Atomic-scale microstructures of Zr2Al3C4 and Zr3Al3C5 ceramics

Acta Materialia 54 (2006) 3843–3851
www.actamat-journals.com
Atomic-scale microstructures of Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 ceramics
Z.J. Lin a,b , M.J. Zhuo a,b , L.F. He a,b , Y.C. Zhou a, *, M.S. Li a , J.Y. Wang a
a Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,
72 Wenhua Road, Shenyang, Liaoning 110016, China
b Graduate School of Chinese Academy of Sciences, Beijing 100039, China
Received 11 December 2005; received in revised form 19 February 2006; accepted 24 February 2006
Available online 27 June 2006
Abstract
The microstructures of bulk Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 ceramics have been investigated using transmission electron microscopy and
scanning transmission electron microscopy. These two carbides were determined to have a point group 6/mmm and a space group
P6 3 /mmc using selected-area electron diffraction and convergent beam electron diffraction. The atomic-scale microstructures of Zr 2 Al 3 C 4
and Zr 3 Al 3 C 5 were investigated through high-resolution imaging and Z-contrast imaging. Furthermore, intergrowth between Zr 2 Al 3 C 4
and Zr 3 Al 3 C 5 was identified. Stacking faults in Zr 3 Al 3 C 5 were found to result from the insertion of an additional Zr–C layer. Cubic ZrC
was occasionally identified to be incorporated in elongated Zr 3 Al 3 C 5 grains. In addition, Al may induce a twinned ZrC structure and lead
to the formation of ternary zirconium aluminum carbides.
Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: TEM; Carbides; Layered structures; STEM
1. Introduction
* Corresponding author. Tel.: +86 24 23971765; fax: +86 24 23891320.
E-mail address: yczhou@imr.ac.cn (Y.C. Zhou).
Binary transition metal carbides (TMCs), such as TiC,
NbC, and ZrC, exhibit excellent mechanical and chemical
properties, which make them suitable for diverse applications
[1]. These carbides possess a high melting point, high
hardness, high strength, high wear resistance, and chemical
inertness. However, the poor oxidation resistance and
intrinsic brittleness restrict their extensive applications in
high-temperature environments [1]. The toughness and
high-temperature oxidation resistance of TiC and NbC
can be greatly improved by forming ternary aluminum carbides,
such as Ti 3 AlC 2 ,Ti 2 AlC, and Nb 2 AlC [2–6]. Furthermore,
a series of new ternary aluminum carbides in
Ti–Al–C, Nb–Al–C, Zr–Al–C, and Hf–Al–C systems have
been successfully synthesized using transition metals, aluminum,
and graphite as starting materials [7,8]. Successes
with the Ti–Al–C and Nb–Al–C systems highlighted a possible
way to overcome difficulties in the applications of
ZrC.
Ternary aluminum carbides in Ti–Al–C and Nb–Al–C
systems, such as Ti 3 AlC 2 and Nb 2 AlC, have been identified
to crystallize in P6 3 /mmc symmetry [2,8]. Their crystal
structures can be described as non-stoichiometric TiC x
(or NbC x ) units in NaCl-type structure being intercalated
and mirrored by close-packed Al atomic planes. These ternary
aluminum carbides have attractive properties, such as
easy machinability, damage tolerance, excellent high-temperature
oxidation resistance, and good electrical and thermal
conductivities, due to their nano-laminated structures
[6]. Moreover, these ternary carbides properly conserve
the desired high moduli and strength of their binary
counterparts.
Two ternary carbides, i.e. Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 (previously
reported with the chemical formula of Zr 2 Al 3 C 5 x
and ZrAlC 2 x , respectively [8]), were determined to have
hexagonal symmetry in the ternary Zr–Al–C system
[9,10]. Compared with those well-studied ceramics in the
Ti–Al–C and Nb–Al–C systems, ternary Zr–Al–C compounds
have been less investigated. The reasons can be
1359-6454/$30.00 Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.actamat.2006.02.052

3844 Z.J. Lin et al. / Acta Materialia 54 (2006) 3843–3851
attributed to difficulties in synthesizing bulk materials and
lack of knowledge of the microstructural characteristics
and mechanical and chemical properties.
