Materials Transactions, Vol. 44, No. 10 (2003) pp. 2035 to 2041
Special Issue on Nano-Hetero Structures in Advanced Metallic Materials
#2003 The Japan Institute of Metals
OVERVIEW
Atomic-Scale Characterization of Nanostructured Metallic Materials
by HAADF/Z-contrast STEM
Eiji Abe
Materials Engineering Laboratory, National Institute for Materials Science, Tsukuba 305-0047, Japan
We demonstrate that high-angle annular-dark-field scanning transmission electron microscopy (HAADF-STEM) with a finely-focused
electron probe (0:15 nm) is very powerful technique to provide direct information on a local chemistry of nano-materials at atomic scale. This
is due to an atomic-number (Z) sensitive nature of the HAADF contrast (Z-contrast). We describe the microstructures of some crystalline,
quasicrystalline and amorphous metallic materials with focusing on their local chemical environments. Not only the chemical Z-contrast,
HAADF-STEM possesses more substantial advantages compared to the conventional phase-contrast high-resolution imaging, so that it would be
one of the most powerful methods for total characterization of nano-structured materials.
(Received April 23, 2003; Accepted June 19, 2003)
Keywords: scanning transmission electron microscope, high-angle annular-dark-field imaging, atomic-resolution Z-contrast, local chemistry,
aluminium- and magnesium-alloys
1.
Introduction
Nano- or even atomic-scale microstructural control has
become a key technique to develop super functional
materials. For metallic materials, this fine-scale structure
control may be achieved through alloying with a small
amount of additional elements into the matrix, which leads to
a solid solution, local clustering or nano-scale precipitates
that improve the material’s properties. This is originated
from the solibility or non-soichiometry nature intrinsic for
most metallic phases. Besides, there are various structural
forms available for metallic phases; crystal, quasicrystal and
amorphous states, some of which are realized through a
successful non-equilibrium processing. Because of these
various flexibilities in terms of matrix structures and
chemical environments (alloy compositions), metallic materials are still strongly promising candidates that would realize
some superior properties via microstructural controls at
extremely fine-scale. For the success of this project, a precise
characterization of the local element distribution as well as
local atomic configurations is essential to understand the
microscopic origins of the alloy properties.
Annular dark-field scanning transmission electron microscope (ADF-STEM) provides the atomic-resolution images
by effectively illuminating each atomic column one-by-one
as a finely focused electron probe (< 0:2 nm) scans across
the specimen, generating an intensity map at the annular
detector as shown in Fig. 1.1) When detecting high-angle
scatterings, the intensity reaching the detector is almost
proportional to the square of the atomic number, Z2 , so that
the strong chemical contrast (Z-contrast) can be obtained by
high-angle ADF-STEM (HAADF-STEM). A typical
HAADF contrast observed in an Al-Ag alloy is shown in
Fig. 2. More importantly, the high-angle scattered electrons
are basically incoherent to a good approximation, and hence
the HAADF-STEM images are the direct representation of
the specimen scattering power1) (weighted by Z2 ). The
incoherent HAADF-STEM image can therefore be directly
inverted to the object function representing atomic positions
Fig. 1 Schematic illustration showing principle of atomic-resolution
imaging with annular-dark-field scanning transmission electron microscope.
and species, without the image simulations. This is in sharp
contrast to the fact that the conventional coherent highresolution phase-contrast images suffer from the phase
problem which means that they cannot be directly converted
to the atomic positions; in principle, simulations are
necessary for their interpretations. In addition, STEM is the
only viable means for obtaining the spectroscopic analysis at
near atomic-resolution. Energy-dispesive x-ray spectroscopy
(EDS) and electron energy loss spectroscopy (EELS)
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Fig. 2 Chemically sensitive Z-contrast clearly reveals the Z-shaped 0 Ag2 Al platelet precipitates in the Al-Ag alloy. Since the atomic number of
Ag (Z=47) is much larger than that of Al (Z=13), Ag-rich regions reveal
significant Z-contrast. Particles showing relatively weak Z-contrast are the
sphere-shaped GP zone of Ag atoms embedded in the Al matrix.
measurements of local compositions and electronic structures
are possible by locating the atomic-sized electron probe on an
individual atomic column.2)
Because of these substantial advantages compared to
conventional high-resolution transmission electron microscopy (HRTEM), atomic-resolution STEM would be one of
the most powerful methods for total characterization of nanostructured materials to understand the microscopic origins of
their physical and chemical properties.2) In this paper, we
briefly describe some of our recent results that demonstrate
the powerful use of Z-contrast to investigate the local
chemistry at very high-spatial resolution.
