Tungsten carbides and W-C phase diagram

Article abstract of DOI:10.1134/S0020168506020051

The crystal structures of the tungsten monocarbide δ-WC and the disordered lower carbide β-W₂C are studied. Using magnetic susceptibility measurements, the hexagonal carbide δ-WC is shown to be stable from 300 to 1200 K. The sequence of phase t

Full text of DOI:10.1134/S0020168506020051

ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 2, pp. 121–127. © Pleiades Publishing, Inc., 2006.
Original Russian Text © A.S. Kurlov, A.I. Gusev, 2006, published in Neorganicheskie Materialy, 2006, Vol. 42, No. 2, pp. 156–163.
Tungsten Carbides and W–C Phase Diagram
A. S. Kurlov and A. I. Gusev
Institute of Solid-State Chemistry, Ural Division, Russian Academy of Sciences,
Pervomaiskaya ul. 91, Yekaterinburg, 620219 Russia
e-mail: gusev@ihim.uran.ru
Received May 17, 2005; in final form, September 8, 2005
Abstract—The crystal structures of the tungsten monocarbide δ-WC and the disordered lower carbide β-W C
2
are studied. Using magnetic susceptibility measurements, the hexagonal carbide δ-WC is shown to be stable
from 300 to 1200 K. The sequence of phase transformations associated with β-W C ordering is analyzed. The
2
temperature and composition stability limits of the cubic carbide γ-WC1 – x are evaluated, and the first data are
presented on the variation of its lattice parameter with composition. An optimized W–C phase diagram is pro-
posed which takes into account detailed structural and phase-equilibrium data for tungsten carbides.
DOI: 10.1134/S0020168506020051
INTRODUCTION
accurately assessing the phase equilibria in the W–C
system.
Group IV–VI transition metal carbides offer the
highest melting points and hardness values among
known compounds [1–3]. Owing to this, they are
widely used in the production of structural and tool
materials capable of working at high temperatures, in
aggressive environments, and under high loads. The
hardness of WC is sufficiently stable and decreases rel-
atively little compared to other carbides as the temper-
ature is raised from 300 to 1200–1300 K. In addition,
WC has a factor of 1.5–2 higher elastic modulus and a
factor of 1.5–2 smaller thermal expansion coefficient in
comparison with other transition metal carbides. It is
this combination of properties and their thermal stabil-
ity which underlie the wide use of WC in the production
of wear-resistant hard alloys, which are basic compo-
nents of all tool materials. The first synthesis of tung-
sten carbide was reported by Moissan in 1893 [4], and
commercial-scale production of WC-based hard alloys
began just 20–25 years after the discovery of WC and
continues to this day. The practical importance of the
W–C system has led to intense studies of the W–C
phase diagram and the structure and properties of tung-
sten carbides.
EXPERIMENTAL
The structure of tungsten carbides was investigated
by x-ray diffraction (XRD) (CuKα1, 2 radiation, 2θ =
0°–140°, step-scan mode with a step size ∆(2θ) =
.03° and a counting time of 2 s per data point).
Magnetic susceptibility χ was measured by the
Faraday method on a Domenicalli balance in a vacuum
of 10 Pa and magnetic fields of 576, 640, and
04 kA/m at temperatures from 300 to 1200 K.
1
0
–
3
7
RESULTS AND DISCUSSION
The major phase in the W–C system is the higher
tungsten carbide δ-WC (WC). Its XRD pattern is dis-
played in Fig. 1. According to our results, WC has a
1
3
hexagonal structure (sp. gr. P6 m2 = D ) with lattice
h
parameters a = 0.2906 nm and c = 0.28375 nm. Both
the W and C atoms in WC form simple hexagonal sub-
lattices. The carbon atoms sit in the center position of
The compounds existing in the W–C system are trigonal-prismatic interstices in the tungsten sublattice.