Hashimoto et al. [11] fabricated Zr 3 Al 3 C 5 -based composites
and reported that the composites displayed satisfactory
oxidation resistance. Recently, Leela-adisorn et al.
[12,13] fabricated bulk Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 from the
corresponding compound powders which were prepared
by solid-state reaction of ZrC, Al, and graphite. The fracture
strength and Vickers hardness were tentatively characterized
for Zr 3 Al 3 C 5 . He et al. [14] synthesized Zr 3 Al 3 C 5
powders using Zr–Al intermetallics and graphite as starting
materials. Oxidation tests indicated that Zr 3 Al 3 C 5 powders
displayed higher oxidation resistance than ZrC [14].
Despite these achievements, the structural information
for Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 is very limited and confused.
Using X-ray diffraction analysis, the space group of
Zr 2 Al 3 C 4 was reported to be P31 c (C 4 3vÞ by Schuster and
Nowotny [8], P6 3 /mmc by Parthé and Chabot [10], and
P6 3 mc by Fukuda et al. [15]. Similarly, Zr 3 Al 3 C 5 has been
reported to have P6 3 /mmc [8,9] and P6 3 mc symmetry [16].
Therefore, it is necessary to clarify the symmetry of both
carbides. Furthermore, atomic-scale microstructures are
valuable in understanding the structures and properties
of these two ternary carbides.
In this work, the microstructural characterizations of
two ternary Zr–Al–C ceramics were carried out by means
of transmission electron microscopy (TEM) and scanning
transmission electron microscopy (STEM). The symmetry
of both carbides was determined using selected-area electron
diffraction (SAED) and convergent beam electron diffraction
(CBED). Atomic-scale microstructures of both
carbides were characterized and are discussed in this paper.
which was equipped with a high-angle annular dark-field
(HAADF) detector in the STEM system and a postcolumn
Gatan imaging filter system, was used for highresolution
imaging and Z-contrast STEM imaging. Fast
Fourier transformation (FFT) was carried out using a
DigitalMicrograph software package.
3. Results and discussion
3.1. Symmetry determination of Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5
Fig. 1(a) and (b) show the crystal structures of Zr 2 Al 3 C 4
and Zr 3 Al 3 C 5 , respectively [8,10]. The atomic arrangements
of these two carbides are further illustrated in Fig. 1(c) and
(d) using two 2 · 2 · 1 slices on the ð1 210Þ plane. As
described by Gesing and Jeitschko [16] and by Fukuda
et al. [15], Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 can be viewed as inter-
2. Experimental
Bulk Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 ceramics were prepared
through a hot pressing method using elemental powders
of zirconium (99% purity, 400 mesh), aluminum (99%
purity, 200 mesh), and graphite (99% purity, 200 mesh)
as starting materials. Powder mixtures of appropriate stoichiometry
were homogenized using ethanol as a mixing
medium in a planetary ball mill for 12 h and then dried
at room temperature for 24 h. Afterwards, the mixed powders
were uniaxially pressed under a pressure of 10 MPa in
a BN-coated graphite mold of 25 mm in diameter. The
compacted mixture was heated to 1700 °C at a heating rate
of 15 °C/min and then hot pressed for 1 h in a flowing Ar
atmosphere. The applied pressure was 35 MPa during the
synthesis of both carbides. The surface layers of the samples
were machined off using a SiC grinding wheel to
remove contaminants arising from the graphite mold.
