Journal
J. Am. Ceram. Soc., 89 [12] 3765–3769 (2006)
DOI: 10.1111/j.1551-2916.2006.01303.x
r 2006 The American Ceramic Society
Microstructures and Theoretical Bulk Modulus of Layered Ternary
Tantalum Aluminum Carbides
Zhijun Lin,z Mujin Zhuo,z Yanchun Zhou,*,w Meishuan Li, and Jingyang Wang
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,
Shenyang 110016, China
hensive understanding of the microstructures and mechanical
properties of ternary Ta–Al–C carbides is still less developed.
In the present work, microstructural characterizations of
ternary Ta–Al–C carbides were conducted using scanning electron microscopy (SEM), transmission electron microscopy
(TEM), and scanning transmission electron microscopy
(STEM). Atomic-scale microstructure of Ta2AlC, a previously
unknown layered ternary Ta6AlC5 carbide, as well as intergrown Ta2AlC–Ta4AlC3 and Ta4AlC3–Ta6AlC5 structures were
characterized. Lattice parameters for the identified Ta–Al–C
compounds are presented, and the theoretical bulk modulus of
some Ta–Al–C carbides was computed using density-functional
calculations in order to illustrate the microstructures–properties
relationship.
Direct atomic resolution observations of the layered stacking
characteristics of TaCx slabs and Al atomic planes in ternary
Ta–Al–C carbides were achieved. Layered ternary Ta–Al–C
compounds have diverse structures. A previously unknown
Ta6AlC5 carbide, as well as intergrown Ta2AlC–Ta4AlC3 and
Ta4AlC3–Ta6AlC5 structures were identified. Theoretical lattice
parameters and bulk modulus of Ta2AlC, Ta3AlC2, Ta4AlC3,
and Ta6AlC5 are presented. Furthermore, the Ta–C bonds are
much stronger than the Ta–Al bonds in ternary Ta–Al–C carbides, which accounts for the enhancement of bulk modulus with
increasing Ta–C layers.
I. Introduction
B
transition metal carbides (TMCs) such as TiC, NbC,
TaC, ZrC, and HfC are of scientific and technological importance because of their unique high performances.1–4 TMCs
display excellent mechanical and chemical properties including
high melting point, high hardness, good high-temperature
strength, and chemical inertness. TaC is of particular interest
due to its extremely high melting point (4256 K).2 The application of TaC, however, is currently limited by its intrinsic brittleness.3,4 Recently, the toughness and high-temperature
oxidation resistance of binary TiC compound have been greatly improved by forming ternary aluminum carbides, such as
Ti3AlC2 and Ti2AlC.5–9 Furthermore, a series of new ternary
aluminum carbides in Zr–Al–C, Hf–Al–C, Nb–Al–C, and Ta–
Al–C systems were successfully synthesized.10
Owing to the unique layered crystal structures, Ti3AlC2 and
Ti2AlC display attractive properties, such as easy machinability,
damage tolerance, and good electrical and thermal conductivities.7–9,11 Moreover, Al2O3 scales could grow on the Ti3AlC2
substrate following two specific sets of orientation relationships
during high-temperature oxidation in air.12 The as-formed
Al2O3 scales efficiently protect the Ti3AlC2 substrate and account for its excellent oxidation resistance.
Inspired by the achievements in the Ti–Al–C system, synthesizing layered ternary Ta–Al–C compounds may be a possible
means to overcome difficulties for the applications of TaC.
