Transmission Electron Microscopy Sample Research Articles

Microstructure Evolution and Fretting Wear Mechanisms of Steels Undergoing Oscillatory Sliding Contact in Dry Atmosphere.

Variations in the microstructure and the dominant fretting wear mechanisms of carbon steel alloy in oscillatory sliding contact against stainless steel in a dry atmosphere were evaluated by various mechanical testing and microanalytical methods. These included scanning electron microscopy and energy dispersive spectrometry with corresponding elemental maps of the wear tracks, in conjunction with cross-sectional transmission electron microscopy of samples prepared by focused ion beam machining to assess subsurface and through-thickness changes in microstructure, all as a function of applied load and sliding time. Heavily dislocated layered microstructures were observed below the wear tracks to vary with both the load and sliding time. During the accumulation of fretting cycles, the subsurface microstructure evolved into stable dislocation cells with cell walls aligned parallel to the surface and the sliding direction. The thickness of the damaged subsurface region increased with the load, consistent with the depth distribution of the maximum shear stress. The primary surface oxide evolved as Fe2O3 and Fe3O4 with increasing sliding time, leading to the formation of a uniform oxide scale at the sliding surface. It is possible that the development of the dislocation cell structure in the subsurface also enhanced oxidation by pipe diffusion along dislocation cores. The results of this study reveal complex phase changes affecting the wear resistance of steels undergoing fretting wear, which involve a synergy between oxidative wear, crack initiation, and crack growth along dislocation cell walls due to the high strains accumulating under high loads and/or prolonged surface sliding.

Size-dependent amorphization of cementite lamellae in a tribolayer

The deformation and structural evolution characteristics of cementite in tribolayers significantly impact the friction and wear properties of pearlitic steels. However, the mechanism and size dependency of cementite amorphization remain unclear. Herein, the focused ion beam (FIB) lift-out technique was used to prepare transmission electron microscopy (TEM) samples of tribolayers formed during dry sliding of pearlitic steel. The microstructure of and elemental distributions in the tribolayers were characterized. Furthermore, molecular dynamics (MD) simulations were employed to analyze the behavior of dislocation slip systems in ferrite that extended toward cementite and induced amorphization. The results indicated that cementite amorphization occurred prior to dissolution within the tribolayer. Dislocation slips at the phase interface resulted in C diffusion and cementite amorphization. The transformation of cementite was size dependent. Thin lamellae transformed into amorphous lamellae, while local deformation and shear bands formed in thick lamellae. These research findings could contribute to rational design of cementite and to control of its structural evolution behavior within a tribolayer, thereby enhancing the wear resistance of a material.

Enabling focused ion beam sample preparation for application in reverse tip sample scanning probe microscopy

Focused ion beam (FIB) has become a powerful tool for transmission electron microscopy sample preparation in the nanoelectronics industry and has in recent years also shown its benefits for specific preparation steps in electrical scanning probe microscopy (SPM). Most recently, a novel SPM approach – so-called reverse tip sample (RTS) SPM – has been proposed in which the position of sample and tip are switched compared to standard SPM; in RTS SPM the sample is attached to the end of a cantilever beam. To achieve this configuration, the region of interest must first be extracted from a substrate and then needs to be reliably fixed to the cantilever by FIB. Therefore, we have explored and developed dedicated FIB preparation methods for RTS SPM in this work. Our established procedures ensure a strong mechanical and good electrical connection of the sample to the cantilever for both cross-section and top view sample preparation. Furthermore, we introduce an approach for mounting samples from a full wafer size workflow. This paper presents the developed FIB procedures and discusses the quality and stability of all mounted samples and their electrical evaluation in RTS SPM.

