Conventional EBSD Research Articles

Revisiting the phase transformations involving Cu6Sn5 intermetallic: resolving local domain structures induced by ordering

Cu6Sn5 is one of the most important intermetallics present in solder joints based on Cu and Sn. Due to still uncertain reasons for failure of these joints, detailed microstructural research can help to figure out the underlying mechanisms. For Cu- and as well for Sn-rich environments of the Cu6Sn5 the microstructural occurrence of the phase transformations are investigated. During the formation of stable η′ from the disordered high temperature η phase the formation of metastable modulated incommensurately ordered η′′ takes place. For the copper rich samples long-range ordering and twin-related domain formation within η′′ was observed. Although conventional Hough-based EBSD analysis was not able to identify the correct domain orientation, Kikuchi pattern matching using simulated patterns was able to resolve the correct microstructure. As ordered intermetallics tend to be brittle in nature, the current microstructure research provides a basis to investigate unresolved problems in the initiation of failure.

The influence of parent austenite characteristics on the intervariant boundary network in a lath martensitic steel

The influence of the parent austenite deformation state on the intervariant boundary network (i.e., population, plane orientation, and connectivity) of a lath martensitic microstructure was investigated using conventional EBSD mapping and five-parameter boundary analysis approach along with quantification of boundary connectivity using homology metrics. The lath martensite largely revealed a bimodal misorientation angle distribution, closely matched with the Kurdjumov–Sachs (K–S) orientation relationship. The application of deformation significantly changed the distribution, gradually reducing the intensity of the 60° misorientation angle peak. This was largely ascribed to substructure development within the parent austenite upon deformation, which stimulates particular variant/s having a habit plane (011)α′ closely parallel to the primary (111)γ and/or secondary (1overline{1 }1)γ slip plane. The interaction of these variants eventually promoted specific intervariant boundaries (e.g., 60°/[111], 10.5°/[011], and 49.5°/[110]) at the expense of 60°/[011]. The application of deformation in the parent austenite did not change the intervariant boundary plane character distribution, which mostly exhibited an anisotropic character terminated on {110} planes because of the displacive nature of the martensitic transformation. However, the extent of anisotropy progressively decreased with increasing strain in the austenite prior to transformation. The grain boundary network connectivity was markedly altered due to the local variant selection induced by the deformation. Deformation in the austenite regime generally decreased the connectivity of boundaries having a {110} plane orientation. The intervariant boundaries with the {110} twist character also displayed a similar trend, though the connectivity of {110} tilt boundaries progressively enhanced with increasing strain. The former was closely matched with a decrease in the population of 60°/[110] intervariant boundaries with the strain. The current findings suggest that the intervariant boundary network of lath martensite can be manipulated through changes in the parent austenite deformation state which, in turn, can be used to enhance key final product properties such as toughness.Graphical abstract

A new approach for determining GND and SSD densities based on indentation size effect: An application to additive-manufactured Hastelloy X

• A new approach to determine GND and SSD densities for FCC metals was established. • The new approach was based on indentation size effect and classical strengthening theories. • Results of new approach were verified by conventional Hough-based EBSD and XRD CMWP methods. • Due to low SFE of L-PBF Hastelloy X, edge-GNDs contribute greatly the increment of strengthening. • Dislocation distribution shows grain orientation-dependent: low in large <101> grains, high in fine <001> grains. • This method may construct a firm basis for future quantitative work on BCC and HCP metals. Dislocation plays a crucial role in controlling the strength and plasticity of bulk materials. However, determining the densities of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs) is one of the classical problems in material research for several decades. Here, we proposed a new approach based on indentation size effect (ISE) and strengthening theories. This approach was performed on a laser powder bed fused (L-PBF) Hastelloy X (HX), and the results were verified by the Hough-based EBSD and modified Williamson–Hall (m-WH) methods. Furthermore, to better understand the new approach and essential mechanisms, an in-depth investigation of the microstructure was conducted. The distribution of dislocations shows a clear grain orientation-dependent: low density in large <101> preferentially orientated grains while high density in fine <001> orientated grains. The increment of strengthening in L-PBF HX is attributed to a huge amount of edge-GNDs. Planar slip is the main operative deformation mechanism during indentation tests, and the slip step patterns depend mostly on grain orientations and stacking fault energy. This study provides quantitative results of GND and SSD density for L-PBF HX, which constructs a firm basis for future quantitative work on other metals with different crystal structures.