2.
Scanning Transmission Electron Microscope
For atomic-resolution HAADF-STEM observations, we
used a 200 kV field-emission transmission electron microscope (JEOL: JEM-2010F) equipped with a scanning unit.
An objective lens is designed as an ultrahigh resolution type
pole-piece (a spherical aberration coefficient Cs ¼ 0:5 mm),
providing a minimum probe of approximately 0:15 nm with
a convergence angle of 10 mrad. A resolution of STEM is
determined by a probe size, since the incoherent image can be
represented, to a good approximation, by a simple convolution between the projected potential and the probe intensity
profile. For the HAADF imaging, the annular detector was set
to collect the electrons scattered at angles larger than
40 60 mrad, sufficiently high to reveal a chemical sensitive
Z-contrast.
3.
Results and Discussions
3.1 Al-Li-Cu alloy
In Al-Li-Cu alloys, two types of precipitates occur, which
are a famous Guinier-Preston (GP) zone of Cu atoms and a 0 Al3 Li with L12 structure.3) In spite of the fact that these two
precipitates are technologically important to control the
desired mechanical properties, their precipitation mechanisms are still not fully understood. One of the reasons is due
to a lack of structure characterization of these precipitates in
atomic level. Recently, the early stages of precipitation
behaviors of aged Al-Li-Cu alloys have been investigated
using both the HRTEM and HAADF-STEM.3) Here we
describe the microstructural details revealed by atomicresolution Z-contrast.
Figure 3(a) shows the HAADF-STEM image of the Al6.0 at%Li-1.3 at%Cu alloy, taken along the [100] axis of the
Al matrix. One clearly notices the strong Z-contrast from the
GP-I zone structure, a monolayer of Cu atoms lying on the
{100} planes of Al crystal.4) It may also be noticeable that the
background contrast is somewhat weaker around the GP-I
zones, such as indicated by arrows. This is due to 0 -Al3 Li
precipitates formed in these regions. In the corresponding
atomic-resolution Z-contrast image (Fig. 3(b)), the Al, 0 Al3 Li and Cu monolayer structures are definitely identified
from the contrast. The intensity profile (Fig. 3(c)) generates
fairly well the alternate (002) planes of Al lattice and (001)
planes of 0 -Al3 Li with L12 structure. Note that the nearestneighbor lattice planes next to the Cu monolayer can be
deduced to be Al layers from the sequence of alternate Al and
(Al+Li) layers in Z-contrast, although they are not resolved
due to a spread background effect of strong contrast at the Cu
layer. Anti-phase relationship between the L12 superlattice
domains across the Cu layer3) can be also recognized as the
relative shift of alternate (020) lattice planes with a sequence
of Al and (Al+Li) layers, as indicated by arrows in the Zcontrast image of Fig. 3(d). From these results, the
corresponding local atomic configuration can be constructed
(Fig. 3(e)) without any help of image simulation.
Note that try-and-error comparisons between the simulation of the model structure and observed image have been
essentially necessary, if one attempts to derive the atomic
configuration around the GP zone from phase-contrast
HRTEM imaging that is based on many-beam interference.5)
This is because that various kinds of image contrast occur
depending on imaging conditions, for which the defocus
values and the specimen thickness are the dominant factors. It
is also noted that local strain field around the GP zone may
seriously affect and hamper the clear atomic-resolution
imaging in conventional HRTEM, while the ADF-STEM is
expected to be much less sensitive to the strain field effects
when detecting sufficient high-angle scatterings.
