Heidenreich RD (2004). Electron microscope and diffraction study of metal crystal textures by means of thin sections. J Appl Phys 20(10), 993–1010. https://doi.org/10.1063/1.1698264

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Phillips PJ, Mills MJ & De Graef M (2011b). Systematic row and zone axis STEM defect image simulations. Philos Mag 91(16), 2081–2101. https://doi.org/10.1080/14786435.2010.547526

Understanding the mechanical properties of crystalline materials relies heavily on using transmission electron microscopy (TEM) to characterize dislocations (Hirsch & Whelan, 1960; Howie, et al., 1962), which are among the most important defects in engineering materials for structural applications. Recent studies (Phillips et al., 2011,a; Phillips et al., 2011,b; Agudo Jácome et al., 2013) have shown that conventional g⋅b invisibility criteria for dislocation imaging in diffraction contrast TEM are applicable in scanning transmission electron microscopy (STEM) when the experimental setting is appropriate, offering specific advantages. These advantages include a higher signal-to-noise ratio and reduced thickness fringes, bend contours, imaging defects in thicker samples, and other dynamical contrast effects. In nuclear material research, irradiation can induce dislocation loop formation (Lin et al., 2021), which can be visualized using the same g⋅b invisibility criteria applied to dislocations (Yao et al., 2013). Investigating dislocations and dislocation loops in irradiated materials is crucial for understanding the effects of irradiation on material properties and evaluating a material's lifetime in extreme nuclear reactor environments (Zinkle, 2020).

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Häussermann F, Katerbau KH, Rühle M & Wilkens M (1973). Calculations and observations of the weak-beam contrast of small lattice defects. J Microsc 98(2), 135–154. https://doi.org/10.1111/j.1365-2818.1973.tb03816.x

Lin YR, Ho CY, Hsieh CY, Chang MT, Lo SC, Chen FR & Kai JJ (2014). Atomic configuration of irradiation-induced planar defects in 3C-SiC. Appl Phys Lett 104(12), 12. https://doi.org/10.1063/1.4869829

According to electron diffraction theory (Hirsch & Whelan, 1960), the half-width of a TEM-imaged dislocation line is ∼ ξg/3 (where ξg is the extinction distance) under a two-beam approximation with vector g excited, typically ∼10 nm. Consequently, determining the dislocation loop type solely with on-zone BF STEM images may be challenging when the loop diameter is less than ∼10 nm, making them appear as tiny “black dot” defects. Moreover, unlike the model alloys used in this study for fundamental research, nuclear materials typically comprise complex material systems in which dislocation loops can coexist with primitive dislocation lines, precipitates, and other irradiation-induced defects. For instance, it is common to observe dislocation loops nucleating around preexisting dislocations (Wen et al., 2005; Hernández-Mayoral et al., 2016). When different features overlap in the TEM/STEM-projected images, this complexity poses a challenge in identifying and quantifying the loop type based on loop morphology and orientation in on-zone STEM images (Xiu et al., 2021). Hence, employing WBDF STEM methods for dislocation loop analysis is beneficial for isolating information on the type and nature of dislocation loops by tilting the sample to different g vectors.

Sandström R, Melander A & Eriksson L (1974). Influence of non-systematic reflexions on weak-beam and high-resolution bright-field images in high-voltage electron microscopy. Physica Status Solidi (a) 26(1), 273–284. https://doi.org/10.1002/pssa.2210260128

Employing STEM for defect analysis offers numerous advantages, with the added benefit of suppressing dynamical diffraction contributions to background contrast in the image. Moreover, STEM mode imaging can be executed along zone axes. However, it is essential to acknowledge potential limitations, such as the possibility of “dirty” BF STEM or ADF images resulting from using two-beam conditions without an aperture or overlapping CBED discs. Additionally, on-zone STEM imaging may be limited when the defect diameter is less than 10 nm. This study introduces WBDF STEM methods, which aim to address these challenges and extend the advantages of STEM for defect analysis. These methods enable the isolation of defect information to identify the dislocation loop type, showcasing narrow dislocation lines for small loop analysis and providing inside–out contrasts to identify the dislocation loop nature. Furthermore, as analytical electron microscopes combine TEM and STEM with EDXS and/or EELS, elemental analysis and/or thickness mapping can be performed within the same STEM imaging area. Moreover, combining STEM defect analysis methods with 4D STEM techniques allows for obtaining strain mapping information of defects. This holistic approach enhances the depth and precision of defect analysis in materials science.