Dislocation structure of deformed olivine single crystals from conventional EBSD maps

Dislocations, linear defects in a crystalline lattice characterized by their slip systems, can provide a record of grain internal deformation. Comprehensive examination of this record has been limited by intrinsic limitations of the observational methods. Transmission electron microscopy reveals individual dislocations, but images only a few square upmum of sample. Oxidative decoration requires involved sample preparation and has uncertainties in detection of all dislocations and their types. The possibility of mapping dislocation density and slip systems by conventional (Hough-transform based) EBSD is investigated here with naturally and experimentally deformed San Carlos olivine single crystals. Geometry and dislocation structures of crystals deformed in orientations designed to activate particular slip systems were previously analyzed by TEM and oxidative decoration. A curvature tensor is calculated from changes in orientation of the crystal lattice, which is inverted to calculate density of geometrically necessary dislocations with the Matlab Toolbox MTEX. Densities of individual dislocation types along with misorientation axes are compared to orientation change measured on the deformed crystals. After filtering (denoising), noise floor and calculated dislocation densities are comparable to those reported from high resolution EBSD mapping. For samples deformed in [110]c and [011]c orientations EBSD mapping confirms [100](010) and [001](010), respectively, as the dominant slip systems. EBSD mapping thus enables relatively efficient observation of dislocation structures associated with intracrystalline deformation, both distributed, and localized at sub-boundaries, over substantially larger areas than has previously been possible. This will enable mapping of dislocation structures in both naturally and experimentally deformed polycrystals, with potentially new insights into deformation processes in Earth’s upper mantle.

The role of thermomechanical processing routes on the grain boundary network of martensite in Ti–6Al–4V

The influence of thermomechanical processing (TMP) of a Ti–6Al–4V alloy on the transformation texture and intervariant boundary network were investigated by conventional EBSD mapping along with the five-parameter boundary analysis approach. The texture characteristics of Ti–6Al–4V subjected to deformation in the β regime followed by the β→α martensitic transformation were examined using visco-plastic self-consistent simulation and forward calculation of the transformation texture. Comparison of the simulated and experimental texture characterisitcs revealed that the transformed α′ texture was dominated by the variant selection associated with substructure development in the β parent phase and the occurrence of specific self-accommodating α′ variants in the microstructure, promoting the quadrilateral and/or V shape variant arrangement. This resulted in a progressive increase in the 63.26°/[10‾553‾] intervariant boundaries with the strain increment, at the expense of 60°/[112‾0]. Moreover, the grain boundary network for all conditions was dominated by the triple junctions (grain boundary network) terminating on 63.26°/[10‾553‾] and 60°/[112‾0] intervariants. It is shown that the elastic interactions among the variants during the martensitic transformation is the dominant parameter affecting the grain boundary network, despite the presence of dislocation based variant selection.

Adaptive domain misorientation approach for the EBSD measurement of deformation induced dislocation sub-structures

In the current work a novel domain misorientation approach is introduced, which can resolve sub-grains and dislocation cells using conventional EBSD. The measurement principle utilises measurement domains that are grown radially until a specified misorientation value has been reached. This enables stochastic analysis of local misorientation to be carried out within individual sub-grains and dislocation cells. The sub-structural boundaries are classified according to the total misorientation across the boundary region, the thickness of which can vary from approximately one hundred nanometres to several hundred nanometres. Sub-grain boundaries with a total misorientation larger than 2° are resolved effectively for as-measured Hough-based EBSD data. De-noising of the EBSD data allows small dislocation cells to be resolved, typically having a misorientation of 0.4° – 1.0°. The developed approach is applied to various deformed metals, showing a significant increase in the level of detail resolved compared to the conventional kernel misorientation approach. The developed adaptive domain misorientation approach and the EBSD datasets measured for this publication are provided as open access.