W C and WC. Both compounds have several polymor- Below 1300 K, δ-WC has no homogeneity range. The
2
phic modifications, stable in different temperature and present results agree well with earlier data [3, 5–8].
composition ranges. Although only a limited number of Conclusive experimental evidence that the hexagonal
groups have studied phase equilibria in the W–C sys- carbide δ-WC exhibits no deviations from stoichiome-
tem, some of the data available in the literature are try has been recently reported by Rempel et al. [9]: their
inconsistent and even mutually exclusive. Moreover, positron annihilation results indicate that δ-WC in a
different designations were sometimes used for the normal state contains no carbon vacancies.
same phase, which added confusion to phase-diagram
Our magnetic susceptibility measurements demon-
data.
strate that WC is a weak paramagnet (Fig. 2) with
–
6
3
This led us to study the crystal structure and some χ(300) ꢀ 0.058 × 10 cm /g. This agrees with the
properties of tungsten carbides with the aim of more report by Klemm and Schüth [10] that the 293-K sus-
1
21

1
22
KURLOV, GUSEV
WC
β-W C (WC0.48)
2
4
0
60
80
100
120 2θ, deg
Fig. 1. XRD patterns of the hexagonal tungsten monocarbide δ-WC (WC1.0) (sp. gr. P6 m2, a = 0.2906 nm, c = 0.28375 nm) and
the hexagonal lower tungsten carbide β-W C (WC ) (sp. gr. P6 /mmc, a = 0.2996 nm, c = 0.4724 nm) with disordered carbon
2
0.48
3
and vacancy distributions.
–
6
3
ceptibility of WC is about 0.07 × 10 cm /g. As the form an hcp sublattice in which half of the octahedral
temperature is raised from 300 to 1250 K, the suscepti- interstices are occupied by carbon atoms. Depending
bility of WC increases steadily, exhibiting Pauli-para- on the arrangement of carbon atoms, W C may be dis-
2
2
magnetic behavior, χ(T) = χ(0) + BT , with χ(0) ꢀ ordered (at high temperatures) or ordered (at low tem-
–
6
3
–14
3
2
peratures).
The XRD pattern of the high-temperature phase
β-W C of the lower carbide WC is displayed in
0
.0565 × 10 cm /g and B ꢀ 1.3 × 10 cm /(g K ).
The absence of anomalies in χ(T) implies that, in the
range 300–1200 K, the electronic structure of WC
remains unchanged, and hexagonal WC undergoes no
phase transformations. This agrees with earlier reports
2
0.48
Fig. 1. WC0.48 has a hexagonal structure (sp. gr.
4
6h
P6 /mmc = D ) of the L'3 type with a disordered
arrangement of carbon atoms and vacancies. Its lattice
[
3
5–8] that hexagonal WC is stable between 300 and
3
030–3050 K.
The lower tungsten carbide β-W C (W C) was ini-
2
2
tially believed to be stable from 300 K to its melting
point. It was shown, however, later that, below 1523 K,
W C does not exist in thermodynamic equilibrium,
2
0
.10
undergoing solid-state decomposition. Lander and
Germer [11] reported the synthesis of cubic W C with
2
0.08
a = 0.416 nm via deposition from tungsten hexacarbo-
nyl vapor. Using a spark discharge between WC elec-
trodes immersed in oil, Lautz and Schneider [12]
obtained a cubic carbide with a = 0.425 nm and tenta-
0
.06
.04
0
tively identified it as W C.
2
According to current views, the lower tungsten car-
4
00
600
800
1000
1200
bide W C exists in three polymorphs—low-tempera-
2
T, K
ture (β''), intermediate (β'), and high-temperature (β)—
which were also designated α-, β-, and γ-W C, respec-
2
Fig. 2. Magnetic susceptibility of hexagonal WC as a func-
tion of temperature.