Jeitschko et al.5 firstly identified hexagonal Ta2AlC in the
1960s. Schuster and Nowotny10 confirmed the presence of
Ta2AlC and detected Ta5Al3C during the investigations of the
complex ternary Ta–Al–C carbides. Very recently, the authors
characterized Ta4AlC3, a new layered ternary carbide with P63/
mmc symmetry, in the Ta–Al–C system.13 However, a compreINARY
II. Experimental Procedure and Theoretical Method
(1) Experimental Procedure
Bulk Ta–Al–C ceramics were fabricated through a hot-pressing
method using elemental Ta, Al, and graphite powders as starting
materials. Elemental Ta, Al, and C powders were homogenized
using a ball-milling method for 10 h. Then, the powder mixtures
were placed in a graphite mold whose inner surface had been
sprayed with a BN layer. The hot-pressing reaction synthesis
and simultaneous densification process was performed in a flowing argon atmosphere in a furnace using graphite heating elements. Mixed powders were heated to 15001C at a rate of 151C/
min. Ternary compounds were fabricated by hot pressing at
15001C for 1 h under an applied pressure of 30 MPa.
SEM analysis was performed using a LEO Supra 35 SEM
(Zeiss, Oberkochen, Germany). Thin-foil specimens for TEM
observations were prepared by slicing, mechanical grinding,
dimpling, and finally ion milling. A 200 kV JEM-2010 TEM
(JEOL, Tokyo, Japan) was used for selected area electron diffraction analysis. A 300 kV Tecnai G2 F30 TEM (FEI, Eindhoven, the Netherlands), equipped with a high-angle annular dark
field detector in a STEM system, was used for high-resolution
imaging and Z-contrast STEM (FEI, Eindhoven, the Netherlands) imaging. Fast Fourier transformation (FFT) was carried
out using a DigitalMicrograph software package.
(2) Theoretical Method
Density functional calculations were conducted to obtain theoretical crystal structures of Ta2AlC, Ta3AlC2, Ta4AlC3, and
Ta6AlC5.14 The Vanderbilt-type ultrasoft pseudopotential15 and
generalized gradient approximation (GGA–PW91)16 in the
Cambridge Sequential Total Energy Package code were used.
The plane-wave basis set cut-off was 450 eV. The Broyden–
Fletcher–Goldfarb–Shanno minimization scheme17 was used in
geometry optimization. During the geometry optimization process, lattice parameters, including lattice constants and internal
atomic coordinates, were modified independently to minimize
the total energy, interatomic forces, and stresses of unit cell. The
convergence tolerances for geometry optimization were: differ-
I. Tanaka—contributing editor
Manuscript No. 21742. Received April 26, 2006; approved July 20, 2006.
Supported by the National Outstanding Young Scientist Foundation for Y. C. Zhou
under Grant No. 59925208, Natural Sciences Foundation of China under Grant Nos.
50232040, 50302011, and 90403027.
*Member, American Ceramic Society.
w
Author to whom correspondence should be addressed. e-mail: yczhou@imr.ac.cn
z
Graduate School of Chinese Academy of Sciences, Beijing, China
3765
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Journal of the American Ceramic Society—Lin et al.
ence on total energy within 5 106 eV/atom, maximum ionic
Hellmann–Feynman force within 0.01 eV/Å, and maximum
ionic displacement within 5 104 Å and maximum stress within 0.02 GPa.
The elastic coefficients were determined by applying a set of
given homogeneous deformations and calculating the resulting
stress, as implemented by Milman and Warren.18 Two strain
patterns could generate stresses related to all five independent
elastic coefficients for a hexagonal unit cell. The elastic stiffness
was determined from a linear fit of the calculated stress as a
function of strain and the bulk modulus was calculated from the
inverse of the stiffness tensor, S 5 C 1. Details on the calculation of bulk modulus were described in Wang and Zhou.19
Vol. 89, No. 12
Fig. 2. (a) Atomic arrangement of Ta2AlC on a (1210)
plane, (b)
HRTEM image of Ta2AlC taken with the incident beam parallel to
the [1210] direction. This image displays a stacking sequence of ABABAB with a period of 1.38 nm.
