Weak Beam Dark Field Research Articles

Oxidation behavior of β-Nb formed in Zr-1Nb under neutron irradiation in PWR conditions

This work focuses on neutron irradiated Zr-1Nb alloy, using High Resolution Transmission Electron Microscopy (HRTEM) to investigate the oxidation behavior of β-Nb at different distances from the Oxide /Metal (O/M) interface within the oxide film. Results show that β-Nb was initially oxidized to T-NbO2 at 0 nm at O/M interface, then into a complex morphology of T-NbO2, M-Nb2O5, and O-Nb2O5 within 600nm. Finally, it was completely oxidized to M-Nb2O5 within 800nm. β-Nb in this study did not exhibit amorphous morphology within observed distances. In addition, Inverse Fast Fourier Transformation (IFFT) and Weak Beam Dark Field (WBDF) techniques are employed to characterize the dislocation density and distribution in the oxide film, results indicate that the distribution of dislocations generated by neutron irradiation in the oxide film is relatively uniform and neutron irradiation is not the primary reason affecting the oxidation behavior of β-Nb.

Application of Weak-Beam Dark-Field STEM for Dislocation Loop Analysis†.

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.

Characterization of Vacancy Defects Using TEM in Heavy-Ion-Irradiated Tungsten Foils

The nature and type of defects formed due to heavy-ion irradiation in tungsten foils are analyzed using transmission electron microscopy. The recrystallized tungsten foils were irradiated by 80 MeV gold ions at room temperature for a fluence of 1.3 ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes $$\\end{document} 1014\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{14}$$\\end{document} ions/cm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^2$$\\end{document} that amounts to a net displacement per atom (dpa) of 0.22. The defect structures were analyzed using bright-field and weak-beam dark field imaging at two different depths to understand the depth profile of the defects. It is found that the defect clusters formed during the irradiation, both at the near-surface and at 2 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu $$\\end{document}m depth are of vacancy type that is confirmed by the local strain analysis and supports the findings of vacancy clusters in positron lifetime measurements. The analysis also shows that the dislocation-lines were of pure edge, pure screw and mixed. The fraction of mixed dislocation is found to increase during irradiation at the expense of pure edge and screw dislocations.

Identifying the activated slip modes and twin variants in compressive creep deformation of Mg-8.0Al-1.0Nd-1.0Gd

The compressive creep deformation mechanism of as-cast Mg-8.0Al-1.0Nd-1.0Gd alloy at 180 °C under constant stress of 60 MPa and 90 MPa was novelly analyzed through the distribution characteristics of in-grain misorientation axes (IGMA). Activated slip modes in the creep process was accurately identified and the structure of dislocations is revealed by the weak beam dark-field (WBDF) technology. The intensities of IGMA are concentrated on 011̅0 and 1̅21̅0 axis demonstrates that 0001112̅0(basal slip) and 112̅2112̅3̅ (pyramidal slip) dominate the deformation mechanism. Tensile twins 101̅21̅011 were extensively activated in compression-creep process and the 101̅21̅011 variants with a twinning trace angle close to 90 deg are 1̅012101̅1 and 101̅21̅011 with para-position. The creep mechanism of the alloy crept at 180°C/60 MPa is dominated by grain boundary sliding triggered by the precipitation of β-Mg17Al12, and pipe diffusion-controlled boundary sliding is the dominant creep mechanism of the alloy under 180 °C/90 MPa. In addition, the results reveal that tensile twins induced by dislocation slip contribute to coordinate the grain c-axis deformation, release the stress concentration caused by dislocation accumulation at the grain boundary.