tively [5, 7]. In the three W C polymorphs, the W atoms
2
INORGANIC MATERIALS Vol. 42 No. 2 2006

TUNGSTEN CARBIDES AND W–C PHASE DIAGRAM
123
Ò
Ò
Ò
(
100)C6
(
001)C6
–
(
(
002)ortho
100)L'3
β"-W C (C6 type, sp. gr. P3m1)
2
(
110)ortho
(111)ortho
β'-W C (ζ-Fe N type, sp. gr. Pbcn)
2
2
(a)
(b)
(c)
1
2
3
β-W C (L'3 type, sp. gr. P6 /mmc)
2
3
2
0
25
30
35
2
Fig. 4. Arrangement of carbon atoms in the structure of the
lower carbides β-W C, β'-W C, and β"-W C (the carbon
2
2
2
θ, deg
planes are normal to the c axis of the parent, disordered
structure of the L'3 type; W atoms are not shown): (a) L'3
type (random arrangement of the carbon atoms and vacan-
Fig. 3. XRD patterns of the lower carbides β-W C, β'-W C,
2
2
and β"-W C. The only differences are observed at low 2θ
2
cies), (b) ζ-Fe N type (ordered arrangement of the carbon
2
angles. The intensity of the (110)ortho and (111)ortho reflec-
tions, characteristic of the orthorhombic carbide β'-W C,
and that of the (001)C6 reflection, characteristic of β"-W2C
C6 structure), are at the background level, less than 0.5%
of the intensity of the strongest reflection (101)L'3 , (101)C6
or (121)ortho
atoms and vacancies in each carbon plane of β'-W C, (c) C6
2
2
type (ordered arrangement of the carbon atoms and vacan-
cies in alternating carbon and vacancy planes of β"-W C);
2
(
(1) interstitial positions occupied at random by carbon
atoms with a site occupancy of 1/2, (2) carbon atoms,
(3) carbon vacancies.
,
.
parameters are a = 0.2996 nm and c = 0.4724 nm. In an ture to the ordered molybdenum carbide Mo C (ζ-Fe N
2
2
ideal unit cell of the lower carbide β-W C, the two W type) [13], but they failed to accurately determine the
2
atoms sit in positions 2c (1/3 2/3 1/4 and 2/3 1/3 3/4) atomic position coordinates of W in the structure of
and the C atom occupies positions 2a (0 0 0 and 0 0 1/2) β'-W C. Our analysis of the arrangement of atoms in
2
at random. The partially occupied (half of the sites), the orthorhombic unit cell, derived from the hexagonal
disordered carbon layers are stacked along the c axis. cell of the L'3 structure, and calculation of the XRD pat-
According to earlier reports [5–7], the disordered hex- tern for β'-W C with the ζ-Fe N structure (Fig. 3) indi-
2
2
agonal phase β-W C has a homogeneity range from
2
cate that the atoms in the orthorhombic unit cell have
the following position coordinates: W in 8d (1/4 ꢀ1/8
1/2) and C in 4c (0 ꢀ3/8 1/4). The β'-W C phase with
WC0 to WC0.52 and is stable in the range from 2670–
.34
2
720 K to its melting point (3000–3050 K).
ꢀ
2
the ζ-Fe N structure (Fig. 4b) has an ordered carbon
The XRD patterns of the high-temperature
2 2
2
sublattice, in contrast to the high-temperature hexago-
(
(
β-W C), intermediate (β'-W C), and low-temperature
nal phase β-W C (Fig. 4a). The orthorhombic phase
β"-W C) phases differ very little because these phases
2
2
β'-W C has a homogeneity range from WC
to WC0.49
have the same hexagonal tungsten sublattice, whereas
the atomic scattering factor of carbon is many times
lower than that of tungsten. For this reason, changes in
2
0.34
and exists in the temperature range from 2370 to 2670–
750 K [5]. It cannot be ruled out however that β-W C
2
2
the arrangement of carbon atoms in W C are only and β'-W C are in fact the same, hexagonal phase (sp.
2
2
detectable at low diffraction angles (Fig. 3). This led us gr. P6 /mmc) with a disordered carbon sublattice.
3
to critically evaluate the diffraction data available in the
The low-temperature phase β"-W C has a hexago-
literature before discussing the structures of β'-W C
2
2
3
and β"-W C.
