Synergistic Thermal and Electron Wind Force-Assisted Annealing for Extremely High-Density Defect Mitigation.

This study investigates the effectiveness of combined thermal and athermal stimuli in mitigating the extremely high-density nature of dislocation networks in the form of low-angle grain boundaries in FeCrAl alloy. Electron wind force, generated from very low duty cycle and high current density pulses, was used as the athermal stimulus. The electron wind force stimulus alone was unable to remove the residual stress (80% low-angle grain boundaries) due to cold rolling to 25% thickness reduction. When the duty cycle was increased to allow average temperature of 100 °C, the specimen could be effectively annealed in 1 min at a current density of 3300 A/mm2. In comparison, conventional thermal annealing requires at least 750 °C and 1.5 h. For specimens with 50% thickness reduction (85% low-angle grain boundaries), the electron wind force was again unable to anneal the defects even at 3300 A/mm2 current density and average temperature of 100 °C. Intriguingly, allowing average concurrent temperature of 200 °C eliminated almost all the low-angle grain boundaries at a current density of 700 A/mm2, even lower than that required for the 25% thickness reduced specimens. Comprehensive electron and X-ray diffraction evidence show that alloys with extremely high defect density can be effectively annealed in less than a minute at approximately 200 °C, offering a substantial improvement over conventional high-temperature annealing.

Characterisation of the interplay between microstructure and opto-electronic properties of Cu(In,Ga)S2solar cells by using correlative CL-EBSD measurements.

Cathodoluminescence and electron backscatter diffraction have been applied to exactly the same grain boundaries (GBs) in a Cu(In,Ga)S2solar absorber in order to investigate the influence of microstructure on the radiative recombination behaviour at the GBs. Two different types of GB with different microstructure were analysed in detail: random high angle grain boundaries (RHAGBs) and Σ3 GBs. We found that the radiative recombination at all RHAGBs was inhibited to some extent, whereas at Σ3 GBs three different observations were made: unchanged, hindered, or promoted radiative recombination. These distinct behaviours may be linked to atomic-scale grain boundary structural differences. The majority of GBs also exhibited a small spectral shift of about ±10 meV relative to the local grain interior (GI) and a few of them showed spectral shifts of up to ±40 meV. Red and blue shifts were observed with roughly equal frequency.

Delineating the Ultra-Low Misorientation between the Dislocation Cellular Structures in Additively Manufactured 316L Stainless Steel.

Sub-micro dislocation cellular structures formed during rapid solidification break the strength-ductility trade-off in laser powder bed fusion (LPBF)-processed 316L stainless steel through high-density dislocations and segregated elements or precipitates at the cellular boundaries. The high-density dislocation entangled at the cellular boundary accommodates solidification strains among the cellular structures and cooling stresses through elastoplastic deformation. Columnar grains with cellular structures typically form along the direction of thermal flux. However, the ultra-low misorientations between the adjacent cellular structures and their interactions with the cellular boundary formation remain unclear. In this study, we revealed the ultra-low misorientations between the cellular structures in LPBF-processed 316L stainless steel using conventional electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD), and transmission electron microscopy (TEM). The conventional EBSD and TKD analysis results could provide misorientation angles smaller than 2°, while the resolution mainly depends on the specimen quality and scanning step size, and so on. A TEM technique with higher spatial resolution provides accurate information between adjacent dislocation cells with misorientation angles smaller than 1°. This study presents evidence that the TEM method is the better and more precise analytical method for the misorientation measurement of the cellular structures and provides insights into measuring the small misorientation angles between adjacent dislocation cells and nanograins in nanostructured metals and alloys with ultrafine-grained microstructures.

Automatic Texture Alignment by Optimization Method.

Microstructure analysis via electron backscatter diffraction has become an indispensable tool in materials science and engineering. In order to interpret or predict the anisotropy in crystalline materials, the texture is assessed, e.g. via pole figure diagrams. To ensure a correct characterization, it is crucial to align the measured sample axes as closely as possible with the manufacturing process directions. However, deviations are inevitable due to sample preparation and manual measurement setup. Postprocessing is mostly done manually, which is tedious and operator-dependent. In this work, it is shown that the deviation can be calculated using the contour of the crystal orientations. This can also be utilized to define the axis symmetry of pole figure diagrams through an objective function, allowing for symmetric alignment by minimization. Experimental textures of extruded profiles and synthetically generated textures were used to demonstrate the general applicability of the method. It has proven to work excellently for deviations of up to 5∘, which are typical for careful manual sample preparation and mounting. While the performance of the algorithm is reduced with increasing misalignment, good results have also been obtained for deviations up to 15∘.