Thin-foil specimens for TEM observations were prepared
by slicing, mechanical grinding to 20 lm, dimpling
down to 10 lm, and ion milling at 4.0 kV. A 200 kV
JEM-2010 TEM instrument was used for SAED and
CBED analysis. A 300 kV Tecnai G 2 F30 TEM instrument,
Fig. 1. Crystal structures of: (a) Zr 2 Al 3 C 4 and (b) Zr 3 Al 3 C 5 . Atomic
arrangements of two 2 · 2 · 1 slices on the ð1 210Þ plane of (c) Zr 2 Al 3 C 4
and (d) Zr 3 Al 3 C 5 . The main difference between these two carbides centers
on the (ZrC) m units.

Z.J. Lin et al. / Acta Materialia 54 (2006) 3843–3851 3845
growth structures consisting of two kinds of layers. One is
the non-stoichiometric (ZrC) m (m = 2 or 3) slab in the
NaCl-type structure and the other consists of Al 3 C 2 units
in an arrangement similar to that of the binary aluminum
carbide Al 4 C 3 . The structural difference is that m = 2 for
Zr 2 Al 3 C 4 , while m = 3 for Zr 3 Al 3 C 5 .
The combination of SAED and CBED has been successfully
used in determining the symmetry of ternary carbides
such as Ti 3 SiC 2 [17] and Ti 3 AlC 2 [18]. Therefore, these two
methods were used to identify the symmetry of ternary Zr–
Al–C carbides in this work. Fig. 2(a)–(c) show SAED patterns
which were indexed as [0001], ½1 210Š, and ½1 100Š
zone axes, respectively, of the hexagonal Zr 2 Al 3 C 4 . From
these patterns of low-indices basic zone axes, the lattice
parameters were derived as a = 0.33 nm and c = 2.24 nm,
which were consistent with those determined from powder
X-ray analysis [8,14,15]. These SAED patterns were also
informative for learning extinction rules. It is noted that
all reflections in the ½1 210Š pattern appeared, but the
(000l) (l = odd) reflections in the ½1 100Š pattern were
absent, implying the existence of a c glide plane. The
appearance of the {0 00l} (l = odd) reflections in the
½1 210Š pattern can be attributed to double diffraction
[18–20]. Fig. 2(d) shows an SAED pattern whose orientation
was positioned between ½1 210Š and ½1 100Š. As shown
in Fig. 2(d), the (000l) reflections with l = odd were also
absent which indicated that there was a 6 3 screw axis along
the [0001] axis [18,19]. Fig. 2(e) shows a CBED pattern
obtained from a Zr 2 Al 3 C 4 grain along the [0001] zone axis.
A sixfold axis of rotational symmetry as well as two independent
and mutually perpendicular mirror planes (each of
which is reproduced every 60° by the action of the sixfold
axis) were observed. These two types of mirror planes are
parallel to the sixfold axis. The symmetry was determined
to be 6mm. In addition, the symmetry shown in the SAED
½1 210Š pattern (Fig. 2(b)) was 2mm. A6mm symmetry and
a2mm symmetry indicate a unique 6/mmm point group for
Zr 2 Al 3 C 4 [17–19]. Fig. 2(f) shows a CBED pattern showing
the existence of a mirror plane on (0001). A 6mm symmetry
and a mirror plane on (0001) further confirm the
6/mmm point group for Zr 2 Al 3 C 4 . A combination of the
information derived from these SAED and CBED patterns
indicate that Zr 2 Al 3 C 4 has a space group P6 3 /mmc, which
agrees well with the results of Parthé and Chabot [10].
Fig. 2. (a–c) SAED patterns of hexagonal Zr 2 Al 3 C 4 with the electron beam parallel to the directions of [0001], ½1 210Š, and ½1 100Š, respectively. (d) An
SAED pattern with the orientation being positioned between ½1 210Š and ½1 100Š. (e) A CBED pattern showing the symmetry of a sixfold rotation about
the [0001] axis and also the symmetry of the mirror reflection across two independent planes. (f) A CBED pattern showing the existence of a mirror plane
on (0001).

