An Experimental Protocol Development of Three-Dimensional Transmission Electron Microscopy Methods for Ferrous Alloys: Towards Quantitative Microstructural Characterization in Three Dimensions

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The majority of engineering steels are ferromagnetic and structurally inhomogeneous on scales ranging from nanometers to micrometers, and their physical properties depend on the three-dimensional (3D) features in their microstructures. Thus, obtaining a 3D image with a large field of view is desirable for transmission electron microscopy (TEM) based microstructure characterization in order to establish the relationship between the microstructure and the physical properties with a reasonable statistical relevancy. Here, we use a conventional sample preparation process, i.e., mechanical polishing followed by electropolishing, and optimizing experimental protocols for electron tomography (ET) of ferromagnetic materials, to carry out microstructural characterization of engineering steel. We determined that the sample thickness after the mechanical polishing step is a critical experimental parameter affecting the success rate of tilt-series image acquisitions. For example, for ferritic heat-resistant 9Cr steel, mechanical thinning down to 30 μm or less was necessary to acquire an adequate tilt-series image of the carbide precipitates in the annular dark-field scanning TEM (ADF-STEM) mode. However, acquiring tilt-series images of dislocation structures remains a challenge due to an unavoidable, significant electron beam deflection during specimen tilt, even with a thinned sample. To overcome the electron beam deflection problem, we evaluated several relatively accessible approaches including the “Low-Mag STEM and Lorentz TEM” modes. Although rarely used for ET, both modes reduce or even zero the objective lens current, likely weakening the magnetic interference between the ferromagnetic specimen and the objective lens magnetic field. The advantages and disadvantages of these experimental components are discussed.

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ISIJ International, Vol. 55 (2015), No. 3ISIJ International, Vol. 55 (2015), No. 3, pp. 623–631

# An Experimental Protocol Development of Three-Dimensional Transmission Electron Microscopy Methods for Ferrous Alloys: Towards Quantitative Microstructural Characterization in Three Dimensions

Satoshi HATA, 1) - Kazuhisa SATO, 2) Mitsuhiro MURAYAMA, 3) Toshihiro TSUCHIYAMA 4) and
Hideharu NAKASHIMA 1)

1) Faculty of Engineering Sciences, Kyushu University, 6-1, Kasugakoen, Kasuga, Fukuoka, 816-8580 Japan.
2) Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi, 980-8577 Japan.
3) Department of Materials Science, Virginia Tech, Blacksburg, VA, 24061 U.S.A.
4) Faculty of Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka, 819-0395 Japan.

( _Received on July 28, 2014; accepted on November 20, 2014; originally published in Tetsu-to-Hagané,_
_Vol. 100, 2014, No. 7, pp. 889–896_ )

The majority of engineering steels are ferromagnetic and structurally inhomogeneous on scales ranging
from nanometers to micrometers, and their physical properties depend on the three-dimensional (3D)
features in their microstructures. Thus, obtaining a 3D image with a large field of view is desirable for
transmission electron microscopy (TEM) based microstructure characterization in order to establish the
relationship between the microstructure and the physical properties with a reasonable statistical relevancy.
Here, we use a conventional sample preparation process, _i.e._, mechanical polishing followed by electropolishing, and optimizing experimental protocols for electron tomography (ET) of ferromagnetic materials, to carry out microstructural characterization of engineering steel. We determined that the sample
thickness after the mechanical polishing step is a critical experimental parameter affecting the success
rate of tilt-series image acquisitions. For example, for ferritic heat-resistant 9Cr steel, mechanical thinning
down to 30 μ m or less was necessary to acquire an adequate tilt-series image of the carbide precipitates
in the annular dark-field scanning TEM (ADF-STEM) mode. However, acquiring tilt-series images of dislocation structures remains a challenge due to an unavoidable, significant electron beam deflection during
specimen tilt, even with a thinned sample. To overcome the electron beam deflection problem, we evaluated several relatively accessible approaches including the “Low-Mag STEM and Lorentz TEM” modes.
Although rarely used for ET, both modes reduce or even zero the objective lens current, likely weakening
the magnetic interference between the ferromagnetic specimen and the objective lens magnetic field. The
advantages and disadvantages of these experimental components are discussed.

KEY WORDS: electron tomography; transmission electron microscopy (TEM); scanning transmission electron microscopy (STEM); magnetism; precipitate; dislocation.

1. Introduction

In order to quantitatively analyze material microstructures, three-dimensional (3D) observation using various
microscopy methods has been performed to rationalize
relationships between the microstructure and material
properties. The objects under 3D observation vary from
atomic clusters at a sub nanometer scale to macroscopic
bulk materials at a sub millimeter scale. This study focuses
on 3D transmission electron microscopy (TEM) observation
that covers a sub nanometer to micrometer length scale with
the capability to clarify assorted microstructural information, including morphology, crystal structures, and chemi

- Corresponding author: E-mail: hata.satoshi.207@m.kyushu-u.ac.jp
DOI: http://dx.doi.org/10.2355/isijinternational.55.623

cal compositions, for a particular region of interest in a
sample.