EBSD investigation of microstructure and microtexture evolution on additively manufactured TiC-Fe based cermets-Influence of multiple laser scanning.

Sustainable TiC-Fe-based cermets have been fabricated by adopting an Additive Manufacturing route based on laser powder bed fusion technology (L-PBF). The objective is to produce crack-free cermet components by employing novel multiple laser scanning techniques with variations in laser process parameters. Electron backscatter diffraction analysis (EBSD) was used to study the microstructure and microtexture evolution with variations in laser process parameters. The investigation revealed that adjusting the preheating scan speed (PHS) and melting scan speed (MS) influenced the growth and nucleation of TiC phases. Lowering these speeds resulted in grain coarsening, while higher scan speeds led to grain refinement with larger sub-grain boundaries. Moreover, a high scanning speed increases the degree of dislocation density and internal stress in the fabricated cermet parts. Notably, it is revealed that decreasing the laser scan speed enhanced the proportion of high-angle grain boundaries in the cermet components, signifying an increase in material ductility.

The Performance of Niobium-Microalloying Ultra-High-Strength Bridge Cable Steel during Hot Rolling.

This study focuses on exploring the effects of niobium (Nb)-microalloying on the properties of steel for ultra-high-strength bridge cables during hot-rolling processes. We employed a combination of dual-pass compression tests, stress-strain curve analysis, and Electron Backscatter Diffraction (EBSD) techniques to investigate the influence of Nb-microalloying on the static recrystallization behavior and grain size of the steel. The key findings reveal that Nb-microalloying effectively inhibits static recrystallization, particularly at higher temperatures, significantly reducing the volume fraction of recrystallized grains, resulting in a finer grain size and enhanced deformation resistance. Secondly, at a deformation temperature of 975 °C, Nb-containing steel exhibited finer grain sizes compared to Nb-free steel when held for 10 to 50 s; however, the grain size growth accelerated when the hold time exceeded 50 s, likely linked to the increased deformation resistance induced by Nb. Lastly, this research proposes optimal hot-rolling process parameters for new bridge cable steel, recommending specific finishing rolling temperatures and inter-pass times for both Nb-containing and Nb-free steels during the roughing and finishing stages. This study suggests optimal hot-rolling parameters for both Nb-containing and Nb-free steels, providing essential insights for improving hot-rolling and microalloying processes in high-carbon steels for bridge cables.

Probing the Intrinsic Strain in Suspended Graphene Films Using Electron and Optical Microscopy.

Quantifying the intrinsic properties of 2D materials is of paramount importance for advancing their applications. Large-scale production of 2D materials merits the need for approaches that provide direct information about the role of growth substrate on 2D material properties. Transferring the 2D material from its growth substrates can modify the intrinsic properties of the asgrown 2D material. In this study, suspended chemical vapor deposition (CVD) graphene films are prepared directly on their growth substrates in a high-density grid array. The approach facilitates the quantification of intrinsic strain and doping in suspended CVD graphene films. To achieve this, transmission electron microscopy and large-area Raman mapping are employed. Remarkably, the analysis reveals consistent patterns of compressive strain (≈-0.2%) both in the diffraction patterns and Raman maps obtained from these suspended graphene films. By conducting investigations directly on the growth substrates, the potential influences introduced during the transfer process are circumvented effectively. Consequently, the methodology offers a robust and reliable means of studying the intrinsic properties of 2D materials in their authentic form, uninfluenced by the transfer-induced alterations that may skew the interpretation of their properties.

Weighted Burgers Vector analysis of orientation fields from high-angular resolution electron backscatter diffraction.

