Combined Electron Backscatter Diffraction Research Articles

A combined EBSD and machine learning study of predicting deformation twinning in BCC Fe81Ga19 alloy

Compared to face-centered cubic (FCC) metals, body-centered cubic (BCC) metals demonstrate more intricate deformation behavior and microstructural characteristics because of the influence of intrinsic unstable stacking faults and defect structure. To investigate the unique phenomenon of deformation twinning in Fe81Ga19 alloy with BCC structure at moderate temperature and low strain rate, four twin nucleation models were constructed using a combination of electron backscatter diffraction (EBSD) technique and machine learning. These models are aimed to predict twin nucleation of grains and explore the relationship between twin nucleation and microstructural attributes. The study revealed that amongst the 235 grains analyzed during in-situ compressive deformation, 32 grains experienced twinning. Nine attributes influencing twin nucleation were selected and ranked according to their importance. Grain area, average image quality, neighboring grain count, and the Schmid factor of twinning systems exhibited significant influence on twin nucleation. Decision tree and tree ensemble (XGBoost) achieved 94% and 96% accuracy, respectively, on the test set, showcasing robust generalization capability. Due to the optimization of hyperparameters, support vector machine (SVM) and artificial neural network (ANN) exhibited outstanding performance. The ANN model achieved an accuracy of 97% and an F1 score of 0.91 on the test set, while the SVM model achieved an accuracy of 98% and an F1 score of 0.94, indicating superior performance. Furthermore, the study revealed that the twinning stress in Fe81Ga19 alloy exhibited weak responsiveness to temperature during deformation, and the intrinsic factors of grains were identified as crucial factors influencing twin nucleation.

Laser shock peening strengthens additively manufactured high-entropy alloy through novel surface grain rotation

Additive manufacturing has flourished as an advanced technique to process metals and alloys. However, this strategy usually introduces undesired defects that deteriorates the mechanical performance of structural materials. Herein laser shock peening (LSP) is proposed as an efficient strengthening approach to reshape the surface morphology of a prototypical dual-phase AlCoCrFeNi high-entropy alloy (HEA) after additive manufacturing, in which remarkable strengthening is achieved. Combined electron back scatter diffraction and transmission electron microscope characterizations reveal that the mechanical enhancement is attributed to the grain refinement and accumulation of dislocations at the impact surface. In extreme condition of LSP, the grain refinement is not accommodated by the conventional dynamic recrystallization anymore, but a novel mechanism of parental columnar grain rotation which can be rationalized by a continuum-level theory from a geometrical perspective. The new mechanism is verified by large-scale atomistic simulations which further recognizes the critical role of multiple unstable dislocation slip and amorphization in formation of smaller grains under shock. Our strategy offers a promising pathway toward polishing morphology of HEAs and thus, prohibiting the potential intrinsic defected induced-mechanical degradation of the additively manufactured metals and alloys via novel microscopic mechanism.

Advancing analytical electron microscopy methodologies to characterise microstructural features in superalloys

Electron backscatter diffraction (EBSD) generally links crystallographic orientation to the microstructure of crystalline materials. EBSD datasets are now commonly used to identify phases, grains, and their orientations using off-the-shelf software, although substantial additional information may be extracted. Due to the lack of commercially available software, advanced analyses are often done manually and provide only localised information, lacking statistical significance. Here we introduce novel automated methodologies for advanced analyses of microstructural features in Ni-based superalloys. Our methodologies provide additional insights into the characteristics of these features and their underlying physical phenomena. We showcase how to correct wrongly indexed γ/γ’ interface artefacts in combined EBSD and energy-dispersive X-ray spectroscopy (EDS) measurements, how to classify recrystallised grains based on their location, how to assess and visualise grain boundary planes, and how to study the evolution of Σ3 twins during hot deformation. We further demonstrate how phase fractions and grain sizes are more accurately determined in combined EBSD-EDS measurements. The classification of recrystallised grains into different groups enables individual analyses, facilitating the straightforward identification of the underlying recrystallisation mechanism. Our grain boundary plane analysis provides insights into the coherence of Σ3 twins and the potential boundary planes of incoherent Σ3 boundaries. The current paper is a tutorial-style guide for these methodologies. The algorithms are made freely available and, although demonstrated here on Ni-based superalloys, can also be applied to other systems.

Facet type determination based on combined atomic force microscopy and electron backscatter diffraction.