About the reliability of EBSD measurements: Data enhancement

An extensive set of information about the diffracting volume is carried by EBSD patterns: the crystal lattice, the reciprocal lattice, the crystal structure, the crystal symmetry, the mean periodic number of the diffracting phase, the source point from where it has been projected (projection centre), the crystal orientation, the sample topography (local tilt), the (preparation) quality or defect density of the crystal, and possible pattern overlaps. Some of this information is used regularly in conventional EBSD analyses software while others are still waiting for a more widespread application. Despite the wealth of information available, the accuracy and precision of the data that are presently extracted from conventional EBSD patterns are often well below the actual physical limits. Using a selection of example applications, we will demonstrate the gain in angular resolution possible using relatively low-resolution patterns of approximately 20k pixels in combination with pattern matching (PM) approaches. In this way, fine details in a microstructure can be revealed which would otherwise be hidden in the orientation noise.

Effect of the severe plastic deformation by ECAP on microstructure and phase transformations in Ti-15Mo alloy

Ti-15Mo alloy in a metastable β solution treated condition was processed by equal channel angular pressing (ECAP) at an elevated temperature of 250 °C. The resulting microstructure is highly deformed, contains twins and shear bands, but is not ultra-fine grained. Both the initial solution treated material and the material after ECAP were subjected to ageing at 400 °C and 500 °C in order to study the effect of deformation on phase transformations, namely the α phase precipitation. The phase composition was studied by X-ray diffraction measurement; the microstructure was investigated using conventional EBSD and an advanced method of transmission Kikuchi diffraction (TKD). It was shown that the α phase precipitation is accelerated in areas with higher density of lattice defects, which provide a dense net of preferred sites for nucleation and also fast diffusion paths necessary for accelerated growth. Upon further annealing, discontinuous lamellar coarsening occurs, which had not been previously reported in metastable β-Ti alloys. The microhardness is governed mainly by the formation of ω phase particles. The fraction of ω phase increases during annealing at 400 °C and the specimen aged at 400 °C/16 h shows the highest value of microhardness of 520 HV for both ECAP and undeformed material. Upon annealing at 500 °C, the microhardness is significantly lower.

Nanoscale crystallographic characterization of nanoporous catalyst by TKD

To better understand the structure-reactivity relationships of self-supported nanoporous catalysts, a nanoscale characterization of the materials is required. Transmission Kikuchi Diffraction (TKD), or transmission electron backscatter diffraction (t-EBSD), is a recent technique used to analyze the grain crystallography and morphology of thin films and nanometer-scale materials. Using a conventional EBSD detector in a FIB-SEM instrument, we investigate nanoporous gold (npAu) samples with ligament size in the 50–70 nm range. Grains with diameters below 50 nm can be clearly resolved, but also well-defined grain boundaries extending over 3 to 4 pixels, proving the high resolution of the method. The presence of contaminations during sample preparation by FIB milling is also discussed. The results provide deeper insight than conventional EBSD technique can alone.

Transmission EBSD (t-EBSD) as Tool to Investigate Nanostructures in Superconductors

The transmission electron backscatter diffraction (t-EBSD) technique has proven to be an indispensable tool for the analysis of microstructures of superconducting samples, both high-Tc samples (YBa2Cu3Oy, Bi2Sr2CaCu2O8) as well as MgB2 or iron-based materials. The knowledge of the grain boundary properties (misorientation, length, width) is essential for the further optimization of sample performance. Any addition of secondary phase(s) to improve the flux pinning properties is required to be of nanometer dimensions, so the higher achievable resolution and the better imaging properties are important to obtain reasonably high image quality to enable automated orientation mapping. The orientation maps reveal not only the location and the shape of the inclusions within the superconducting matrix or at the grain boundaries but also their influence on the surrounding superconducting matrix, which also plays an important role in flux pinning. In the case of sintered MgB2 bulk samples, the demand for higher critical current densities leads to MgB2 grains in the 100-nm range, which is already difficult to be studied by means of conventional EBSD. Furthermore, t-EBSD is useful for the analysis of specific microstructures of unconventional superconductors like superconducting foams or superconducting nanowire networks.