The Weighted Burgers Vector (WBV) method can extract information about dislocation types and densities present in distorted crystalline materials from electron backscatter diffraction (EBSD) maps, using no assumptions about which slip systems might be present. Furthermore, high-angular resolution EBSD (HR-EBSD) uses a cross-correlation procedure to increase the angular precision of EBSD measurements by an order of magnitude compared to conventional EBSD. However, the WBV technique has not previously been applied to HR-EBSD data and therefore it remains unclear as to which low-angle substructures can be reliably characterised by WBV analysis of conventional EBSD data and which require additional HR-EBSD processing. To establish some practical examples that can be used to guide future data-acquisition strategies, we compare the output of the WBV method when applied to conventional EBSD data and HR-EBSD data collected from the most common minerals in Earth's lower crust (plagioclase feldspar) and upper mantle (olivine). The results demonstrate that HR-EBSD and WBV processing are complementary techniques. The increase in angular precision achieved with HR-EBSD processing allows low-angle (on the order of 0.1°) structures, which are obscured by noise in conventional EBSD data, to be analyzed quantitatively using the WBV method. Combining the WBV and HR-EBSD methods increases the precision of calculated WBV directions, which is essential when using information about active slip systems to infer likely deformation mechanisms from naturally deformed microstructures. This increase in precision is particularly important for low-symmetry crystals, such as plagioclase, that have a wide range of available slip systems that vary in relative activity with changing pressure, temperature and differential stress. Because WBV directions are calculated using no assumptions about which slip systems may be present, combining this technique with HR-EBSD to refine the precision of lattice orientation gradients is ideal for investigating complex natural materials with unknown deformation histories.

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.

Microstructure of the Advanced Titanium Alloy VT8M-1 Subjected to Rotary Swaging.

In this study, the microstructural behavior of the advanced Ti-5.7Al-3.8Mo-1.2Zr-1.3Sn-0.15Si (VT8M-1) alloy during rotary swaging (RS) was investigated. VT8M-1 has increased heat resistance and is considered a replacement for the Ti-6Al-4V alloy. It was shown that, during RS, the evolution of the primary a phase is characterized by the formation of predominantly low-angle boundaries according to the mechanism of continuous dynamic recrystallization. The density of low-angle boundaries increases three times: from 0.38 µm-1 to 1.21 µm-1 after RS. The process of spheroidization of the lamellar (a + b) component is incomplete. The average size of globular a and b particles was 0.3 µm (TEM). It is shown that the microstructures after RS (ε = 1.56) and equal-channel angular pressing (ECAP) (ε = 1.4) are significantly different. The temperature-velocity regime and the predominance of shear deformations during ECAP contributed to a noticeable refinement of the primary a-phase and a more complete development of globularization of the lamellar (a+b) component. EBSD studies have shown that RS leads to the formation of a structure with a higher density of low- and high-angle boundaries compared to the structure after ECAP. The results are useful for predicting alloy microstructure in the production of long rods that are further used in forging operations.

Statistical Comparison of Substructures in Pure Aluminum Before and After Creep Deformation, Based on EBSD Image Data.

Electron backscatter diffraction (EBSD) images of extruded pure aluminum were statistically analyzed to investigate creep-induced subgrain structures after applying two different levels of creep stress, corresponding to the power law (PL) and power-law breakdown (PLB) regimes. Kernel average misorientation analysis of EBSD measurements revealed 2D morphologies, which were subdivided by a multi-step segmentation procedure into subgranular arrangements. Various descriptors were employed to characterize the "subgrains" quantitatively, including their size, shape, spatial arrangement, and crystallographic orientation. In particular, the analysis of the orientations of subgrains was conducted by neglecting rotations around the loading axis. This approach facilitated the individual investigation of the {001} and {111} subgrain families with respect to the loading axis for two investigated stress levels plus a reference specimen. For the PL regime, the statistical analysis of subgrain descriptors computed from segmented image data revealed a similar degree of strain accumulation for {111} and {001} subgrains. In contrast, for the PLB regime, the analyzed descriptors indicate that {111} subgrains tend to accumulate significantly more strain than {001} ones. These observations suggest that the mechanisms leading to PLB may be associated with strain localization dependent on intergranular stress, hindering the recovery process within {111} grains.

Structural modification of WC-Co cutting tools by laser doping treatment.