Microstructure Study on Very High Cycle Fatigue of an Additively Manufactured Aluminium Alloy via Advanced Characterization Methods

The engineering application of additively manufactured (AM) metallic materials is quite limited by their fatigue behaviors, which are very inconsistent with that of conventionally wrought or cast ones. Here, based on advanced material characterization techniques, such as focused ion beam (FIB), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), the microstructures underneath fracture surfaces were thoroughly investigated in an AM aluminum (AlSi10Mg) alloy with horizontal and vertical building orientation enduring very high cycle fatigue (VHCF) loading under the stress ratios R = −1, 0, and 0.5. Two VHCF failure specimens A and B were representatively selected to further examine SEM and TEM sample preparation via FIB milling. Specimen A was horizontally printed and failed at R = −1; specimen B was vertically printed and failed at R = 0. TEM samples A1 and B1 were lifted from locations near the crack initiation sites on the fracture surfaces of specimens A and B; The locations of TEM samples A2 and B2 kept away from the crack origin sites but still within the “fish-eye” region of crack steady growth. TEM observations show that there was no characteristic microstructure induced by VHCF in different oriented specimens and under various R values.

The characterisation of dental enamel using transmission Kikuchi diffraction in the scanning electron microscope combined with dynamic template matching

The remarkable physical properties of dental enamel can be largely attributed to the structure of the hydroxyapatite (HAp) crystallites on the sub-micrometre scale. Characterising the HAp microstructure is challenging, due to the nanoscale of individual crystallites and practical challenges associated with HAp examination using electron microscopy techniques. Conventional methods for enamel characterisation include imaging using transmission electron microscopy (TEM) or specialised beamline techniques, such as polarisation-dependent imaging contrast (PIC). These provide useful information at the necessary spatial resolution but are not able to measure the full crystallographic orientation of the HAp crystallites. Here we demonstrate the effectiveness of enamel analyses using transmission Kikuchi diffraction (TKD) in the scanning electron microscope, coupled with newly-developed pattern matching methods. The pattern matching approach, using dynamic template matching coupled with subsequent orientation refinement, enables robust indexing of even poor-quality TKD patterns, resulting in significantly improved data quality compared to conventional diffraction pattern indexing methods. The potential of this method for the analysis of nanocrystalline enamel structures is demonstrated by the characterisation of a human enamel TEM sample and the subsequent comparison of the results to high resolution TEM imaging. The TKD – pattern matching approach measures the full HAp crystallographic orientation enabling a quantitative measurement of not just the c-axis orientations, but also the extent of any rotation of the crystal lattice about the c-axis, between and within grains. Results presented here show how this additional information highlights potentially significant aspects of the HAp crystallite structure, including intra-crystallite distortion and the presence of multiple high angle boundaries between adjacent crystallites with rotations about the c-axis. These and other observations enable a more rigorous understanding of the relationship between HAp structures and the physical properties of dental enamel.

A simple coordinate transformation method for quickly locating the features of interest in TEM samples.

We developed a simple coordinate transformation method for quickly locating features of interest (FOIs) of samples in transmission electron microscope (TEM). The method is well suited for conducting sample searches in aberration-corrected scanning/transmission electron microscopes (S/TEM), where the survey can be very time-consuming because of the limited field of view imposed by the highly excited objective lens after fine-tuning the aberration correctors. For implementation, a digital image of the sample and the TEM holder was captured using a simple stereo-optical microscope. Naturally presented geometric patterns on the holder were referenced to construct a projective transformation between the electron and optical coordinate systems. The test results demonstrated that the method was accurate and required no electron microscope or specimen holder modifications. Additionally, it eliminated the need to mount the sample onto specific patterned TEM grids or deposit markers, resulting in universal applications for most TEM samples, holders and electron microscopes for fast FOI identification. Furthermore, we implemented the method into a Gatan script for graphical-user-interface-based step-by-step instructions. Through online communication, the script enabled real-time navigation and tracking of the motion of samples in TEM on enlarged optical images with a panoramic view.