The stereoscopic imaging method using TEM has been
used as a conventional 3D observation method. However,
the electron tomography (ET) method has recently shown
significant promise for providing better quality of quantitative 3D information than the stereoscopic imaging method.
The basic concept of ET is the same as that of X-ray
computed tomography (CT). In ET observation, a sample
is intermittently tilted in an electron microscope with the
object’s image in the sample subsequently acquired at each
specimen tilt angle (tilt-series acquisition), as illustrated
in Fig. 1 . After the tilt-series dataset acquisition, a 3D
representation of the object is reconstructed and visualized
using an algorithm such as the back-projection method.
Recent advancements in ET technique enable a variety

ISIJ International, Vol. 55 (2015), No. 3

for tilt-series data sets of ferromagnetic samples are discussed, and finally, the optimum conditions for ET observation of precipitates and dislocations in steel samples are
proposed.

2. Experimental

Fig. 1. Basic procedure of electron tomography. Step 1, tilt-series

dataset acquisition, is carried out on a transmission electron microscope. A dedicated specimen holder which is
capable of tilting the specimen up to more than 60° is necessary. Step 2, 3D volume reconstruction, is performed in
a computer.

of 3D observations, such as 3D atomic arrangements and
nanoparticle morphology, 1–3) morphology and spatial distribution of precipitates, 4–6) and dislocations 7–10) in crystalline
materials.

However, ET observation of steel has some limitations.
Steels, except for the austenite-based steels, are ferromagnetic at room temperature, and using TEM to observe a
thin-foil steel sample results in image distortion, deflection of the electron beam, and temporary bending of the
sample due to the magnetism of the ferrous iron phase.
The magnetism influence of a thin-foil steel sample makes
tilt-series acquisition difficult, and the resultant 3D reconstruction from the tilt-series dataset is usually not reliable.
One solution for reducing the magnetism influence is the
fabrication of a small thin-foil sample of size 10–20 μm
using a focused ion beam (FIB). However, sample bending
from the magnetism can occur even in these small FIBproduced samples during the tilt-series acquisition. Therefore, thus far, there is no fundamental solution to overcome
the magnetism problems in the ET observation of steel
samples.

The present study proposes an experimental procedure for
ET observation of a steel sample with a field of view larger
than several hundreds of square micrometers. Obtaining a
3D representation across a large field of view is desirable
in TEM microstructural characterization to determine the
relationship between the microstructure and the physical
properties with a reasonable statistical relevancy. First, for
preparation of the large field of view samples described
above, millimeter-sized ferromagnetic iron alloy samples
are thinned using a conventional twin-jet electropolishing
method. Second, in the tilt-series acquisition, the feasibility of several imaging modes in both TEM and scanning
transmission electron microscopy (STEM) are examined. In
addition to the conventional TEM/STEM imaging modes,
the Low-Mag (LM) and LM Lorentz modes are employed,
with the objective lens current tuned to small values or
zero in order to minimize the magnetic field applied to a
sample. Third, the 3D reconstruction algorithms suitable

2.1. Sample Preparation
This study utilized three kinds of iron alloys that are ferromagnetic at a room temperature. The first sample is 9Cr
ferritic heat-resistant steel in which _M_ 23C6 metal-carbide
particles were precipitated at the martensite lath boundaries. The second and third samples are alloys of Fe with 2
mass% Cu that forms ε-Cu phase precipitates and Fe with
0.94 mass% V, 0.19 mass% C that forms V-C compounds
in the ferrous α-Fe matrix.

These alloy samples were cut into 3-mm-diameter disks
or smaller rectangular plates about 1 mm × 2 mm, and then
mechanically polished to 30–100 μm thickness by following
a standard polishing method. 11–13) SiC powder coated polishing papers No. 1 000–4 000 and alumina powders approximately 0.3 μm in diameter were used for the mechanical
polishing. The rectangular-plate sample was fixed on a
3-mm-diameter Mo single-hole grid using a thermosetting
resin to ensure deformation-free mechanical polishing. After
the mechanical polishing, an electron transparent region was
obtained using twin-jet electropolishing. The electropolished surface was cleaned by low-energy argon ion milling
at an acceleration voltage of 300 V.

Square thin-foil specimens smaller than 20 μm were also
prepared using FIB milling in order to examine the influence of sample shape on the tilt-series observation. These
FIB milled specimens were finished by low-energy argon
ion milling to remove the damaged surface layer created by
the FIB milling.