3.2 Mg-Zn-Y nanocrystalline alloy
A nanocrystalline Mg-1 at%Zn-2 at%Y alloy (denoted as
Mg97 Zn1 Y2 ) was successfully developed by warm extrusion
of rapidly solidified powders at 573 K, showing excellent
mechanical properties including maximum tensile yield
strength 600 MPa and elongation 5% at room temperature.6,7) HRTEM investigation of the microstructure showed
a formation of a novel long-period hexagonal structure,7)
which is believed to play an important role in strengthening
the alloy. However, details of atomic configurations and
especially the distribution of Zn and Y in the novel structure
Atomic-Scale Characterization of Nanostructured Metallic Materials by HAADF/Z-contrast STEM
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Fig. 3 (a) HAADF-STEM image obtained from the Al-6.0 at%Li-1.3 at%Cu alloy aged at 473 K for 2 days. (b) Enlargement of the
rectangle area in (a), showing the atomic-resolution Z-contrast image of the precipitates. (c) One-dimensional Z-contrast intensity profile
across X-Y obtained by integrating over the framed area in (b). (d) Fourier-filtered image of (b), obtained by low-pass filter mask to
remove the background noise. (e) Atomic model directly derived from the incoherent Z-contrast image.
were not clarified.
Figures 4(a) and (b) show many-beam bright-field TEM
and HAADF-STEM images of the fine-lamellar grains
composed of the novel long-period stacking structure. Some
extra diffraction spots show up along the c -direction with
strong streaks (Fig. 4(a)), revealing a correlation length of
6 d(002)Mg as marked by an arrow in the diffraction pattern.
In the HAADF-STEM image, one finds an interesting feature
where the lamellae appear as bright lines due to the
significant Z-contrast, directly representing the distributions
of the heavier constituent elements in the fine-lamellar
grain.8) That is, Zn and/or Y atoms are located not randomly
as a solid solution in the whole Mg crystal, but at the specific
positions of the lamellar structure. In fact, the bright Zcontrast lamellae are arrayed with an interval of approximately 1.56 nm (¼ 6 d(002)Mg ) at some places, which
means a certain chemical order with this correlation-length in
the long-period structure.
Further structural details of the long-period six-layer
structure can be derived from the atomic-resolution HRTEM
and HAADF-STEM images (Figs. 4(c)-(e)).8) The HRTEM
image, in which bright dots represent projected atomic
columns under this observation condition (Fig. 4(c)), shows
that the long-period six-layer structure is basically constructed by a stacking sequence of ABCBCB (6H-type) with
accompanying lattice distortion from an ideal 6H-type. The
corresponding atomic-resolution HAADF image clearly
reveals strong Z-contrast at two close-packed planes (Fig.
4(b)) that are A- and one of the B-layers in the ABCBCB
stacking-structure inserted in Fig. 4(e). This directly shows a
chemical order configuration, and the present long-period
ordered structure is represented as ABCBCB0 where A and B0
layers are remarkably enriched by Zn and/or Y. It is
noteworthy here that, in the HRTEM image of Fig. 4(a), the
brightest spots occur at different layers from those in the Zcontrast, so that it is difficult to deduce a possible chemical
order configuration solely based on the HRTEM image.
Since the Z-contrast method cannot distinguish whether
both of the Zn and Y or only one of them enrich the
significant bright Z-contrast layers, EDS analysis with the
sub-nanometer (0:3 nm) probe was performed for the
regions X, Y in Fig. 4(d).8) It was found that both the Zn
and Y are significantly enriched at the bright Z-contrast
layers, leading to the atomic structural model shown in Fig.
4(f). The results demonstrate that the additional elements of a
few atomic percent to Mg lead to formation of a long-period
chemical-ordered as well as stacking-ordered structure, as
directly revealed by a unique Z-contrast method.
