreover, the reconstruction technique serves as a data augmentation method that expands 342,225 raw patterns to 5,475,600 reconstructed patterns. The resulting Siamese network achieves unprecedented accuracy and robustness over a wide range of experimental setup parameters including pattern center, camera elevation and exposure time. Compared to the cross-correlation based high-resolution EBSD (HR-EBSD) analysis, it shows improved robustness against large rotations and achieves accurate measurements even if the rotation angle is as large as 25°. Further investigation also indicates the Siamese network trained on pure aluminum can be generalized at copper, nickel and 316 stainless steel without retraining, suggesting broad and desirable applicability.

The inclusion and role of micro mechanical residual stress on deformation of stainless steel type 316L at grain level

Validating crystal plasticity models requires careful consideration of all aspects. The initial conditions are important when comparing a model and experiment, as the initial residual stresses within the material can be significant but are often overlooked due to experimental limitations. Therefore, their inclusion has the potential to improve model predictions. This work explores the efficacy of using type-III residual elastic stresses, measured using high resolution electron backscatter diffraction (HR-EBSD),as pre-test stress distributions in 316L stainless steel. Two methods of processing the stresses collected (direct and using a least squares solver) were used. A existing method was used to incorporate the experimental measurements as initial residual stresses; The model results were compared with each other, a model containing no residual stress and the experiment. The least squares method helped remove extreme stresses calculated by the cross-correlation method at points with high mean angular error. The modelled stress distributions from both the direct and least squares model did not fully match the experimentally observed stresses but similarities were seen within some grains.After loading, little difference was seen between the models, with and without the inclusion of residual stress, implying a negligible effect on the deformation response of the material. However, the predicted stress distribution did not match with the experimentally measured stress distribution after deformation, suggesting further physical effects must be accounted for in crystal plasticity models.

Correlation between local mechanical properties and corresponding microstructures in a friction stir processed Monel alloy

Friction stir processing (FSP), an innovative local microstructural modification technique for bulk material, has been used to process Monel alloy. A detailed microstructural analysis was conducted by acquiring high-resolution electron backscatter diffraction (EBSD). The local tensile properties and hardness nanoindentation were obtained for the different FSP area zones. The coarse equiaxed grain structure of 18.9 μm average grain size at the base material (BM) has been dynamically recrystallized refined equiaxed grains of an average grain size of 3.6 μm in the stir zone (SZ) where the thermomechanically affected zone (TMAZ) has an average grain size of 10.7 μm. The grain structure at the interface of the SZ and TMAZ consisted of mixed fine recrystallized grains and coarse deformed grains. The SZ is dominated by significantly refined grains, with the highest fraction of high-angle grain boundaries (HAGBs) of about 71%. The point-to-point misorientation measured by Kernel Average Misorientation (KAM) indicates low values in the SZ compared with the highest values at the interface between SZ and TMAZ. In terms of tensile properties, the yield strength of TMAZ and SZ improved to 380 and 440 MPa, respectively. The ultimate tensile strength (UTS) also increased by 18% from 499 to 593 MPa in TMAZ and 37% from 499 to 699 MPa in the SZ. The maximum nano hardness found in the TMAZ of about 7.13 ± 1.92 GPa due to the high density of the substructure. However, the SZ slightly increased to 3.88 ± 0.22 GPa from 3.33 ± 0.17 GPa of the BM.

Shape memory and mechanical properties of a Fe-Mn-Si-based shape memory alloy: Effect of crystallographic texture generated during additive manufacturing

The fcc-γ → hcp-ε → bcc-α' martensitic transformation of a Fe-Mn-Si-based shape memory alloy fabricated by laser powder bed fusion (LPBF) is studied for the first time by in-situ neutron diffraction, in-situ high-resolution electron backscatter diffraction and digital image correlation during uniaxial tensile testing. The comparison between horizontal specimens (fabricated with no inclination on the build platform) and vertical specimens (fabricated with 90° inclination on the build platform) shows that sample orientation affects the deformation and transformation behavior of the material, considerably influencing alloy's shape memory and mechanical properties. The reason for the variation in the alloy's behavior is found in the distinct crystallographic texture generated during the LPBF process. In the vertical samples, the preferential 〈110〉 orientation of grains along the loading direction facilitates the martensitic transformation, resulting in enhanced shape memory properties and pronounced work hardening.

Investigation of quasi-cleavage in a hydrogen charged maraging stainless steel

Slow strain rate tests on hydrogen-containing specimens of PH13–8Mo maraging stainless steel showed H-assisted subcritical quasi-cleavage cracking, accelerating material failure. Fractographic analysis revealed that quasi-cleavage is composed of flat brittle areas and rougher areas. Electron backscatter diffraction (EBSD) was performed on a secondary subcritically grown crack. High resolution EBSD showed significant crystal lattice rotation, hence consequential plastic deformation, concentrated between the main crack tip and the cracks located ahead of it. Quasi-cleavage consisted of {100} cleavage cracks connected by ductile ridges, suggesting a discontinuous mechanism, involving re-initiation of new cleavage cracks ahead the main crack tip.