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Phillips PJ, Brandes MC, Mills MJ & De Graef M (2011a). Diffraction contrast STEM of dislocations: Imaging and simulations. Ultramicroscopy 111(9), 1483–1487. https://doi.org/10.1016/j.ultramic.2011.07.001

Maher DM & Joy DC (1976). The formation and interpretation of defect images from crystalline materials in a scanning transmission electron microscope. Ultramicroscopy 1(3), 239–253. https://doi.org/10.1016/0304-3991(76)90038-3

It is important to note that an appropriate setup, including the use of an objective aperture, is necessary to obtain “clean” WBDF STEM images. Without an objective aperture in the WBDF STEM method, an ADF detector may receive multiple nonsystematic diffraction beams. TEM calculations and experiments have demonstrated that the dislocation images would be altered if nonsystematic diffraction beams are contributed to the resultant image (Häussermann et al., 1973; Sandström et al., 1974). As shown in Figure 3k, dark-contrast loops at the top of the image were formed with signals from the direct beam T, whereas bright-contrast loops at the bottom primarily originate from the 2g beam signal. Although the middle region of Figure 3k appeared to be defect-free, other images revealed the presence of loops in this region. Loops in the middle region were invisible or “canceled” out because the ADF detector detected signals from both the direct beam T and the 2g beam. This suggests that when multiple beams hit one detector simultaneously, some information can be lost. Therefore, it is advisable for the operator to align the microscope properly and confirm the optic axis, CBED pattern, objective aperture, and BF/ADF detectors are all in the desired position before recording WBDF STEM images. As a setup verification example (Fig. 4), when employing an optimal configuration of WBDF STEM Method 1, the g disc is covered by the objective aperture. The objective aperture should effectively block all diffracted and direct beams, excluding the g beam. Consequently, no signal can be collected by the BF STEM detector (Figs. 4c, 4d).

Comparison of dislocation thickness in two-beam and WBDF STEM conditions with g=[110]⁠: (a,b) two-beam condition, (c) WBDF STEM (−g, g) condition, (d) WBDF STEM (g, 3g), (e,f) corresponding CBED patterns for the WBDF images.

A critical aspect of dislocation loop analysis involves determining their nature, whether interstitial or vacancy type. Traditionally, loop nature analysis depended on the inside–outside contrast technique (Jenkins, 1994), which can be challenging when dealing with complicated background contrast and small dislocation loops. Figure 5 clearly demonstrates that WBDF STEM images can reveal the inside–outside contrast of the small edge-on dislocation loops under diffraction conditions g=[011] or [011¯] with sg>0 (STEM Method 1 in Fig. 3). Figure 5 also shows that the [010] and [001] edge-on dislocation loops appear as loop strings aligned along the 001 planes, rather than forming a single large dislocation loop. Similar observations were evident in plan-view [100] loops, exemplified by the “flower shape” loop near the bottom of Figures 1a and 1b.

Spadotto JC, Burke MG & Solórzano IG (2020). On the morphology of grain boundary discontinuous reactions and phase identification in an advanced Cr–Fe–Ni alloy. J Mater Sci 55(23), 10221–10241. https://doi.org/10.1007/s10853-020-04690-8

Cockayne DJH (1973). The principles and practice of the weak-beam method of electron microscopy. J Microsc 98(2), 116–134. https://doi.org/10.1111/j.1365-2818.1973.tb03815.x

Lin Y-R, Chen W-Y, Li M, Henry J & Zinkle SJ (2021). Dynamic observation of dual-beam irradiated Fe and Fe-10Cr alloys at 435°C. Acta Mater 209, 116793. https://doi.org/10.1016/j.actamat.2021.116793

where sg and ξg are the excitation error and the extinction distance of the diffraction beam g, respectively. Cockayne (1973) established these conditions in TEMs at 100 kV, and calculations demonstrated validity at higher voltages (Sandström, 1973). Cockayne proposed two WBDF setups: (1) exciting a higher-order g spot, such as (g, 3g) or (2) a lower-order negative g spot, such as (−g, g). Typically, TEM operators more commonly choose condition (1) with +n likely because it intuitively aligns with the requirement that |sg| ≥ 0.2 nm−1. Based on weak-beam microscopy theory (Williams & Carter, 2009) and lattice parameters in the Inorganic Crystal Structure Database (ICSD; Hellenbrandt, 2004), Tables 3 and 4 list the |sg| = 0.2 nm−1 condition for several materials with different g vectors with accelerating voltages of 200 and 300 kV. Here, n and –n refers to the values where the Ewald sphere intersects. From Tables 3 and 4, two conclusions could be drawn: (1) for the same absolute value of s, the sum of +n and −n is 2, irrespective of the chosen excitation error. (2) Compared to WBDF with +g, achieving WBDF with −g of the same magnitude of s is easier and has a lower likelihood of exciting other nonsystematic diffraction spots. In other words, for the ideal WBDF condition with the narrowest dislocation lines, (g, 3g) may not always be the best condition.

STEM mode also offers direct accessibility when combined with energy-dispersive X-ray spectroscopy (EDXS) and electron energy loss spectroscopy (EELS) techniques, enabling elemental or thickness mapping within the same imaging area. This integrated approach provides a detailed understanding of solute segregation or element enrichment near or on the dislocations. However, when conducting STEM–EDXS or EELS analyses on specimens oriented along major crystallographic zone axes, it is necessary to consider electron-channeling effects, which can affect electron-induced X-ray emission (Taftø & Spence, 1982) and the intensity of the incident electron wave (Taftø & Krivanek, 1982). In addition, the synergy between STEM defect analysis techniques and 4D STEM strain mapping techniques (Zeltmann, et al., 2019; Yu, et al., 2024) can be beneficial for studying the strain field of the defects. By combining these approaches, valuable information related to the defects can be obtained, providing a more comprehensive understanding of the crystalline material's behavior under various conditions.