HiPIMS-grown AlN buffer for threading dislocation reduction in DC-magnetron sputtered GaN epifilm on sapphire substrate

Gallium nitride (GaN) epitaxial films on sapphire (Al2O3) substrates have been grown using reactive magnetron sputter epitaxy with a liquid Ga target. Threading dislocations density (TDD) of sputtered GaN films was reduced by using an inserted high-quality aluminum nitride (AlN) buffer layer grown by reactive high power impulse magnetron sputtering (R-HiPIMS) in a gas mixture of Ar and N2. After optimizing the Ar/N2 pressure ratio and deposition power, a high-quality AlN film exhibiting a narrow full-width at half-maximum (FWHM) value of the double-crystal x-ray rocking curve (DCXRC) of the AlN(0002) peak of 0.086° was obtained by R-HiPIMS. The mechanism giving rise the observed quality improvement is attributed to the enhancement of kinetic energy of the adatoms in the deposition process when operated in a transition mode. With the inserted HiPIMS-AlN as a buffer layer for direct current magnetron sputtering (DCMS) GaN growth, the FWHM values of GaN(0002) and (10 1‾ 1) XRC decrease from 0.321° to 0.087° and from 0.596° to 0.562°, compared to the direct growth of GaN on sapphire, respectively. An order of magnitude reduction from 2.7 × 109 cm−2 to 2.0 × 108 cm−2 of screw-type TDD calculated from the FWHM of the XRC data using the inserted HiPIMS-AlN buffer layer demonstrates the improvement of crystal quality of GaN. The result of TDD reduction using the HiPIMS-AlN buffer was also verified by weak beam dark-field (WBDF) cross-sectional transmission electron microscopy (TEM).

Temperature-dependent stacking fault energy, deformation behavior, and tensile properties of a new high-entropy alloy

The stacking fault energy (SFE), deformation behavior, and tensile properties of a new high-entropy alloy (HEA), Fe35Mn35Co10Cr10Ni10 (in at. %), were investigated at room temperature (RT) and −100 °C. Deformation substructure evolution during tensile loading was studied using electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). A transition in the slip configuration occurred from fully wavy to completely planar with the decrease in temperature from room temperature (RT) to −100 °C. The studied alloy revealed the onset of deformation twinning only during deformation at −100 °C and the same was absent during RT deformation. SFE of the studied alloy at −100 °C was ∼34.2 (±4) mJ/m2. SFE determination required the value of the distance between Shockley partial dislocations and the same was determined employing the transmission electron microscopy (TEM) based weak-beam dark field (WBDF) technique. SFE of the studied alloy could not be estimated at RT due to the presence of closely spaced partial dislocations (perhaps due to very high SFE) and therefore partial separation by the presently employed WBDF technique couldn’t be achieved. The role of friction stress was limited and SFE was revealed as the main factor in defining the active slip mode in HEAs containing no apparent short-range ordering and similar shear moduli. Activation of planar slip-induced dislocation features such as Taylor lattices, microbands, and twinning at −100 °C provided greater work hardening resulting in improved tensile properties compared to RT.

In situ EBSD/HR-DIC-based investigation on anisotropy mechanism of a near α titanium plate with strong transverse texture

The effect of strong transverse(T) texture on anisotropy deformation mechanism of Ti60 plate was in-situ investigated using the EBSD and the high resolution-digital image correlation (HR-DIC) technique. The yield strength and the ultimate tensile strength is in descending order: TD > RD > 45°, and the ductility shows the opposite trend. The tensile anisotropy related to the discrepancy of activated slip systems in RD, 45°, and TD. In yielding stage, prism<a> slip dominated in RD, basal<a> slip dominated in the 45° direction, while pyram <c+a > slip dominated in TD. The anisotropy coefficient χ was proposed to evaluate the resistance discrepancy based on all the activated slip systems, which well explained the trend of the tensile strength anisotropy. The ductility anisotropy has been analyzed through slip transmission and slip-GB interaction using HR-DIC technology, which is determined by residual dislocation under different tensile directions. The slip transmission can be easily achieved when the Burgers vector of incoming grain is a [2‾ 110] and the Burgers vector of outcoming grain is a [112‾ 0] in 45° direction, which can effectively weaken stress concentration on the grain boundary leading to the greatest ductility among three tensile directions. The dislocation structures in three directions have been analyzed under two-beam condition and weak beam dark field (WBDF) images.