Discrimination of dynamically and post‐dynamically recrystallized grains based on EBSD data: application to Inconel 718

The importance of angular resolution in EBSD analyses is discussed based on an Inconel 718 sample containing several populations of recrystallized grains, with subtle differences in dislocation contents. Classical EBSD analyses (with angular resolution in the range of 0.5-1°) do not allow for distinguishing recrystallized grains grown dynamically or post-dynamically. The angular resolution of EBSD orientation and misorientation data can be significantly improved (down to about 0.1-0.2°) either using more sophisticated Kikuchi pattern indexing methods and/or using the recently proposed LLASS denoising filter (Local Linear Automatic Smoothing Splines). Then the coexistence of both dynamically and post-dynamically recrystallized grains in the sample can be confirmed and quantified. ECCI images unambiguously confirm the conclusions drawn from the analysis of improved angular resolution EBSD data, and furthermore reveal the presence of thermal stress induced dislocations with typical patterns in water quenched Inconel 718 recrystallized grains. LAY DESCRIPTION: EBSD is widely used to study recrystallization phenomena. Conventional EBSD is nevertheless not able to distinguish dynamic recrystallized grains from post-dynamic recrystallized grains which differ by subtitle differences in dislocation contents. In this paper, we show that improving the orientation precision of EBSD data by means of different methods allows distinguishing these two recrystallized grains populations. Analyses and discussion are based on an Inconel 718, a famous Nickel-based superalloy in aeronautic.

ICHORD-SI combination as an alternative to EDS-EBSD coupling for the characterization of γ-γ′ nickel-based superalloy microstructures

Because they exhibit very similar diffraction patterns, the γ and γ′ phases of nickel-based superalloys cannot be distinguished by the conventional EBSD technique alone. This paper presents an original method which discriminates those phases on an orientation map by exploiting their chemical differences. Performed with a Focused Ion Beam (FIB) microscope, the method is the combination of the ion CHanneling ORientation Determination technique (iCHORD) – recently developed by Langlois et al. [1] – with the chemical information obtained from Secondary Ion (SI) images. The performances of the method are compared to the EDS-EBSD coupling which also discriminates phases based on their chemical compositions. Applied filters, angular resolution, spatial resolution of the phase discrimination and acquisition time are discussed. It results that the iCHORD–SI combination offers an orientation map with an angular resolution slightly decreased compared to that of the EBSD technique, but with a good phase resolution (down to 150 nm) and within a reasonable acquisition time. Thus, the iCHORD–SI combination appears to be an interesting method for the crystallographic systems where phases are difficult to discriminate by the EBSD technique alone but present a significant chemical contrast.

Selectively Electron-Transparent Microstamping Toward Concurrent Digital Image Correlation and High-Angular Resolution Electron Backscatter Diffraction (EBSD) Analysis.

Digital image correlation (DIC) in a scanning electron microscope and high-angular resolution electron backscatter diffraction (HREBSD) provide valuable and complementary data concerning local deformation at the microscale. However, standard surface preparation techniques are mutually exclusive, which makes combining these techniques in situ impossible. This paper introduces a new method of applying surface patterning for DIC, namely a urethane microstamp, that provides a pattern with enough contrast for DIC at low accelerating voltages, but is virtually transparent at the higher voltages necessary for HREBSD and conventional EBSD analysis. Furthermore, microstamping is inexpensive and repeatable, and is more suitable to the analysis of patterns from complex surface geometries and larger surface areas than other patterning techniques.