Parish CM, Field KG, Certain AG & Wharry JP (2015). Application of STEM characterization for investigating radiation effects in BCC Fe-based alloys. J Mater Res 30(9), 1275–1289. https://doi.org/10.1557/jmr.2015.32

Liu J (2021). Advances and applications of atomic-resolution scanning transmission electron microscopy. Microsc Microanal 27(5), 943–995. https://doi.org/10.1017/S1431927621012125

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Conflict of Interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Zhu P, Zhao Y, Lin Y-R, Henry J & Zinkle SJ (2024). Defect-specific strength factors and superposition model for predicting strengthening of ion irradiated Fe18Cr alloy. J Nucl Mater 588, 154823. https://doi.org/10.1016/j.jnucmat.2023.154823

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Humphreys CJ (1981). Fundamental concepts of stem imaging. Ultramicroscopy 7(1), 7–12. https://doi.org/10.1016/0304-3991(81)90017-6

Taftø J & Krivanek OL (1982). Site-Specific valence determination by electron energy-loss spectroscopy. Phys Rev Lett 48(8), 560–563. https://doi.org/10.1103/PhysRevLett.48.560

Note: This manuscript has been authored by UT Battelle, LLC, under contract no. DE-AC05-00OR22725 with the US Department of Energy. The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript or allows others to do so. The Department of Energy will provide public access to these results with full access to the published paper of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Schematic illustration of various setup and corresponding BF and WBDF micrographs of dislocation loops in irradiated Fe-5Cr, captured through various TEM and STEM techniques: (a–d) schematic diagram illustrating the imaging conditions; (e) BF TEM and (i) WBDF TEM micrographs (column 1); (f) BF STEM and (j) WBDF STEM micrographs using STEM Method 1 (column 2); (g) BF STEM and (k) \WBDF STEM micrographs using STEM Method 2 (column 3); (h) BF STEM and (l) WBDF STEM micrographs using STEM Method 3 (column 4). T is the direct beam and g is the diffraction beam with g = 011.

Hernández-Mayoral M, Heintze C & Oñorbe E (2016). Transmission electron microscopy investigation of the microstructure of Fe–Cr alloys induced by neutron and ion irradiation at 300°C. J Nucl Mater 474, 88–98. https://doi.org/10.1016/j.jnucmat.2016.03.002

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Figure 3 shows a compilation of BF and WBDF micrographs of dislocation loops in irradiated Fe-5Cr through diverse TEM and STEM methods, as outlined in Table 1 and illustrated in Figures 3a to 3d. All images are recorded from the identical region of the TEM foil to enable a direct comparison of contrast variations. The cropped selected area electron diffraction (SAED) and CBED patterns in the lower right of each micrograph indicate the positions of the optic axis, direct beam (T), diffraction beam (⁠g=[011]⁠), detector (BF or ADF), and OL aperture. In BF TEM (Fig. 3e) and WBDF TEM (Fig. 3i) micrographs, discernible thickness fringes and background contrasts are evident. It was noted that all BF and WBDF micrographs obtained through the three STEM methods exhibited a suppression of thickness fringes (Fig. 3). Notably, the contrast of the images using STEM Method 2 was inverted between detectors; that is, ADF images were obtained using the BF detector (Fig. 3g) and BF images were obtained using the ADF detector (Fig. 3k). Furthermore, in Figure 3k, the dislocation loops in the upper half of the image display dark contrast, whereas in the lower half, the loops exhibit white contrast. Lastly, in the comparison between STEM Method 1 and Method 3, the results were generally aligned, with minor differences in contrast intensity and the thickness of the loop's contour line.

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Yu Z, Lin Y-R, Zachman MJ, Zinkle SJ & Xu H (2024). The role of stacking fault tetrahedra on void swelling in irradiated copper. Commun Mater 5(1), 53. https://doi.org/10.1038/s43246-024-00491-7

Appearance of dislocation loops imaged under various g vectors with and without objective apertures: (a–d) images recorded using the BF STEM detector and (e–h) ADF STEM detector. In the four columns (left-to-right), images were acquired on-zone, weak-beam g=[002] without an objective aperture, and weak-beam g=[002] and [011] with an objective aperture. The zone axis was near or along [100]⁠.

Taftø J & Spence JCH (1982). Atomic site determination using the channeling effect in electron-induced x-ray emission. Ultramicroscopy 9(3), 243–247. https://doi.org/10.1016/0304-3991(82)90207-8

This research was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory (M.G.B. and Y.-R.L.) and the Fusion Energy Sciences, US Department of Energy (Y.R.L.) under contract DE-AC05-00OR22725 with UT Battelle, LLC. Support was also provided by the Office of Fusion Energy Sciences from grant # DE-SC0023293 with the University of Tennessee (Y.L. and S.J.Z.).