3.3 Al-Ni-Co quasicrystalline compound
The Al72 Ni20 Co8 decagonal phase is thermodynamically
stable at 1100 K and well characterized as a nearly-perfect,
ideal quasiperiodic structure. Since the most quasicrystalline
phases form as an intermetallic compound with a chemicallyordered structure, Z-contrast is effective for specifying the
heavy atom position in the structure.9) Z-contrast images of
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Fig. 4 (a) Many-beam bright-field TEM image together with the corresponding electron diffraction pattern and (b) HAADF-STEM image
obtained from the fine-lamellar grains formed in the nanocrystalline Mg97 Zn1 Y2 alloy. Atomic-resolution (c) HRTEM and (d) HAADFSTEM images of the novel long-period ordered structure. (e) Fourier-filtered Z-contrast image of (b), obtained by applying a low-pass
filter mask to remove the background noise level. (f) Atomic model of the long-period chemically-ordered as well as stacking-ordered
Mg-(Zn, Y) structure.
the Al72 Ni20 Co8 structure successfully revealed a striking
feature of transition metal (TM; Ni or Co) arrangements that
breaks tenfold symmetry of the decagonal cluster,10–13) which
is a structural unit to construct the entire quasiperiodic
structure and had been believed to be tenfold symmetric. This
discovery of local symmetry-breaking lead to a new
paradigm of the quasi-unit-cell picture.12)
Apart from the chemical Z-contrast, ADF-STEM may
provide information on a local thermal vibration anomaly at
atomic-scale,14) because high-angle scattering is dominated
by phonon scattering events, namely the thermal diffuse
scattering (TDS)1) that is sensitive to the Debye-Waller (DW)
factor. We describe the direct observation of local anomalies
of the DW factor in the Al72 Ni20 Co8 , through an in-situ hightemperature ADF-STEM observation.14) In the observation at
room temperature, 300 K (Fig. 5(a)), the ADF-STEM image
highlights the TM positions relative to the Al due to the Zcontrast. Some decagonal clusters with a diameter of 2 nm are
outlined in Fig. 5(a). When the sample is heated and held at
approximately 1100 K, we find a remarkable change in the
relative contrast; compare Fig. 5(a) with Fig. 5(b). It is
evident that a significant enhancement in contrast appears at
some specified places that can be well represented by the
pentagonal quasiperiodic lattice (tiling) with an edge-length
of 2 nm, as drawn in Fig. 5(b). Further, we notice that the
anomalous contrast occurs at cores of the decagons, as shown
in Figs. 5(c) and (d). Viewing carefully the interior contrast
of the decagons, increase of intensities is found to be
significant at the positions indicated by arrowheads, which
are the Al sites in the decagonal cluster model (Fig. 5(e)). We
confirmed that this temperature-dependence contrast change
is reversible; that is, after cooling down to 300 K from
1100 K, the anomalous contrast regions become darker again.
This temperature-dependence of ADF-contrast is fairly well
explained by assuming the anomalous DW factor at the
specific Al atomic sites, demonstrating the first direct
observation of a local vibration anomaly in a solid. The
local DW anomalies imply significant anharmonicity at these
specific Al sites, providing an important hint on the issue of
phason - the phason is an extra elastic degree of freedom
specific to the quasicrystals (in addition to the usual phonons
in crystals) and has been theoretically predicted to cause
Atomic-Scale Characterization of Nanostructured Metallic Materials by HAADF/Z-contrast STEM
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Fig. 5 Atomic-resolution ADF-STEM images of Al72 Ni20 Co8 along the tenfold symmetry axis, taken at (a) 300 K and (b) 1100 K.
Decagonal clusters with a diameter of about 2 nm, whose image contrast at (c) 300 K and (d) 1100 K. (e) Atomic model of the cluster.
localized fluctuations.
The results described here demonstrate the additional
capabilities and advantages of ADF-STEM - by doing the
angle-resolved14) and/or in-situ heating/cooling experiments, it now can be used to see not only the atomic
structure but also the significant local fluctuations, which is
caused by slight atomic displacement with a order of
0:01 nm.