2
nal structure of the C6 type (sp. gr. P3 m1 = D ) [14].
3
d
The carbon atoms and vacancies in this phase are
arranged in the form of ordered layers perpendicular to
the c axis. The carbon planes are sandwiched between
A and B tungsten layers, and the vacancy planes,
between B and A tungsten layers (Fig. 4c). In an ideal
The β'-W C phase possesses orthorhombic symme-
2
try, even though it has an hcp tungsten sublattice, just as
the high-temperature phase β-W C. The structure of the
2
orthorhombic phase β'-W C can be assigned to the
2
1
4
PbO structure type (sp. gr. Pbcn = D ). Rudy and structure of β"-W C, the two W atoms sit in position 2d
2
2h
2
Windisch [5] assumed that β'-W C is similar in struc- (1/3 2/3 z and 2/3 1/3 –z, with z = 0.25), the C atom sits
2
INORGANIC MATERIALS Vol. 42 No. 2 2006

1
24
KURLOV, GUSEV
in position 1a (0 0 0), and the vacancy is located in posi- found the cubic carbide γ-WC1 – x on the surface of
tion 1b (0 0 1/2). Actually, the c-axis spacing between spark-processed, melted samples of δ-WC and WC–Co
tungsten layers separated by a vacancy layer is slightly hard alloys. Given the processes that were successfully
narrower than that between tungsten layers separated used to prepare cubic tungsten carbide [8, 16–20], it
by a carbon layer, and z slightly exceeds 0.25. Compar- seems likely that Lander and Germer [11] and Lautz
ison with the structure of the disordered carbide β-W C and Schneider [12] also obtained the cubic phase
2
indicates that ordering splits position 2a, occupied at γ-WC1 – x rather than the lower carbide W C with a
2
random by C atoms with a site occupancy of 1/2, into cubic structure.
positions 1a and 1b, one of which is fully occupied by
Single-phase γ-WC1 – x exists in the composition
carbon, and the other is vacant. At ꢀ2300 K, the homo-
range WC0.58 to WC0.65 at temperatures above 2790–
geneity range of β"-W C extends from WC
to
2
0.34
2
810 K [6, 7, 15]. Moreover, γ-WC1 – x may exist in
WC0.48. With decreasing temperature, the homogeneity
range narrows down, to the extent that, at 1523 K, there
is no homogeneity range, and the composition of this
phase is WC0.48 [5, 15]. Below 1523 K, the lower car-
equilibrium with β-W C or δ-WC. According to studies
2
of cubic tungsten carbide containing 51.0 at % C [7],
the DTA curve obtained during cooling of the melt
from ꢀ3270 K shows peaks at ꢀ2990 and ꢀ2200 K,
which are due to the eutectic transformation L ⇔
bide β"-W C decomposes into W and the higher car-
2
bide δ-WC. The ordering-induced changes in the
γ-WC
+ δ-WC and eutectoid decomposition
1
– x
arrangement of the carbon atoms and vacancies in the
γ-WC1 – x ⇔ β-W C + δ-WC. It follows from the results
reported by Rudy and Hoffman [7] that the cubic phase
2
lower carbide W C are illustrated in Fig. 4.
2
In the composition range between the lower tung- γ-WC1 – x forms rather readily from the melt, but the
sten carbide β-W C and the higher carbide δ-WC, there
2
direct solid-state transformation δ-WC ⇔ γ-WC1 – x is
hindered and requires that the temperature be varied
exists the cubic phase γ-WC1 – x (also designated β-WC
[
8], α-WC1 – x [7], or simply WC1 – x). This phase was slowly near the transformation temperature T . Another
tr
first described in [11, 12, 16] and was assumed initially possibility is that the temperature stability range of the
to be yet another polymorph of the lower carbide W C. cubic carbide is very narrow.