2.2. Electron Microscopy Observation
In order to study the feasibility of various measurement
conditions for 3D imaging of ferrous alloys, we used the
following TEMs: the FEI Tecnai F20 Super Twin (ST)
and the FEI Titan 80–300 ST equipped with and without a
CEOS TEM image corrector for spherical aberrations. The
Tecnai F20 microscope is equipped with a double-condenser
lens system operated at a 200-kV accelerating voltage, and
the Titan 80–300 microscopes were operated at 300 kV.
The three-stage condenser-lens system of the Titan 80–300
enables different illumination modes such as parallel and
condensing illumination. These FEI microscopes can operate in conventional TEM/STEM modes as well as the
LM-TEM, LM-STEM, and LM-TEM Lorentz modes. The
present study investigated the feasibility of these LM imaging modes for observing precipitates and dislocations in
ferromagnetic steels. Table 1 summarizes the lens settings
for the different imaging modes. The lens currents in Table
1 are represented as percentages of the maximum current.
Here, we focus on the objective-lens currents. In the case
of the Lorentz TEM mode, where the main objective is
observation of the magnetic image contrast, the objectivelens current can be set to 0% for visualizing the sample’s
original magnetic microstructures such as magnetic domains
and walls. Furthermore, the Lorentz TEM mode can alter

ISIJ International, Vol. 55 (2015), No. 3

Table 1. Lens configurations of electron microscopes used in the present study under various imaging modes.

FEI Tecnai F20 Super
Apparatus

Twin 200 kV

FEI Titan 80–300 Super Twin

300 kV

Observation mode TEM LM-TEM Lorentz TEM STEM LM-STEM LM-STEM μ-probe STEM

Condenser lens 1 (CL1)

13.9% 13.9% 13.9% (Valuable) 21.2% 21.2% 36.6% 36.6%
current (%)

Condenser lens 2 (CL2)

Variable Variable 43.0% 32.0% 37.2% 29.1% 38.0%
current (%)

Condenser mini-lens 0% (Variable for

83.9% 83.9% −83.9% 83.9% 97.9% 66.1%
current (%) magnetizing sample)

Objective lens current

(%)

0% (Variable for
~91.3% ~6.0% 91.9% 6.0% 4.2% 88.8%
magnetizing sample)

Objective mini-lens 0–100% (Change

0% 0% 0% 0% 0% 0%
current (%) with focusing)

Magnification (times) 6 200–910 000 19–1 850 110–44 000 10 000–330 000 000 150–155 000 140 000 (Max.) 10 000 (Min.)

Camera length  -  -  - 30 mm (Min.) 5.13 m (Min.) 9.1 m (Min.)

Incident beam conver- 0.2 mrad (70 μm

-              -              -              -              gence angle CL2 aperture)

1.6 mrad (70 μm
CL2 aperture)

both the objective-lens currents and the condenser minilens currents to observe the magnetization processes in a
sample. In comparison, the objective-lens currents for the
LM-TEM and LM-STEM modes were set close to zero, at
approximately 5% (1 000 Oe magnetic field). Although this
5% objective-lens current is much smaller than that (~90%)
used for conventional TEM/STEM modes (including the
“μ-probe STEM” mode), it should be noted that this 5%
objective-lens current is sufficient to magnetize the α-Fe
ferromagnetic phase.

Nevertheless, we examined the feasibility of using an
objective-lens-less imaging method (LM-TEM/STEM) for
ET observation of ferromagnetic steel samples based on
their promising aspects. First, the magnetic interference
between the sample and objective lens would be weak to
negligible for objective-lens-less imaging modes. Therefore, it is expected that the volume reduction process can
be omitted for a magnetic sample. Second, the low spatial
resolution in the objective-lens-less imaging modes is not
necessarily a serious problem. For the electron microscopes
used in this study (Tecnai F20 and Titan 80–300 equipped
with and without a TEM image corrector), the nominal spatial resolution for the objective-lens-less mode is
approximately 2 nm for TEM and 10 nm for STEM. These
values of the spatial resolution are suitable for quantitative
analyses of the spatial distribution and the quantities of
precipitates and dislocations in some steel samples, as these
microstructural analyses do not require an ultrahigh (atomicscale) resolution. Furthermore, if the electron microscope
is equipped with a TEM spherical aberration corrector, the
spatial resolution of the objective-lens-less TEM modes is
improved.

2.3. Electron Tomography (ET) Observation
The ET observation consists of two steps: (1) acquisition of a tilt-series dataset and (2) reconstruction of the 3D
volume, as shown in Fig. 1. Since the samples in this study
are ferromagnetic, the projection requirement 4) could have
serious violations affecting the collected TEM/STEM tilt

series datasets. The projection requirement is an essential
imaging condition for reconstructing a reliable 3D image.
Figure 2 illustrates the relationships between the image
intensity and the specimen thickness; the projection requirement is satisfied for cases A and B, but is violated for case
C. The violation of the projection requirement is mainly
due to diffraction contrast such as thickness fringe, bend
contour, and strain contrast. Because these changes in the
diffraction contrast are sensitive to the diffraction conditions, we minimized these changes by using an incident
beam-tilt function while collecting the tilt-series acquisition.
The total acquisition time for a tilt-series dataset was 1–2
hours depending on the number of images acquired. Several
images for which the projection requirement was seriously
violated were excluded from the tilt-series dataset, and a 3D
image was reconstructed using the weighted back projection
(WBP) 3,4,14,15) or the simultaneous iterative reconstruction
technique (SIRT). 3,15,16)

3. Results and Discussion

First, we introduce the optimized conditions for ET

Fig. 2. Schematic explanation of the projection requirement. For

accurate tomographic 3D reconstruction, the image intensity should be monotonic functions of physical properties
of the specimen such as density, thickness, and degree of
order.