III. Results and Discussions
(1) Microstructure of Hexagonal Ta2AlC
Ta2AlC was determined to crystallize in the Cr2AlC-type structure with P63/mmc symmetry using X-ray diffraction (XRD)
analyses.5 Jeitshko and coworkers also proposed the possible
crystal structure of Ta2AlC. TEM enables morphological and
crystallographic analysis of small domains and thus provides
useful information besides that obtained from XRD. Figure 1(a)
shows a bright-field TEM image of Ta2AlC. It can be seen that
the Ta2AlC grains display interesting features, i.e., the grains
generally had elongated morphologies ranging from 2 to 25 mm
in the length scale and 50–400 nm in width in the perpendicular
direction. SAED analysis revealed that the [0001] direction of
Ta2AlC was perpendicular to the elongated direction of the
Ta2AlC grains, as indicated using white arrows in Fig. 1(a).
Figures 1(b)–(d) show typical SAED patterns that were indexed
as [0001], [1210], and [1100] zone axes, respectively. From these
patterns of low-index basic zone axes, the lattice parameters
were derived as a 5 0.308 nm and c 5 1.38 nm, which were consistent with those determined from XRD analysis.5 It is noted
that all reflections in the [1210] pattern appeared, but the (000l)
(l 5 odd) reflections in the [1100] pattern were absent. The appearance of (000l) (l 5 odd) reflections in the [1210] pattern resulted from double diffraction.20
Figure 2(a) shows the atomic arrangements of Ta2AlC projected on a (1210) plane.5 The layer stacking sequence of Ta and
Al atoms along the [0001] direction is
Fig. 1. (a) Bright-field transmission electron microscopy image of
Ta2AlC. (b)–(d) SAED patterns of Ta2AlC with the electron beam parallel to the directions of [0001], [1210], and [1100], respectively. The c
direction of Ta2AlC is indicated using white arrows.
ABABAB
(1)
where the underlined letters refer to Al layers and the rest to Ta
layers. The carbon atoms occupy the interstitial sites of Ta6 octahedra. The layered stacking characteristics are clearly illustrated in the (1
210) plane. The stacking sequence described by
Eq. (1) can be observed in an HRTEM image with the electron
beam parallel to the [1
210] zone axis of Ta2AlC, as shown in
Fig. 2(b). The bright spots follow a layered stacking sequence of
ABABAB along the [0001] direction with a periodicity of 1.38
nm. This type of stacking sequence represents the stacking of
Ta and Al atoms of Ta2AlC and coincides with the previously
proposed crystal structure.5 Although HRTEM analysis can
provide atomic-scale microstructure, identifying specific atomic
positions of Ta and Al remains a very difficult task because the
conventional HRTEM uses phase-contrast imaging.
Z-contrast STEM imaging can efficiently distinguish different
atoms because this technique uses high-angle inelastic electrons,
which removes coherent effects and leads to a strong atomic
number, Z, contrast. Z-contrast STEM imaging has been applied to determine a series of useful atomic-scale microstructural
information in ceramics such as Si3N4 doped with various rareearth elements21 and the atomic positions of Cr and Al in a
layered ternary Cr2AlC ceramic.22 Z-contrast STEM imaging is
more directly interpretable than conventional phase-contrast
imaging and it was used to determine the positions of Al and
Ta in ternary Ta–Al–C carbides. Figure 3(a) shows a Z-contrast
STEM image of Ta2AlC viewed along the [1
210] direction. The
noise was filtered out of the image using FFT in the Digitalmicrograph software package, and the corresponding filtered
image is displayed as Fig. 3(b). The intensity of a Z-contrast
image is approximately proportional to Z2, the square of the
atomic number. The atomic numbers of Ta (ZTa) and Al (ZAl)
are, respectively, 73 and 13, yielding a (ZTa/ZAl)2 5 31.5. Therefore, the Al atomic columns appear dark compared with the
bright columns of Ta. The alternative stacking of two bright
Fig. 3. (a) High-resolution Z-contrast scanning transmission electron
microscopy image taken along the [1210] zone of Ta2AlC, (b) fast Fourier transformation filtered image of (a).