2
Given, however, that all of the octahedral interstices in
Sara [8] investigated in detail the phase equilibria in
its hcp tungsten sublattice can be occupied by carbon
the W–C system with the participation of the cubic
atoms, resulting in the stoichiometry γ-WC , it is
1
.0
phase γ-WC1 – x. Microstructural examination of sam-
ples with 45 at % carbon quenched in the solid state
from 2988–3023 K or liquid-quenched from 3043–
more correct to consider it a cubic polymorph of the
higher tungsten carbide WC. Data on the temperature
and composition stability limits of the cubic carbide
γ-WC1 – x are the most contradictory.
3
063 K revealed the presence of the cubic tungsten car-
bide γ-WC1 – x. According to Sara [8], the tungsten-rich
phase boundary of the cubic carbide γ-WC1 – x is
WC0.59, independent of temperature, whereas its car-
bon-rich phase boundary shifts to higher carbon con-
tents with increasing temperature and may reach the
composition WC1.00. On the whole, according to his
results the cubic tungsten carbide γ-WC1 – x has a
broader homogeneity range at temperatures from 3030
to 3055 K, from WC0.59 to WC0.98–1.00, than was
reported by Rudy et al. [6, 7, 15]. Additional evidence
that the carbon-rich phase boundary of γ-WC1 – x
reaches the stoichiometric composition WC is the high
5
h
γ-WC1 – x has the B1 structure (sp. gr. Fm3 m = O ),
typical of nonstoichiometric Group IV and V transition
metal carbides and nitrides with the general formula
MX . Goldschmidt and Brand [16] obtained cubic
tungsten carbide by spark processing of W foil. Evi-
dence for the existence of cubic tungsten carbide was
also reported by Sara [8]. Ronsheim et al. [17] identi-
fied γ-WC1 – x with the B1 structure among the products
of the arc plasma (Ar–CH ) synthesis of tungsten car-
bide from W powder. The reaction product contained
4 to 96% cubic tungsten carbide ranging in particle
y
4
1.0
7
superconducting transition temperature of this com-
size from 2 to 16 nm. The chemical composition of the
cubic phase γ-WC1 – x synthesized in [17] was not
reported. In studies by Willens et al. [18, 19], con-
pound reported by Willens et al. [18, 19]: T = 10 K.
c
According to Goldschmidt and Brand [16], the lat-
cerned with the superconducting transition tempera- tice parameter of γ-WC1 – x is aB1 ꢀ 0.427 nm. The lattice
tures of cubic MoC, WC, and carbide solid solutions,
cubic tungsten carbide was obtained by rapid quench-
ing from the liquid state. According to Willens and
parameter of cubic WC0.82 was reported to be
.4215 nm [8]. Ronsheim et al. [17] found the lattice
0
parameter of cubic tungsten carbide of unknown com-
position to be 0.4229 nm. According to Krainer and
Robitsch [20], a near-stoichiometric cubic tungsten car-
Buehler [18], the stoichiometric cubic carbide γ-WC1.0
has a relatively high T of 10 K, whereas the T of
WC0.85 is 9 K. Note that the superconducting transition bide has a = 0.4248 nm. The lattice parameters of cubic
temperatures of the hexagonal carbides β-W C and δ- WC1.0 and WC0.85 were reported to be 0.4266 and
c
c
2
WC, 3.6 and 0.3 K, respectively [18], are substantially 0.4252 nm, respectively [18]. In a sample containing, in
lower than the T of γ-W1 – x. Krainer and Robitsch [20] addition to cubic tungsten carbide, the lower carbide
c
INORGANIC MATERIALS Vol. 42 No. 2 2006

TUNGSTEN CARBIDES AND W–C PHASE DIAGRAM
125
a , nm
cubic carbide γ-WC1 – x is a linear function of carbon
B1
content, a = 0.424 nm corresponds to the composition
B1
WC0.71–0.72. Rudy et al. [7, 15] determined the lattice
parameter of cubic WC0.61 to be 0.4220 nm. Extrapolat-
ing this value, Krainer and Robitsch [21] obtained a =
0.4265 nm for a defect-free cubic tungsten carbide. As
seen in Fig. 5, the most consistent data on the lattice
parameter of the cubic carbide γ-WC1 – x were reported
in [7, 15, 18–21]. Those data demonstrate that aB1
increases with carbon content and are well represented
0
0
0
0
0
.426
.425
.424
.423
.422
[7, 15]
[
[
[
[
18, 19]
8]
20]
2
by a (y) = 0.4015 + 0.0481y – 0.0236y [nm].
