3D Electron Backscatter Diffraction Research Articles

Quantification of melt pool dynamics and microstructure during simulated additive manufacturing

The dynamics of laser spot melting and metallic alloy solidification are investigated through synchrotron x-ray radiography, 3D electron backscatter diffraction data, and computational fluid dynamics (CFD) simulations on a model NiMoAl alloy. Solidification velocities are measured from in-situ images to validate a CFD model of the spot melt. TriBeam tomography is used to reconstruct the melt pool in 3D, characterize the microstructure, and validate the CFD model melt pool dimensions. 3D geometrically necessary dislocation (GND) density calculations reveal extensive deformation at the sample surface. GND calculations integrated with the CFD model reveal that the halo of small, equiaxed grains adjacent to the fusion line is a result of recrystallization that occurred in the heat affected zone. By combining in-situ and ex-situ data modalities across a variety of temporal and spatial length scales, the microstructure formation mechanisms and their relationship to melt pool solidification are quantified.

Q-RBSA: high-resolution 3D EBSD map generation using an efficient quaternion transformer network

Gathering 3D material microstructural information is time-consuming, expensive, and energy-intensive. Acquisition of 3D data has been accelerated by developments in serial sectioning instrument capabilities; however, for crystallographic information, the electron backscatter diffraction (EBSD) imaging modality remains rate limiting. We propose a physics-based efficient deep learning framework to reduce the time and cost of collecting 3D EBSD maps. Our framework uses a quaternion residual block self-attention network (QRBSA) to generate high-resolution 3D EBSD maps from sparsely sectioned EBSD maps. In QRBSA, quaternion-valued convolution effectively learns local relations in orientation space, while self-attention in the quaternion domain captures long-range correlations. We apply our framework to 3D data collected from commercially relevant titanium alloys, showing both qualitatively and quantitatively that our method can predict missing samples (EBSD information between sparsely sectioned mapping points) as compared to high-resolution ground truth 3D EBSD maps.

Comparison of Laboratory Diffraction Contrast Tomography and Electron Backscatter Diffraction Results: Application to Naturally Occurring Chromites.

Understanding how minerals are spatially distributed within natural materials and their textures is indispensable to understanding the fundamental processes of how these materials form and how they will behave from a mining engineering perspective. In the past few years, laboratory diffraction contrast tomography (LabDCT) has emerged as a nondestructive technique for 3D mapping of crystallographic orientations in polycrystalline samples. In this study, we demonstrate the application of LabDCT on both chromite sand and a complex chromitite sample from the Merensky Reef (Bushveld Complex, South Africa). Both samples were scanned using LabDCT and Electron Backscatter Diffraction (EBSD), and the obtained results were rigorously evaluated using a comprehensive set of qualitative and quantitative characterization techniques. The quality of LabDCT results was accessed by using the "completeness" value, while the inaccuracies were thoroughly discussed, along with proposed potential solutions. The results indicate that the grain orientations obtained from LabDCT are comparable to that of 2D EBSD but have the advantage of collecting true 3D size, shape, and textural information. This study highlights the significant contribution of LabDCT in the understanding of complex rock materials from an earth science perspective, particularly in characterizing mineral texture and crystallography in 3D.

A large-volume 3D EBSD study on additively manufactured 316L stainless steel

Most microstructure characterization tools are based on the observation of two-dimensional (2D) sections through a material. The results allow, for microstructures with homogeneous and often equiaxed grain shapes, a sufficiently good interpretation of microstructure formation and materials properties. Such microstructures are typically recrystallized grains or homogeneously deformed grains in plastically deformed materials. In contrast, metal parts fabricated by additive manufacturing (AM) exhibit microstructures that differ significantly from classical materials as they frequently develop very heterogeneous and often nonregular grain structures. Orientation microscopy-based electron backscatter diffraction (EBSD) is probably the most quantitative materialographic technique for 2D microstructure characterization. By combination with serial sectioning the technique is extended to a powerful 3D characterization technique. Here we use a large-volume 3D EBSD system (ELAVO 3D) to analyze an AM-built 316L stainless steel sample. The results show unique “tree-like” grain morphologies and related textures. We highlight issues invisible when using 2D EBSD.

