On the use of transmission electron microscopy to quantify dislocation densities in bulk metals

doi:10.1016/j.scriptamat.2019.11.011

Scripta Materialia

Volume 178, 15 March 2020, Pages 161-165

https://doi.org/10.1016/j.scriptamat.2019.11.011 Get rights and content

Abstract

Transmission electron microscopy (TEM) is used to study dislocations, but image stresses introduced during sample preparation may induce slip, potentially significantly modifying the microstructure. The present study quantifies these effects by simulating sample preparation in Al, Fe, Mg, and Zr using dislocation dynamics. Significant decreases in total density are seen, which average nearly 40% for all four metals. The extent of microstructure loss during sample preparation depends on crystal orientation in the HCP systems, and also exhibits substantial statistical variations between cases.

Graphical abstract

Section snippets

Declaration of Competing 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.

Acknowledgments

This work was funded by the United States Department of Energy (DOE) Office of Basic Energy Sciences (BES) Project FWP 06SCPE401.

References (55)

R.J. Asaro et al.
Acta. Metallurgica.
(1985)
N.A. Fleck et al.
Acta. Metallurgica. et Materialia.
(1994)
U.F. Kocks et al.
Prog. Mater. Sci.
(2003)
E. Rizzi et al.
Int. J. Plast.
(2004)
T. Hasegawa et al.
Mat. Sci. Eng.
(1975)
P.S. Bate et al.
Acta. Metall.
(1986)
I.J. Beyerlein et al.
Int. J. Plast.
(2007)
F.F. Lavrentev et al.
Mat. Sci. Eng.
(1975)
L.A. Giannuzzi et al.
Micron
(1999)
A. Akhtar
Acta. Metall.
(1973)

D. Gerlich et al.

J. Phys. Chem. Solids

(1969)

J. Cho et al.

Int. J. Plast.

(2017)

K. Dang et al.

Acta. Mater.

(2019)

X.J. Shi et al.

J. Nucl. Mater.

(2015)

I.J. Beyerlein et al.

Int. J. Plast.

(2008)

A. van den Beukel

Phys. Stat. Sol.

(1975)

A. Abel et al.

Philos. Mag. A

(1972)

M. Wilkens

phys. stat. sol.

(1970)

I. Groma et al.

J. Appl. Crystallogr.

(1988)

T. Ungar et al.

J. Appl. Crystallogr.

(1989)

F. Barra et al.

Int. J. Bifurcat. Chaos

(2009)

R. Keller et al.

J. Mater. Res.

(1998)

M.A. Haque et al.

Proc. Natl. Acad. Sci.

(2004)

D. Kiener et al.

Adv. Eng. Mater.

(2012)

J.R. Greer et al.

Phys. Rev. B

(2006)

A.T. Jennings et al.

Phys. Rev. Lett.

(2010)

D. Kiener et al.

Nano Lett.

(2011)

Cited by (18)