B1
Thus, among tungsten carbides, only the hexagonal
phase WC (δ-WC) has an insignificant homogeneity
range, whereas the three polymorphs of the lower car-
21]
bide W C and the cubic carbide γ-WC
have rather
2
1 – x
0
.7
0.8
0.9
1.0
broad homogeneity ranges.
Û = 1 – x = C/W
The broad homogeneity range of the cubic carbide
γ-WC1 – x [8], extending to the composition WC , fits
Fig. 5. Lattice parameter aB1 as a function of carbon content
1.0
for the cubic tungsten carbide γ-WC1 – x ≡ γ-WC (B1 struc-
well with the results reported in [16–20]. At the same
time, according to the W–C phase diagram reported by
Sara [8] the stoichiometric cubic carbide WC1.0 exists
only at ꢀ3058 K, and the phase boundary of γ-WC1 – x
y
ture) in its homogeneity range. The dashed line represents
the best fit to the data reported in [7, 15, 18–21]: aB1(y) =
2
0
.4015 + 0.0481y – 0.0236y [nm].
for (1 – x)
1 is such that, as the temperature is low-
ered from 3058 to 3028 K, WC must undergo two
1
.0
W C and a large amount of free carbon, the lattice
parameter of the cubic phase was 0.4240 nm [18]. tectoid, which appears unlikely. Moreover, one of the
Under the assumption that the lattice parameter of the phase-equilibrium lines in that phase diagram is drawn
2
consecutive transformations, peritectic and then peri-
Û = C/W
0
.2
0.4
0.6 0.8 1.0
1.5
T, K
500
3
W + L
L
L + C
γ + C
γ + L
γ-WC1 – x
058 ± 10
3028 ä
3
3058 ± 5
2
988 ± 5
3
2
2
000
500
000
β
+
γ
β-W C
γ + δ
δ-WC
2993 ± 5
W + β-W C
2
2
2789 K
ꢀ
2673 K
2370 K
β-W C + δ-WC
2
2
657 K
β'-W C
β'-W C + δ-WC
ꢀ
2
2
β''-W C (α-W C)
β''-W C + δ-WC
δ-WC + C
2
2
2
1
523 K
1
500
W
W + δ-WC
10
20
30
40
50
60
at % C
Fig. 6. Phase diagram of the W–C system.
INORGANIC MATERIALS Vol. 42 No. 2 2006

1
26
KURLOV, GUSEV
Table 1. Structural data for phases existing in the W–C system above 1300 K
Phase
Composition
range, at % C
Structure type
and symmetry
Lattice parameters, nm
Sp. gr.