ISIJ International, Vol. 55 (2015), No. 3

observation of ferromagnetic samples. Second, we present the results of preliminary 3D observation of precipitates and dislocations under the optimum observation
conditions.

3.1. Effect of Sample Shape on ET Observation
In order to fabricate a thin-foil sample using the FIB microsampling technique, a rectangular-plate sample 10–20 μm
in size was cut from the bulk sample, and then thinned by
a Ga 2 + ion beam to a sample thickness that was transparent
to the electron beam. The thin-foil specimens obtained by
FIB milling were wedge-shaped to some extent, resulting
in a varying thickness in the electron transparent region
from several micrometers to several tens of micrometers. In
other words, the specimen thickness was not uniform in a
wide field of view, even though the FIB milled sample had
a small total volume.

In contrast, for the thin-foil specimens prepared using
electropolishing as the final thinning process, in general 3
mm disks of thickness 50–100 μm were collected from the
bulk sample by mechanical processes, and then, the central
portion of the disks were thinned by twin-jet electropolishing or argon ion milling. These thin-foil samples were
considerably larger in volume than the FIB milled samples,
which is believed to be disadvantageous in ferromagnetic
sample observation because the magnitude of magnetization
is proportional to the sample volume. However, if appropriate mechanical polishing was performed for the flat sample
preparation and optimized electropolishing conditions such
as optimized voltage, current, temperature, and flow-rate
of the solution were used, we could obtain a wider field of
view in the electropolished sample than a sample prepared
using the FIB microsampling technique. The electropolished
ferromagnetic material sample with a wide field of view and
uniform thickness was advantageous for low-magnification
imaging of the microstructure. Furthermore, we found that
the magnetic interactions between the electron beam and the
electropolished sample prepared under optimized conditions
remained constant during the tilt-series acquisition. Actually
for the 3 mm disc sample thinned down to as thin as 30 μm
by mechanical polishing prior to electropolishing, STEM

Fig. 3. LM-STEM dark-field images of a 9Cr ferritic heat-resis
tant steel sample acquired at 300 kV (Titan 80–300
equipped with a TEM image corrector). The sample was
mechanically polished down to 30 μm and subsequently
electropolished. A low-magnification image (a) visualizes
a wide electron transparent field of view and its magnified
view (b) shows various sizes of lath martensites. This
specimen was capable of acquiring annular dark-field
-STEM tilt-series datasets, as shown in Fig. 9.

tilt-series acquisition of the dispersed precipitate microstructure in a ferritic steel sample was possible even in the strong
objective-lens field of the normal STEM mode. Figure 3
shows an LM-STEM image of the 9Cr ferritic heat-resistant
steel for which the STEM tilt-series dataset was acquired, as
demonstrated in Fig. 9 . Martensite laths of various widths
are visible in a field of view exceeding 100 μm × 100 μm,
indicating that the electropolished sample is beneficial for
quantitative analysis of the ferritic steel sample microstructure to show the micron scale inhomogeneity.

As for sample preparation using a FIB, Kacher _et al._ 17)
succeeded in collecting tilt-series of dislocation substructures in nonmagnetic austenitic steel using a TEM sample
preparation procedure, where the FIB microsampling and
thinning in the thin-foil portion of an electropolished sample
produced a micrometer-sized thin foil normal to the desired
crystallographic orientation. This method is effective for
tilt-series acquisition under a constant diffraction condition,
although its feasibility for similar tilt-series observation of
dislocation substructures in ferromagnetic samples has not
been reported. This final treatment using the FIB microsampling technique is capable of reducing the final sample
volume drastically, although an FIB mill may not be easily
accessible for everyone.

3.2. Observation Mode
Commercially available TEM/STEM instruments provide the following imaging modes: (1) normal TEM/
STEM modes capable of high-magnification imaging; (2)
LM-TEM/STEM modes for which the objective-lens current is set to zero or near zero; and (3) LM-TEM Lorentz
modes for which the objective lens current is definitively
at zero. Although optional functions, such as the free lens
control, enable setting various imaging modes by changing the individually applied currents for all lenses, these
customized lens settings are not generally applicable for
microstructural material observation using conventional
electron microscopes. In this study, we attempted to use the
LM-STEM and LM-TEM Lorentz modes for ET observation of ferromagnetic samples, and we examined whether
the use of these imaging modes coupled with no or weak
magnetic interference between the sample and the objective
lens are applicable to 3D microstructural observation in ferritic steels with a limited resolution.

3.2.1. LM-STEM Mode
First, we describe a notable characteristic of the LMSTEM mode. The electron beam-convergence angle (α)
for a normal STEM mode using the objective-lens field is
α= 5–20 mrad, optimized to a reasonable spatial resolution
and contrast. Conversely, the LM-STEM mode without the
objective-lens field can reduce α to ~0.2 mrad, enabling a
parallel-beam STEM observation. As a result, the parallelbeam LM-STEM mode gives approximately 10 times the
focus depth of the normal STEM modes. This is advantageous for tilt-series acquisition because one can avoid the
out-of-focus conditions for high tilt angles that arise from
long beam penetration distances in a specimen.