Registration between DCT and EBSD datasets for multiphase microstructures

The ability to characterise the three-dimensional microstructure of multiphase materials is essential for understanding the interaction between phases and their associated materials properties. Here, laboratory-based diffraction-contrast tomography (lab-based DCT), a recently-established materials characterization technique that can determine grain phases, morphologies, positions and orientations in a voxel-based reconstruction method, was used to map part of a dual-phase steel alloy sample. To assess the resulting microstructures produced by the lab-based DCT technique, an electron backscatter diffraction (EBSD) map was collected within the same sample volume. To identify the two-dimensional (2D) slice of the three-dimensional (3D) lab-based DCT reconstruction that best corresponded to the 2D EBSD map, a novel registration technique based solely on grain-averaged orientations was developed – this registration technique requires very little a priori knowledge of dataset alignment and can be extended to other techniques that only recover grain-averaged orientation data such as far-field 3D X-ray diffraction microscopy. Once the corresponding 2D slice was identified in the lab-based DCT dataset, comparisons of phase balance, grain size, shape and texture were performed between lab-based DCT and EBSD techniques. More complicated aspects of the microstructural morphology such as grain boundary shape and grains less than a critical size were poorly reproduced by the lab-based DCT reconstruction, primarily due to the difference in resolutions of the technique compared with EBSD. However, lab-based DCT is shown to accurately determine the centre-of-mass position, orientation, and size of the large grains for each phase present, austenite and martensitic ferrite. The results reveals a complex ferrite grain network of similar crystal orientations that are absent from the EBSD dataset. Such detail demonstrates that lab-based DCT, as a technique, shows great promise in the field of multi-phase material characterization.

Three-dimensional study of grain scale tensile twinning activity in magnesium: A combination of microstructure characterization and mechanical modeling

Tensile twinning is a main deformation mode in hexagonal close packed structure metals, so it is important to comprehensively understand twinning mechanisms which are not fully disclosed using 2D or small volume 3D characterization techniques. A large area 3D electron backscatter diffraction (EBSD) measurement and crystal plasticity modeling were carried out to investigate the tensile twinning behaviors in a magnesium (Mg) alloy. The results showed that tensile twinning activity was underestimated using conventional 2D EBSD scans. When compressed to yield point, the examined twin frequency with 2D was lower than that using 3D EBSD. The effects of Schmid factor (SF) on twinning were investigated. Almost all high Schmid factor (SF>0.35) grains were twinned. A surprising high twin frequency of 82% in middle SF (0.35>=SF>=0.15) grains was observed, which was unexpected since the middle SF grains were believed to be unfavorable for twinning. The twin frequency in low SF (SF<0.15) grains was slightly increased from 2D to 3D EBSD due to the small volume of twins. The shear stress maintained a high level and was homogeneously distributed in high SF grains, facilitating twin nucleation and growth. The shear stress was distributed heterogeneously within the middle SF grains, and twins were nucleated within areas with positive shear stress. The shear stress in low SF grains was not favorable for twinning and twins occurred in the vicinity of stress accumulation. Twinning activities in the same grain varied on different layers. It was attributed to the stress fluctuation derived from grain environment changes.

Slip localization behavior at triple junctions in nickel-base superalloys

Incipient slip localization in the vicinity of hundreds of grain boundary triple junctions (TJs) in a lightly deformed nickel-base superalloy IN718 is studied using a combination of three-dimensional (3D) crystal plasticity finite element (CPFE) modeling, high resolution digital image correlation (HR-DIC) and 3D electron back-scatter diffraction tomography (3D EBSD). A 3D reconstruction method enables identification of thousands of TJs and correspondence of any observed slip bands with their originating TJ lines below the specimen surface. We present a large-scale CPFE model of the experimental 3D microstructure composed of high-fidelity representation of the TJ lines and the boundaries and interiors of the parent grains and use it to calculate the local micromechanical response and slip activity of all TJs at the onset of macroscopic yielding. Statistical analysis of the calculated quantities reveal TJs develop larger stress concentration and grain-average re-orientation than grain interiors and grain boundaries, however no substantial differences in cumulative slip were found among these microstructural regions. We find that TJs with observed slip bands generate lower grain-average re-orientation, fewer active slip systems, and more localized slip on a single system than those without. The distinctions in the reorientation and slip activity are stronger in TJs that experience more intense slip.