designation
designation
in Fig. 6 in earlier reports
9
h
W
W
0–1.0
ꢀ25.5–34.0
ꢀ29.5–33.0
ꢀ29.5–32.5
a = 0.3165
Im3 m (O )
cI2 A2 (W type), fcc
a = 0.3002,
c = 0.475–0.476 [15]
4
6h
L'3 (W C type),
2
β-W C
γ-W C [5, 7]
P6 /mmc (D ) hP3
2
2
3
hexagonal
a = 0.4728, b = 0.6009,
c = 0.5193 [5]
14
2h
PbO or Mo C (ζ-Fe N)
2
2
2
β'-W C
β-W C [5, 7]
Pbcn (D
)
–
2
2
type, orthorhombic
a = 0.2985, c = 0.4717
3
3d
C6 (CdI type),
2
β''-W C
α-W C [5, 7]
P3 m1 (D
)
hP3
2
2
(WC0.41) [15]
hexagonal
a = 0.3001, c = 0.4728
WC0.50) [15]
(
5
h
γ-WC1 – x
δ-WC
C
α-WC1 – x [7]
β-WC [8, 17]
WC [7]
ꢀ37.0–39.5 [7] a = 0.4266 (WC1.0),
Fm3 m (O )
cF8 B1 (NaCl type), cubic
ꢀ37.1–50.0 [8] a = 0.4252 (WC0.85) [18]
1
3
Bh (WC type),
hP2
ꢀ50
a = 0.2906,
P6 m2 (D
)
h
hexagonal
α-WC [8]
C
c = 0.2837 [15]
a = 0.142, c = 0.339
4
A9 (C, graphite),
hexagonal
100
P6 /mmc (D ) hP4
3
6h
Table 2. Special points in the phase diagram of the W–C system (Fig. 6) at T > 1300 K
Reaction
at % C in the phases involved
T, K
Reaction type
L ⇔
W
0
0
–
3755 ± 5
3058 ± 10
3058 ± 5
2988 ± 5
3028 ± 5
3008 ± 5
2993 ± 5
2798 ± 5
2673 ± 10
2768 ± 10
2657 ± 10
2370 ± 15
1523 ± 5
Melting
L ⇔ β-W C
ꢀ30.6
ꢀ42.0
ꢀ23.5
ꢀ37.0
ꢀ49.3
50.0
ꢀ30.6
100.0
ꢀ1.2
–
Congruent melting
2
L + C ⇔ γ-WC1 – x
50.0
ꢀ25.5
ꢀ37.8
–
Peritectic
L ⇔ W + β-W C
Eutectic
2
L ⇔ β-W C + γ-WC
ꢀ34.3
ꢀ49.3
ꢀ49.8
ꢀ34.0
ꢀ0.7
Eutectic
2
1 – x
γ-WC1 – x
γ-WC1 – x
γ-WC1 – x
⇔
⇔
⇔
δ-WC
Polymorphic transformation
Eutectoid decomposition
Eutectoid decomposition
Eutectoid decomposition
Disorder–order transformation
Eutectoid decomposition
Order–order transformation
Eutectoid decomposition
δ-WC + C
100.0
ꢀ49.5
ꢀ29.7
–
β-W C + δ-WC
ꢀ38.2
ꢀ28.6
ꢀ31.6
ꢀ33.5
2
β-W C ⇔ W + β'-W C
2
2
β-W C ⇔ β'-W C
ꢀ31.6
ꢀ32.8
2
2
β-W C ⇔ β'-W C + δ-WC
ꢀ49.8
–
2
2
β'-W C ⇔ β''-W C
30.0–32.5 30.0–32.5
ꢀ32.6
2
2
β''-W C ⇔ W + δ-WC
0
50.0
2
improperly: the horizontal representing the invariant lar (inflection point). This, however, would reduce the
eutectic transformation L ⇔ β-W C + γ-WC1 – x gradu- temperature range of the two-phase field L + γ-WC1 – x
2
ally passes into a monovariant curve, which is physi- and would raise its lower boundary, which is at variance
cally impossible and incorrect from the viewpoint of with the DTA and microstructural analysis results
geometrical thermodynamics. Such a phase equilib- reported by Sara [8]. The inconsistencies in question
rium would be possible if the point common to the can be eliminated by assuming that the cubic carbide
eutectic horizontal and monovariant curve were singu- WC1.0 has at least a narrow temperature stability range
INORGANIC MATERIALS Vol. 42 No. 2 2006

TUNGSTEN CARBIDES AND W–C PHASE DIAGRAM
127
and that the two consecutive phase transformations
occur at different compositions.
6. Rudy, E., Compendium of Phase Diagram Data. Ternary
Phase Equilibria in Transition Metal–Boron–Carbon–
Silicon Systems, Report AFML TR-65-2, Wright–Patter-
son Air Force Base (Ohio), 1969.
Figure 6 shows the W–C phase diagram which takes
into account the temperature stability range of the cubic
carbide WC , the experimental data reported in [5–8,
7
. Rudy, E. and Hoffman, J.R., Phasengleichgewichte im
Bereich der kubischen Karbidphase im System Wol-
fram–Kohlenstoff, Planseeber. Pulvermetall., 1967,
vol. 15, no. 3, pp. 174–178.