Second, we describe the effects of observation conditions
on the LM-STEM images. In this study, we operated electron microscopes at acceleration voltages of 200 kV and 300

ISIJ International, Vol. 55 (2015), No. 3

kV. As the electron penetration depth is proportional to the
root of the acceleration voltage, the penetration depth at 300
kV is 20% greater than that at 200 kV. From the viewpoint
of the electron wavelength, the spatial resolution of the
LM-STEM image at 300 kV is expected to also be approximately 20% higher than that at 200 kV, under a constant
spherical-aberration coefficient of the probe-forming lens
system. Furthermore, the condenser-lens systems should be
discussed; the Tecnai F20 microscope has a two-stage condenser lens system that focuses the electron beam using the
second condenser lens. On the other hand, the Titan 80–300
microscope has a three-stage condenser-lens system, where
the third condenser lens focuses the electron beam, allowing
the minimum size of the focused beam on the specimen to
be smaller than that for the two-stage condenser-lens system. In contrast to TEM observation, for STEM, the size of
the electron beam scanning the specimen strongly influences
the spatial resolution. Indeed, the LM-STEM image in Fig.
4 (a) acquired using the Titan 80–300 at 300 kV shows a
higher spatial resolution than the LM-STEM image in Fig.
4(b) acquired using the Tecnai F20 at 200 kV. In addition,
the size of the condenser-lens aperture is also an important
parameter that strongly influences the spatial resolution
of the LM-STEM image. When we selected a smaller
condenser-lens aperture, the focused electron beam size
decreased, also decreasing the electron dose and resultant
image contrast. Therefore, the size of the condenser-lens

aperture needs to be optimized for LM-STEM. In this study,
the optimized condenser-lens aperture size was 50–70 μm.
Bright- and dark-field LM-STEM images show different
characteristic image contrasts compared to normal STEM
images. Figure 4 indicates that dislocations are recognized
as dark lines in both the dark-field (a) and bright-field (b)
LM-STEM images. This distinctive dislocation image contrast in LM-STEM is different from that in normal STEM/

Fig. 4. LM-STEM bright-field image of an Fe–V–C alloy sample

acquired at 300 kV (Titan 80–300 equipped with a TEM
image corrector) (a) and LM-STEM dark-field image of a
9Cr ferritic heat-resistant steel acquired at 200 kV (Tecnai
F20) (b). Both images visualize interactions between dislocations and precipitates. The spatial resolution of the 300
kV image appears to be higher than that of the 200 kV
image, regardless of the imaging modes.

Fig. 5. A part of LM-STEM tilt-series dataset of an Fe–Cu alloy sample. ε-Cu precipitates are recognized in α-Fe

matrix. Bright-field and dark-field images are intermixed due to the electron beam deflection as a function of the
specimen tilt angle.

ISIJ International, Vol. 55 (2015), No. 3

TEM because dislocations are observed as bright lines in
normal STEM/TEM dark-field images.

3.2.2. ET Observation Using LM-STEM
In this study, we acquired LM-STEM tilt-series datasets
of ferritic steel samples at a direct 2 × 10 4 x magnification.
When the LM-STEM image exhibited a sudden focus or
contrast change at a specific specimen tilt angle, we subsequently checked the imaging conditions before and after
the specimen tilt.

Figure 5 shows a part of the LM-STEM tilt-series dataset
of an electropolished Fe-Cu alloy sample. The acquisition
conditions were as follows: the Titan 80–300 equipped with
a TEM image corrector was operated at 300 kV; the second
condenser-lens aperture was 50 μm; the beam-convergence
angle (α) was 0.35 mrad; the camera length was 9.1 m; and
samples were measured in dark-field mode using an annular
detector. The LM-STEM tilt-series shows several “contrastreversals” with varying specimen tilt angles. For example,
when the specimen tilt angle varied between 0°, −5°, and
−10°, the image contrast appeared to be dark-field (DF),
bright-field (BF) and DF again, respectively. This mixture of
the BF and DF images was found to be caused by electron

Fig. 6. A schematic illustration shows the direct beam translating

across the annular STEM detector during the tilt-series
acquisition in Fig. 5. Such a movement of the direct beam
causes a mixture of bright-field and dark-field images in a
tilt-series dataset, as shown in Fig. 5.

beam deflection with the specimen tilt angle. Figure 6 illustrates the locus of the direct beam ( _hkl_ = 000) on the annular
detector observed in the LM-STEM mode. When the ferromagnetic sample was tilted on the objective-lens pole piece,
the magnetic field changed, deflecting the electron beam
below the sample due to the Lorentz force and shifting it
from the center of the annular detector, as illustrated in Fig.
6. The BF images appeared when the direct beam reached
the annular detector, and the DF images reappeared when
the direct beam moved further from the annular detector
edge. We recognized that the magnitude of this beam shift
depended on the specimen tilt angle and the locus of the
beam shift was reproducible.