An evaluation of Ar ion milling in TEM sample preparation by energy-filtered TEM technique

Irradiated materials impose unique challenges when it comes to sample preparation for transmission electron microscopy (TEM) analysis. After the preparation of TEM samples by electrolytic polish, Ar ion milling was usually used to remove the surface contamination, as well as to thin the samples to desired thickness. Therefore, it is necessary to evaluate the advantages and limitations of Ar ion milling technique. In this work, the optimum milling condition with minimal surface artifacts was determined by Ar ion milling with various incident angles and beam energies, and the high-resolution TEM (HR-TEM) examinations confirmed the optimum condition to be 4.0keV Ar ions with incident angles of ±4°. In addition, Ar ion milling and subsequent thickness measurement by energy-filtered TEM (EF-TEM) were conducted to determine the thinning rate. It is found that electrolytic polished TEM lamella has a wedge-shape structure with a very sharp edge, and the thinning rates varied from 9.7±1.4nm/minute to 5.0±2.3nm/minute across the TEM lamella for 4.0keV Ar ions. However, the thinning rates for 2keV and 1keV Ar ions were under the examination limit, and they were supposed not sufficient to thin the tungsten samples. Even though the exact thinning rate varies for different materials, the general trends for the Ar ion milling observed in this study can be used as a guidance.

Visualization of Engineered M13 Phages Bound to Bacterial Targets by Transmission Electron Microscopy.

The filamentous phage M13 is one of the most well-studied and characterized phages, particularly since it was introduced as a scaffold for phage display, a technique to express and evolve fusion proteins on the M13 phage's coat to study protein or peptide binding interactions. Since phages can be engineered or evolved to specifically bind to a variety of targets, engineered M13 phages have been explored for applications such as drug delivery, biosensing, and cancer therapy, among others. Specifically, with the rising challenge of antimicrobial resistance among bacteria, chimeric M13 phages have been explored both as detection and therapeutic agents due to the flexibility in tuning target specificity. Transmission electron microscopy (TEM) is a powerful tool enabling researchers to directly visualize and characterize binding of phages to bacterial surfaces. However, the filamentous phage structure poses a challenge for this technique, as the phages have similar morphology to bacterial structures such as pili. In order to differentiate between bacterial structures and the filamentous phages, here we describe a protocol to prepare TEM samples of engineered M13 phages bound to bacterial cells, in which the phage virions have been specifically labeled by decoration of the major capsid proteins with gold nanoparticles. This protocol enables clear visualization and unambiguous identification of attached filamentous phages within the context of bacterial cells expressing numerous pili.

Fabrication and Electrical Characterization of GaAs/GaN Junctions

GaAs/GaN heterojunctions are likely to be an ideal platform for high-frequency/high-output-power electron devices because of excellent electrical properties of GaAs and GaN as well as the similarity in their coefficients of thermal expansion. We have to note, however, that heteroepitaxial growth of GaAs/GaN junctions is quite difficult because of difference in crystal structures (in case of GaAs/wurtzite GaN junctions) and large mismatch in lattice constants (GaAs/zincblende GaN junctions). In this work we fabricate p+-GaAs/n-GaN heterojunctions by bonding GaAs and GaN epi wafers to each other using surface-activated-bonding technologies [1,2], selectively etching off the GaAs substrate used in the epitaxial growth, making ohmic contacts on the exposed surface of GaAs layer, and forming GaAs mesas. Ohmic contacts on the backside of GaN epi wafer are formed before bonding. The junctions are annealed at 400 ℃ in contact formation on GaAs surfaces. The temperature is much lower in comparison with the temperature in a wafer-fusion process of GaAs and GaN (750 ℃), which were reportedly applied for fabricating group-III arsenide/nitride heterojunction bipolar transistors [3]. Observation using cross sectional transmission electron microscope (TEM) of as-bonded and 400-℃ annealed GaAs/GaN interfaces shows that no voids are formed at the GaN/GaAs interfaces by the 400-℃ annealing. We also fabricate n+-GaAs/n-GaN heterojunctions for comparison.We measure the capacitance-voltage characteristics and current-voltage characteristics for reverse-bias voltages of the p+-GaAs/n-GaN and n+-GaAs/n-GaN junctions and find that the characteristics of the two junctions are close to each other, which suggests that the band profiles of GaN layers in the p+-GaAs/n-GaN and n+-GaAs/n-GaN junctions are almost the same, i.e., the Fermi-level pinning occurs at the GaAs/GaN interfaces. More importantly, the reverse-bias characteristics are measured up to -60 V. The reverse-bias voltage corresponds to an electric field of as high as ~2 MV/cm, which is comparable to the breakdown field of GaN (3-4 MV/cm) [4]. We also excite minority electrons in the p+-GaAs layer using a 488-nm Ar laser and successfully observe the photo current due to the transport of minority electrons across the reverse-biased GaAs/GaN interfaces.Acknowledgement: TEM samples were fabricated under the Inter-University Cooperative Research in IMR of Tohoku University. Epi wafers used in the work were provided from Sciocs (present Sumitomo Chemical) Co., Ltd.[1] H. Takagi, et al. Appl. Phys. Lett. 68, 2222 (1996). [2] S. Yamajo, et al., Jpn. J. Appl. Phys. 57, 02BE02 (2018). [3] C. Lian, et al. Appl. Phys. Lett. 91, 063502 (2007). [4] T. Maeda, et al., IEEE Electron Device Lett., 43, 96 (2022).