We have previously shown that sharpening the cutting edge of a cemented carbide tool by chemical-mechanical polishing (CMP) improved the cutting speed by approximately 150% and reduced the wear on the flank surface by approximately 50% compared to a commercial tool when cutting a heat-resistant alloy. In addition, the cutting edges of carbide tools treated by laser doping (LD) using boron nitride as doping material achieved approximately 100 times longer cutting distance in glass machining than the edges of carbide tools treated with CMP grinding wheels. In this study, LD was conducted on a tool base material (WC-Co) to investigate and understand the crystal structure changes of the base material upon treatment. Electron backscatter diffraction and X-ray diffraction results show that the effect of LD was observed in the region 50 nm below the surface. LD improved the strength by approximately 11.7% without destroying the surface crystal structure. Thus, doping can be performed on tool tips while maintaining the WC structure to improve the performance of WC-Co cutting tools.

Grain Structure Evolution in 6013 Aluminum Alloy during High Heat-Input Friction-Stir Welding.

This work was undertaken to evaluate the influence of friction-stir welding (FSW) under a high-heat input condition on microstructural evolution. Given the extreme combination of deformation conditions associated with such an FSW regime (including the highest strain, temperature, and strain rate), it was expected to result in an unusual structural response. For this investigation, a commercial 6013 aluminum alloy was used as a program material, and FSW was conducted at a relatively high spindle rate of 1100 rpm and an extremely low feed rate of 13 mm/min; moreover, a Ti-6Al-4V backing plate was employed to reduce heat loss during welding. It was found that the high-heat-input FSW resulted in the formation of a pronounced fine-grained layer at the upper weld surface. This observation was attributed to the stirring action exerted by the shoulder of the FSW tool. Another important issue was the retardation of continuous recrystallization. This interesting phenomenon was explained in terms of a competition between recrystallization and recovery at high temperatures. Specifically, the activation of recovery should reduce dislocation density and thus retard the development of deformation-induced boundaries.

Transmission Kikuchi Diffraction Mapping Induces Structural Damage in Atom Probe Specimens.

Measuring local chemistry of specific crystallographic features by atom probe tomography (APT) is facilitated by using transmission Kikuchi diffraction (TKD) to help position them sufficiently close to the apex of the needle-shaped specimen. However, possible structural damage associated to the energetic electrons used to perform TKD is rarely considered and is hence not well-understood. Here, in two case studies, we evidence damage in APT specimens from TKD mapping. First, we analyze a solid solution, metastable ß-Ti-12Mo alloy, in which the Mo is expected to be homogenously distributed. Following TKD, APT reveals a planar segregation of Mo among other elements. Second, specimens were prepared near Σ3 twin boundaries in a high manganese twinning-induced plasticity steel, and subsequently charged with deuterium gas. Beyond a similar planar segregation, voids containing a high concentration of deuterium, i.e., bubbles, are detected in the specimen on which TKD was performed. Both examples showcase damage from TKD mapping leading to artefacts in the distribution of solutes. We propose that the structural damage is created by surface species, including H and C, subjected to recoil from incoming energetic electrons during mapping, thereby getting implanted and causing cascades of structural damage in the sample.

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.

On the Degree of Plastic Strain during Laser Shock Peening of Ti-6Al-4V.

Laser shock peening (LSP) is an innovative technique that is used to enhance the fatigue strength of structural materials via the generation of significant residual stress. The present work was undertaken to evaluate the degree of plastic strain introduced during LSP and thus improve the fundamental understanding of the LSP process. To this end, electron backscatter diffraction (EBSD) and nano-hardness measurements were performed to examine the microstructural response of laser-shock-peened Ti-6Al-4V alloy. Only minor changes in both the shape of α grains/particles and hardness were found. Accordingly, it was concluded that the laser-shock-peened material only experienced a small plastic strain. This surprising result was attributed to a relatively high rate of strain hardening of Ti-6Al-4V during LSP.

A Denoising Autoencoder for Improved Kikuchi Pattern Quality and Indexing in Electron Backscatter Diffraction.