As summarized in Table 2, identifying loop types necessitates a minimum of two noncollinear diffraction vectors with g⋅b=0⁠. This is because, in three-dimensional space, a vector is determined by the cross product of two noncollinear vectors. To investigate whether the classic g⋅b invisibility criteria in WBDF TEM condition are applicable to WBDF STEM methods, we focused on a higher-magnification area, tilting the sample along different g vectors to verify the appearance of distinct types of loops (Table 2). In Figure 4, all the images were taken near or along the [100] zone axis. Figures 4a and 4e are the BF STEM and ADF STEM images acquired in the on-zone condition, respectively. Consistent with previous on-zone STEM studies (Parish et al., 2015; Xiu et al., 2021), all types of dislocation loops were visible along the [100] zone axis. Edge-on defects corresponded to vertical [010] or horizontal [001] dislocation loop types, small plan-view loops were [100] loops, and several ½<111> elliptical loops were also evident. When tilting the sample toward the g=[002] direction to set up the (g, 3g) weak-beam condition without an objective aperture on the g diffraction disc (Figs. 4b, 4f), vertical [010] loops, which should not be present according to the g[002]⋅b[010]=0 criteria, still exhibited loop contrast with reduced intensity. After inserting an appropriately sized objective aperture covering only the g diffraction disc, the vertical [010] loops in WBDF STEM images were eliminated, as indicated by the rectangular box in Figure 4g. The plan-view [100] loops, marked by the smaller square in Figure 4g, also vanished following the g[002]⋅b[100]=0 criteria. Additionally, no signal was detected by the BF detector when the objective aperture was inserted and appropriately covered the g diffraction disc, as shown in Figure 4c. Similar results were observed when tilting the sample toward the g=[011] direction, impacting the ½<111> loop contrast, as marked by the bigger square in Figure 4h.

Wen M, Ghoniem NM & Singh BN (2005). Dislocation decoration and raft formation in irradiated materials. Philos Mag 85(22), 2561–2580. https://doi.org/10.1080/14786430500154281

Haley JC, Briggs SA, Edmondson PD, Sridharan K, Roberts SG, Lozano-Perez S & Field KG (2017). Dislocation loop evolution during in-situ ion irradiation of model FeCrAl alloys. Acta Mater 136, 390–401. https://doi.org/10.1016/j.actamat.2017.07.011

Iwata H & Saka H (2017). Resolving individual Shockley partials of a dissociated dislocation by STEM. Philos Mag Lett 97(2), 74–81. https://doi.org/10.1080/09500839.2017.1282634

Comparison of background contrast in TEM and STEM modes CL aperture sizes ranging from 10 to 70 µm: (a) SAED pattern in TEM mode from (e). (b–d) CBED patterns from (f–h). (e) BF TEM image. (f–h) BF STEM images. All images were captured in the identical area along the [001] zone axis. For each CL aperture utilized in this study, the corresponding convergence angle, α, is indicated in the lower left corner of (b), (c), and (d).

Yao Z, Jenkins ML, Hernández-Mayoral M & Kirk MA (2010). The temperature dependence of heavy-ion damage in iron: A microstructural transition at elevated temperatures. Philos Mag 90(35–36), 4623–4634. https://doi.org/10.1080/14786430903430981

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In addition to the advantages for using STEM for defect analysis, employing WBDF STEM methods for dislocation loop analysis provides an effective means to isolate defect information efficiently, ensuring a clearer understanding of dislocation loop types. Furthermore, these methods produce narrow dislocation lines, enabling in-depth analysis of small loops, and permit inside–out contrast analysis for precise identification of dislocation loop nature. The WBDF STEM methods can be used not only to improve dislocation loop imaging but also to analyze dislocation lines in various material systems. Any dislocation analysis conducted using the WBDF method in TEM mode in the past can take advantage of the WBDF STEM methods. For example, the Burgers vector can be determined for dislocation types such as perfect, Shockley partial, or Frank partial dislocations. It is necessary to perform a full g⋅b analysis to determine the Burgers vector of defects, as well as conduct slip trace analysis across other zone axes to identify the slip plane. Additionally, by extending the concept of using objective apertures to select signals from specific g discs in STEM mode, defects and/or second-phase analysis, such as for centered dark-field conditions (Spadotto et al., 2020) and diffraction-selected on-zone STEM (Kozuka et al., 2024), may also derive potential benefit when selecting precipitate or superlattice reflections for imaging.

As shown in Figures 3 to 6, WBDF STEM is a superior technique for imaging finer details of dislocations loops. To achieve a significant reduction in the half-width (thickness of the dislocation line), the excitation error of the g beam must be sufficiently large. The criteria for optimal WBDF condition are met with (Cockayne, 1973):

STEM on-zone and WBDF images demonstrating the inside–outside contrast of dislocation loops: (a–c) BF STEM images and (d–f) ADF STEM. The orientation of the g vector is indicated by the arrow.

