J. Am. Ceram. Soc., 91 [4] 1357–1360 (2008)
DOI: 10.1111/j.1551-2916.2008.02279.x
r 2008 The American Ceramic Society
Journal
New MAX-Phase Compounds in the V–Cr–Al–C System
Yanchun Zhou,*,w,z Fanling Meng,z,y and Jie Zhangz,y
z
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,
Shenyang 110016, China
y
Graduate School of Chinese Academy of Sciences, Beijing 100039, China
solid solution strengthening by substituting V with Cr was
predicted by ab initio calculations.20–22
New layered compounds, (V0.5Cr0.5)3AlC2, (V0.5Cr0.5)4AlC3,
and (V0.5Cr0.5)5Al2C3 were synthesized by reactive hot pressing
V, Cr, Al, and graphite powders. The crystal structures of these
new phases were determined using a combination of X-ray
diffraction and scanning transmission electron microscopy.
(V0.5Cr0.5)3AlC2 is isotypical to Ti3AlC2; while (V0.5Cr0.5)4AlC3
has the Ti4AlN3 or a-Ta4AlC3-type crystal structure.
(V0.5Cr0.5)5Al2C3 is formed by periodically stacking of halfunit cells of (V0.5Cr0.5)2AlC and (V0.5Cr0.5)3AlC2.
II. Experimental Procedure
The starting materials for the synthesis of new compounds in the
Cr—V–Al–C systems were powders of vanadium (99%, 200
mesh), chromium (99%, 200 mesh), aluminum (99%, 300
mesh), and graphite (99%, 200 mesh). The powders with near
stoichiometric ratios of V:Cr:Al:C 5 1:1:1.1:1, V:Cr:Al:C 5
1.5:1.5:1.1:2, and V:Cr:Al:C 5 2:2:1.1:3 were weighed, ball
milled for 24 h in resin jar with stainless-steel balls coated
with polyurethane and then sieved. Because amount of aluminum was consumed during the synthesis procedure, the composition rich in Al was chosen. The mixed powders were uniaxially
pressed at 10 MPa to green compacts in a BN-coated graphite
die. To obtain different phases, the green compacts were heated
in a furnace with graphite as a heating element in flowing Ar
atmosphere to temperatures between 14001–16001C and held at
the target temperatures for 1 h under a pressure of 30 MPa.
After removing the surface contaminations, the as-prepared
samples were examined by X-ray diffraction (XRD) (Rigaku
D/max-2400, Tokyo, Japan) with CuKa radiation. Atomic-scale
microstructures of the different phases in V–Cr–Al–C system
were observed by transmission electron microscopy (TEM). Thinfoil specimens for TEM investigations were prepared by slicing,
mechanical grinding to B50 mm, dimpling down to B10 mm,
and ion beam polishing at 4.0 kV. A 300 kV Tecnai G2 F30
TEM (FEI, Eindhoven, the Netherlands), which was equipped
with a high-angle annular dark field (HAADF) detector in a
scanning transmission electron microscopy (STEM) system, was
used for Z-contrasting STEM imaging. The Digitalmicrograph
software was used for fast Fourier transformation (FFT) of
Z-contrast images.
I. Introduction
T
HE Mn11AXn phases (where M is an early transition metal,
A is an A group element, X is C and/or N, and n 5 1–3),
also called the MAX phases, are layered carbides or nitrides
with crystal structures of the hexagonal symmetry. These materials attracted the attention of material scientists, physicists, and
chemists due to the fascinating properties that they display.1,2
Up to now, more than 50 M2AX compounds (the so-called 211
phase) have been found. For M3AX2 compounds (312 phase)
three members, i.e., Ti3AlC2, Ti3SiC2, and Ti3GeC2, were identified before 1995; while Ta3AlC23–5 and Ti3SnC26 were added to
this group in 2006 and 2007. For M4AX3 compounds (413
phase), only Ti4AlN3 was synthesized in 20007,8. Then, metastable Ti4SiC39 and Ti4GeC310 were found during thin film synthesis in 2004 and 2005, respectively. Very recently,
Ta4AlC3,3,11–14 Nb4AlC3,15 and V4AlC316,17 were discovered
and their crystal structures were determined by different groups.