Deformation microstructure and thermomechanical processing maps of homogenized AA2070 aluminum alloy

The hot deformation behavior of homogenized AA2070 Al–Li–Cu alloy was studied using isothermal compression in a temperature range of 300 °C–500 °C and a strain rate range of 0.001 s-1 to 1 s-1. Deformation microstructure was characterized using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) including weak beam dark field STEM imaging to correlate the microstructure evolution with hot deformation mechanism. TEM/STEM results show that a high density of fine T1 (Al2LiCu) phase forms in the microstructure through dynamic precipitation at compression temperatures lower than 400 °C, especially at low strain rates. Constitutive analysis shows that back stress due to dynamic precipitation of T1 phase decreases with increasing compression temperature and increasing strain rate. EBSD characterization indicates that dynamic recovery is the main softening mechanism at temperatures below 400 °C. At 400 °C and above, intermetallic phases including T1, Al20Cu2Mn3 and Al3Cu2 in the microstructure gradually dissolve, and dynamic recrystallization becomes the dominant softening mechanism, resulting in texture weakening and grain refinement. Three dynamic recrystallization mechanisms including discontinuous dynamic recrystallization (DDRX) at large angle grain boundaries, particle stimulated nucleation (PSN) and continuous dynamic recrystallization (CDRX) were observed. Based on hot work efficiency, deformation instability and microstructure evaluation, processing maps were constructed using both conventional method and Garofalo sinh equation, providing optimum processing conditions (temperature >400 °C and strain rate <0.1 s-1) for industrial forging, extrusion and rolling processes.

Quantitative characterization of frank loops and their effects on the plastic degradation in heavy ion irradiated alloy 800H

Alloy 800H has recently received significant attention owing to its potential applications in molten salt reactors. However, its irradiation tolerance and plastic degradation after irradiation are not well understood. In this study, Alloy 800H was irradiated by 3 MeV Cu+ ions with irradiation damage up to 10 dpa. The microstructure changes were characterized using two Rel-rods cases and weak beam dark field technology by performing transmission electron microscopy. The plastic mechanical properties before and after the irradiation were extracted from P-h curves derived by performing nanoindentation at loading rates of 0.05, 0.1, and 0.5 nm/s. The microscopy studies demonstrate that the characterization and evolution of the two variations of Frank loops are distinct and the 13⟨1‾11⟩ Frank loops firstly nucleate and coarsening. The nanoindentation results demonstrate that the measured yield stress increases with the increasing irradiation damage and decreasing loading rate. The good agreement on yield stress between the calculated and measured values suggests that using the characteristics of two variations of Frank loops to calculate the contribution of yield stress is appropriate. In addition, the decreasing measured values with increasing loading rate are attributed to the increased geometrically necessary dislocations, which are correlated with the indent size effect.

Temperature dependent deformation behavior and stacking fault energy of Fe40Mn40Co10Cr10 alloy

The variation in stacking fault energy (SFE) with the change in temperature has been evaluated experimentally for Fe40Mn40Co10Cr10 high entropy alloy. The distance between partial dislocations was measured using transmission electron microscopy (TEM) based weak-beam dark field (WBDF) technique. SFE of the system was found to be 37.7 (±7) mJ/m2 and 19.5 (±5) mJ/m2 at room temperature (RT) and -100°C, respectively. Owing to the decrease in SFE, a transition in the deformation behavior occurred from limited twin formation and slip dominance at RT to mixed-mode consisting of FCC→HCP transformation and twinning at -100°C. Simultaneous occurrence of twins and deformation-induced martensitic transformation led to superior strength-ductility combination in the specimen deformed at -100°C. SFE of the studied alloy at RT was 42% higher than that of the equiatomic FeMnCoCrNi alloy. This increase in SFE, despite removal of Ni can be understood by considering the effect of the alloy chemistry re-adjustment on ΔGγ→ε.