The distribution of facet types affects the functionality of the surfaces of polycrystalline films. However, we are not aware of a previously published convenient method to determine their distribution. This work describes and demonstrates a process to determine and map the Miller indexes (hkl) of crystal facets exposed at the surfaces of polycrystalline films. To find facet types in non-trivial cases, one must know the orientation of the crystal and the direction in which the facet is facing. The method presented here combines the crystal orientations obtained with electron backscatter diffraction with the topography of the same sample area measured with atomic force microscopy. A challenging step is to transfer the data from the two instruments into a common coordinate system. The sequence of steps in the data processing is presented, with methods to verify the results. The process is illustrated with the analysis of an etched copper clad laminate (CCL) and an electroless Cu film deposited on the CCL. This example relates to facet selection in electroless and galvanic plating processes in printed circuit board production, where an uncontrolled transition from epitaxial to non-epitaxial growth can lead to surfaces with unacceptable roughness.

Mapping the kinetics of hydrogen evolution reaction on Ag via pseudo-single-crystal scanning electrochemical cell microscopy

AbstractElucidating the structure-activity relationship in electrocatalysis is of fundamental interest for electrochemical energy conversion and storage. However, the heterogeneity in the surface structure of electrocatalysts, including the presence of various facets, poses an analytical challenge in revealing the true structure-activity relationship because the activity is conventionally measured on ensemble, resulting in an averaged activity that cannot be unequivocally associated with a single structural motif. Scanning electrochemical cell microscopy (SECCM) [1] combined with colocalized electron backscatter diffraction (EBSD) offers a direct way to reveal the correlative local electrochemical and structural information. Herein, we measured the hydrogen evolution reaction (HER) activity on Ag and its dependence on the crystal orientation. From the combined EBSD and SECCM mapping, it is found that Ag grains closer to {111} show a higher exchange current density, while those closer to {110} show a lower Tafel slope. The Tafel slope is also found to decrease with the step density increase. The ability to measure the electrocatalytic activity under a high mass-transfer rate allows us to reveal the activity difference at a high current density (up to 200 mA/cm2). The approach reported here can be expanded to other systems to reveal the nature of active sites of electrocatalysis.

In-situ study of microstructural evolution and strain distribution during single axis tension of 316L stainless steel prepared by selective laser melting

A combined digital image correlation (DIC) and electron backscatter diffraction (EBSD) study of the multi-scale strain distribution and microstructure evolution during tensile loading has been carried on 316L stainless steel samples prepared by selective laser melting (SLM). The initial material shows a range of microstructural heterogeneity. Macro- and micro-DIC results reveal a non-uniform distribution of plastic strain over several length scales, including the formation of localized slip-bands at the intra-grain level. No obvious correlation is found between the heterogeneity in plastic strain and either grain size, grain morphology or the orientation of the cellular structures formed during SLM. Intragranular orientation subdivision and the evolution of local misorientation have also been investigated by EBSD during tension, where a complex pattern of heterogeneity in both microstructure and plastic strain is found.

Nanotwinning-assisted dynamic recrystallization at high strains and strain rates.

Grain refinement is a widely sought-after feature of many metal production processes and frequently involves a process of recrystallization. Some processing methods use very high strain rates and high strains to refine the grain structure into the nanocrystalline regime. However, grain refinement processes are not clear in these extreme conditions, which are hard to study systematically. Here, we access those extreme conditions of strain and strain rate using single copper microparticle impact events with a laser-induced particle impact tester. Using a combined dictionary-indexing electron backscatter diffraction and scanning transmission electron microscopy approach for postmortem characterization of impact sites, we systematically explore increasing strain levels and observe a recrystallization process that is facilitated by nanotwinning, which we term nanotwinning-assisted dynamic recrystallization. It achieves much finer grain sizes than established modes of recrystallization and therefore provides a pathway to the finest nanocrystalline grain sizes through extreme straining processes.

A practical guide to characterizing irradiated nuclear fuels using FIB tomography

Focused ion beam (FIB) tomography with combined electron backscatter diffraction (EBSD) and energy dispersive x-ray spectroscopy (EDS) is a technique capable of statistically characterizing the microstructure and spatial compositional variation of nuclear fuel in three-dimensions (3D). The 3D visualization from FIB tomography provides a comprehensive picture of the interconnected microstructural and compositional features that can impact fuel performance. While these features are often characterized with surface examination, the complexity and relationship of fission products and grain boundary networks may not fully be captured by these 2D methods. This work presents a practical guide to FIB tomography that is tailored to nuclear fuel characterization. The steps used to collect and process the data are provided along with the scripts used to process the data. Additionally, suggestions for future characterization efforts utilizing this approach are given.