Development of a new, fully automated system for electron backscatter diffraction (EBSD)-based large volume three-dimensional microstructure mapping using serial sectioning by mechanical polishing, and its application to the analysis of special boundaries in 316L stainless steel.

We report the development of a fully automatic large-volume 3D electron backscatter diffraction (EBSD) system (ELAVO 3D), consisting of a scanning electron microscope (ZEISS crossbeam XB 1540) with a dedicated sample holder, an adapted polishing automaton (Saphir X-change, QATM), a collaborative robotic arm (Universal Robots UR5), and several in-house built devices. The whole system is orchestrated by an in-house designed software, which is also able to track the process and report errors. Except for the case of error, the system runs without any user interference. For the measurement of removal thickness, the samples are featured with markers put on the perpendicular lateral surface, cut by plasma focused ion beam (PFIB) milling. The individual effects of both 1μm diamond suspension and oxide polishing suspension polishing were studied in detail. Coherent twin grain boundaries (GBs) were used as an internal standard to check the removal rates measured by the side markers. The two methods for Z-spacing measurements disagreed by about 10%, and the inaccurate calibration of the PFIB system was found to be the most probable reason for this discrepancy. The angular accuracy of the system was determined to be ∼2.5°, which can be significantly improved with more accurate Z-spacing measurements. When reconstructed grain boundary meshes are sufficiently smoothed, an angular resolution of ±4° is achieved. In a 3D EBSD dataset of a size of 587 × 476 × 72μm3, we focused on the investigation of coincidence site lattice ∑9 GBs. While bearing predominantly a pure tilt character, ∑9 GBs can be categorized into three groups based on correlative 3D morphologies and crystallography.

Large Volume 3D Electron Backscatter Diffraction Characterization of Lath Martensite in 13%Cr-4%Ni Stainless Steel by Xe Plasma FIB

Large Volume 3D Electron Backscatter Diffraction Characterization of Lath Martensite in 13%Cr-4%Ni Stainless Steel by Xe Plasma FIB

Elimination of the charge effect on zirconia coatings using a focused ion beam for characterization of the three-dimensional microstructure

How to improve the acquisition efficiency of electron back scatter diffraction (EBSD) and meanwhile avoid the drifting of images caused by the charge effect is still an issue that needs to be solved urgently for three-dimensional (3D) EBSD characterization of non-conductive ceramic materials. The purpose of this study is to solve the problem based on an experimental and theoretical basis using the yttria-stabilized zirconia (YSZ) coating as the research subject. To eliminate the charge effect, the charge equilibrium point of the YSZ coating was determined using the Duane–Hunt limit method. When the incident energy is equal to the E2 value (3.06 keV), the charge effect on the scanning electron microscopy images of the YSZ coating disappears. However, the incident energy of 3.06 keV exhibits lower EBSD collection efficiency for a 3D EBSD result because of the long exposure time; therefore, increasing the incident energy to 7 keV under the large beam current is a practical way to improve EBSD collection efficiency, which inevitably results in the charge problem again. Furthermore, Ga ion implantation caused the charge effect on the YSZ coating at 7 keV under the large beam current to disappear, which can be attributed to the decrease in bandgap widths and the improvement of electrical conductivity of the t-YSZ phase after Ga ion implantation based on first principles. Calculations of the cohesive energies reveal that Ga ions are more likely to occupy the lattice positions of the YSZ coating after Ga ion implantation. Finally, a high-resolution 3D EBSD image of the YSZ coating was obtained.