Beam broadening in transmission and conventional EBSD

Transmission electron backscatter diffraction (t-EBSD) has become a routine technique for crystal orientation mapping when ultrahigh resolution is needed and has demonstrated advantages in the characterization of nanoscale and micron-sized samples (Babinsky et al., 2015). In this work, we use experimental measurements and simulations to compare the resolution of the transmission and conventional reflection EBSD techniques across a range of sample volumes and characterization conditions. Monte Carlo simulations of electron trajectories provide the opportunity to estimate beam size and effective resolution, as well as electron flux, as a function of sample thickness or incident beam energy in t-EBSD. Increasing incident beam energy is shown to negatively impact beam diameter in some cases, and the effect of thinning a sample for conventional EBSD is shown to improve characterization resolution but dramatically decrease the number of high-loss electrons backscattered to the detector. In addition to considering spatial resolution when implementing EBSD techniques, it is found that maintaining a high yield of diffracted electrons to the detector is also of critical importance, which is supported by experimental results. Consequently, this work provides key insights into the nature of electron scattering and probe volume for the practical implementation of both transmission and reflection EBSD techniques.

Investigation of Fe-rich fragments in aluminum-steel friction stir welds via simultaneous Transmission Kikuchi Diffraction and EDS

An attempt has been made to investigate Fe-rich fragments and surrounding intermetallic compounds in aluminum-steel friction stir welds using conventional EBSD + EDS as well as advanced Transmission Kikuchi Diffraction (TKD). Results indicate that the fragments are covered by a layer of orthorhombic Al5Fe2. This layer is surrounded by a mixed region composed of sub-micrometer Al3Fe crystals in a matrix of Al nano grains. Employing high beam current TKD on electron-transparent samples enables investigation of sub-micrometer grains in the mixed region with a higher spatial resolution than conventional EBSD. As a result, nano-twinning in Al in the mixed region has been observed. Microstructural and texture results imply that Fe-rich fragments travel through the Al matrix under extreme shear forces, severely refining Al grains, and inducing nano-twinning. Due to extreme shear forces and Fe outward diffusion, fine steel fragments dissolve in the surroundings leading to the nucleation of nano Al3Fe crystals particularly along Al grain boundaries. Al3Fe crystals consume the Al matrix as they grow in size and fraction.

Geometrically necessary dislocation densities in olivine obtained using high-angular resolution electron backscatter diffraction

Dislocations in geological minerals are fundamental to the creep processes that control large-scale geodynamic phenomena. However, techniques to quantify their densities, distributions, and types over critical subgrain to polycrystal length scales are limited. The recent advent of high-angular resolution electron backscatter diffraction (HR-EBSD), based on diffraction pattern cross-correlation, offers a powerful new approach that has been utilised to analyse dislocation densities in the materials sciences. In particular, HR-EBSD yields significantly better angular resolution (<0.01°) than conventional EBSD (~0.5°), allowing very low dislocation densities to be analysed. We develop the application of HR-EBSD to olivine, the dominant mineral in Earth's upper mantle by testing (1) different inversion methods for estimating geometrically necessary dislocation (GND) densities, (2) the sensitivity of the method under a range of data acquisition settings, and (3) the ability of the technique to resolve a variety of olivine dislocation structures. The relatively low crystal symmetry (orthorhombic) and few slip systems in olivine result in well constrained GND density estimates. The GND density noise floor is inversely proportional to map step size, such that datasets can be optimised for analysing either short wavelength, high density structures (e.g. subgrain boundaries) or long wavelength, low amplitude orientation gradients. Comparison to conventional images of decorated dislocations demonstrates that HR-EBSD can characterise the dislocation distribution and reveal additional structure not captured by the decoration technique. HR-EBSD therefore provides a highly effective method for analysing dislocations in olivine and determining their role in accommodating macroscopic deformation.

Effect of microscope parameter and specimen thickness of spatial resolution of transmission electron backscatter diffraction.