Characterization of Anomalous Grains in FeCo Magnetic Alloys.

Heat-treated FeCo-based magnetic alloys were characterized using a suite of electron microscopy techniques to gain insight into their structural properties. Electron channeling contrast imaging (ECCI) in the scanning electron microscope (SEM) found unique grains towards the outer edge of a FeCo sample with nonuniform background contrast. High-magnification ECCI imaging of these nonuniform grains revealed a weblike network of defects that were not observed in standard uniform background contrast grains. High-resolution electron backscattered diffraction (HR-EBSD) confirmed these defect structures to be dislocation networks and additionally found subgrain boundaries within the nonuniform contrast grains. The defect content within these grains suggests that they are unrecrystallized grains, and ECCI can be used as a rapid method to quantify unrecrystallized grains. To demonstrate the insight that can be garnered via ECCI on these unique grains, the sample was imaged before and after micro indentation. This experiment showed that slip bands propagate throughout the material until interacting with the dislocation networks, suggesting that these specific defects provide a barrier to plastic deformation. Taken together, these results show how ECCI can be used to better understand failure mechanisms in alloys and provides further evidence that dislocation networks play a critical role in the brittle failure of FeCo alloys.

Statistical properties of fractal type dislocation cell structures

The dislocation microstructure developing during plastic deformation strongly influences the stress-strain properties of crystalline materials. Resent theoretical investigations based on the 2D continuum theory of straight parallel edge dislocations were able to predict a periodic dislocation microstructure. The results obtained, however, can only be considered as a very first step toward the understanding of the origin of dislocation patterning. One of the most challenging problems is the modeling of the formation of the fractal like dislocation microstructure. So, it is crucial to determine the statistical properties of such a structure developing at ideal multiple slip orientation. In the paper, by x-ray line profile analysis and the method of high resolution electron backscatter diffraction (HR-EBSD) a complex experimental characterization of dislocation microstructure developing in uniaxially compressed Cu single crystals is presented. With these methods, the maps of the internal stress, the Nye tensor, and the geometrically necessary dislocation (GND) density were determined at different load levels. It is found from the fractal analysis of the GND maps that the fractal dimension of the cell structure is decreasing with increasing average spatial dislocation density fluctuation. Moreover, it is shown that the evolution of different types of dislocations can be successfully monitored with the HR-EBSD-based technique.

Determining Stress in Metallic Conducting Layers of Microelectronics Devices Using High Resolution Electron Backscatter Diffraction and Finite Element Analysis.

Delayed failure due to stress voiding is a concern with some aging microelectronics, as these voids can grow large enough to cause an open circuit. Local measurements of stress in the metallic layers are crucial to understanding and predicting this failure, but such measurements are complicated by the fact that exposing the aluminum conducting lines will relieve most of their stress. In this study, we instead mechanically thin the device substrate and measure distortions on the thinned surface using high resolution electron backscatter diffraction (HREBSD). These measurements are then related to the stresses in the metallic layers through elastic simulations. This study found that in legacy components that had no obvious voids, the stresses were comparable to the theoretical stresses at the time of manufacture (≈300 MPa). Distortion fields in the substrate were also determined around known voids, which may be directly compared to stress voiding models. The technique presented here for stress determination, HREBSD coupled with finite element analysis to infer subsurface stresses, is a valuable tool for assessing failure in layered microelectronics devices.

Friction Stir Welding of AA5754-H24: Impact of Tool Pin Eccentricity and Welding Speed on Grain Structure, Crystallographic Texture, and Mechanical Properties