Ordering effects on deformation substructures and strain hardening behavior of a CrCoNi based medium entropy alloy

A CrCoNi based medium entropy alloy with small additions of Ti, Al and Nb (denoted as (CrCoNi)93Al4Ti2Nb) in the as-quenched condition, exhibits tensile properties comparable to those of the equiatomic CrCoNi alloy at room temperature. Dark field transmission electron microscopy (TEM), atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) together with atom probe tomography (APT) show that spatially-localized long range ordering (LRO) L12 domains exist in this alloy. The evolution of deformation substructure with plastic deformation in this alloy was characterized using electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and STEM based techniques including the recently developed weak beam dark field STEM imaging. Plastic deformation occurs by the slip of a/2<110>dislocations, which are narrowly dissociated into Shockley partial dislocations on {111} slip planes. Their dissociation distances in the (CrCoNi)93Al4Ti2Nb alloy are much smaller than the widths of the corresponding partials in the equiatomic CrCoNi alloy due to one or more of the minor alloying elements (Al, Ti, Nb). Dislocation slip in this alloy has a pronounced planar character. The leading dislocations in slip bands glide as pairs due to the existence of LRO domains. Multipoles were formed through the slip of dislocations with opposite signs on adjacent {111} slip planes. Those multipoles serve as building blocks for the formation of subgrain structures consisting of fine slip bands. The distances between slip bands were continuously refined during plastic deformation and dynamic refinement of slip bands plays a crucial role in strain hardening. The effects of LRO domains on planar dislocation slip, the deactivation of deformation twinning and strain hardening of this alloy are discussed.

High Resolution Imaging of Dislocations Using Weak Beam Dark Field STEM

An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

Ordering Effects on Deformation Substructures and Strain Hardening Behavior of a CrCoNi Based Medium Entropy Alloy

A CrCoNi based medium entropy alloy with a small addition of Ti, Al and Nb (denoted as (CrCoNi)93Al4Ti2Nb) in the as-quenched condition, exhibits tensile properties comparable to those of the equiatomic CrCoNi alloy at room temperature. Atom probe tomography (APT) together with electron diffraction show that nanosized L12-type ordered γʹ domains exist in this alloy, which may be formed through spinodal decomposition during the quenching process. The evolution of deformation substructure with plastic deformation in this alloy was characterized using electron backscattered diffraction (EBSD) and scanning transmission electron microscopy (STEM) based techniques including the recently developed weak beam dark field STEM imaging. Plastic deformation occurs by the slip of a/2 dislocations, which are narrowly dissociated into Shockley partial dislocations on {111} slip planes. Their dissociation distances in the (CrCoNi)93Al4Ti2Nb alloy are much smaller than the widths of the corresponding partials in the equiatomic CrCoNi alloy due to one or more of the minor alloying elements (Al, Ti, Nb). Dislocation slip in this alloy has a pronounced planar character. The leading dislocations in slip bands glide as pairs due to the existence of ordered γʹ domains. Multipoles were formed through the slip of dislocations with opposite signs on adjacent {111} slip planes. Those multipoles serve as important building blocks for the formation of subgrain structures consisting of fine slip bands. The distances between slip bands were continuously refined during plastic deformation and dynamic refinement of slip bands plays a crucial role in strain hardening. The effects of ordered γʹ domains on planar dislocation slip, the deactivation of deformation twinning and strain hardening of this alloy are discussed.