December 2006
Microstructures and Modulus of Ta–Al–C Compounds
3767
systems,20,23–25 a generalized orientation relationship between
binary cubic MX carbides/nitrides and layered ternary MxAyXz
phases carbides/nitrides is proposed as: [1
10]MX//
[1
210]MxAyXz and (111)MX//(0001)MxAyXz (where M is an
early transition metal, A is a IIIA and IV group element, X is
carbon or nitrogen, and x, y, and z are natural numbers). The
extension of x, y, and z is based on the observation of Ti5Si2C3
and Ti7Si2C5 in the Ti–Si–C system26 and new ternary Ta–Al–C
compounds in this work, which will be shown below.
Fig. 4. Scanning electron microscopy micrograph of the fractured surfaces of Ta2AlC.
columns and a dark one reflects the layered stacking feature of
Ta2C slabs and Al atomic planes in Ta2AlC.
Ternary Ta–Al–C carbides display laminated features not
only at nano-scale but also at micro-scale. For example, Fig. 4
shows a SEM micrograph of typical fractured surfaces of
Ta2AlC wherein the laminated feature is clearly illustrated.
The fractured surfaces of Ta4AlC3 displayed essentially the
same characteristics and are not shown here for brevity. The
plate-like grains consisted of a number of thin slices. Laminated
grain morphologies were also observed in layered ternary carbides such as Ti3AlC2 and Ti2AlC.7,8 It has been demonstrated
that delamination, buckling, and kinking of the unique layered
grains account for the deformation mechanism of the layered
ternary carbides and result in their high toughness and damage
tolerance. Consequently, it is predicted that ternary Ta–Al–C
carbides would display mechanical properties similar to other
layered ternary carbides.
In addition, a minor amount of cubic TaC was occasionally
observed in Ta2AlC. In order to determine the crystallographic
relationship between these two carbides, high-resolution imaging was conducted. Figure 5(a) shows an HRTEM image of
the interfacial structure between Ta2AlC and TaC. It can be seen
that TaC formed a coherent interface with Ta2AlC and the
crystallographic orientation relationship between TaC and
Ta2AlC could be described as (111)TaC//(0001)Ta2AlC and
[110]TaC//[1210]Ta2AlC. In order to better understand the
structural relationship between TaC and Ta2AlC, an interfacial
structural model based on the observed orientation relationship
is proposed and shown in Fig. 5(b). The Ta–C units of Ta2AlC
are locally the same as those of TaC, which ensures a coherent
interfacial structure between TaC and Ta2AlC. Combining the
orientation relationships between binary cubic carbides and layered ternary carbides identified in the Ti–Si–C and Ti–Al–C
Fig. 5. (a) HRTEM image of the interfacial structure between TaC and
Ta2AlC. The electron beam is parallel to the [110] and [1120] zone axes
of TaC and Ta2AlC, respectively. The arrow indicates the position of the
interface. (b) Interfacial structural model between TaC and Ta2AlC.
(2) Intergrown Structures in a Ta–Al–C System
Z-contrast STEM imaging was used to determine the atomicscale microstructures of ternary Ta–Al–C carbides. Figure 6(a)
shows an intergrown structure between Ta2AlC and Ta4AlC3,
with the corresponding FFT filtered image being displayed in
Fig. 6(b). The numbers in the image denote the Ta layers being
separated by close-packed Al atomic planes. As shown in Fig. 6,
the number of Ta layers in one slab could be either 2 or 4, which
corresponds to Ta2AlC and Ta4AlC3, respectively. An intriguing
feature is that stackings with six Ta layers in one Ta–C slab were
also observed, which are shown in Figs. 7(a) and (b). Such a
microstructural feature suggests the presence of a previously
unknown phase with a formula Ta6AlC5. Furthermore,
Ta6AlC5 formed an intergrown structure with Ta4AlC3 (Fig. 7).