3D electron backscatter diffraction characterization of fine α titanium microstructures: collection, reconstruction, and analysis methods

3D electron backscatter diffraction (3D-EBSD) is a method of obtaining 3-dimensional crystallographic data through serial sectioning. The recent advancement of using a Xe+ plasma focused ion beam for sectioning along with a complementary metal-oxide semiconductor based EBSD detector allows for an improvement in the trade-off between volume analyzed and spatial resolution over most other 3D characterization techniques. Recent publications from our team have focused on applying 3D-EBSD to understand microstructural phenomena in Ti-6Al-4V microstructures as a function of electron beam scanning strategies in electron beam powder bed fusion additive manufacturing. The microstructures resulting from this process have fine features, with α laths as small as 1 μm interwoven in a highly complex fashion, presenting a significant challenge to characterize. Over the course of these fundamental works, we have developed best-practice 3D-EBSD collection protocols and advanced methods for 3D data reconstruction and analysis of such microstructures which remain unpublished. These methods may be of interest to the 3D materials characterization community, especially considering the lack of standard commercial software tools. Thus, the current paper elaborates on the methods and analysis used to characterize fine titanium microstructures using 3D-EBSD and presents a detailed description of the new algorithms developed for probing the unique features therein. The new analyses include algorithms for identifying intervariant boundary types, classifying three-variant clusters, assigning grains to variants, and quantifying interconnectivity of branched α platelets.

Formation and 3D morphology of interconnected α microstructures in additively manufactured Ti-6Al-4V

Three-dimensional characterization methods, such as 3D electron backscatter diffraction (3D-EBSD), have been used to reveal phase transformation and microstructural evolution mechanisms in multi-phase materials such as steel or titanium alloys. While 3D techniques have enabled many findings in steels, fine dual phase microstructures in titanium alloys such as the basketweave structure have been challenging to resolve. Now, advances in 3D-EBSD methods using sectioning with a plasma focused ion beam have allowed in-depth analyses of fine α microstructures. We apply 3D-EBSD to investigate the microstructures formed in Ti-6Al-4V by electron powder bed fusion (E-PBF) using different scanning strategies. Basketweave, acicular, and colony microstructures are produced from linear, Dehoff, and random scanning strategies, respectively. Different types of 3D interconnectivity were revealed in each microstructure including within clusters of platelets in the basketweave microstructure, within a grain boundary allotriomorph in the acicular microstructure, and between platelets in colonies. These observations are discussed in terms of the formation mechanisms of interconnectivity, including sympathetic nucleation, impingement, and morphological instability. Morphological instability was found to potentially play a role in both the basketweave and colony structures while the interconnectivity in the acicular structure likely forms via sympathetic nucleation or impingement. This information allows for a more complete description of the phase evolution of Ti-6Al-4V during thermal cycling in E-PBF than previously available and represents new insights into the complex branching reported in different titanium microstructures.

Slip localization in Inconel 718: A three-dimensional and statistical perspective

The slip localization behavior of the polycrystalline nickel base superalloy Inconel 718 during monotonic tensile loading at room temperature, is investigated for the first time in relation to the 3D microstructure. Multi-modal data merging tools are used to recombine high resolution digital image correlation (HR-DIC) data with 3D electron back-scatter diffraction tomography (3D EBSD), over a wide region of interest. This procedure enables reconstruction of the slip band planes in the 3D microstructure. Statistical analyses conducted over 500 individual slip bands reveal strong correlations between their location and specific microstructure configurations. In particular, over half of the slip bands emanate from triple junction lines (3D lines defined by the junction of three crystals). Moreover, the most intense and longest slip bands, which would become critical fatigue crack nucleation sites during cyclic loading, are located close and parallel to particular annealing twin boundaries and are simultaneously connected to triple junction lines. Crystal plasticity finite elements calculations are performed on the experimental microstructure to identify the slip activity that results in the formation of high intensity slip bands (localized plasticity) or zones of high lattice rotation (non-localized plasticity) in these particular microstructure regions.

A multivariate grain size and orientation distribution function: derivation from electron backscatter diffraction data and applications.