The spatial resolution of transmission electron backscatter diffraction (t-EBSD) with a standard conventional EBSD detector was evaluated quantitatively based on the calculation of the correlation coefficient of transmission patterns which were acquired across a twin boundary in the sample of austenitic steel. The results showed that the resolution of t-EBSD improved from tens of nanometres to below 10 nm with increasing accelerating voltage and thinning of specimen thickness. High voltage could enhance the penetration depth and reduce the scattering angle. And the thinning of specimen thickness would result in decreasing of the scattering events according to the theory of thermal diffuse scattering (TDS). In addition, the transmission patterns were found to be weak and noisy if the specimen was too thin, because of the decreasing intensity detected by the screen. Consequently, in this work, the best spatial resolution of 7 nm was achieved at 30 kV and 41 nm thickness. Moreover, the specimen thickness range was also discussed using Monte-Carlo simulation. This approach was helpful to account for the differences of measured spatial resolutions, by t-EBSD, of lamellas with different thickness.

A comparison of EBSD based strain indicators for the study of Fe-3Si steel subjected to cyclic loading

The current work aims at proposing an EBSD-based indicator for fatigue damage of a Fe-3Si steel. At the same time direct observation of dislocation structures is provided by electron channelling contrast imaging (ECCI). The investigation consisted in processing the EBSD data from patterns collected on specimen subjected to low cycle fatigue. It revealed two different regimes depending on the applied total strain variation which is explained by the identification of the dislocations structures and their evolution. At low strain variation, strain accommodation occurs by planar glide of dislocations uniformly distributed throughout the grains. No misorientation evolution is observed. At higher strain variation, the vein-channel structure is observed within the grain and the wall-channel structure in the vicinity of grain boundaries. The misorientation between these two dislocation structures is evaluated at about 0.7° which is detected by the EBSD analyses and explains the increase of the different misorientation based criteria. The EBSD study enables also the prediction of crack initiation mode. Finally, this study points out the limits of the EBSD technique as no misorientation evolution is detected at small strain variation. Indeed, the lattice distortion is too weak to be detected by conventional EBSD.

Analysis of Complex Steel Microstructures by High-Resolution EBSD

High-resolution electron backscattered diffraction (HR-EBSD) is a powerful tool to describe microstructures at the sub-micrometric scale that achieves a higher degree of angular accuracy compared with conventional EBSD. However, such an EBSD technique is time-consuming and requires data-intensive computing to save and postprocess the results obtained after each scan. In the current work, a simple strategy to transform conventional results into high-resolution results is put forward in an averaging statistical layout. This makes it possible to measure the misorientations more precisely and, subsequently, the geometrically necessary dislocations by lowering the typical noise generated from Hough transformation-based conventional EBSD. Different steel microstructures are analyzed in light of this strategy. The calculated dislocation densities for those microstructures are used as input values for evaluating the initial dislocation density contribution to the yield strength in a newly developed mechanical model.

Three-dimensional microstructural characterization of porous cubic zirconia

A set of cubic zirconia samples were investigated using 3-dimensional electron backscatter diffraction (3D EBSD) to analyze the grain structure, grain boundary networks and pore morphology. 3D EBSD is a variation of conventional EBSD, whereby a focused ion beam (FIB) is used in a dual beam scanning electron microscope (SEM) i.e. FIB–SEM to mill away material and to create ‘serial sections’ through the material being analyzed. Each new surface revealed is subject to an EBSD scan, which continues sequentially until a desired volume of material has been removed. In this manner, many consecutive 2D EBSD scans can be rendered in 3D to gain a greater insight of microstructural features and parameters.The three samples were examined in order to determine the effect of differences in the manufacturing process used for each. For each sample, a volume of ca. 15,000μm3 was studied. The analysis of several microstructure parameters revealed a strong dependence on manufacturing conditions. Subsequently, the results of 3D EBSD analysis were compared to conventional 2D EBSD. Significant differences between the values of microstructure parameters determined by 2D and 3D EBSD were observed.