Experimental measurement of dislocation density in metallic materials: A quantitative comparison between measurements techniques (XRD, R-ECCI, HR-EBSD, TEM)
2023, Materials Characterization
The dislocation densities were measured on the same samples using transmission electron microscopy (TEM), scanning electron microscopy (electron channeling contrast imaging (ECCI) and high-angular-resolution-electron backscattered diffraction (HR-EBSD)), and X-ray diffraction (XRD). Notably, these different methods do not observe the same types of dislocations, i.e., statistically stored dislocations (SSDs) and/or geometrically necessary dislocations (GNDs). ECCI and TEM imaging are direct-measurement techniques, whereas HR-EBSD and XRD are indirect methods. Therefore, a quantitative comparison of the measurements obtained using these four techniques on undeformed and deformed duplex steels is proposed. For low deformation, where the dislocation density is quite small (1 − 5 × 10¹³ m⁻²), imaging methods are rather performant, whereas XRD measurements suffer from high uncertainty levels. HR-EBSD measurements show results that are in good agreement with the other methods for these deformation levels. For higher deformation levels (with dislocation densities above 1 − 3 × 10¹⁴ m⁻²), imaging methods are no longer relevant because of the increasing uncertainty arising from local contrast variation and overlapping of dislocations. The different results obtained highlight the necessity of taking a step back on each method used. Correctly defining what is to be measured (SSDs or GNDs), in which condition (solid material or thin plate) as well as the parameters (pixel size, area, etc.) and their bias is essential, especially if the objective is to use the measurement in a micromechanical model.
A model experiment on nano-particle stability in 12Cr-ODS steel using high-resolution high voltage microscopy
2022, Acta Materialia
Citation Excerpt :
Kohnert et al. reported a 40% decrease of dislocation density in Al, Fe, Mg, and Zr during a simulation thin film sample (100 nm) preparation compared with bulk microstructure. According to Kohnert's data, the retention of the dislocation loop is not linear with sample thickness, the retention rate decreases faster in the thinner sample [36]. The observation region in Du's research is 120–150 nm which is almost triple of this work, suggesting a much lower number density in current works.
In this work, the microstructure stability of 12Cr-ODS ferritic steel under irradiation is studied using a high-resolution high voltage electron microscope up to 1.5 dpa at 300 and 500 °C. Coherent YTiO₃ nanoparticles (NPs) undergo coarsening during electron irradiation and the coarsening rate is positively correlated to irradiation temperature (the third power of NPs size is proportional to the irradiation dose). He pre-implantation can significantly reduce the coarsening rate. A model coupling Ostwald ripening and irradiation enhanced diffusion is used to explain this coarsening. The fitting results between NPs size and irradiation dose show that pre-implanted He causes a lower diffusion of Y in 12Cr-ODS steel compared with the non-implanted sample. In response to the irradiation, plate-like radiation-induced precipitates appear which result in diffraction streaks in fast fourier transform (FFT) pattern. Precipitations are paralleled to [100] or [110] direction of Fe matrix and located near the interface between NPs and matrix. The hypothesis that coherent Cr precipitate along NPs/matrix interface caused diffraction streaks is proposed. Besides, He pre-implantation can also delay this precipitation.
Understanding radiation effects in friction stir welded MA956 using ion irradiation and a rate theory model
2022, Journal of Nuclear Materials
Citation Excerpt :
This was an effect of decreasing network and loop sink strength after an initial growth period from 0 dpa as dislocations interact and coalesce. Again, some variation could be explained by the theoretical loss of dislocations during FIB sample preparation [72] already noted in Section 4.3, but as this happened at each temperature, it is likely a genuine effect. Using the steady state rate theory calculations summarized in Table 7, Cv is always greater than Ci and both Cv and Ci are larger for SZ-H relative to SZ-M, regardless of temperature or dose, likely due to the of the overall lower sink strength.
An outstanding challenge in the manufacturing and joining of oxide dispersion strengthened steels is retaining the nanofeatures in the alloy throughout the fabrication and welding process. MA956 was friction stir welded with two different sets of welding parameters, resulting in a medium and high heat input. After welding, 5 MeV Fe⁺⁺ ion irradiations were performed at doses ranging from 50 to 200 dpa in the temperature range of 400 to 500 °C. Post-irradiation characterization was performed with scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy to investigate the Y-Al-O dispersoids, voids, and dislocations. After welding, the dispersoid microstructure coarsened, resulting in fewer and larger dispersoids regardless of heat input. After irradiation, the dispersoid behavior in the welded material was sensitive to temperature, exhibiting growth behavior attributed to Ostwald coarsening at 500 °C but a mixture of nucleation and more muted growth at 400 and 450 °C, attributed to competing mechanisms of radiation-enhanced diffusion and Ostwald coarsening. Void swelling correlated to heat input; being more prevalent in the welded conditions occurring at lower doses and in higher values relative to the base material. The low values of swelling despite microstructure coarsening caused by welding demonstrate the excellent swelling resistance of MA956, even after welding with the highest swelling values of 0.5% noted in the stir zone high heat input condition at 450 °C, 200 dpa. The dislocation behavior was inconsistent: the strongest trend was that network density was higher for welded versus base material, and an increase in loop diameter with temperature was observed. A rate theory model based on the observed microstructure suggests at high temperature interstitial loss to sinks was more likely to be dominant compared to mutual annihilation via point defect recombination, because of an increase of the radiation diffusion coefficient with temperature regardless of initial welded microstructure.
Insights from microstructure and mechanical property comparisons of three pilgered ferritic ODS tubes
2022, Materials and Design
Citation Excerpt :
From the high number of lamellar boundaries noted in the Fig. 4 EBSD maps, it is unsurprising that the XRD values are thus considerably higher. The other factor that could explain the larger discrepancy is that when using ions to thin TEM specimens, significant losses in dislocations (∼40%) have been shown to occur due to dislocation slip induced by stresses associated with sample preparation [45]. Important repercussions for this difference in dislocation density are discussed later in Section 4.1.