The identification of these new phases enriches the knowledge of
MAX phases and provides more opportunities to tailor the
properties of these technologically important ceramics. Thus,
the search for new MAX compounds is important and we are
intrigued in finding new materials with fascinating properties. In
this work, we present the crystal structure and atomic-scale
microstructure of three new compounds ((V0.5Cr0.5)3AlC2,
(V0.5Cr0.5)4AlC3, and (V0.5Cr0.5)5Al2C3) belonging to the
MAX phases. V–Cr–Al–C system was chosen because of the
following reasons. First, besides the 211 phase, a new 413 phase
has been found in V–Al–C system,16,17 indicating the possibility
to find a variety of MAX compounds in this system. Second,
although only one 211 phase was identified in Cr–Al–C system,
previous work demonstrated that the compounds in this system
had good high-temperature oxidation and hot corrosion resistance.18,19 Third, Cr has one more valence electron than V and
III. Results and Discussion
All MAX phases crystallize in the hexagonal symmetry with the
space group P63/mmc. The major difference between 211, 312,
and 413 phases is the number of M–X layers separating each A
layer,9,10 i.e., 2, 3, and 4 M–X layers separating each A layer for
211, 312, and 413 phases, respectively. Thus, the coexistence of
these phases and synthesis of specific phases with n 5 1, 2, 3 in a
single system are possible. In our attempts to synthesize solid
solutions in V–Cr–Al–C system at 14501C using the composition
of (V1xCrx):Al:C 5 2:1:1, only the 211 phase, i.e., (V1xCrx)2AlC
with a minor amount of a transition metal carbide (V1xCrxC)
was obtained when x was close to 0 or 1. The peak positions
in XRD patterns shifted to higher angles for both the
(V1xCrx)2AlC and V1xCrxC when x increased from 0 to 1,
indicating that the lattice constants decreased with the increasing x, which is attributed to the fact Cr is smaller than V. When
x was close to 0.5, reflections that corresponded to the 312 and
P. Chartier—contributing editor
Manuscript No. 23870. Received October 16, 2007; approved November 30, 2007.
This article was financially supported by the National Outstanding Young Scientist
Foundation (No. 59925208 for Y. C. Zhou), Natural Sciences Foundation of China under
Grant Nos. 50232040, 50302011, 90403027, High-Tech Bureau of the Chinese Academy of
Sciences, and French Atomic Energy Commission.
*Member, The American Ceramic Society.
w
Author to whom correspondence should be addressed. e-mail: [email protected]
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Fig. 1. X-ray diffraction pattern of a sample prepared at 15001C
using initial composition of V:Cr:Al:C 5 1:1:1.1:1. (V0.5Cr0.5)2AlC,
(V0.5Cr0.5)3AlC2, and (V0.5Cr0.5)4AlC3 phases are labeled as 211, 312,
and 413 in the figure, respectively. Peaks marked with % correspond to
the strongest peaks of each phase.
Table I. Structure Parameters of (V0.5Cr0.5)2AlC,
(V0.5Cr0.5)3AlC2, and (V0.5Cr0.5)4AlC3
Compounds
(V0.5Cr0.5)2AlC
Lattice constants
a (Å)
2.885
c (Å)
12.94
Space group
P63/mmc
Stacking
ABABAB
sequence for
(V, Cr) and
Al atoms along
the [0001]
direction
(V0.5Cr0.5)3AlC2
(V0.5Cr0.5)4AlC3
2.892
2.905
17.73
22.39
P63/mmc
P63/mmc
ABCBCBABABC ABABACBCBC
413 phases were also identified in the XRD pattern. It was
also interesting to note that the newly identified 312
((V0.5Cr0.5)3AlC2) and 413 ((V0.5Cr0.5)4AlC3) phases appeared
at temperatures higher than 15001C and their amount increased
with the preparation temperature (the intensity of the reflections
increased with temperature), which agreed well with the previous
work on the synthesis of Ta4AlC3 and Nb4AlC3.15,23 Figure 1
shows the XRD pattern of a sample prepared at 15001C using
Vol. 91, No. 4
a initial composition of (V0.5Cr0.5):Al:C 5 2:1.1:1. Obviously,
the new 312 and 413 phases coexist with the 211 phase
((V0.5Cr0.5)2AlC) in V–Cr–Al–C system. Using the peak positions shown in Fig. 1, the lattice constants of the
(V0.5Cr0.5)2AlC, (V0.5Cr0.5)3AlC2, and (V0.5Cr0.5)4AlC3 phases
were calculated and given in Table I. It is worth noting that beside the increase of lattice constant c, the lattice parameter a also
continuously increases from (V0.5Cr0.5)2AlC to (V0.5Cr0.5)4AlC3.