3.4 Al-Ni-Cu-Ce amorphous alloy
Al-base amorphous alloys in the Al-TM-RE (TM: transition metal, RE: rare-earth metal) systems possess excellent
mechanical properties including maximum strength
1000 MPa.15,16) Interestingly, their strength can be further
improved by having nano-scale precipitation of fcc-Al
crystals within the amorphous matrix. The early stage of Al
nanocrystal formation was investigated to understand why a
very high density of nanosrystals can be formed, and it was
proposed that heavy RE atom behaviors may determine the
growth and thermal stability of the nanocrystals.17)
We describe microstructures of an Al87 Ni7 Cu3 Ce3 (at%)
amorphous alloy18) through a preliminary Z-contrast observation, and test whether or not the heavy atom positions in the
amorphous matrix can be detected. Since the atomic number
of Ce (Z=58) is much larger than the other constituent
elements (Al: Z=13, Ni: Z=28, Cu: Z=29), we would expect
sufficient Z-contrast from the Ce atoms even though they are
just added to the alloy by a few atomic percent. Figure 6(a)
shows the HAADF-STEM image of the Al87 Ni7 Cu3 Ce3
amorphous structure. In the image, one notices that some
brightest spots occur, some of which are indicated by small
circles. Note that there are no definite atomic columns in the
amorphous structure; in the case of zone axis imaging of
crystalline or quasicrystalline materials described previously,
all the bright spots represent the projected atomic columns.
Therefore, the brightest spots observed in Fig. 6(a) are highly
likely to be individual Ce atoms embedded in the amorphous
matrix. Some Ce atoms seem to form clusters as indicated by
large circles.
Under the zone axis illumination condition for the
crystalline specimen, a successful atomic-resolution STEM
imaging is described by the strong electron channeling
effects along the atomic columns.1,2) Although there are no
such columns, we nevertheless obtain atomic-scale Zcontrast images from the amorphous structure composed of
a small amount of heavy atoms. In this case, the situation may
be similar to the imaging of a single, isolated heavy atom
located on an amorphous carbon support. In this sense,
projected potential concept can be applied to explain the
successful imaging of individual heavy atoms embedded in
the amorphous matrix.
The advantageous Z-contrast imaging is well demonstrated after nanocrystallization takes place (Fig. 6(b)). Since the
nanocrystals precipitate in almost pure fcc-Al,17) these are
clearly identified as significantly darker regions in the Zcontrast image of Fig. 6(b), whereas it is rather difficult and
not straightforward to find the Al nanocrystals in the HRTEM
image of Fig. 6(c). As nanocrystallization proceeds, the
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Fig. 6 HAADF-STEM images of (a) as rapidly-solidified and (b) subsequently annealed (at 553 K for 30 sec) samples of an
Al87 Ni7 Cu3 Ce3 alloy. In this alloy, a high density of Al nanocrystals precipitates in the amorphous matrix by short time annealing.
(c) HRTEM phase-contrast image of the annealed Al87 Ni7 Cu3 Ce3 alloy.
amorphous matrix becomes enriched by the solute atoms
rejected from the growing Al nanocrystals, and therefore the
matrix reveals brighter Z-contrast in Fig. 6(b). Even with this
situation, we still find the brightest spots within the
amorphous matrix as indicated by small circles in Fig. 6(b);
these can be attributed to the Ce atoms.
As demonstrated in the present work, atomic-resolution Zcontrast imaging is also powerful to investigate the structure
of multi-component amorphous alloys. Further detailed
analysis regarding the Ce atom distribution in the
Al87 Ni7 Cu3 Ce3 amorphous and nanocrystalline states is still
in progress, and the results will be published elsewhere.
4.
Summary
We have demonstrated that HAADF-STEM provides
direct information about local atomic structure including its
chemical environments through the atomic-number sensitive
contrast (Z-contrast), which is shown to be effectively
applicable for crystalline, quasicrystalline and even amorphous metallic materials. Since there are few possible
artifacts for HAADF imaging compared to the phase-contrast
imaging, HAADF imaging gives a straightforward interpretation on the atomic structures. Combining with the spectroscopic analysis such as EELS or EDS, ADF-STEM will be
one of the most powerful methods for total characterization
of nano-structured materials.
Acknowledgements
This paper is based on the several collaborating projects.
The author would like to thank to the following collaborators:
Dr. M. Murayama and Dr. K. Hono for the Al-Ag alloy, Prof.
T. J. Konno and Prof. K. Hiraga for Al-Li-Cu alloys, Prof. Y.
Kawamura and Prof. A. Inoue for Mg-Zn-Y nanocrystalline
alloy, Dr. A. P. Tsai and Dr. S. J. Pennycook for Al-Ni-Co
quasicrystal, and Dr. A. P. Tsai for Al-Ni-Cu-Ce amorphous
alloys.
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