Surface patterning allowing combined EBSD and DIC investigations during in-situ deformation experiments

A simple method for surface patterning using silica nanoparticles is presented and it is shown how under appropriate patterning conditions digital image correlation (DIC) can be carried out with sub-micrometer resolution, while still allowing collection of electron backscatter diffraction (EBSD) data over the same area with negligible effect on the EBSD indexing during multi-step in-situ deformation experiments. A demonstration based on in-situ tensile deformation of aluminum reveals significant heterogeneities in several EBSD- and DIC-derived parameters, both between, and within individual grains, with only a weak correlation between these parameters on the scale of the dislocation patterning.

Correlative statistical microstructural assessment of precipitates and their distribution, with simultaneous electron backscatter diffraction and energy dispersive X-ray spectroscopy

Modern engineering alloys have bespoke microstructures, where features such as precipitates are used to control properties. In many Ni-based alloys, carbo-nitride precipitates are introduced to strengthen and improve performance. These precipitates can be distributed throughout the microstructure and niobium rich carbides are often found at grain boundaries. In this work, we used combined energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) to characterise the population of these precipitates. Processing of the EDS signal is used to label the Mo/Nb-rich precipitates, and their size and location are measured from maps using a circular Hough transform. This label map is combined with the grain boundary network (from EBSD analysis). Statistical analysis, using ANOVA testing, reveals differences in chemistry between carbides found in Ni-rich matrix grain interiors, on random high angle boundaries and on special boundaries (Σ3 and Σ9). These results are compared between wrought and power metallurgy product forms. These distributions are discussed in the context of their performance within demanding environments, such as reactor core internals.

Surface patterning for combined digital image correlation and electron backscatter diffraction in-situ deformation experiments

A considerable amount of information can be gained from combined collection of both digital image correlation (DIC) measurements, and of electron backscattered diffraction (EBSD) data from the same area during in-situ deformation experiments. Such data collection remains a challenge, however, due the influence of the surface markers required for DIC on the EBSD data quality. In this work we demonstrate that by careful control of the sample preparation process, a two-step electropolishing method can be used to generate excellent surface markers for DIC where the markers are integral features on the surface. The method does not require any special equipment and can be used to collect high quality EBSD data and DIC data with sub-micron resolution on the same area during repeated loading as part of in-situ deformation experiments. Combined EBSD and DIC measurements are illustrated in this work for samples of Al deformed in tension to a strain of 10%. Due to the integral nature of the surface markers we demonstrate also that it is possible to perform DIC measurements up to large plastic strains, including the necked region within a tensile sample. A comparison of EBSD and DIC data collected from the same area of a tensile-deformed sample shows that although the EBSD data provide excellent information on the lattice rotations developed during deformation, these data cannot be directly used as a measure of local plastic strain on the scale of the deformation microstructure. The ability to collect both EBSD and DIC data during an in-situ experiment provides, therefore, an opportunity to explore further the complex relationship between local plastic strain heterogeneity and EBSD-based measures of crystal lattice rotation.

A combined Raman spectroscopy, cathodoluminescence, and electron backscatter diffraction study of kyanite porphyroblasts from diamondiferous and diamond‐free metamorphic rocks (Kokchetav massif)

AbstractA series of precise nondestructive analytical methods (Raman spectroscopy, cathodoluminescence, and EBSD—electron backscatter diffraction) has been employed to investigate the internal textures of kyanite porphyroblasts from diamondiferous and diamond‐free ultrahigh‐pressure metamorphic rocks (Kokchetav massif, Northern Kazakhstan). Such internal kyanite characteristics as twinning, radial fibrous pattern, and spotty zoning were identified by means of Raman and cathodoluminescence imaging, whereas an intergrowth of two kyanite crystals was distinguished only by Raman imaging. The EBSD analysis recorded an ~10–25° changing of orientations along the elongation in the investigated kyanite porphyroblasts. The absence of a radial fibrous pattern and a spotty zoning on the EBSD maps indicates that these textures are not related to variations in crystallographic orientation. The absence of clear zoning patterns (cores, mantles, and rims) on the Raman, cathodoluminescence, or EBSD maps of the kyanite porphyroblasts indicates the rapid single‐stage formation of these porphyroblasts near the peak metamorphic conditions and the lack of recrystallization processes. The obtained results provide important implications for deciphering of mineral internal textures, showing that the data obtained by cathodoluminescence mapping can be clearly reproduced by Raman imaging, with the latter method occasionally being even more informative. This observation is of significant importance for the study of minerals that are unexposed on a thin section surface or Fe‐ and Ni‐rich minerals that do not show luminescence emission. The combination of the Raman spectroscopic, cathodoluminescence, and EBSD techniques may provide better spatial resolution for distinguishing different domains and textural peculiarities of mineral than the selective application of individual approaches.