Two of the microstructural parameters most influential in the properties of polycrystalline materials are grain size and crystallographic texture. Although both properties have been extensively studied and there are a wide range of analysis tools available, they are generally considered independently, without taking into account the possible correlations between them. However, there are reasons to assume that grain size and orientation are correlated microstructural state variables, as they are the result of single microstructural formation mechanisms occurring during material processing. In this work, the grain size distribution and orientation distribution functions are combined in a single multivariate grain size orientation distribution function (GSODF). In addition to the derivation of the function, several examples of practical applications to low carbon steels are presented, in which it is shown how the GSODF can be used in the analysis of 2D and 3D electron backscatter diffraction data, as well as in the generation of representative volume elements for full-field models and as input in simulations using mean-field methods.

Co-deformation and dynamic annealing effects on the texture development during alpha–beta processing of a model Zr-Nb alloy

The in-service properties and performance of dual-phase Zr and Ti alloys depend on their crystallographic texture, which develops during hot-working and is affected by interactions between the α and β phases during deformation, annealing and phase transformation. Recent work on hot-rolled Zr-2.5Nb has shown that the texture of the two phases are related, with coupled strengthening of the α near {112¯0}〈101¯0〉, which produces strong 0002 pole intensities along the transverse direction, and β with {001}〈110〉 rotated cube, particularly when the relative volume fraction is around 50:50. To investigate the origin of this texture coupling, we studied a hot-rolled model Zr alloy with 7 wt.% Nb, in which the as-deformed α + β microstructure is preserved on cooling. The alloy was hot-rolled to different reductions at 725∘C, which corresponds to a relative α:β volume fraction of 30:70, where the characteristic textures are known to develop quickly at first and then weaken with further reduction. The rolled material was characterised using both 2D and 3D electron backscatter diffraction (EBSD). This analysis uncovered evidence that both recrystallization and phase transformation cause the disappearance of specific α variants during rolling, favouring the formation of “soft” primary α grains flattened in 〈112¯0〉 and elongated along 〈101¯0〉 during rolling, which in turn has an effect on surrounding β orientations, promoting the stronger rotated cube component. At higher reductions, these elongated α-grains start to break up, as does the β surrounding it, forming bands of characteristic coupled textures. These observations imply that non-plasticity effects should be included in models of texture evolution during processing of α + β Zr and Ti alloys.

Wire-arc additive manufacturing of nickel aluminum bronze/stainless steel hybrid parts – Interfacial characterization, prospects, and problems

Hybrid parts of nickel aluminum bronze (NAB) and 316L stainless steel were fabricated using a commercially available wire-arc additive manufacturing (WAAM) technology to evaluate the feasibility and cracking tendency. Focused Ion beam (FIB) based Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Electron Backscatter Diffraction (EBSD), and Transmission Electron Microscopy (TEM) were used to characterize the built (NAB)-substrate (SS) interfacial characteristics. FIB extracted a selected region of the interface, and the spatial distribution of the interface across several sections was characterized by using the state-of-the-art technique for 3D EBSD mapping. A metallurgically bonded interface without any pores and cracks, with the inter-diffusion region in a thickness of 2 μm, was formed, which was further confirmed by a video with the results of 3D reconstructed EBSD maps. The interface did not exhibit any strong texture orientation owing to the control of the thermal gradient as NAB is more conductive than 316L. EDS elemental mapping confirmed that Fe3Al intermetallic was formed at the NAB/SS bimetallic-joint interface. Occasional liquation cracks on the grain boundaries in the heat-affected zone (HAZ) of 316L substrate were observed. Fe-Al based intermetallic formation, along with the penetration of copper along the HAZ cracks, was noticed. The problems associated were highlighted, and remedial measures were suggested to open up the possibilities of additive manufacturing to fabricate NAB-Stainless steel hybrid parts for industrial repair and maintenance applications.

3D electron backscatter diffraction study of α lath morphology in additively manufactured Ti-6Al-4V

Titanium alloys exhibit complex, multi-phase microstructures which form during liquid-solid and solid-solid phase transformations. These phase transformations govern the microstructural evolution and are potentially more complex during additive manufacturing due to large thermal gradients and inhomogeneities. The prototypical fundamental unit of titanium microstructures are the α laths, and investigations into their three-dimensional morphology may provide new insights. A prior β-grain boundary, 3-variant clusters and interconnected laths were studied in 3D in electron-beam printed Ti-6Al-4V using a plasma FIB. These key features are of interest for studying variant selection in additive manufacturing.