3846 Z.J. Lin et al. / Acta Materialia 54 (2006) 3843–3851
The symmetry of Zr 3 Al 3 C 5 was also determined using
SAED and CBED. The lattice parameters obtained were
a = 0.33 nm and c = 2.76 nm, which are in good agreement
with previously reported data [9,16]. Zr 3 Al 3 C 5 was also
determined to have a point group 6/mmm and a space
group P6 3 /mmc, which is consistent with that reported by
Mikhalenko et al. [8,9]. The SAED and CBED patterns
of Zr 3 Al 3 C 5 were similar to those of Zr 2 Al 3 C 4 and are
therefore not shown for brevity.
3.2. Microstructure of hexagonal Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5
Fig. 3(a) and (b) display the low-magnification brightfield
TEM images of Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 , respectively.
The overall characteristics of the microstructures of the assynthesized
ceramics can be observed. Extensive TEM
observations have shown that the Zr 2 Al 3 C 4 grains had salient
features, i.e. the grains generally had elongated morphologies
ranging from 2 to 20 lm in length and 30 to
500 nm in width in the perpendicular direction. SAED
analysis revealed that the crystallographic direction
[0001] of Zr 2 Al 3 C 4 was perpendicular to the elongated
direction of the Zr 2 Al 3 C 4 grains. Rotations between grains
were found to be along the [0001] axis and along the direction
perpendicular to [0001]. Most of the grain boundaries
were large-angle grain boundaries; and grains with misorientation
of only several degrees were also observed. In contrast,
the Zr 3 Al 3 C 5 grains were less regular as displayed in
Fig. 3(b). In addition, some grains were found to crystallize
in the form of elongated slabs, which were similar to those
in Zr 2 Al 3 C 4 .
It is generally believed that elongated grains with large
aspect ratio would benefit the toughness of ceramics. For
example, elongated Si 3 N 4 grains were effective in toughening
a ceramic by crack deflection and crack bridging
[21,22].SoZr 2 Al 3 C 4 with elongated grains may display better
toughness than Zr 3 Al 3 C 5 . The difference of grain morphologies
between Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 may originate
from the different liquid phase content during the synthesizing
processes. It is possible that the relatively higher
liquid phase during the synthesis of Zr 2 Al 3 C 4 favors the
formation of elongated grains. During the synthesis of
Si 3 N 4 , Perera et al. [22] have shown that sintering additives
strongly influence the grain morphologies of the ceramics
by controlling the liquid phase content. Their work may
support our hypothesis.
Fig. 4 shows a high-resolution TEM (HRTEM) image
of Zr 2 Al 3 C 4 obtained with the incident beam parallel to
the ½1 210Š direction. The image fringes with a periodicity
of 2.24 nm along the [0001] direction can be clearly seen.
The C atoms cannot be resolved in the microscopes
because of its weak diffraction power. The bright spots
can be described as a layered stacking with a sequence of
ABCBACBABC along the [0001] direction. This type of
stacking sequence corresponds to the Zr and Al atoms in
the Zr 2 Al 3 C 4 structure and is consistent with the structure
previously proposed by Parthé and Chabot [10].
Unfortunately, the atomic positions of Zr and Al cannot
be distinguished using conventional HRTEM because this
technique uses phase-contrast imaging.
Z-contrast STEM imaging, which was developed by
Pennycook and co-workers [23,24], can efficiently
distinguish different atoms because this technique uses
high-angle inelastic electrons. This technique removes the
coherent effects of diffraction and leads to strong atomic
number, Z, contrast. The intensity of the obtained image
is proportional to Z 2 , the square of atomic number.
Therefore, Z-contrast STEM images are more directly
Fig. 3. Typical TEM bright-field images of: (a) Zr 2 Al 3 C 4 and (b) Zr 3 Al 3 C 5 . The Zr 2 Al 3 C 4 grains generally have elongated morphologies while the
Zr 3 Al 3 C 5 grains are irregular.