This study investigates the effect of tool pin eccentricity and welding speed on the grain structure, crystallographic texture, and mechanical properties of friction stir welded (FSWed) AA5754-H24. Three tool pin eccentricities of 0, 0.2, and 0.8 mm at different welding speeds ranging from 100 mm/min to 500 mm/min and a constant tool rotation rate of 600 rpm were investigated. High-resolution electron backscattering diffraction (EBSD) data were acquired from each weld's center of the nugget zone (NG) and processed to analyze the grain structure and texture. In terms of mechanical properties, both hardness and tensile properties were investigated. The grain structure in the NG of the joints produced at 100 mm/min, 600 rpm, and different tool pin eccentricities showed significant grain refining due to dynamic recrystallization with average grain sizes of 18, 15, and 18 µm at 0, 0.2, and 0.8 mm pin eccentricities, respectively. Increasing the welding speed from 100 to 500 mm/min further reduced the average grain size of the NG zone to 12.4, 10, and 11 µm at 0, 0.2, and 0.8 mm eccentricity, respectively. The simple shear texture dominates the crystallographic texture with both B¯/B texture component with the C component at their ideal positions after rotating the data to align the shear reference frame with the FSW reference frame in both the PFs and ODF sections. The tensile properties of the welded joints were slightly lower than the base material due to the hardness reduction in the weld zone. However, the ultimate tensile strength and the yield stress for all welded joints increased by increasing the friction stir welding (FSW) speed from 100 to 500 mm/min. Welding using the pin eccentricity of 0.2 mm resulted in the highest tensile strength; at a welding speed of 500 mm/min, it reached 97% of the base material strength. The hardness profile showed the typical W shape with a reduction in the hardness of the weld zone and a slight recovery of the hardness in the NG zone.

On the influence of crystallographic orientation on superelasticity - Fe-Mn-Al-Ni shape memory alloys studied by advanced in situ characterization techniques

The superelasticity of iron-based shape memory alloys is strongly influenced by microstructural features such as crystallographic orientation, size and morphology of grains, character of grain boundaries as well as volume fraction of precipitates. Furthermore, elementary processes such as formation of dislocations, interaction of dislocations with the austenite/martensite phase boundary, and the interaction of different martensite variants result in functional degradation. Advanced in situ characterization techniques like digital image correlation (DIC) and infrared thermography (IR-TG) corroborated by ex situ high resolution electron backscatter diffraction (HR-EBSD) and electron contrast channeling imaging (ECCI) were used to investigate the functional stability of the superelasticity of Fe–Mn–Al–Ni single crystals and oligocrystals under tensile and compressive loading. The early stabilization of martensite during repeated loading was attributed, besides other mechanisms, to the interaction of different martensite variants leading to the formation of dislocations, locally pinning the austenite/martensite interface.

Microstructure evolution and deformation behaviors of pulse electro-assisted deformation in M50 bearing steel

M50 bearing steel, as a typical high-carbon and high-alloy steel, exhibits low cold working capacity with high susceptibility to produce voids during the ring rolling process. In this paper, the deformation behavior and microstructure evolution during the pulse electro-assisted tensile of M50 bearing steel was studied. The results show that the maximum flow stress of the pulse electro-assisted deformation (PEAD) specimen decreases by 24.4% compared with that of the traditional deformation (TD) specimen. However, the section shrinkage rate of the PEAD specimen is increased by 32.2%, which is even higher than that of the hot deformation (HD) specimens. This indicates the positive role of pulse current on the increase of plasticity. Besides, a constitutive model of the PEAD process was established by fitting the experimental data. With the assistance of pulse current, the deformation resistance and work hardening are reduced, so the number of voids is significantly reduced due to the easier metal flow. Moreover, the energy barrier of recrystallized grain nucleation is decreased, while the nucleation rate is increased under pulse current. As a result, the occurrence of recrystallization leads to the reduction of average grain size from 4.51 μm to 1.62 μm. According to the high-resolution transmission electron microscopy (HRTEM) and electron backscatter diffraction (EBSD) characterizations, the dislocation density decreases by 24% under the applied current, which should be ascribed to the increase of atomic flux and lattice diffusion coefficient. This work provides a new idea for solving the plastic deformation problem of M50 bearing steel.

Resolving local microstructure variations in Zr-2.5Nb CANDU pressure tubes using high spatial resolution electron backscatter diffraction and imaging

The alloy Zr-2.5Nb is used for manufacturing the pressure tubes installed in the core of Canada Deuterium Uranium (CANDU) nuclear reactors. This alloy consists of α-zirconium with hexagonal close-packed (HCP) crystals and a minor β-zirconium phase with body centered cubic (BCC) crystals that are mainly located in between the α-grains. Due to the significant elastic and plastic anisotropy of HCP crystals as well as the development of strong textures during manufacturing, these tubes exhibit significant mechanical anisotropy. However, both local microstructure and texture can change, affecting the local patterning of stress fields and the subsequent hydrogen embrittlement rate. Resolving the local microstructure variation of Zr-2.5Nb is challenging due to the development of nano-sized grains during manufacturing. This paper focuses on developing a procedure for the consistent characterization of such local variations using high spatial resolution electron backscatter diffraction (EBSD) and imaging. Microstructure variations along the radial and transverse directions of the transverse-radial plane, as well as along the axial direction of the transverse-axial and radial-axial planes are studied. It is shown that microstructure variations are non-negligible. For the locations that the transverse-radial planes are studied, pole figures rotate, and Kearns factors decrease along the radial direction. In addition, the morphologies of both α- and β-crystals change significantly along the radial direction. With the procedure used for characterization, some β-crystals are indexed and their orientation relationship with their neighboring α-grains are examined.