Modeling the Local Deformation and Transformation Behavior of Cast X8CrMnNi16-6-6 TRIP Steel and 10% Mg-PSZ Composite Using a Continuum Mechanics-Based Crystal Plasticity Model

A Transformation-Induced Plasticity (TRIP) steel matrix reinforced with magnesium-partially stabilized zirconia (Mg-PSZ) particles depicts a superior energy absorbing capacity during deformation. In this research, the TRIP/TWIP material model already developed in the framework of the Düsseldorf Advanced Material Simulation Kit (DAMASK) is tuned for X8CrMnNi16-6-6 TRIP steel and 10% Mg-PSZ composite. A new method is explained to more accurately tune this material model by comparing the stress/strain, transformation, twinning, and dislocation glide obtained from simulations with respective experimental acoustic emission measurements. The optimized model with slight modification is assigned to the steel matrix in 10% Mg-PSZ composite material. In the simulation model, zirconia particles are assigned elastic properties with a perfect ceramic/matrix interface. Local deformation, transformation, and the twinning behavior of the steel matrix due to quasi-static tensile load were analyzed. The comparison of the simulation results with acoustic emission data shows good correlation and helps correlate acoustic events with physical attributes. The tuned material models are used to run full phase simulations using 2D Electron Backscatter Diffraction (EBSD) data from steel and 10% Mg-PSZ zirconia composites. Form these simulations, dislocation glide, martensitic transformation, stress evolution, and dislocation pinning in different stages of deformation are qualitatively discussed for the steel matrix and ceramic inclusions.

3D Crystallographic Analysis of Grain Boundaries of Austenite Transformed from Ferrite on Heating in Fe-Mn-C Alloy

To control the ferrite and martensite microstructure in dual-phase steels, the phase transformation from ferrite (α) to austenite (γ) on heating is essential. For a comprehensive understanding of the γ formation in the early stage of the α → γ phase transformation, the orientation relationships and the boundaries between α and γ grains were investigated using a 3D electron backscatter diffraction analysis. It was found that, when the orientations of the α and γ grains hold the K–S relationship or the N–W relationship, the orientation of the boundary does not correspond to the exact close-packed planes {011}α or {111}γ, but deviates within the vicinity of these crystal planes based on a variety of misorientations of up to 40 deg. Most of the boundary planes with the K–S relationship are parallel to the crystal planes between {011}α and {111}α of the α grain, and {111}γ and {012}γ of the γ grain. Focusing on the crystallographic feature of the boundary planes in austenite phase, the mechanism of the α → γ transformation is discussed.

Estimation of geometrically necessary dislocation density from filtered EBSD data by a local linear adaptation of smoothing splines

An implementation of smoothing splines is proposed to reduce orientation noise in electron backscatter diffraction (EBSD) data, and subsequently estimate more accurate geometrically necessary dislocation (GND) densities. The local linear adaptation of smoothing splines (LLASS) filter has two advantages over classical implementations of smoothing splines: (1) it allows for an intuitive calibration of the fitting versus smoothing trade-off and (2) it can be applied directly and in the same manner to both square and hexagonal grids, and to 2D as well as to 3D EBSD data sets. Furthermore, the LLASS filter calculates the filtered orientation gradient, which is actually at the core of the method and which is subsequently used to calculate the GND density. The LLASS filter is applied on a simulated low-misorientation-angle boundary corrupted by artificial orientation noise (on a square grid), and on experimental EBSD data of a compressed Ni-base superalloy (acquired on a square grid) and of a dual austenitic/martensitic steel (acquired on an hexagonal grid). The LLASS filter leads to lower GND density values as compared to raw EBSD data sets, as a result of orientation noise being reduced, while preserving true GND structures. In addition, the results are compared with those of filters available in theMTEXtoolbox.