(3) Theoretical Lattice Parameters and Bulk Modulus of
Ta–Al–C Carbides
The crystal structures of Ta2AlC, Ta3AlC2, and Ta4AlC3 are
displayed in Figs. 8(a)–(c). Although Ta3AlC2 was rarely observed using Z-contrast STEM imaging (not shown for brevity),
theoretical crystal structure and bulk modulus of Ta3AlC2 are
also included to complement this study. The crystal structure of
Ta6AlC5 was not shown because of its relatively high c/a ratio
(11.3). The crystal structures of these layered carbides are basically made up of two units: a nonstoichiometric TaCx
(x 5 0.50, 0.67, 0.75, and 0.83 for Ta2AlC, Ta3AlC2, Ta4AlC3,
and Ta6AlC5, respectively) slab and an Al atomic plane. The
TaCx units in Ta4AlC3 and Ta6AlC5 display a zig–zag configuration. Geometry optimization was conducted to obtain the
theoretical lattice parameters of these compounds. The lattice
parameters of ternary Ta–Al–C carbides are summarized in
Table I. It can be seen that the lattice parameter, a, of these
compounds is close with deviations within 1%, which further
confirms the crystal structure similarity of these layered ternary
carbides. Hence, the diversity of the structures in Ta–Al–C
(Figs. 3, 6, and 7) may result from the crystal structural similarity of these layered ternary compounds.
Theoretical bulk modulus was calculated to illustrate the relationship between the layered stacking characteristics and the
mechanical properties of Ta–Al–C carbides. The calculated bulk
moduli of Ta2AlC, Ta3AlC2, and Ta4AlC3 are also listed in
Table I. The reported bulk modulus of Ta2AlC computed by
first-principles calculations yielded 220 GPa,27 which supports
Fig. 6. (a) High-resolution Z-contrast scanning transmission electron
microscopy image of an intergrown structure between Ta2AlC and
Ta4AlC3, (b) fast Fourier transformation filtered image of (a). The numbers ‘‘2’’ and ‘‘4’’ denote the Ta–C layers separated by the Al atomic
planes.
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Journal of the American Ceramic Society—Lin et al.
Fig. 7. (a) High-resolution Z-contrast scanning transmission electron
microscopy image of an intergrown Ta4AlC3–Ta6AlC5 structure, (b)
corresponding fast Fourier transformation filtered image of (a).
Fig. 9. (a)–(c) Valence electron density of slices on the (1120) plane in
2 2 1 supercells of Ta2AlC, Ta3AlC2, and Ta4AlC3, respectively.
Fig. 8. (a)–(c) Crystal structures of Ta2AlC, Ta3AlC2, and Ta4AlC3,
respectively.
our result. It is seen that the bulk modulus of ternary Ta–Al–C
compounds is related to the atomic-scale microstructure, i.e., the
bulk modulus enhances with increasing number of Ta layers per
Al layer. For example, the bulk modulus of Ta3AlC2 is 13.3%
higher than that of Ta2AlC, and the bulk modulus of Ta4AlC3
increases by 23.3% compared with that of Ta2AlC. The bulk
modulus of Ta6AlC5 is 35.7% higher than that of Ta2AlC.
Bulk modulus measures the resistance of a material to a volume change and reflects the internal bonding characteristics. So
the changes in bulk modulus originate from the atomic bonding
within the layered ternary compounds. In order to understand
the mechanism of the change of bulk modulus, the bonding
properties of layered Ta–Al–C carbides were investigated. Valence electron density distributions on the (11
20) plane in
2 2 1 supercells of Ta2AlC, Ta3AlC2, and Ta4AlC3 are displayed in Figs. 9(a)–(c), respectively. The most distinguished
feature is that the Ta–(C–Ta)m–C–Ta (m 5 0, 1, and 2 for
Ta2AlC, Ta3AlC2, and Ta4AlC3, respectively) atomic chains display a strong covalent bonding. The adjacent Ta–(C–Ta)m–C–
Ta units are interleaved and mirrored by Al atomic planes. The
electron density is relatively low in the regions between the Ta–
(C–Ta)m–C–Ta atomic chains and Al. In other words, the Ta–C
bonds are much stronger than the Ta–Al bonds, which explains
the enhancement of bulk modulus for ternary Ta–Al–C compounds with increasing number of Ta layers per Al layer.