Measurement of the pattern shifts for HR-EBSD with larger lattice rotations

High-resolution electron backscattering diffraction (HR-EBSD) was used to measure rotations and elastic strains by matching diffraction patterns based on cross-correlation. However, the subset-based phase correlation algorithm was unable to determine pattern shifts accurately when large rotations occurred. In this paper, a new matching algorithm was proposed to measure pattern shifts and recover the elastic strain and lattice rotation with finite deformation theory. The algorithm was implemented in two steps: (a) Integral pixel matching: The pixel-related information of the Kikuchi patterns was mapped to the original three-dimensional sphere to obtain the image projected in parallel by using the feature points as the pattern center through the transformation of its spatial coordinates. The correlation between the images projected in parallel before and after deformation was then obtained. The locations of the integral pixels were determined by the peaks of the surface of correlation obtained by traversing all pixels in the search area. (b) subpixel refinement: the locations of subpixels were obtained by FAGN with an appropriate shape function involving rotation and translation. The algorithm was applied to dynamic simulated test sets, and its results were compared with those of the first-pass cross-correlation and the second-pass cross-correlation method with remapping. The proposed method was more robust in the case of rotation and solved the problem that displacement vectors could not be accurately measured when a larger lattice rotation occurred. The mean errors of the measured displacement, rotation, and strain components were 0.02 pixel, 0.5×10−4rad, and 1×10−4, respectively. Compared with the second-pass cross-correlation method, the angle of rotation was more precisely extracted.

Quantification of Slip Band Distribution in Polycrystals: An Automated Fast Fourier Transform Decomposition Approach.

Plastic deformation is accumulated in slip bands in a wide variety of engineering alloys. Multiple material and loading conditions impact their distribution and degree of slip localization, but these effects are rarely quantified. To tackle this, the current work introduces a fast Fourier transform (FFT) decomposition method and applies it to a tensile-loaded polycrystalline nickel-based superalloy imaged via high-resolution digital image correlation and electron backscatter diffraction. This approach identifies active slip planes over the FFT images of individual grains and performs inverse transforms such that slip band traces with shared orientations are isolated. This technique enabled the largest quantification of slip band spacings and in-plane strains to date, with a total of 6,557 slip bands detected. The results show that the slip band spacings increase with grain size, with no evident dependence on grain orientation and Schmid factor. Slip bands are found to develop similar spacings along different octahedral planes and continue to spread over larger regions of the grain as the resolved shear stress of the active slip system increases. The FFT decomposition technique, which could be employed with multiple microscopy techniques, will allow for much-needed large-scale quantitative studies of slip localization.

Time-strain evolution of shear zones from petrographically constrained Rb–Sr muscovite analysis

Some isotopic ratios in minerals like monazite, apatite, and mica can record the thermal response to mylonitization, but such systems may fail to track the time of low- to medium temperature fabric realignment (recrystallization). New analytical methods that allow spatially resolved measurement isotopic ratios in distinct microstructures offer the potential to characterize the time-strain evolution of mid-crustal shear zones over a temperature range conducive to capturing the growth timing of those structures. Here we present high spatial resolution electron backscatter diffraction (EBSD), mineral chemistry, and in situ Rb–Sr dates from two texturally distinct muscovite populations in a mid- to upper-greenschist facies granitic mylonite from Top Up Rise, Western Australia. Coarse-grained muscovite fish retain a primary magmatic composition, displaying microstructural evidence of mechanical (kink and folds) and crystal-plastic deformation. The muscovite fish yield a Rb–Sr isochron date of 1614±75 Ma that either records the crystallization of the granitic protolith or cooling from high-grade metamorphic overprinting as constrained by in situ garnet Lu–Hf dates in related metapelites (1696±43 Ma and 1670±36 Ma). Conversely, fine-grained muscovite texturally associated with shear bands is chemically distinct, display microstructural evidence of fluid-assisted dynamic recrystallization and yield a distinctly younger isochron date of 609±14 Ma. The Rb–Sr dates from the coarse-grained muscovite fish cannot be unequivocally linked to mylonitization as they are indistinguishable from regional metamorphic processes. Muscovite grains behaved as rigid porphyroclasts during later medium-temperature mylonitization, leading to fish geometries, whilst maintaining intact Rb–Sr systematics. However, recrystallized/neoblastic fine-grained muscovite records the timing of medium-temperature mylonitization, establishing a direct time–strain relationship. These results highlight the potential for significant diachronicity in mineralogical components of mylonitic fabrics, demonstrating the need to establish direct time-strain relationships in order to accurately reconstruct deformation histories.