Note: This manuscript has been authored by UT Battelle, LLC, under contract no. DE-AC05-00OR22725 with the US Department of Energy. The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript or allows others to do so. The Department of Energy will provide public access to these results with full access to the published paper of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

In the early development of STEM defect imaging, the theory relied on the principle of reciprocity (Humphreys, 1981), where αs = βc and αc = βs⁠, with α, β, c, and s denoting incident angle, collection angle, TEM, and STEM, respectively. To optimize STEM advantages, such as smearing out thickness fringes and bending contours, a condition of βs being larger than αc is desirable. Previous studies (Maher & Joy, 1976; Phillips et al., 2011,b) demonstrated that for fine details of defects to be visible in STEM images, it is essential to have βs ≈ 10 αc⁠. Precise control of the αc-to-βs ratio, achieved by adjusting the camera length and condenser aperture size, enabled defect micrographs in STEM mode with suppressed thickness fringes and bending contours to be obtained (Zhu et al., 2018). However, setting up the optimal WBDF STEM imaging condition with background-suppressed (suppression of dynamical contrast effect in the matrix) images requires consideration of the best combination of camera length, CL aperture size, OL aperture size, and collection angles for BF/ADF detectors, which may vary between microscopes and different materials or g vectors. In Figure 2, although the 50 μm CL aperture showed the best background-suppressed BF STEM images (Fig. 2g), the diffraction discs were too large and overlapped for BCC Fe along the [100] zone axis (Fig. 2c), thus making it challenging to set up the WBDF STEM condition because the diffraction discs overlapped. A possible solution was to use a smaller CL aperture, slightly sacrificing background contrast suppression, as demonstrated in this study with the smaller 10 μm CL aperture (Fig. 2d) in the JEOL 2100 for WBDF STEM setting. Another potential solution can involve the careful use of the JEOL “free lens control” that permits users to adjust the convergence angle for a given condenser aperture.

The WBDF STEM method 3 proposed in this study was inspired by Cockayne's −n WBDF setup (Cockayne, 1973). As shown in Figure 7, both the (g, 3g) and (−g, g) conditions for WBDF STEM (Figs. 7c, 7d) yielded similar results with narrower dislocation lines compared to the images acquired under a two-beam condition (Figs. 7a, 7b). In Figure 3, all three variants of WBDF STEM methods presented similar and optimal WBDF images. Among these methods, the (−ng, g) condition (see WBDF STEM Method 3 in Table 1) may be considered to be a more efficient method with a lower likelihood of error, as it generally requires less sample tilting compared to the setup of the (g, ng) condition. Note that in STEM mode, unlike TEM mode, deflecting the direct beam to meet the Bragg condition for WBDF imaging is not possible. The WBDF STEM Method 2 (see Table 1) is simply shifting the CBED pattern with the projection system without affecting the excitation of a specific g disc, unlike the beam deflect (or beam tilt) in TEM mode.

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Nanoscale dislocation loops formed by irradiation can significantly contribute to both irradiation hardening and embrittlement of materials when subjected to extreme nuclear reactor environments. This study explores the application of weak-beam dark-field (WBDF) scanning transmission electron microscopy (STEM) methods for quantitative irradiation-induced defect analysis in crystalline materials, with a specific focus on dislocation loop imaging and analysis. A high-purity Fe-5 wt% Cr model alloy was irradiated with 8 MeV Fe2+ ions at 450°C to a fluence of 8.8 × 1019 m−2, inducing dislocation loops for analysis. While transmission electron microscopy (TEM) has traditionally been the primary tool for dislocation imaging, recent advancements in STEM technology have reignited interest in using STEM for defect imaging. This study introduces and compares three WBDF STEM methods, demonstrating their effectiveness in suppressing background contrasts, isolating defect information for dislocation loop type classification, providing finer dislocation line images for small loop analysis, and presenting inside–outside contrast for identifying loop nature. Experimental findings indicate that WBDF STEM methods surpass traditional TEM approaches, yielding clearer and more detailed images of dislocation loops. The study concludes by discussing the potential applications of WBDF STEM techniques in defect analysis, emphasizing their adaptability across various material systems beyond nuclear materials.

TEM and STEM WBDF images demonstrated the inside–outside contrast of an ½<111> interstitial dislocation loop, indicated by the dashed lines: WBDF TEM images with (a) g=[011] and (b) g=[011¯]⁠; WBDF STEM images with (a) g=[011] and (b) g=[011¯]⁠. The zone axis was near [100]⁠.

Trinkaus H, Singh BN & Foreman AJE (1997). Segregation of cascade induced interstitial loops at dislocations: Possible effect on initiation of plastic deformation. J Nucl Mater 251, 172–187. https://doi.org/10.1016/S0022-3115(97)00246-8