This phenomenon was previously observed in V–Al–C16 and
Nb–Al–C15 systems. Etzkorn et al.16 attributed the increase of
lattice constant a from V2AlC to V4AlC3 to the fact that the
transition metal carbide unit between Al atomic layers are
approaching to the structure of the binary VC.
To further confirm the phases labeled in Fig. 1, selected area
electron diffraction (SAED) and high-resolution STEM were
conducted. 211, 312, or 413 phases can be distinguished using
SAED patterns with the electron beam being parallel to ½1
210
direction.24–26 Figure 2 shows the SAED patterns of the three
phases. From the difference of spacings for {000l} reflections in
the three SAED patterns, one can conclude that Figs. 2(a)–(c)
correspond to 211, 312, and 413 phases, respectively. To directly
reveal the atomic stacking sequence of the different phases, highresolution Z-contrast observations were conducted. Figure 3
shows the high-resolution Z-contrast images viewed along the
½1
210 direction of the three V–Cr–Al–C phases. Because the
intensity of Z-contrast image is proportional to the square of
atomic number, Z2,24 and the atomic number of V, Cr, and Al
are 23, 24, and 13, respectively; the bright and dark layers in
Fig. 3 correspond to (V, Cr) and Al layers accordingly (V and Cr
cannot be distinguished due to the close atomic number). In
Fig. 3(a), every two bright layers of (V, Cr) are separated by a
dark layer of Al, the atomic arrangement along [0001] direction
is described as ABABAB (the underling letters refer to Al atom),
which is a typical 211 phase of the Cr2AlC type structure.24,25 In
Fig. 3(b), every three bright layers of (V, Cr) are separated by a
dark layer of Al, the atomic arrangement along [0001] direction
is described as ABCBCBABABC, which is a typical 312 phase
of the Ti3AlC2 type structure.26 In Fig. 3(c), every four bright
layers of (V, Cr) are separated by a dark Al layer, the layered
stacking sequence along [0001] direction is described as ABABACBCBC, which is a typical structure of the Ti4AlN3 type.
It should be mentioned that in recent discovered 413
phases, both V4AlC3 and Nb4AlC3 crystallize in Ti4AlN3type structure, while Ta4AlC3 crystallizes in either Ti4AlN3type structure (a-Ta4AlC3) or b-Ta4AlC3 type structure.14–17
The structure difference between a-Ta4AlC3- and b-Ta4AlC3type structure lies in the stacking difference, i.e., the atomic
arrangement of a-Ta4AlC3 is ABABACBCBC; while that of
b-Ta4AlC3 is ABABABABAB. Our present high-resolution
STEM observations clearly show that (V0.5Cr0.5)4AlC3 has the
Ti4AlN3- or a-Ta4AlC3-type structure. The atomic positions of
Fig. 2. Selected area electron diffraction patterns of (V0.5Cr0.5)2AlC (a), (V0.5Cr0.5)3AlC2 (b), and (V0.5Cr0.5)4AlC3 (c) phases. The electron beam was
parallel to the ½1
210 direction.
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Fig. 3. High-resolution Z-contrast images of (V0.5Cr0.5)2AlC (a), (V0.5Cr0.5)3AlC2 (b), and (V0.5Cr0.5)4AlC3 (c) phases viewed along ½1210 direction.
Fig. 4. Nonfiltered (a) and filtered (b) high-resolution Z-contrast images showing the alternative stacking of two and three transition metal carbides
layers in one slab, forming an ordered structure of (V0.5Cr0.5)5Al2C3.