The effects of loop size on the unfaulting of Frank loops in heavy ion irradiation

When stainless steels are exposed to high irradiation dose, a fraction of Frank loops will unfault and transform into perfect loops. After unfaulting, perfect loops can glide and impinge into other loops to form dislocation networks, and dislocation networks will act as sinks for point defects and solute elements. Besides, unfaulting will influence the extent of irradiation hardening as well. Based on previous theories, the size of Frank loops was considered to be a key factor in unfaulting. To better understand the unfaulting process, 316L stainless steel model alloy was irradiated to 5dpa at 350 °C, 400 °C and 450 °C by 3 MeV Ni ions. Method was developed to quantitatively compare the size and density between Frank loops and perfect loops through relrod and weak-beam dark field techniques. Results showed that unfaulting barely occurred at 350 °C but was distinct at 400 °C and 450 °C. The majority of perfect loops were in the size range of 8–16 nm at 400 °C and were in the size range of 12–20 nm at 450 °C. The existence of a critical loop size for unfaulting was not supported by our results. Moreover, by our results, the energy of Frank loops does not play a decisive role in unfaulting, and the simple approximation of geometric constraint in the rate theory to calculate unfaulting diameter needs to be revisited. Irradiation hardening was also analyzed by nano-indention. It was found that loop unfaulting played an important role in the hardening decrease from 350 °C to 450 °C.

Toward high-throughput defect density quantification: A comparison of techniques for irradiated samples

Transmission electron microscopy (TEM) is an established tool used for the investigation of defects in materials. Traditionally, diffraction contrast techniques—two-beam bright-field and weak-beam dark-field—have been used to image defects due to contrast sensitivity from weak lattice strains. Use of these methods entail an intricate tilt series of imaging using different diffracting vectors, g, to verify the g•b invisibility criterion relative to the different defect types and habit planes inherent to the material. Recently, the addition of down-zone imaging and STEM imaging has also proven to be effective imaging techniques for defect density analysis. Interest in nanocrystalline (NC) materials, spurred by their conjectured superior properties compared to their coarse-grain counterparts, has been thriving and the investigation of their defect morphologies is essential. Maneuvering within NC samples in the TEM adds another layer of difficulty making the aforementioned techniques not practical for application to specimens with complex microstructures. For this reason, we have devised a protocol for identifying NC grains optimally oriented for quantitative analysis using NanoMegas ASTAR automated crystal orientation mapping (ACOM) in the TEM. In this work, we conduct a series of experiments assessing the effectiveness of conventional two-beam bright-field, weak-beam dark-field, and down-zone STEM imaging. We also evaluate an ACOM-assisted multibeam imaging method and compare defect density results obtained using each technique in an irradiated nanocrystalline Au sample.

Ion-irradiation hardening accompanied by irradiation-induced dissolution of oxides in FeCr(Y, Ti)-ODS ferritic steel

Irradiation effects on hardness and phase stability were investigated for an FeCr(Y, Ti)-ODS ferritic steel strengthened by Y-Ti-O nano-particles after irradiation with 6.4 MeV Fe3+ at room temperature (RT) up to nominal damages of 2, 10 and 50 dpa. With increasing local displacement damage up to ∼20 dpa, nano-sized oxide particles slightly shrank, while the corresponding number density drastically decreased by almost two orders of magnitude compared to that of before irradiation. It is considered that ballistic dissolution should be responsible for such reductions in the particle size and number density. Dislocation loops consisting of 1/2<111> type (>80%) and <100> type were observed under weak beam dark field (WBDF) imaging condition in the specimen irradiated to the nominal damage of 50 dpa. The average size and number density of all the dislocation loops were 2.8 ± 0.7 nm and (4.1 ± 0.7) × 1022 m−3, respectively, at the local damage of ∼72 dpa. Although the oxide particles were almost completely dissolved, nanoindentation hardness measurements revealed that the hardening went up continuously with increasing displacement damage and was estimated to be 1.63 ± 0.39 GPa by the Nix-Gao model at the nominal damage of 50 dpa. The irradiation hardening accompanied by the dissolution of oxide particles was interpreted in terms of loss of oxide particles, solid solution hardening and formation of fine dislocation loops. The contribution of dislocation loops observed by transmission electron microscopy (TEM) to the hardening was insufficient to overcome the loss of strengthening by dissolution, suggesting the importance of solid solution hardening and the larger strength factor of dislocation loops as a hardening contributor.