Proton Radiation Effect on Iron Thermal Oxides: Point Defects, Microstructure & Corrosion Electrochemistry

Iron(Fe)-based alloys, such as austenitic stainless steels, form protective surface oxide layers when exposed to oxygen, aqueous electrolyte, and water vapor, which effectively inhibit corrosion [1]. One attribute that underpins the electrochemical reactivity of surface oxide layers is the presence and properties of charged point defects (e.g. vacancies and interstitials charge compensated by electrons or holes), which govern the rate and mechanism of ionic transport when exposed to an oxidizing environment such as an aqueous electrolyte [2,3,4]. However, the effect of radiation on the relationship between point defects, oxide microstructure, and oxidation behavior is poorly understood. Structural materials used in nuclear applications are particularly susceptible to radiation damage, which can degrade the underlying surface oxides. To investigate this phenomenon, thermally oxidized pure Fe was selected as a model material system to study radiation-induced defects in iron oxide scales [3]. Iron oxides were fabricated by thermal oxidation of pure Fe metal in air at 400°C, 600oC, and 800°C for 1 hour. Each oxides layer exhibits an outer hematite (α-Fe2O3) and inner magnetite (Fe3O4)/wüstite (FeO) layers with varying thickness depending on the oxidation conditions. These oxides were then irradiated with 200 keV, 0.03 dpa protons at room temperature. The oxide microstructure and phase compositions were characterized with transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS). These results were corroborated with X-Ray diffraction (XRD) and Raman spectroscopy analysis. The relative vacancy concentrations and size distributions in the oxide layers were investigated using positron annihilation spectroscopy (PAS) and positron annihilation lifetime spectroscopy (PALS) techniques. Lastly, these oxides were exposed to a 0.1 M borate buffer solution (pH 9.2) at applied potential where corrosion reactivity was governed by interfacial reactions and ionic defects to regulate the growth processes observed. The potentials were cycled over the ranges where oxides growth and reduction occurred involving Fe/Fe(II)/Fe(III) reactions at a pH which suppressed chemical reactions at the oxide/electrolyte interface and would lead to solubilization of Fe species. The subsequent oxidation, reduction, and semiconductor properties of the oxides were evaluated using AC and DC electrochemical methods, both with and without prior irradiation. Current results demonstrate that radiation damage can manifest itself in the form of vacancy clustering and possible maghemite (γ-Fe2O3) formation. These phenomena were corroborated with donor density and diffusivity values calculated using Mott-Schottky analysis and electrochemical impedance spectroscopy (EIS) analysis and equivalent circuit model (ECM) interpretation, respectively. Radiation damages to the outer rate-regulating α-Fe2O3 layer correlates with enhanced anodic current density corresponding to Fe/Fe(II)/Fe(III) reactions. However, proton radiation may also interact with the metal/oxide and oxide/oxide interfaces depending on the oxide structure and H+ beam penetration depth, leading to either degradation or improvement in the Fe-oxide corrosion responses. Nevertheless, the damage persists and can continue to impact corrosion mechanisms long after the irradiation source is removed. The multiple techniques adopted, each characterizing a material property of a different length scale (from meso-, micro-, to macroscopic) enable us to hypothesize the roles that point defects played after irradiation and during corrosion and how the radiation-induced changes in the defect characteristics influence corrosion electrochemistry. Acknowledgment Research primarily supported as part of the fundamental understanding of transport under reactor extremes (FUTURE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Utilization of the Malvern-Panalytical Empyrean diffractometer were supported by Nanoscale Materials Characterization Facility with National Science Foundation under award CHE-2102156. The TEM sample preparation and characterization was performed at the Analytical Instrumentation Facility at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (award number ECCS-1542015). LANL is operated by Triad National Security, LLC, for the National Nuclear Security Administration of the U.S. DOE (Contract No. 89233218CNA000001).