Z.J. Lin et al. / Acta Materialia 54 (2006) 3843–3851 3847
Fig. 4. HRTEM image of Zr 2 Al 3 C 4 obtained with the incident beam
parallel to the ½1 210Š direction.
interpretable than conventional phase-contrast images and
this method was used to determine the positions of Al and
Zr in ternary Zr–Al–C carbides. Fig. 5(a) displays a raw
high-resolution Z-contrast STEM image of Zr 2 Al 3 C 4
viewed along the ½1 210Š direction. The main experimental
parameters were a probe size of 0.17 nm and detector semiangles
of 36–100 mrad. An inner detector semi-angle of
36 mrad was used for the following two reasons. Firstly,
Pennycook and co-workers [25] proposed that the detector
semi-angle depends on the electron wavelength and point
resolution. A criterion of semi-angle to obtain incoherent
images can be described by the following equation:
b > 1:22k=DR
ð1Þ
where b is the semi-angle, k is the electron wavelength, and
DR is the point resolution of the TEM. In this work, a
Tecnai G 2 F30 TEM instrument working at 300 kV was
used for Z-contrast imaging. k and DR are 0.0197 and
1.7 Å, respectively. As a result, 1.22k/DR = 14.1 mrad.
Consequently, incoherent images can be obtained with an
inner angle larger than 14.1 mrad in the present system.
Secondly, the atomic number of Zr (Z = 40) is relatively
large; in order to reduce the coherent effect of Zr atomic
columns, an inner detector semi-angle of 36 mrad was used.
The atomic positions of Zr and Al were clearly characterized
in the Z-contrast image since the intensity of a spot reflects
the atomic number of the corresponding atom. As
shown in Fig. 5(b), an important feature of Zr 3 Al 3 C 5 is that
the three Zr layers separate the Al 3 C 2 units, as well as two
Zr layers in Zr 2 Al 3 C 4 . This is consistent with the crystal
structures shown in Fig. 1.
Fig. 6(a) shows an intergrown structure between
Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 . The corresponding FFT filtered
image of Fig. 6(a) is displayed in Fig. 6(b) The intergrowth
between these two carbides can be understood as follows.
The lattice parameters a of Zr 2 Al 3 C 4 (0.3346 nm) [10]
and Zr 3 Al 3 C 5 (0.3347 nm) [9] are essentially equal, which
ensures an intergrowth with negligible lattice misfit; in
addition, Zr 2 Al 3 C 4 shares a similar crystal structure with
Zr 3 Al 3 C 5 with the only difference centering on the (ZrC) m
units. Locally, enrichment of Zr in Zr 2 Al 3 C 4 may lead to
the formation of Zr 3 Al 3 C 5 . Similar intergrown structures
have been identified in the ternary Ti–Al–C [18,20] and
Ti–Si–C [26] systems.
Fig. 5. High-resolution Z-contrast STEM images viewed along the ½1 210Š direction of: (a) Zr 2 Al 3 C 4 and (b) Zr 3 Al 3 C 5 .

3848 Z.J. Lin et al. / Acta Materialia 54 (2006) 3843–3851
Fig. 6. Z-contrast STEM image of the intergrown structure of Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 : (a) raw image and (b) FFT filtered image. There can be either two
or three Zr layers separating the Al 3 C 2 units.
Besides the intergrown structure of Zr 2 Al 3 C 4 and
Zr 3 Al 3 C 5 , stacking faults were also identified in these two
ternary carbides. Fig. 7(a) shows a low-magnification
bright-field image of stacking faults in Zr 3 Al 3 C 5 . Bright
and dark fringes were observed. In order to clarify the
microstructure of the stacking faults, Z-contrast STEM
images were obtained. As displayed in Fig. 7(b), the periodicity
of the stacking sequence was not identical. The
atomic-scale microstructure of the stacking faults is further
illustrated in Fig. 7(c) and (d). The high-resolution Z-
contrast image indicated that the stacking faults resulted
from the insertion of an additional Zr–C layer. Yu et al.
[27] also reported similar stacking faults with the insertion
of additional Ti–C layers in the ternary Ti–Si–C system.