We attempted LM-STEM tomography observation for
an electropolished Fe-V-C alloy sample. Figure 7 shows
the typical LM-STEM images acquired under the following
conditions: the Titan 80–300 equipped with a TEM image
corrector was operated at 300 kV; the condenser-lens aperture was 50 μm; the beam-convergence angle (α) was 0.2
mrad; the camera length was 9.1 m; and measurements were
obtained in the DF using an annular detector. V-C type precipitates in a size range of several tens of nanometers can be
clearly recognized. An LM-STEM energy-dispersive X-ray
spectroscopy (EDS) elemental map confirmed the existence
of vanadium in the precipitates.

The acquired LM-STEM tilt-series dataset of the Fe–C–
V alloy sample was again a mixture of BF and DF images,
as shown in Fig. 5. We excluded the BF images from the

Fig. 7. LM-STEM dark-field images of an Fe–V–C alloy sample.

The images were taken at (a) a low-magnification and (b) a
medium-magnification. V–C precipitates are observed.

Fig. 8. Three-dimensional views of precipitates in an Fe–V–C alloy sample reconstructed from a LM-STEM tilt-series

dataset. V–C precipitates are reconstructed in the 3D volume of 1407 nm × 1848 nm × 693 nm and the pixel size
of 5.25 nm/px.

ISIJ International, Vol. 55 (2015), No. 3

tilt-series dataset and reconstructed a 3D image from the
remaining 52 DF image frames in the angular specimen tilt
angle range of −52° to + 58°, as shown in Fig. 8 . Although
the morphology of the V-C precipitates is not clear, it is
possible to recognize their 3D spatial distribution. However, the scattered electron detection angle at the annular
detector was not constant during the data acquisition due
to the beam-deflection angle varied with the specimen tilt
angle. This infers that the projection requirement (Fig. 2) is
not satisfactory in the LM-STEM tilt-series dataset. Therefore, the 3D microstructure in Fig. 8 should be analyzed
qualitatively.

3.2.3. LM-TEM and LM-TEM Lorentz Modes
The conventional LM-TEM mode without using the
objective lens field provides a few thousands times the
maximum direct magnification, and its spatial resolution
power is insufficient for visualizing fine precipitates in
steels. However, in the case of the LM-TEM Lorentz mode
using a Lorentz lens (objective mini lens), the maximum
direct magnification reaches several tens of thousands. Since
the LM-TEM Lorentz mode was originally developed for
observing magnetic microstructures, it is possible to avoid
sample magnetization (zero the objective-lens field). Therefore, in a TEM spherical aberration corrector with an available LM-TEM Lorentz mode, one can expect to observe, for
example, fine precipitates several nanometers in size. There
are two kinds of Lorentz microscopy methods: the Fresnel
method that visualizes magnetic domain boundaries using
large defocus distances around 1 mm, and the Foucault
method that visualizes magnetic domains as a DF image

contrast by selecting one of the separated diffracted beams
from the electron beam deflection. The Foucault method
is suitable for this study to visualize fine precipitates and
dislocations under minimal influence of the objective-lens
field, because it is operated near an in-focus condition,
capable of selecting a desirable diffracted beam using a
DF imaging. However, we were unable to perform LMTEM Lorentz tilt-series acquisition for the ferromagnetic
steel sample using the Foucault method. The main reason
for this was that it was not possible to align the deflected
electron beam in a constant ray path using the alignment
coils.

3.2.4. Optimum Conditions for ET Observation of Precipitates and Dislocations
As demonstrated in Fig. 8, LM-STEM tomography is
capable of 3D visualization of spatially dispersed precipitates several tens of nanometers in size. On the other hand,
LM-STEM tomography was not appropriate for 3D imaging
of dislocations in our experiments. At the beginning of this
study, we expected LM-STEM to be a promising candidate
for low-magnification ET observation of both precipitates
and dislocations. This was because LM-STEM provides
an enhanced image contrast with a large focus depth as a
result of the high brightness and a parallel electron beam
on a specimen, compared to the case of the normal STEM
mode, which uses the objective-lens field for focusing the
electron probe. For 200 kV imaging with the conventional
two-stage condenser-lens system (Tecnai F20), the spatial
resolution of LM-STEM was not satisfactory for 3D tomographic observation of precipitates and dislocations in steel

Fig. 9. A part of ADF-STEM tilt-series dataset of a 9Cr ferritic heat-resistant steel sample. Specimen tilt angles are

denoted in the images.

ISIJ International, Vol. 55 (2015), No. 3

samples. However, the LM-STEM mode operated at 300
kV with the three-stage condenser-lens system is promising
for 3D tomographic observation, as demonstrated in Figs.
5(a), 7 and 8.
The most serious issue with LM-STEM tomography
observation of ferromagnetic samples is that the deflected
electron beam with specimen tilt cannot be aligned in a
constant optical axis by the beam-alignment coils. This
makes dislocation tilt-series acquisition under a constant
diffraction condition difficult. We should also mention that
the objective-lens field is not necessarily zero even in the
LM-STEM mode, as seen in Table 1. For example, in the
case of the Titan 80–300, the LM-STEM mode objectivelens current is 4%, corresponding to a magnetic field of 10 3
Oe applied to a sample on the objective lens pole piece. This
magnetic field is sufficient to magnetize the α-Fe phase,
resulting in the magnetic interference between the tilted
ferromagnetic sample and the objective-lens field generating
a large electron beam deflection in the LM-STEM mode.