To develop advanced nuclear fuel claddings, two oxide dispersion strengthened (ODS) alloys were designed, manufactured, and evaluated. First, 12 %Cr alloy, is an ODS-version of ferritic steel, and second, 10 %Cr-6 %Al alloy, is a high-strength version of accident tolerant iron-chrome-aluminum alloy. Their properties and performance were compared with “classical” ODS material, 14 %Cr alloy 14WYT. Thin-walled (∼500 µm wall thickness) tubes were manufactured successfully using the pilgering technique. For all alloys, axial tensile specimens exhibited high tensile strength (>1 GPa) and reasonable plastic strains (10–17%). Ring tensile specimens, conversely, showed limited ductility (∼1%) with similar strengths to those measured in the axial orientation. The grain size, precipitate dispersion characteristics, and dislocation densities were then used to estimate yield strengths that were compared against room temperature axial and ring-pull tensile test data. The strengthening models showed mixed agreement with experimentally measured values due to the highly anisotropic microstructures of all three ODS tubes. These results illustrate the need for future model optimization to accommodate non-isotropic microstructures associated with components processed using rolling/pilgering approaches. In all cases, atom probe tomography and energy-filtered transmission electron microscopy demonstrated that ODS structure survived multiple pilgering operations, and precipitate microstructure evolution matched well the state-of-the-art nanoprecipitate coarsening models.
Crystalline defect analysis in epitaxial Si<inf>0.7</inf>Ge<inf>0.3</inf> layer using site-specific ECCI-STEM
2021, Micron
Citation Excerpt :
The sample was then directly observed in STEM mode. It is worth mentioning that the image stress relaxation can lead to a dislocation density drop of ∼40 % according to simulations using 3D discrete dislocation dynamics (Kohnert et al., 2020). However, in this study, the investigated sample differs from the simulation in the strain origin, the initial strain level, and the relaxation mechanisms.
Electron channeling contrast imaging (ECCI) is a powerful technique to characterize the structural defects present in a sample and to obtain relevant statistics about their density. Using ECCI, such defects can only be properly visualized, if the information depth is larger than the depth at which defects reside. Furthermore, a systematic correlation of the features observed by ECCI with the defect nature, confirmed by a complementary technique, is required for defect analysis. Therefore, we present in this paper a site-specific ECCI-scanning transmission electron microscopy (STEM) inspection. Its value is illustrated by the application to a partially relaxed epitaxial Si_0.7Ge_0.3 on a Si substrate. All experiments including the acquisition of ECCI micrographs, the carbon marking and STEM specimen preparation by focused ion beam, and the in-situ-subsequent-STEM-in-scanning electron microscopy (SEM) characterization were executed in one SEM/FIB-based system, thus significantly improving the analysis efficiency. The ECCI information depth in Si_0.7Ge_0.3 has been determined through measuring stacking fault widths using different beam energies. ECCI is further utilized to localize the defects for STEM sample preparation and in-situ-subsequent-STEM-in-SEM investigation. This method provides a correlative 2.5D defect analysis from both the surface and cross-section. Using these techniques, the nature of different line-featured defects in epilayers can be classified, as illustrated by our study on Si_0.7Ge_0.3, which helps to better understand the formation of those detrimental defects.
The effects of free surfaces on deformation twinning in HCP metals
2021, Materialia
Citation Excerpt :
Molecular dynamics simulations have also shown that the stress required for dislocation nucleation near free surfaces are lower than the bulk [27]. Dislocation dynamics simulations have also been implemented in order to capture image force effects on near-surface dislocations that assist in their fast ejection from the surface [28,29]. Crone et al. explained that voids, another source of free surfaces, in Al provided weaker strengthening effects than predicted by classical calculations of Lothe due to long-range image forces that act on the entire dislocation further away from the free surface [30,31].
Deformation twinning is a predominant mode of plastic deformation for hexagonal close packed metals, like Mg and Ti. The heterogenous microstructure and the local stresses associated with twinning play a key role in their mechanical response and fracture. Surface analyses, like electron microscopy, are frequently employed to spatially map microstructure and micromechanical fields in order to study twinning behavior. However, these measurements are inherently influenced by the vicinity of the free surface. Here, an elasto-visco-plastic fast-Fourier-transform (EVP-FFT) polycrystal modeling approach is employed to investigate the effects of free surfaces on twin development before and after loading. We compare calculated micromechanical fields on free surfaces with those calculated inside the bulk and, in some cases, experimental surface measurements. The results indicate that the creation of free surfaces can promote twin propagation and growth and can influence twin morphology by causing a twin lamella to become larger, more blunted and irregular. The structure along the twin boundaries are also affected, due to the higher driving stresses that extend prismatic-basal and basal-prismatic facets. Furthermore, free surfaces invoke different slip activities in the twin and the surrounding parent crystal by enhancing basal, prismatic and pyramidal slip in some localized regions, while reducing slip in others. We demonstrate that the simulated free-surface effects lead to better qualitative and quantitative agreement with experimental measurements from scanning electron microscopy and digital image correlation.

View all citing articles on Scopus

View full text

On the use of transmission electron microscopy to quantify dislocation densities in bulk metals

Abstract

Graphical abstract

Section snippets

Declaration of Competing Interest

Acknowledgments

Acta. Metallurgica.

Acta. Metallurgica. et Materialia.

Prog. Mater. Sci.

Int. J. Plast.

Mat. Sci. Eng.

Acta. Metall.

Int. J. Plast.

Mat. Sci. Eng.

Micron

Acta. Metall.

J. Phys. Chem. Solids

Int. J. Plast.

Acta. Mater.

J. Nucl. Mater.

Int. J. Plast.

Phys. Stat. Sol.

Philos. Mag. A

phys. stat. sol.

J. Appl. Crystallogr.

J. Appl. Crystallogr.

Int. J. Bifurcat. Chaos

J. Mater. Res.

Proc. Natl. Acad. Sci.

Adv. Eng. Mater.

Phys. Rev. B

Phys. Rev. Lett.

Nano Lett.