A Stepped Mesh Host for Lithium Metal Batteries Inspired by Transmission Electron Microscopy Sampling Grids

A Stepped Mesh Host for Lithium Metal Batteries Inspired by Transmission Electron Microscopy Sampling Grids

Optimized procedure for conventional TEM sample preparation using birefringence

Specimens for quality transmission electron microscopy (TEM) analyses must fulfil a range of requirements, which demand high precision during the prior preparation process. In this work, an optimized procedure for conventional TEM specimen preparation is presented that exploits the thickness-dependence of interference colors occurring in birefringent materials. It facilitates the correct estimation of specimen thickness to avoid damage or breaking during mechanical thinning and reduces ion-milling times below 30minutes. The benefits of the approach are shown on sapphire and silicon carbide cross-section samples. The presented method is equally suitable for assessing specimen thickness during dimpling and wedge-polishing, and is particularly useful at thicknesses below 20 μm, where the accuracy of mechanical techniques is insufficient. It is precise enough to be employed for a visual thickness estimation during the thinning process, but can be additionally optimized by analyzing the RGB spectrum of the occurring interference colors.

FIB-shield: a structure to safely transfer and precisely mount beam-sensitive TEM specimens under focused ion beam

Focused ion beam (FIB) has been adopted extensively for transmission electron microscope (TEM) sample preparation during the past decades. However, ion beam- and deposition-induced damage during transferring and mounting of the sample cannot be effectively avoided, limiting the application of FIB in ion-beam illumination-sensitive samples. A transferring device called the FIB shield is designed and fabricated to greatly reduce the damage and contamination of the sample during transfer and mounting under Ga+ beam imaging, milling and Pt deposition. Nearly damage-free transfer and precise positioning and attachment of beam-sensitive in situ TEM nanoindentation samples are achieved. The effectiveness of the shielding plate to block Ga+ radiation damage during Ga+ beam imaging and milling and that of the buffer region to alleviate sputtering damage during Pt deposition are verified by corresponding experiments.

Acoustic shock wave-induced short-range ordered graphitic domains in amorphous carbon nanoparticles and correlation between magnetic response and local atomic structures

Over the years, a wide range of spectral research has been carried out on various carbon forms to explore deeply their structure-property relationship. The structural stability of carbon allotropies remains to be a challenging endeavor yet to be achieved under extreme conditions, especially under acoustical shocked conditions. From the experimental findings, we report the acoustic shock wave response of the amorphous carbon nanoparticles and the resultant degree of crystalline nature, morphological characteristics and magnetic response. Moreover, analytical techniques such as X-ray diffractometry (XRD), Raman spectrometer, High resolution transmission electron microscopy (HR-TEM) and vibrational sample magnetometer (VSM) have been employed to arrive at the relationships of structure – morphology – magnetic properties under shocked conditions. Interestingly, under shocked conditions (0, 250, 500 and 750 shocks), compelling evidence has been found for the short-range amorphous pockets which turn into spatially short-range ordered graphitic domains within the sphere-shaped amorphous nanoparticles whereby the obtained values of the ratio ID/IG of the samples are 0.94, 0.93, 0.95 and 1.01 for 0, 250, 500 and 750 shocked conditions, respectively. Moreover, the saturation magnetization is found to have reduced in the base of the local atomic ordering and the obtained values are found to be 1, 0.64, 0.41, 0.725 emu/g for 0, 250, 500 and 750 shocked conditions, respectively. The structure-property relationship is to be elaborated in the upcoming sections.