In comparison, the normal STEM mode of the Titan
80–300 has a typical operation objective-lens current of
89%, producing a magnetic field of approximately 2.1 × 10 4
Oe (this value changes to some extent depending on the
imaging and alignment conditions). This objective lens
magnetic field is much stronger than that of the magnetized
steel sample. Therefore, for the normal STEM mode, the
electron beam-deflection angle with specimen tilt on the
objective-lens pole piece is roughly one order of magnitude
smaller than the beam-deflection angle for the LM-STEM
mode. As a result, a mixture of BF and DF images in the
tilt-series dataset, shown in Fig. 5, did not occur frequently
in the normal STEM mode. In other words, the strong
magnetic field of the objective lens system in the normal
STEM mode is valuable from the viewpoint of suppressing
the electron beam deflection during the tilt-series acquisition
for a ferromagnetic sample.

Figure 9 shows a portion of an annular dark-field (ADF)STEM tilt-series dataset of the 9Cr ferritic heat-resistant
steel sample of thickness 30 μm after the mechanical polishing as shown in Fig. 3. In the tilt-series dataset acquisition, a high-angle triple-axis specimen holder 10) was used
to align a diffracted beam of _g_ ( _hkl_ ) = 110α-Fe to the Bragg
condition exactly on the specimen tilt axis. As a result, the
ADF-STEM tilt-series images visualize both dislocations
and carbides precipitated at the martensite lath boundaries. However, the influence of magnetic field interference
between the sample and the objective-lens system was still
present, including sudden changes in the image contrast
of the dislocations and martensite matrix for specimen
tilt angles higher than 20°, and the left/right edge areas
of the STEM images becoming out of focus due to the
ineffective dynamic focus alignments and electron beam
astigmatism.

It is inferred that the ADF-STEM tilt-series dataset in Fig.
9 would be capable of 3D visualization of carbide particles.
However, the dislocation line contrast would not be clearly
resolved in a 3D reconstructed image because the dislocation contrast in the ADF-STEM tilt-series images was not
constant (consistently visible as bright lines), caused by
the sudden changes in the diffraction conditions with the
specimen tilt angle. In order to realize high-resolution 3D

visualization of dislocations in such ferromagnetic samples,
a method of tilt-series acquisition combined with correction of any deviation from a constant diffraction condition
is required to eliminate the influence of the electron beam
deflection. One possible method is to use an incident beam
tilt function for precise diffraction alignments. 18) Another
possible method is to use novel 3D reconstruction algorithms such as discrete tomography 19) and compressive
sensing. 20–22) These new algorithms are promising since
reducing the number of images in a tilt-series dataset does
not notably degrade the accuracy of the reconstructed 3D
object morphology. Therefore, acquisition of a set of dislocation images under a suitable diffraction condition from
a tilt-series dataset along with 3D reconstruction using one
of these new algorithms would enable high-resolution 3D
visualization of the dislocation substructures in ferromagnetic steels. After overcoming the technical issues described
above, ET observation of various microstructures in steels
will be possible, allowing a paradigm shift from qualitative
to quantitative understanding of the relationship between
the mechanical properties and dislocation substructures in
steels.

4. Conclusions

The present study focused on the development of an
experimental protocol of ET observation techniques for
ferrous alloys, and investigated the optimal conditions and
methods for each ET process, from sample preparation to
TEM/STEM mode settings and 3D reconstruction algorithms. The conclusions are described as follows:

(1) In the case of a sample with a wide field of view
over several hundreds of square micrometers, the sample
should be thinned to less than 30 μm by mechanical polishing before the final electropolishing. The thinning process
effectively suppresses electron beam deflection and image
quality degradation due to the magnetic field interference
between the tilting ferromagnetic sample and the objectivelens system.

(2) For tilt-series dataset acquisition using a mass-thickness contrast of precipitates in an electropolished sample,
the strong magnetic field of the objective-lens system in the
normal STEM mode suppresses the electron beam deflection
with specimen tilt, resulting in successful acquisition of the
tilt-series dataset.

(3) For tilt-series dataset acquisition using diffraction
contrast of dislocations in an electropolished sample, even
for the normal STEM mode, the small angle of electron
beam deflection results in significant changes in the diffraction conditions with specimen tilt. This degrades the
resolution power of the resultant 3D reconstructed image of
sample dislocations.

(4) We showed the possibility of objective-lens-less
LM-STEM tomography observation in a wide field of view
along with a preliminary 3D reconstruction result for V-C
type particles of several tens of nanometers in size precipitated in the α-Fe matrix.

(5) Because the LM-STEM mode without use of the
objective-lens field is strongly influenced by the electron
beam deflection with a ferromagnetic sample tilt, the normal STEM mode is preferable for ET observation of fer

ISIJ International, Vol. 55 (2015), No. 3

romagnetic samples. Nevertheless, the LM-STEM mode
requires further investigation as a promising 3D imaging
mode, because the high brightness and highly parallel
electron beams in the LM-STEM mode is advantageous for
wide field-of-view tilt-series dataset acquisition for steel
samples.