The rapid collection and indexing of electron diffraction patterns as produced via electron backscatter diffraction (EBSD) has enabled crystallographic orientation and structural determination, as well as additional property-determining strain and dislocation density information with increasing speed, resolution, and efficiency. Pattern indexing quality is reliant on the noise of the collected electron diffraction patterns, which is often convoluted by sample preparation and data collection parameters. EBSD acquisition is sensitive to many factors and thus can result in low confidence index (CI), poor image quality (IQ), and improper minimization of fit, which can result in noisy datasets and misrepresent the microstructure. In an attempt to enable both higher speed EBSD data collection and enable greater orientation fit accuracy with noisy datasets, an image denoising autoencoder was implemented to improve pattern quality. We show that EBSD data processed through the autoencoder results in a higher CI, IQ, and a more accurate degree of fit. In addition, using denoised datasets in HR-EBSD cross correlative strain analysis can result in reduced phantom strain from erroneous calculations due to the increased indexing accuracy and improved correspondence between collected and simulated patterns.

Tool for automatic macrozone characterization from EBSD data sets of titanium alloys.

Microtexture heterogeneities are commonly found in titanium forgings because of the thermomechanical processing. Also known as macrozones, these regions can reach millimetres in length, with grains sharing a similar crystallographic orientation leading to less resistance to crack propagation. Since the link between macrozones and the reduction of cold-dwell-fatigue performance on rotative components in gas turbine engines was established, efforts have been put into macrozone definition and characterization. The electron backscatter diffraction (EBSD) technique, widely used for texture analysis, allows for a qualitative macrozone characterization; however, further processing is required to define the boundaries and disorientation spread of each macrozone. Current approaches often use c-axis misorientation criteria, but this can sometimes lead to a large disorientation spread within a macrozone. This article describes the development and application of a computational tool implemented in MATLAB for automatic macrozone identification from EBSD data sets on the basis of a more conservative approach where both the c-axis tilting and rotation are considered. The tool allows for detection of macrozones according to the disorientation angle and density-fraction criteria. The clustering efficiency is validated by pole-figure plots, and the effects of the key parameters defining the macrozone clustering (disorientation and fraction) are discussed. In addition, this tool was successfully applied to both fully equiaxed and bimodal microstructures of titanium forgings.

A new method for calculating interplanar spacing to distinguish between similar phases in EBSD.

Commercial electron backscatter diffraction (EBSD) systems generally use interplanar angle matching for pattern indexing, and thus, they are unable to distinguish between some similar phases with close interplanar angles, such as Al and Si. The interplanar spacing is more diagnostic but generally difficult to apply in pattern indexing because it lacks precision. In this study, we proposed an efficient approach for accurately measuring interplanar spacing by correcting the reciprocal-lattice vector (RLV). The phase discrimination of Al and Si was performed by interplanar spacing matching. The Kikuchi bands were identified automatically by the self-developed method using pattern rotation combined with grey gradient recognition without the help of human eyes. The reliable RLV relationship was extracted by accurately drawing reciprocal-lattice vectors. The lengths of RLVs were corrected, and then the RLVs were used for evaluating lattice spacing. The results of five Kikuchi patterns with different clarity showed that this new method reduced the average error of interplanar spacings by 50.611% and achieved an average accuracy of 1.644% for lattice spacing calculation. The method could distinguish structures with a difference in lattice spacing of at least 3.3%. This method was also effective for fuzzy patterns and partially missing Kikuchi bands and might be used as a new strategy for improving the calculation accuracy of lattice spacing for fuzzy patterns. The method did not have additional requirements concerning the number of detected Kikuchi bands and poles. The accuracy of lattice spacing could be effectively improved by correcting the RLVs based on routine pattern recognition. This method might be used as an auxiliary approach to differentiate between similar phases and is well-adapted to the existing commercial EBSD system.

Use of electron backscatter diffraction patterns to determine the crystal lattice. Part 1. Where is the Bragg angle?

The derivation of a crystal structure and its phase-specific parameters from a single wide-angle backscattered Kikuchi diffraction pattern requires reliable extraction of the Bragg angles. By means of the first derivative of the lattice profile, an attempt is made to determine fully automatically and reproducibly the band widths in simulated Kikuchi patterns. Even under such ideal conditions (projection centre, wavelength and lattice plane traces are perfectly known), this leads to a lattice parameter distribution whose mean shows a linear offset that correlates with the mean atomic number Z of the pattern-forming phase. The consideration of as many Kikuchi bands as possible reduces the errors that typically occur if only a single band is analysed. On the other hand, the width of the resulting distribution is such that higher image resolution of diffraction patterns, employing longer wavelengths to produce wider bands or the use of higher interference orders is less advantageous than commonly assumed.