Previous attempts at modifying the properties of layered ternary phases mainly concentrated on solid solution treatments.
Theoretically, Wang and Zhou19 and Sun et al.28 investigated
the influence of the M-site substitutions in layered ternary phases with the M2AlC formula. Moreover, Grechnev et al.29 predicted that substituting Ti by Nb could form a new metastable
Nb3SiC2 phase with a much higher modulus than that of
Ti3SiC2. Experimentally, substitutions on the M site (such as
(Ti,Nb)2AlC30), A site (e.g., Ti3Al1xSixC231), and X sites (e.g.,
Table I. Calculated Lattice Parameters and Bulk Modulus for Some Ternary Ta–Al–C Carbides
Compounds
Method
Symmetry
a (Å)
c (Å)
c/a
B (GPa)
Ta2AlC
Ta2AlC
Ta2AlC
Ta3AlC2
Ta4AlC3
Ta4AlC3
Ta6AlC5
Calc.
Jeitschko et al.5
Expt.
Calc.
Expt.
Calc.
Calc.
P63/mmc
P63/mmc
P63/mmc
P63/mmc
P63/mmc
P63/mmc
P63/mmc
3.074
3.075
3.079
3.072
3.092
3.070
3.078
13.500
13.830
13.860
18.750
23.708
24.198
34.681
4.392
4.498
4.501
6.104
7.668
7.882
11.267
210
—
—
238
—
259
285
Experimental lattice constants for Ta2AlC and Ta4AlC3 are also included for comparison.
December 2006
Microstructures and Modulus of Ta–Al–C Compounds
Ti2AlC0.5N0.58) have been conducted to modify the properties of
the MAX phases.
Generally speaking, structure and composition determine the
properties of materials. Previous work8,19,28–31 on modifying the
properties of layered ternary phases used compositional control.
Therefore, a second possible approach to modify the properties
of ternary Ta–Al–C carbides and other layered ternary phases is
microstructural control. In other words, controlling the atomicscale microstructure is a possible way to tune the properties of
layered ternary compounds. As far as the Ta–Al–C system is
concerned, ternary carbides have a diverse stacking sequence
and the Ta–C bonds are much stronger than the Ta–Al bonds.
The unique stacking characteristic suggests the possibility of
tailoring the performance of ternary compounds by controlling
their microstructures. We hope that the results presented here
will spur others to study and design the microstructures of the
Ta–Al–C carbides and also other layered ternary compounds for
widespread applications.
IV. Conclusions
In summary, atomic-scale microstructures and theoretical bulk
modulus of Ta–Al–C carbides have been investigated using
TEM and first principles calculations, respectively. Binary cubic TaC shares close structural relationships with ternary Ta–
Al–C carbides. A general crystallographic orientation relationship between binary cubic MX carbides/nitrides and layered
ternary MxAyXz phases carbides/nitrides is proposed as:
[110]MX//[1210]MxAyXz and (111)MX//(0001)MxAyXz.
Ternary Ta–Al–C compounds have diverse microstructures.
A previously unknown ternary Ta6AlC5 carbide, as well as
intergrown Ta2AlC–Ta4AlC3 and Ta4AlC3–Ta6AlC5 structures
were identified. Theoretical calculations demonstrated that the
bulk modulus of ternary Ta–Al–C carbides is related to the
atomic arrangements. The Ta–C bonds are much stronger than
the Ta–Al bonds in these ternary carbides. The diversity of the
microstructures in the ternary Ta–Al–C system provides the opportunity of tuning the properties of Ta–Al–C carbides by
microstructural design.
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