211, 312, and 413 phases were well documented in a recent paper
by Etzkorn et al.,4 thus we will not repeat them here. The above
analyses on XRD pattern, SAED patterns and STEM images
confirm the presence of (V0.5Cr0.5)2AlC, (V0.5Cr0.5)3AlC2, and
(V0.5Cr0.5)4AlC3 phases in the V–Cr–Al–C system. Because the
bulk modulus increases while the electrical conductivity decreases with the number n in Mn11AXn,3,9 the discoveries of the new
(V0.5Cr0.5)3AlC2 and (V0.5Cr0.5)4AlC3 phases provide new possibilities to tailor mechanical and electrical properties of these
materials.
Besides (V0.5Cr0.5)3AlC2 and (V0.5Cr0.5)4AlC3 phases, another
new phase, i.e. (V0.5Cr0.5)5Al2C3 (523 phase) was also identified.
Figure 4 shows the nonfiltered and filtered high-resolution
Z-contrast images of the 523 phase. The stacking sequence is
alternative stacking of two and three (V, Cr)–C layers between
each Al layer. This structure can also be described as a combination of half-unit cells of 211 and 312 phases. Thus, the lattice
constant c of 523 (30.67 Å) is equal to the sum of the value of
those for 211 (12.94 Å) and 312 (17.73 Å) phases. Palmquist
et al.9 identified Ti5Si2C3 (523) and Ti7Si2C5 (725) in Ti–Si–C
system by a combination of XRD and high-resolution TEM.
They also calculated the bulk modulus of these phases and
found that the bulk modulus of the 523 phase is between those
of 312 and 211 phases; while the bulk modulus for the 725 phase
is between those of 413 and 312 phases. They concluded that
other than compounds with the formula of Mn11AXn, MAX
phases with complex structures existed. The identification of
(V0.5Cr0.5)5Al2C3 gives further evidence that there are MAX
phases with more complex structures.9
We also tried to synthesize pure (V0.5Cr0.5)3AlC2 and
(V0.5Cr0.5)4AlC3 phases. Predominantly single-phase bulk
ceramics of both (V0.5Cr0.5)3AlC2 and (V0.5Cr0.5)4AlC3 were
successfully synthesized. Figure 5 shows an XRD pattern of a
sample prepared at 15501C using a starting composition of
V:Cr:Al:C 5 1.5:1.5:1.1:2. It is obvious that (V0.5Cr0.5)3AlC2 is
the dominant phase with minor amount of (V0.5Cr0.5)4AlC3.
Energy dispersive X-ray spectroscopy analysis (see inset in
Fig. 5. X-ray diffraction pattern of a sample prepared at 15501C using
a composition of V:Cr:Al:C 5 1.5:1.5:1.1:2. (V0.5Cr0.5)3AlC2 is the
predominant phase. Inserted energy dispersive X-ray spectroscopy
spectrum is taken from 312 phase, which reveals the atomic ratio of
V/Cr/Al is close to the stoichiometric ratio of (V0.5Cr0.5)3Al.
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Fig. 5) indicated that no other elements were detected in 312
phase and the atomic ratio of V/Cr/Al was close to the stoichiometric ratio of (V0.5Cr0.5)3Al. Similarly, (V0.5Cr0.5)4AlC3
with minor amount of (V0.5Cr0.5)C was also prepared at
16001C using a starting composition of V:Cr:Al:C 5 2:2:1.1:3.
The above results demonstrated that new MAX phases
(V0.5Cr0.5)3AlC2, (V0.5Cr0.5)4AlC3, and (V0.5Cr0.5)5Al2C3 were
synthesized and identified. Although several new Mn11AXn
phases have been discovered recently, it is still an open question why more than 50 compounds were found for n 5 1, while
only a few compounds were discovered for n 5 2 and 3. Therefore, more theoretical and experimental work is needed to
disclose the underlying mechanisms.
IV. Conclusions
New compounds (V0.5Cr0.5)3AlC2, (V0.5Cr0.5)4AlC3, and
(V0.5Cr0.5)5Al2C3, that belong to MAX phases were discovered
in V–Cr–Al–C system using XRD, SAED, and Z-contrast
STEM. Lattice constants and layered stacking features of these
three compounds were characterized. The present results enrich
the number of crystal structures of the MAX phase classification; moreover, they further imply possibility of tailoring the
mechanical and electrical properties by crystal structure control.
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