Acknowledgments

The present study was financially supported by the following: the JFE 21st Century Foundation, Japan; NSSM,
Japan; the ISIJ Research Promotion Grant, Japan; MEXT
Kakenhi, Japan; JSPS Kakenhi, Japan; the JSPS Strategic
Young Researcher Overseas Visits Program, Japan; the
JST “Collaborative Research Based on Industrial Demand”
program, Japan; and the JST “Development of systems and
technology for advanced measurement and analysis” program, Japan. The electron microscopy observation in this
study was performed at the following locations: the Ultramicroscopy Research Laboratory, Kyushu University, Japan;
the 1 MV Electron Microscopy Laboratory, Tohoku University, Japan; and the Nanoscale Characterization and the
Fabrication Laboratory, ICTAS, Virginia Tech, USA. The
authors would like to thank the following people for their
kind support and valuable comments: D. Akama, S. Yamamoto, R. Imamura, T. Yoshimoto, M. Mitsuhara, K. Ikeda,
and S. Takaki (Kyushu University, Japan); N. Monsegue
(Virginia Tech), T. Konno, E. Aoyagi, Y. Hayasaka, and
Y. Kodama (Tohoku University, Japan); K. Sato (JFE Steel,
Japan); G. Shigesato (NSSM, Japan); and B. Freitag (FEI,
Netherlands).

REFERENCES

1) S. Van Aert, K. J. Batenburg, M. D. Rossell, R. Erni and G. van
Tendeloo: _Nature_, 470 (2011), 374.
2) M. C. Scott, C.-C. Chen, M. Mecklenburg, C. Zhu, R. Xu, P. Ercius,
U. Dahmen, B. C. Regan and J. Miao: _Nature_, 483 (2012), 444.
3) K. Sato, K. Aoyagi and T. J. Konno: _J. Appl. Phys._, 107 (2010),
024304.
4) P. A. Midgley and M. Weyland: _Ultramicroscopy_, 96 (2003), 413.
5) K. Inoke, K. Kaneko, M. Weyland, P. A. Midgley, K. Higashida and
Z. Horita: _Acta Mater._, 54 (2006), 2957.
6) K. Kaneko, K. Inoke, K. Sato, K. Kitawaki, H. Higashida, I. Arslan
and P. A. Midgley: _Ultramicroscopy_, 108 (2008), 210.
7) J. S. Barnard, J. Sharp, J. R. Tong and P. A. Midgley: _Science_, 313
(2006), 319.
8) J. S. Barnard, A. S. Eggman, J. Sharp, T. A. White and P. A. Midgley:
_Philos. Mag._, 90 (2010), 14.
9) M. Tanaka, K. Higashida, K. Kaneko, S. Hata and M. Mitsuhara: _Scr._
_Mater._, 59 (2008), 901.
10) S. Hata, H. Miyazaki, S. Miyazaki, M. Mitsuhara, M. Tanaka, K.
Kaneko, K. Higashida, K. Ikeda, H. Nakashima, S. Matsumura, J.
S. Barnard, J. H. Sharp and P. A. Midgley: _Ultramicroscopy_, 111
(2011), 1168.
11) R. D. Schoone and E. A. Fischione: _Rev. Sci. Instrum._, 37 (1966).
12) Practical Electron Microscopy and Its Application to Materials, ed.
by The Japan Institute of Metals and The Iron and Steel Institute of
Japan, Maruzen, Tokyo, (2002).
13) Practical Microscopy Techniques for Materials Development, ed. by
The Japan Institute of Metals, Maruzen, Tokyo, (2006).
14) R. A. Crowther, D. J. de Rosier and A. Klug: _Proc. R. Soc. London_,
A317 (1970), 319.
15) J. Tong, I. Arslan and P. Midgley: _J. Struct. Biol._, 153 (2006), 55.
16) P. Gilbert: _J. Theor. Biol._, 36 (1972), 105.
17) J. P. Kacher, G. S. Liu and I. M. Robertson: _Scr. Mater._, 64 (2011),
677.
18) K. Kimura, S. Hata, S. Matsumura and T. Horiuchi: _J. Electron_
_Microscopy_, 54 (2005), 373.
19) J. R. Jinschek, K. J. Batenburg, H. A. Calderon, R. Kilaas, V.
Radmilovic and C. Kisielowski: _Ultramicroscopy_, 108 (2008), 589.
20) N. Monsegue, X. Jin, T. Echigo, G. Wang and M. Murayama:
_Microsc. Microanal._, 188 (2012), 1362.
21) B. Goris, W. Van den Broek, K. J. Batenburg, H. H. Mezerji and S.
Bals: _Ultramicroscopy_, 113 (2012), 120.
22) Z. Saghi, D. J. Holland, R. Leary, A. Falqui, G. Bertoni, A. J.
Sederman, L. F. Gladden and P. A. Midgley: _Nano Lett._, 11 (2011),
4666.

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