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The distribution of dislocation loops is shown in the STEM images of the ion-irradiated Fe-5Cr cross-section sample, where the ion beam had been injected perpendicular to the surface (top of Figs. 1a, 1b). In BCC Fe systems, ion irradiation can induce ½<111> {111} and {100} dislocation loops, as schematically shown in Figures 1d and 1c. For ion irradiation of a {100} surface with the [100] zone axis, ½<111> elliptical loops align either parallel to [011] or [011¯]⁠. Conversely, [001] and [010] loops exhibit an edge-on orientation, with [001] loops perpendicular to [002] and [010] loops perpendicular to [020]⁠. Finally, [100] loops are visible as plan-view loops. In general, <100> loops in irradiated Fe-Cr alloys dominate at elevated temperatures (Yao et al., 2010), in agreement with the BF STEM and annular dark-field (ADF) images in Figures 1a and 1b, respectively. All STEM images in this study were captured at a camera length of 8 cm, STEM convergence angle of 4 or 13 mrad, BF detector collecting angles from 0 to 11 mrad, the ADF detector collecting angles from 19 to 42 mrad, and dwell time was between 1 and 4 µs. The exposure time for WBDF TEM images was 2 s. Setting up WBDF STEM imaging conditions involved using the objective lens (OL) aperture to select the desired diffraction disk, as depicted in Figure 1f.

The simplest focal length definition is a description of the distance between the center of a lens and the image sensor when the lens is focused at infinity.

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Miao J, Singh S, Tessmer J, Shih M, Ghazisaeidi M, DeGraef M & Mills MJ (2018). Dislocation characterization using weak beam dark field STEM imaging. Microsc Microanal 24(S1), 2202–2203. https://doi.org/10.1017/S1431927618011492

Chen W-Y, Miao Y, Gan J, Okuniewski MA, Maloy SA & Stubbins JF (2016). Neutron irradiation effects in Fe and Fe-Cr at 300°C. Acta Mater 111, 407–416. https://doi.org/10.1016/j.actamat.2016.03.060

Yan-Ru Lin, Yao Li, Steven J Zinkle, Jose’ D Arregui-Mena, M Grace Burke, Application of Weak-Beam Dark-Field STEM for Dislocation Loop Analysis, Microscopy and Microanalysis, Volume 30, Issue 4, August 2024, Pages 681–691, https://doi.org/10.1093/mam/ozae067

Dislocation loops play a significant role in irradiation hardening and embrittlement of materials at low-to-intermediate temperatures, making their analysis central to understanding the materials’ response to irradiation (Klueh et al., 2008). Traditionally, determining the Burgers vector of a loop involves imaging with at least two noncollinear g vectors, a time-consuming process requiring sample tilting. Yao et al. (2013) introduced an efficient method in TEM mode, based on the projected loop orientation and features (e.g., edge-on, plan-view, or elliptical loops) at specific zone axes, to determine loop types (e.g., ½<111> or <100> loops) in irradiated body-centered cubic (BCC) Fe-based steels. This method was later extended to STEM mode by researchers Parish et al. (2015) and Xiu et al. (2021) for examining dislocation loops in BCC and face-centered cubic (FCC) materials under certain zone axes. However, these methods have limitations when the loop diameter is less than 5–10 nm. When the thickness of the loop's contour line is similar to the loop size, the loop appears as a “black dot” loop for every type of loop. This makes it impossible to preserve the crystallographic information of these loops for on-zone imaging in both TEM and STEM modes.

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The Dot Product Calculation Involving All Seven Burgers Vectors and Diffraction Vectors under the Major Zone Axes in the BCC Iron System.

Fung KY, Lin YR, Yu PJ, Kai JJ & Hu A (2018). Microscopic origin of black spot defect swelling in single crystal 3C-SiC. J Nucl Mater 508, 292–298. https://doi.org/10.1016/j.jnucmat.2018.05.054

To address these gaps, this study focuses on applying various STEM methods for dislocation loop imaging and comparing them to traditional TEM methods. Although previous studies have demonstrated the value of bright-field (BF) STEM over traditional TEM methods (Wang & Bentley, 1991; Parish et al., 2015; Xiu et al., 2021), our current investigation uniquely focuses on the advantages of weak-beam dark-field (WBDF) STEM. We outline steps for establishing WBDF conditions for dislocation loop imaging in STEM mode and discuss the advantages of using WBDF STEM for dislocation and dislocation loop analysis. The proposed approach involves using the WBDF STEM method to capture finer details of dislocation loops and determine their Burgers vector and nature (e.g., interstitial or vacancy type). Furthermore, this method proves particularly beneficial in accurately locating dislocation line and loop cores, especially in situations involving bent samples (Phillips et al., 2011,b), small “black dot” loops, or a locally high density of defects (Trinkaus et al., 1997). Importantly, this approach extends beyond irradiated materials, offering a versatile method for defect analysis in other material systems.