1
.0
1
5–20], and the present results. Table 1 summarizes
structural data for the phases of the W–C system. Near
the composition WC1.0 (Fig. 6), reducing the tempera-
ture from 3058 to 2993 K leads first to the peritectic for-
mation of the cubic tungsten carbide, L + C ⇔
γ-WC , and then to the cubic–hexagonal phase trans-
8
9
. Sara, R.V., Phase Equilibrium in the System Tungsten–
Carbon, J. Am. Ceram. Soc., 1965, vol. 48, no. 5,
pp. 251–257.
1
− x
. Rempel, A.A., Würschum, R., and Schaefer, H.-E.,
Atomic Defects in Hexagonal Tungsten Carbide Studied
by PositronAnnihilation, Phys. Rev. B: Condens. Matter,
2000, vol. 61, no. 9, pp. 5945–5948.
formation, γ-WC1 – x
eutectoid decomposition γ-WC1 – x
(
⇔
δ-WC. The temperature of the
δ-WC + C
2993 K) was inferred from the DTA data reported by
Rudy and Hoffman [7] for a carbide sample containing
1 at % C. It is clear from Fig. 6 that, even though the
⇔
1
1
0. Klemm, W. and Schüth, W., Magnetochemische Unter-
suchungen: 3. Über den Magnetismus einiger Carbide
und Nitride, Z. Anorg. Allg. Chem., 1931, vol. 201, no. 1,
pp. 24–31.
5
temperature stability range of γ-WC1 – x is rather nar-
row, the stoichiometric cubic carbide WC1.0 can be liq-
uid-quenched. This correlates with the results reported
by Willens et al. [18, 19]. The phase diagram in Fig. 6
provides a consistent picture of phase equilibria in the
W–C system, which agrees with earlier reports. The
special points in the phase diagram of the W–C system
1. Lander, J.J. and Germer, L.H., Plating Molybdenum,
Tungsten, and Chromium by Thermal Decomposition of
Their Carbonyls, Trans. AIME, 1948, vol. 175,
pp. 661−691.
1
1
1
1
2. Lautz, G. and Schneider, D., Über die Supraleitung in
(Fig. 6) at T > 1300 K are listed in Table 2.
den Wolframkarbiden W C und WC, Z. Naturforsch., A,
2
1
961, vol. 16, no. 12, pp. 1368–1372.
Note that the structural data for tungsten carbides
especially for those expected to be ordered-vacancy
(
3. Parthe, E. and Sadagopan, V., The Structure of Dimolyb-
denum Carbide by Neutron Diffraction Technique, Acta
Crystallogr., 1963, vol. 16, no. 3, pp. 202–205.
compounds) are only tentative. To refine the structure
of nonstoichiometric tungsten carbides to the point of
determining the positions of carbon atoms and vacan-
cies, neutron or synchrotron x-ray diffraction studies
are needed.
4. Butorina, L.N. and Pinsker, Z.G., Electron Diffraction
Study of W C, Kristallografiya, 1960, vol. 5, no. 4,
2
pp. 585–588.
5. Rudy, E., Windisch, S., and Hoffman, J.R., W–C Sys-
tem: Supplemental Information on the Mo–C System.
Ternary Phase Equilibria in Transition Metal–Boron–
Carbon–Silicon Systems (Part I. Related Binary Sys-
tems, vol. VI), Report AFML-TR-65-2, Wright–Patter-
son Air Force Base (Ohio), 1966, pp. 1–50.
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research, project no. 06-03-32047.
1
1
1
6. Goldschmidt, H.J. and Brand, J.A., The Tungsten-Rich
Region of the System Tungsten–Carbon, J. Less-Com-
mon Met., 1963, vol. 5, no. 2, pp. 181–194.
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2
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INORGANIC MATERIALS Vol. 42 No. 2 2006