Beginners guide to sample preparation techniques for transmission electron microscopy

Background purpose: The revolution in microscopy came in 1930 with the invention of electron microscope. Since then, we can study specimens on ultrastructural and even atomic level. Besides transmission electron microscopy (TEM), for which specimen preparation techniques will be described in this article, there are also other types of electron microscopes that are not discussed in this review. Materials and methods: Here, we have described basic procedures for TEM sample preparation, which include tissue sample preparation, chemical fixation of tissue with fixatives, cryo-fixation performed by quick freezing, dehydration with ethanol, infiltration with transitional solvents, resin embedding and polymerization, processing of embedded specimens, sectioning of samples with ultramicrotome, positive and negative contrasting of samples, immunolabeling, and imaging. Conclusion: Such collection of methods can be useful for novices in transmission electron microscopy.

A method for extraction of exosomes from breast tumour cells and characterisation by transmission electron microscopy.

Exosomes can not only be used as markers of tumour metastasis but also be used for noninvasive diagnosis of clinical diseases, which holds significant medical research value. Observing the morphology and distribution of exosomes using transmission electron microscopy (TEM) is highly significant. In this study, we use breast tumour cell (MDA-MB-231) exosomes as a representative sample and focus on the extraction and purification of exosomes, as well as the investigation of optimal conditions for the observation of exosomes using TEM. Through comparative tests, we obtained the optimal dilution concentration and staining method for TEM of exosomes, the best dilution concentration is 100 times, the best negative staining time is 1.5min. Western blotting and scanning electron microscopy (SEM) confirmed the presence of exosome. The particle size experiment shows that the size of exosomes is mainly distributed between 60nm and 150nm. This method provides a reference for TEM sample preparation of subcellular structures and small molecular biomaterials other than exosomes.

Nitriding layer characteristics of modified AISI422 martensitic stainless steel in tempered state

In this study, the nitriding layer of the modified AISI422 martensitic stainless steel was characterized by several method, including optical microscopy (OM), micro-hardness tester, X-ray diffraction (XRD), scanning electron microscopy, and transmission electron microscopy (TEM), to study the phase transformation at the whole nitriding layer during the plasma nitriding. In order to investigate the evolution of the nitriding process the TEM samples of the nitriding layer has been prepared from the white layer to the un-nitriding zone by the focus ion beam method. With nearly 20 h nitriding process, the white layer, vein-like structure and the diffusion zone was clearly observed though the OM. Together, the TEM results show that the γ′-Fe4N and the micro holes existed in the white layer and the thick vein-like structure. And the morphology of the continuous precipitation of CrN was highly related with the depth of the nitriding layer. Excepted the normal coarsening mechanism, the EDS results indicate that the discontinuous precipitation of CrN may also originate from the transformation of the M23C6(M=Cr) during the nitriding process. Meanwhile, the residual stress value is not directly proportionate to the nitriding depth or the N contents. Furthermore, it could be affected by the N contents and the degree of the CrN spinodal decomposition, and these two factors lead the maximum value of the compress stress appears at the depth of 120μm.

The Casting and Hot Forging of Low-Carbon Copper-Bearing Steel and Its Substructural Characterization