The above observations demonstrate that the structural
difference between Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 is the thickness
of (ZrC) m units between the Al 3 C 2 units. In order to illustrate
the influence of this difference on the properties of ternary
Zr–Al–C carbides, theoretical mechanical properties
for Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 are compared. The calculated
bulk modulus B, shear modulus G, and anisotropic
Young’s modulus E of ZrC, Zr 2 Al 3 C 4 , and Zr 3 Al 3 C 5 [28],
together with experimental values for polycrystalline ZrC
[29] are presented in Table 1. It is seen that the calculated
data for ZrC agree well with the experimental values, suggesting
the reliability of the theoretical results. The main
difference in mechanical properties of Zr 2 Al 3 C 4 and
Zr 3 Al 3 C 5 lies in the magnitude of the shear modulus. The
shear modulus of Zr 3 Al 3 C 5 (166 GPa) is 37% higher than
that of Zr 2 Al 3 C 4 (121 GPa). Therefore, the shear modulus
of ternary Zr–Al–C carbides strongly depends on the
atomic-scale arrangements. It is also seen that Zr 3 Al 3 C 5
properly conserves the excellent mechanical properties of
binary carbide ZrC. Consequently, Zr 3 Al 3 C 5 is predicted
to be a promising carbide for ultrahigh-temperature applications.
On the other hand, the low shear modulus may
lead to Zr 2 Al 3 C 4 possessing similar properties to ternary
carbides such as Ti 3 AlC 2 [3]. Therefore, it is very important
to control the microstructure in the synthesized Zr–Al–C
compounds.
3.3. Determination of ZrC inclusions in Zr 3 Al 3 C 5 grains
As shown in a Z-contrast image in Fig. 8(a), inclusions
of rounded grains have been found. The contrasting white
regions showed that the diameter of the circular grains ranged
from 20 to 200 nm. Typical nanobeam diffraction
(NBD) patterns taken from the inclusions are displayed
in Fig. 8(b) and (c). These two NBD patterns were indexed
to be [110] and [111] of cubic ZrC, which indicated that
the inclusions were ZrC particles. Morgiel et al. [30] have
also reported the inclusions of binary carbide TiC in the
ternary Ti 3 SiC 2 ceramic.
The present results for the microstructure are definitely
useful in optimizing the processing and properties of these
ternary carbides. Both carbides can be fabricated from elemental
Zr, Al, and graphite powders at 1700 °C. Although
X-ray diffraction analysis showed that the Zr 3 Al 3 C 5
obtained is predominantly single phase, TEM analysis indicated
that minor amounts of ZrC inclusions were present in
the Zr 3 Al 3 C 5 grains (Fig. 8(a)–(c)). It has been demonstrated
that Zr 3 Al 3 C 5 has higher oxidation resistance than
ZrC [14]. To achieve good high-temperature oxidation
resistance, further optimization is needed to improve the
purity of Zr 3 Al 3 C 5 .
High-resolution imaging of the inclusions was conducted.
A typical FFT pattern of an HRTEM image of
ZrC is shown in Fig. 9(a). The FFT pattern was indexed
to be twinned ZrC with the incident beam parallel to the
[110] direction. Linear diffraction streaks besides the
twinned reflections were observed, suggesting the presence
of two-dimensional defects. Insight into the twinned ZrC

Z.J. Lin et al. / Acta Materialia 54 (2006) 3843–3851 3849
Fig. 7. (a) Low-magnification bright-field image of stacking faults of Zr 3 Al 3 C 5 . (b) Medium-magnification Z-contrast image showing the periodicity of the
stacking faults. (c) Raw high-magnification Z-contrast STEM image of a stacking fault resulting from the insertion of an additional ZrC layer. (d) FFT
filtered image of (c). The arrows in (b) indicate the stacking faults.