Recent advancements in STEM (Liu, 2021), including spherical aberration correction and the development of direct electron and pixelated STEM detectors, have renewed interest in STEM imaging of dislocations and irradiation-induced defects (e.g., cavities, stacking faults, and point defect clusters; Lin et al., 2014; Parish et al., 2015; Fung et al., 2018). Although TEM methods have traditionally dominated dislocation imaging, STEM using convergent beams emerged as a promising alternative as early as the 1970s (Maher & Joy, 1976). Apart from dislocation contrast, diffraction of the electron beam introduces additional dynamical contrast such as bend contours and thickness fringes (Heidenreich, 2004). These can complicate image contrast, potentially obscuring the finer details of dislocations, dislocation loops, or other microstructures/defects under TEM operating conditions. To deal with this unfavorable situation, STEM methods for imaging dislocations with suppressed background contrasts due to dynamical diffraction effects were proposed. This takes advantage of the oscillation of the electron beam in STEM mode, which can significantly cancel out dynamical diffraction contrasts when the size of the convergence semi-angle (α) and the collection angle (β) are well arranged (Zhu, et al., 2018). A “clean background” free of dynamical diffraction effects is especially crucial for analyzing defects and microstructures in irradiated materials. Irradiation-induced defects, like dislocation loops or cavities, can be as small as 1 nm—much smaller than the length of dislocation lines, which can extend to thousands of nanometers. Additionally, when using focused ion beam (FIB) methods to prepare TEM samples, FIB damages can produce “black dot” or “black spot” defects, complicating differentiation from very fine irradiation-induced loops (Zhong et al., 2022). Recent studies demonstrated that the flash electropolishing method minimizes this artifact by removing the thin layer with FIB damage from FIB-produced TEM samples of steel or other alloys (Li et al., 2023).

In this study, we compared three WBDF STEM methods, two from previous studies (Iwata & Saka, 2017; Miao et al., 2018) and one newly proposed herein. The detailed steps for setting up WBDF STEM conditions are summarized in Table 1 and Figures 3b to 3d. Method 1, akin to the conventional WBDF setup in TEM mode, involved exciting the 3 g diffraction vector and detecting the transmitted beam on the BF detector and one diffracted beam on the ADF detector, with an objective aperture used to filter out information from other diffraction signals. Method 2 utilized a shifted CBED pattern via the projector system, moving the g disc to the BF detector, which functioned as both a signal collector and an “aperture.” In Method 3, the sample was tilted to the standard two-beam condition by exciting the g beam with an excitation error near zero, and a proper objective aperture was inserted to allow only the −g beam to be detected on the ADF detector.

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Conflict of Interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Klueh RL, Shiba K & Sokolov MA (2008). Embrittlement of irradiated ferritic/martensitic steels in the absence of irradiation hardening. J Nucl Mater 377(3), 427–437. https://doi.org/10.1016/j.jnucmat.2008.04.002

Howie A, Whelan MJ & Mott NF (1962). Diffraction contrast of electron microscope images of crystal lattice defects. III. Results and experimental confirmation of the dynamical theory of dislocation image contrast. Proc R Soc London. Ser A Math Phys Sci 267(1329), 206–230. https://doi.org/10.1098/rspa.1962.0093

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The study employed a BCC high-purity Fe-5 wt% Cr model alloy. To induce a high proportion of dislocation loops, the Fe-5Cr samples were irradiated with 8 MeV Fe2+ ions (with an ion range of ∼2 μm) at 450°C at the Michigan Ion Beam Laboratory to a total fluence of 8.8 × 1019 m−2 (equivalent to doses of ∼3.7 dpa at midrange, ∼1 μm). Details of the material and irradiation experiments can be found elsewhere (Zhu et al., 2024). Prior to the FIB lift-out process for TEM sample preparation, a Zeiss EVO SEM equipped with an electron backscatter diffraction (EBSD) detector was used to identify grains with a surface plane {100} and a normal close to the 100 zone axes. Subsequently, TEM samples were prepared using a Zeiss Auriga SEM–FIB, employing a 30 kV Ga probe throughout the FIB process with ion beam currents ranging from 12 nA to 50 pA. The flash electropolishing method, as reported in a previous study (Li et al., 2023), was applied to remove FIB-induced damage from the TEM specimen surfaces. All TEM sample preparations were conducted at the Institute for Advanced Materials & Manufacturing lab at the University of Tennessee, Knoxville. TEM and STEM analyses were performed using a JEOL 2100F operated at 200 kV, located at the Low Activation Materials Development and Analysis (LAMDA) laboratory at Oak Ridge National Laboratory.

Xiu P, Bei H, Zhang Y, Wang L & Field KG (2021). STEM characterization of dislocation loops in irradiated FCC alloys. J Nucl Mater 544, 152658. https://doi.org/10.1016/j.jnucmat.2020.152658

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To evaluate the applicability of the inside–outside contrast mechanism with WBDF STEM methods, a specific ½<111> loop was examined by both TEM and STEM with the same imaging condition (Fig. 6). Although the WBDF STEM image exhibited a cleaner background contrast and stronger loop contrast compared to the WBDF TEM images, both WBDF TEM and WBDF STEM images of the loop indicated by the dashed lines in Figure 6 displayed outside contrast when g=[011] and inside contrast when g=[011¯] . Following Föll's convention (Föll & Wilkens, 1975), the loop nature was determined to be of the interstitial type. In the ion-irradiated Fe-5Cr sample, nearly all of the dislocation loops were observed to be interstitial type, in agreement with expectations from prior studies on irradiated Fe alloys (Chen et al., 2016; Haley et al., 2017). It is noted that in BCC Fe systems, irradiation-induced loops are assumed to be perfect loops, obviating the need to consider reverse inside–outside behavior for specific orientations. This demonstrates that the inside–outside contrast technique can be effectively combined with the WBDF STEM method, providing better image quality for accurately determining the dislocation loop nature.