In-situ study of microstructural evolution and local strain distribution during tensile loading of near-micrometre grain size aluminium

A combined digital image correlation (DIC) and electron backscatter diffraction (EBSD) study of local strain distribution and microstructural evolution during tensile loading has been carried on samples of near-micrometre grain size Al prepared by spark plasma sintering. To achieve the required spatial resolution and allow combined DIC and EBSD measurements a method of surface patterning using colloidal silica was developed. The DIC results reveal a wide variety in heterogeneity of plastic strain, both in terms of the average strain experienced by each grain and regarding the extent and nature of strain localization within individual grains. No obvious correlation is found between the heterogeneity in plastic strain and either Schmid factor, grain size or the pattern of local grain rotations. The experiments also allow investigation of the pattern of crystal rotation during tension in near-micrometer grain size samples and reveal differences in the mean grain rotation and the in-grain orientation spread.

Direct observation of dislocation nucleation in pyrite using combined electron channelling contrast imaging and electron backscatter diffraction

AbstractCrystal‐plastic deformation is one of the main mechanisms that can accommodate large amounts of strain within the lithosphere. Despite the requirement of understanding dislocation nucleation and arrangement, the only accepted method for direct observation of dislocations in geological materials so far is transmission electron microscopy. Herein, we present a study using a combination of electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) to visualize and analyse crystal defects in pyrite deforming close to the crystal plastic to brittle transition zone. Structures in focus include (a) dislocation nucleation at crack‐tips and (b) the reactivation of mode I cracks accompanied by the nucleation of dislocations and crystal‐plastic behaviour resulting in the development of complex dislocation structures and low‐angle grain boundaries. EBSD maps reveal an increase in misorientation towards micro‐cracks, consistent with a greater dislocation density along cracks observed by ECCI.

A combined cathodoluminescence and electron backscatter diffraction examination of the growth relationships between Jwaneng diamonds and their eclogitic inclusions

To fully understand the implications of the compositional information recorded by inclusions in diamond it is vital to know if their growth was syn- or protogenetic and the extent to which they have equilibrated with diamond forming agents. The current paradigm is that the majority of inclusions in diamond are syngenetic but recently this assumption has been questioned. This study presents an integrated cathodoluminescence (CL) and electron backscatter diffraction (EBSD) study of 8 diamonds containing eclogitic inclusions: 19 pyrope-almandine garnets, 12 omphacitic clinopyroxenes, 4 sulphides, 1 coesite and 1 rutile from the Jwaneng diamond mine, Botswana. Diamond plates were sequentially polished to expose inclusions at different levels and CL imaging and EBSD were performed to constrain the relationship between diamond and inclusion growth. Despite complex growth and resorption, individual diamonds are single crystals with a homogeneous crystallographic orientation. All individual inclusions have homogeneous crystallographic orientation and no resolvable compositional zonation. The combined CL and EBSD data suggest that epitaxial inclusion-diamond growth is rare (none of 24 inclusions) and that the imposition of cubo-octahedral faces on inclusions does not necessarily result in epitaxy. Individual diamonds contain inclusions that record evidence of both syngentic and protogenetic relationships with the host diamond and in one case an inclusion appears syngenetic to the diamond core but protogenetic to the growth zone that surrounds 70% of the inclusion. These findings emphasise that inclusions in diamonds have multiple modes of origin and that in order to validate the significance of geochronological studies, further work is needed to establish that there is rapid chemical equilibration of protogenetic inclusions with diamond forming agents at mantle temperatures.

Quantification of complex-phase steel microstructure by using combined EBSD and EPMA measurements

We combined electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) techniques to characterize the microstructure of industrial produced complex-phase CP800 steel. First, three major constituents (ferrite, martensite and bainite) are characterized. Then, transformation-induced dislocation zones in ferrite and bainitic regions consisting of different sub-structures are quantified and segmented. A final digitalized component map including plenty of microstructural features including crystal structures, orientations and elemental distribution are successfully achieved. The precise carbon content is measured in bainite and martensite utilizing EPMA line scans, which is around 0.2 wt% and 0.4 wt%, respectively. In the bainite phase of CP800, a typical Bagaryatski orientation relationship between cementite and lath-like bainitic ferrite is revealed based on the select diffraction patterns of transmission electron microscopy measurements.