The casting of metal alloys followed by hot forging is a widely used manufacturing technology to produce a homogeneous microstructure. The combination of mechanical and thermal energy envisages the microstructural properties of metal alloys. In the present investigation, a metal alloy of composition 0.05C-1.52Cu-1.51Mn (in weight %) was cast in an induction furnace using a zirconia crucible. The melt pool was monitored using optical emission spectroscopy (OES) to maintain the desired composition. The as-cast block was then subjected to forging under a pneumatic hammer of 0.5 t capacity so that any casting defects were eliminated. The as-cast block was reheated to a temperature of 1050 °C and held at that temperature for 6 h to homogenize, followed by hammering with a 50% strain using a pneumatic hammer. The microhardness was calculated using a Vickers microhardness testing apparatus. The microstructure characterization of the processed alloy was carried out using an optical microscope, electron backscattered diffraction (EBSD), energy-dispersive X-ray spectroscopy (EDXA), and a transmission electron microscope (TEM). The sample for optical microscopy was cut using a diamond cutter grinding machine and surface polishing was carried out using emery paper. Further, mechanical polishing was performed to prepare the samples for EBSD using a TEGRAPOL polishing machine. The EBSD apparatus was operated at a 20 kV accelerating voltage, 25 mm from the gun, and with a 60 µ aperture size. HKL Technology Channel 5 Software was used for the post-processing of EBSD maps. The procedure of standard polishing for OES and TEM sample preparation was followed. Recrystallization envisages equiaxed grain formation in hot forging; hence, the strain-free grains were observed in the strained matrix. The lower distribution of recrystallized grains indicated that the driving force for recrystallization was not abundant enough to generate a fully recrystallized microstructure. The fractional distribution of the misorientation angle between 15 and 60° confirms the formation of grain boundaries (having a misorientation angle greater than 15°) and dislocations/subgrain/substructures (having a misorientation angle less than 15°). The fraction of misorientation angle distribution was higher between the angles 0.5 and 6.5°; afterwards, it decreased for higher angles. The substructure was observed in the vicinity of grain boundaries. The softening process released certain strains, but still, the dislocation was observed to be deposited mostly in the vicinity of grain boundaries and at the grain interior. The fine precipitates of the microalloying element copper were observed in the range of size in nanometers. However, the densities of these precipitates were limited and most of these precipitates were deposited at the grain interior. The microhardness of 210.8 Hv and mean subgrain size of 1.61 µ were observed the enhanced microhardness was due to the limited recrystallized grains and accumulation of dislocations/subgrain/substructures.

Importance of TEM sample thickness for measuring strain fields

Transmission electron microscopy (TEM) has emerged as a valuable tool for assessing and mapping strain fields within materials. By directly analyzing local atomic spacing variations, TEM enables the precise measurement of local strain with high spatial resolution. However, it is standard practice to use thin specimens in TEM analysis to ensure electron transparency and minimize issues such as projection artifacts and contributions from multiple scattering. This raises an important question regarding the extent of structural modification, such as strain relaxation, induced in thin samples due to the increased surface-to-volume ratio and the thinning process. In this study, we conducted a systematic investigation to quantify the influence of TEM sample thickness on the residual strain field using deformed Fe-based and Zr-based metallic glasses as model systems. The samples were gradually thinned from 300 nm to 70 nm, and the same area was examined using 4D-STEM with identical imaging settings. Our results demonstrate that thinning the sample affects the atomic configuration at both the short-range (SR) and medium-range (MR) scales. Consequently, when the sample is thinned too much, it no longer preserves the native deformation structure. These findings highlight the critical importance of maintaining sufficient TEM sample thickness for obtaining meaningful and accurate strain measurements.

Flash electropolishing of BCC Fe and Fe-based alloys

The preparation of transmission electron microscopy (TEM) samples is a critical step in the characterization of materials, and the focused ion beam (FIB) technique is a commonly used method. However, a significant limitation of this technique is the FIB-induced damages on the foil surfaces, which can obscure the real features of interest, particularly in radiation effects studies. To overcome this limitation, this study presents a detailed description of the flash electropolishing technique, which can be used to remove the FIB damage from samples. The flash electropolishing technique has been successfully applied to a range of materials, including Fe, Fe-based model alloys, commercial Fe-Cr alloys, and advanced Fe-Cr alloys, both in their as-received and ion-irradiated states. Furthermore, the parameters used for Fe-Cr model alloys can be adjusted for commercial and advanced alloys with minimal modifications. The study also examined the effects of electropolishing variables, such as perchloric acid concentration, electropolishing temperature, Cr concentration, and voltage. Qualitatively, a general trend and scoping test strategy is explored in our experiments. Overall, the flash electropolishing technique offers a promising solution to the challenges posed by FIB-induced damages in the preparation of TEM samples.