Table 1
Calculated bulk modulus B, shear modulus G, and anisotropic Young’s
modulus E of ZrC, Zr 2 Al 3 C 4 , and Zr 3 Al 3 C 5 , together with experimental
values for polycrystalline ZrC for comparison
Method B (GPa) G (GPa) E (GPa)
ZrC Calc. [28] 229 170 408
Expt. [29] 223 170 407
Zr 2 Al 3 C 4 Calc. [28] 190 121 E x = 356
E y = 302
Zr 3 Al 3 C 5 Calc. [28] 202 166 E x = 388
E y = 346
microstructure showed that a very thin Al platelet with several
atomic layers was present at the ZrC twin boundaries.
A typical Z-contrast image with three Al atomic layers at a
ZrC twin boundary is displayed in Fig. 9(b) and (c). The
spots with higher intensity correspond to Zr and the rest
denote Al. The structure of this thin Al platelet was identical
to the Al 3 C 2 unit in ternary Zr–Al–C carbides, suggesting
that ternary Zr–Al–C thin platelets could form at the
ZrC twin boundaries.
TEM investigations of bulk ZrC are very limited
because of the difficulties in sintering ZrC ceramic [31].

3850 Z.J. Lin et al. / Acta Materialia 54 (2006) 3843–3851
Fig. 8. (a) Z-contrast image of ZrC inclusions in an elongated Zr 3 Al 3 C 5 grain. (b, c) NBD patterns of cubic ZrC with the electron beam parallel to [110]
and [111] directions.
Fig. 9. (a) FFT image from a region containing ZrC twins. (b) Initial high-resolution Z-contrast STEM image of a twinned ZrC with three Al layers at the
twin boundaries. (c) FFT filtered image of (b).
The microstructural characteristics of TiC, another binary
TMC with NaCl-type structure, have been extensively
investigated [32,33]. Previous investigations showed that
TiC has a high stacking fault energy. Interestingly, impurities
such as Al and Si could stabilize the twinned TiC structure
and induced the formation of ternary Ti–Si(Al)–C
carbides [34,35]. Since ZrC and TiC have the same rocksalt
structure, it is reasonable that Al can also efficiently
reduce the twin boundary energy of ZrC and consequently
lead to the formation of ternary Zr–Al–C carbides. It is
also clear that ZrC serves as the intermediate phase during
the synthesis of ternary Zr–Al–C carbides from Zr, Al, and
graphite elemental powders. This result leads to the interpretation
that ternary Zr–Al–C carbides can be synthesized
using elemental Zr, Al, and graphite powders as in this
work, or ZrC and Al as in previous works [11–13].
4. Conclusions
The microstructural characteristics were investigated
for Zr 2 Al 3 C 4 and Zr 3 Al 3 C 5 ceramics. The point group
and space group of both carbides were determined to
be 6/mmm and P6 3 /mmc, respectively, form SAED
and CBED analyses. The lattice parameters of asprepared
Zr 2 Al 3 C 4 (a = 0.33 nm and c = 2.24 nm) and
Zr 3 Al 3 C 5 (a = 0.33 nm and c = 2.76 nm) were
obtained.
The atomic-scale microstructures of these two carbides
were determined through high-resolution imaging and
Z-contrast STEM imaging. Zr 3 Al 3 C 5 was identified to
form an intergrown structure with Zr 3 Al 3 C 5 . Stacking
faults in Zr 3 Al 3 C 5 resulted from the insertion of an additional
Zr–C layer.

Z.J. Lin et al. / Acta Materialia 54 (2006) 3843–3851 3851
Minor amounts of rounded ZrC inclusions were
observed in the elongated Zr 3 Al 3 C 5 grains. Al was found
to induce a twinned ZrC structure and lead to the formation
of ternary Zr–Al–C carbides.
Acknowledgements
The authors thank Miss Y. Cui for revising this manuscript.
This work was supported by the National Outstanding
Young Scientist Foundation for Y.C.Z. under Grant
No. 59925208, Natural Sciences Foundation of China under
Grant Nos. 50232040, 50302011, and 90403027.
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