Time-resolved, defect-hosted, trace element mobility in deformed Witwatersrand pyrite

The Pb isotopic composition of rocks is widely used to constrain the sources and mobility of melts and hydrothermal fluids in the Earth's crust. In many cases, the Pb isotopic composition appears to represent mixing of multiple Pb reservoirs. However, the nature, scale and mechanisms responsible for isotopic mixing are not well known. Additionally, the trace element composition of sulphide minerals are routinely used in ore deposit research, mineral exploration and environmental studies, though little is known about element mobility in sulphides during metamorphism and deformation. To investigate the mechanisms of trace element mobility in a deformed Witwatersrand pyrite (FeS2), we have combined electron backscatter diffraction (EBSD) and atom probe microscopy (APM). The results indicate that the pyrite microstructural features record widely different Pb isotopic compositions, covering the entire range of previously published sulphide Pb compositions from the Witwatersrand basin. We show that entangled dislocations record enhanced Pb, Sb, Ni, Tl and Cu composition likely due to entrapment and short-circuit diffusion in dislocation cores. These dislocations preserve the Pb isotopic composition of the pyrite at the time of growth (∼3 Ga) and show that dislocation intersections, likely to be common in deforming minerals, limit trace element mobility. In contrast, Pb, As, Ni, Co, Sb and Bi decorate a high-angle grain boundary which formed soon after crystallisation by sub-grain rotation recrystallization. Pb isotopic composition within this boundary indicates the addition of externally-derived Pb and trace elements during greenschist metamorphism at ∼2 Ga. Our results show that discrete Pb reservoirs are nanometric in scale, and illustrate that grain boundaries may remain open systems for trace element mobility over 1 billion years after their formation.

EBSD-assisted Laue microdiffraction for microstrain analysis

The X-ray Laue microdiffraction (µLaue) technique has been establishing itself as a reliable means for microstrain analysis for the past few decades. One problem with this technique is that when the crystal size is significantly smaller than the probed volume and when the diffracting crystals are closely oriented, a large number of individual µLaue patterns are superimposed in a complex way on the recorded diffraction images. In that case, because of the difficulty of isolating unambiguously a single-grain µLaue pattern, a reliable analysis of strains is tedious manually and hardly achievable with current automated methods. This issue is even more severe for low-symmetry crystals or when high-energy X-rays are used, since each single-crystal µLaue pattern already contains a large number of spots. This paper proposes overcoming this challenge through the development of a combined approach coupling µLaue and electron backscatter diffraction (EBSD). The capabilities of this `EBSD-assisted µLaue' automated method are illustrated on a monoclinic zirconia-based specimen and µLaue diffraction patterns are analysed with the crystal orientation input from EBSD. The obtained results are statistically reliable, reproducible and provide a physical insight into the micromechanical characteristics of the material.

Insights into twinning in Mg AZ31: A combined EBSD and machine learning study

To explore the driving forces behind deformation twinning in Mg AZ31, a machine learning framework is utilized to mine data obtained from electron backscatter diffraction (EBSD) scans in order to extract correlations in physical characteristics that cause twinning. The results are intended to inform physics-based models of twin nucleation and growth. A decision tree learning environment is selected to capture the relationships between microstructure and twin formation; this type of model effectively highlights the more influential characteristics of the local microstructure. Trees are assembled to analyze both twin nucleation in a given grain, and twin propagation across grain boundaries. Each model reveals a unique combination of crystallographic attributes that affect twinning in the Mg. Twin nucleation is found to be mostly controlled by a combination of grain size, basal Schmid factor, and bulk dislocation density while twin propagation is affected most by grain boundary length, basal Schmid factor, angle from grain boundary plane to the RD plane, and grain boundary misorientation. The machine learning framework can be readily adapted to investigate other relationships between microstructure and material response.

Quantification of sigma-phase evolution in thermally aged 2205 duplex stainless steel

In this study, the evolution of sigma-phase development in a 2205 duplex steel was studied following thermal ageing in the temperature range 750–850 °C for periods up to 100 h. A suite of materials analysis techniques including combined electron backscatter diffraction–energy-dispersive X-ray imaging and magnetic force microscopy are used to quantify the change in volume fraction of the phases. The experimental results for each ageing condition are compared with the results from other ageing experiments and the predictions from the computer modelling. They show good correlation for the amount of sigma phase formed under a range of ageing conditions (and thus varying quantities of sigma phase). EBSD measurements showed no preferential orientation relationships relative to the parent ferrite for the nucleation of sigma phase, implying that the minimisation of boundary misorientation energy is not significant in determining sigma-phase nucleation sites. The results are discussed with respect to the experimentally measured properties of sigma phase and the kinetics of the precipitation process.