Tailoring the Ti-C nanoprecipitate population and microstructure of titanium stabilized austenitic steels

The present work reports on the microstructural evolution of a new heat of 24% cold worked austenitic DIN 1.4970 (15-15Ti) nuclear cladding steel subjected to ageing heat treatments of varying duration between 500 and 800 °C (by steps of 100 °C). The primary aim was studying the finely dispersed Ti-C nanoprecipitate population, which are thought to be beneficial for creep and swelling resistance during service. Their size distribution and number density were estimated through dark field imaging and bright field Moiré imaging techniques in the transmission electron microscope. Nanoprecipitates formed at and above 600 °C, which is a lower temperature than previously reported. The observed nucleation, growth and coarsening behavior of the nanoprecipitates were consistent with simple diffusion arguments. The formation of nanoprecipitates coincided with significant dissociation of dislocations as evidenced by weak beam dark field imaging. Possible mechanisms, including Silcock's stacking fault growth model and Suzuki segregation, are discussed. Recrystallization observed after extended ageing at 800 °C caused the redissolution of nanoprecipitates. Large primary Ti(C,N) and (Ti,Mo)C precipitates that occur in the as-received material, and M23C6 precipitates that nucleate on grain boundaries at low temperatures were also characterized by a selective dissolution procedure involving filtration, X-ray diffraction and quantitative Rietveld refinement. The partitioning of key elements between the different phases was derived by combining these findings and was consistent with thermodynamic considerations and the processing history of the steel.

Significant internal quantum efficiency enhancement of GaN/AlGaN multiple quantum wells emitting at ~350 nm via step quantum well structure design

Significant internal quantum efficiency (IQE) enhancement of GaN/AlGaN multiple quantum wells (MQWs) emitting at ~350 nm was achieved via a step quantum well (QW) structure design. The MQW structures were grown on AlGaN/AlN/sapphire templates by metal-organic chemical vapor deposition (MOCVD). High resolution x-ray diffraction (HR-XRD) and scanning transmission electron microscopy (STEM) were performed, showing sharp interface of the MQWs. Weak beam dark field imaging was conducted, indicating a similar dislocation density of the investigated MQWs samples. The IQE of GaN/AlGaN MQWs was estimated by temperature dependent photoluminescence (TDPL). An IQE enhancement of about two times was observed for the GaN/AlGaN step QW structure, compared with conventional QW structure. Based on the theoretical calculation, this IQE enhancement was attributed to the suppressed polarization-induced field, and thus the improved electron–hole wave-function overlap in the step QW.

Resolving individual Shockley partials of a dissociated dislocation by STEM

A practical method was developed to image detailed features of defects in a crystal using STEM. This method is essentially a STEM version of the conventional CTEM g/3g weak beam dark field (WBDF) method. The method was successfully applied to resolving individual Shockley partials of a dissociated dislocation in a Cu-6.44at.%Al alloy.

Stacking fault energies in austenitic stainless steels

We measure the stacking fault energy of a set of 20 at% Cr-austenitic stainless steels by means of transmission electron microscopy using the weak beam dark field imaging technique and the isolated dislocations method. The measurements are analyzed together with first principles calculations. The results show that experiment and theory agree very well for the investigated concentration range of Mn (0–8%) and Ni (11–30%). The calculations show that simultaneous relaxation of atomic and spin degrees of freedom is important in order to find the global energy minimum for these materials. Our results clearly show the great potential of the weak beam dark field technique to obtain accurate measurements of the stacking fault energy of austenitic steels and that the reliable predictability of first principles calculations can be used to design new steels with optimized mechanical properties.