Zhu Y, Ophus C, Toloczko MB & Edwards DJ (2018). Towards bend-contour-free dislocation imaging via diffraction contrast STEM. Ultramicroscopy 193, 12–23. https://doi.org/10.1016/j.ultramic.2018.06.001

Yao B, Edwards DJ & Kurtz RJ (2013). TEM characterization of dislocation loops in irradiated bcc Fe-based steels. J Nucl Mater 434(1), 402–410. https://doi.org/10.1016/j.jnucmat.2012.12.002

We thank Drs. Raymond R. Unocic, Lawrence Allard, Chad Parish, Gerd Duscher, and Vincent Hou for their insightful comments and suggestions.

Zeltmann SE, Müller A, Bustillo KC, Savitzky BH, Minor AM & Ophus C (2019). Improved 4D-STEM strain mapping precision using patterned probes. Microsc Microanal 25(S2), 1958–1959. https://doi.org/10.1017/S1431927619010523

Wang ZL & Bentley J (1991). Z-contrast imaging of bulk crystal surfaces in scanning reflection electron microscopy. Ultramicroscopy 37(1), 39–49. https://doi.org/10.1016/0304-3991(91)90005-Q

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Kozuka M, Miyahara Y & Kobayashi T (2024). Simplification of selective imaging of dislocation loops: Diffraction-selected on-zone STEM. Philos Mag Lett 104(1), 2321134. https://doi.org/10.1080/09500839.2024.2321134

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STEM imaging conditions for dislocation loops in ion irradiated Fe-5Cr: (a,c) BF STEM and (b,d) ADF STEM micrographs acquired along the [100] zone axis with an 8 cm camera length. (e) Schematic diagram illustrating the projected loop types along the [100] zone axis, corresponding to the crystal orientation in (d). ½<111> loops are elliptical, whereas <100> loops are denoted by a dashed circle and the line segments. (f) Illustration of the size and collection angles of the BF and ADF detectors, as well as the OL aperture. BF, bright field; ADF, annular dark-field; OL, objective lens.

Sandström R (1973). The weak-beam method in electron microscopy. Physica Status Solidi (a) 19(1), 83–91. https://doi.org/10.1002/pssa.2210190106

Zhong X, Zhou X, Haigh SJ, Withers PJ & Burke MG (2022). Challenges in FIB TEM sample preparation: Damage issues and solutions. Microsc Microanal 28(S1), 60–62. https://doi.org/10.1017/S1431927622001155

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The Dot Product Calculation Involving All Seven Burgers Vectors and Diffraction Vectors under the Major Zone Axes in the BCC Iron System.

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Zinkle SJ (2020). 1.04—Radiation-Induced effects on microstructure⋆. In Comprehensive Nuclear Materials, 2nd ed. Konings RJM & Stoller RE (Eds.), pp. 91–129. Oxford: Elsevier.

Agudo Jácome L, Nörtershäuser P, Heyer JK, Lahni A, Frenzel J, Dlouhy A, Somsen C & Eggeler G (2013). High-temperature and low-stress creep anisotropy of single-crystal superalloys. Acta Mater 61(8), 2926–2943. https://doi.org/10.1016/j.actamat.2013.01.052

To optimize the suppression of dynamical background contrast in the STEM images acquired using the JEOL 2100F, we tested the 70, 50, and 10 μm condenser lens (CL) apertures with a fixed camera length of 8 cm, imaging the same area of interest (Fig. 2). The convergence angles of each CL size, ranging from small to large, correspond to 4, 13, and 33 mrad (Figs. 2b–2d). For TEM imaging, the 70 μm CL aperture was used. In Figures 2b to 2d, the convergent beam electron diffraction (CBED) patterns clearly demonstrate a decrease in diffraction disc size with decreasing condenser aperture sizes. The corresponding incident angles (α) are labeled on the lower left of the CBED patterns. Overall, STEM images (Figs. 2f–2h) exhibited superior suppression of background contrast compared to the TEM image (Fig. 2e). Among the STEM imaging conditions, the 50 μm CL aperture size produced the best BF STEM image with suppressed background contrast (Fig. 2g). This supports the previous study by Zhu et al. indicating that Kossel–Mollenstedt (K–M) fringes can largely “cancel out” when the α angle is reasonably large, and the β angle is comparable to the α angle (Zhu et al., 2018). However, for WBDF STEM methods, the diffraction disc in the CBED patterns needs to be separated to isolate the diffraction signal. Therefore, in this study, the 10 μm CL aperture was used for WBDF STEM imaging (Fig. 2d).

Li Y, Song M, Zhu P, Lin Y-R, Qi Z, Zhao Y, Levine S & Zinkle SJ (2023). Flash electropolishing of BCC Fe and Fe-based alloys. J Nucl Mater 586, 154672. https://doi.org/10.1016/j.jnucmat.2023.154672