Titanium diboride-tungsten diboride solid solutions formed by induction-field-activated combustion synthesis

Article abstract of DOI:10.1111/j.1151-2916.2003.tb03361.x

Solid solutions of titanium diboride-tungsten diboride (TiB₂-WB₂) were synthesized by induction-field-activated combustion synthesis (IFACS) using elemental reactants. In sharp contrast to conventional methods, solid solutions could be formed by the IFACS method within a very short time, ～2 min. Solutions with compositions ranging from 40-60 mol% WB₂ were synthesized with a stoichiometric ratio (Ti + W)/B = 1/2; however, samples with excess boron were also made to counter the loss of boron by evaporation. The dependence of the lattice constants of the resulting solid solutions on composition was determined. The a parameter decreased only slightly with an increase in the WB₂ content, whereas the c parameter exhibited a significant decrease over the range 40-60 mol% WB₂. Solid-solution powders formed by the IFACS method were subsequently sintered in a spark plasma sintering (SPS) apparatus. After 10 min at 1800°C, the samples densified to relative density 86%. XRD analysis showed the presence of only the solid-solution phase.

Full text of DOI:10.1111/j.1151-2916.2003.tb03361.x

J. Am. Ceram. Soc., 85 [4] 706–10 (2003)
journal
Titanium Diboride–Tungsten Diboride Solid Solutions Formed by
Induction-Field-Activated Combustion Synthesis
Masachika Shibuya, Makoto Kawata, and Manshi Ohyanagi*
Department of Materials Chemistry and the High-Tech Research Center, Ryukoku University,
Seta, Otsu 520-2194, Japan
Zuhair A. Munir*
Facility for Advanced Combustion Synthesis (FACS), Department of Chemical Engineering and Materials Science,
University of California, Davis, California 95616
Solid solutions of titanium diboride–tungsten diboride (TiB2–
WB2) were synthesized by induction-field-activated combus-
tion synthesis (IFACS) using elemental reactants. In sharp
contrast to conventional methods, solid solutions could be
formed by the IFACS method within a very short time, ϳ2
min. Solutions with compositions ranging from 40–60 mol%
of highly oriented second phases, resembling fiber-reinforced
structures.5–9
Solid solutions of TiB2–WB2 have been produced by annealing
mixtures of the two borides, a process that requires high temper-
atures (ϳ2000°C) and long times (ϳ8 h).6,7 In recent studies, the
formation of solid solutions of TiB2–WB2–CrB2 by field activa-
tion using the spark plasma sintering (SPS) apparatus has been
investigated.8,9 In this method, synthesis was conducted using
elemental reactants, reacted under the influence of a high, pulsed
dc current and a uniaxial pressure. With this approach, 94% dense
solid solutions of (Ti,W,Cr)B2 were formed by reacting the
powders at 1900°C for 10 min. The use of this form of field
activation has recently been extended to the synthesis of dense
nanometric composites.10,11
In the present work, we investigated the formation of solid-
solution TiB2–WB2 by a variant of field activation, induction-
field-activated combustion synthesis (IFACS).12,13 Conventional
combustion synthesis (CS) experiments14,15 were also made to
compare the results of our IFACS approach. Because the products
of this approach were not dense, a second step was undertaken to
prepare high-density products. For this, we used the SPS method.
The TiB2–WB2 system is a eutectic type, as shown in Fig. 1,
which includes a relatively large region of the solid solution
(Ti,W)B2 that has the AlB2 structure. On the other side, a relatively
limited solubility of titanium in WB2 defines the (W,Ti)B2 phase,
which has the W2B5 structure.5 As can be seen from this figure,
extensive solid solubility (to ϳ63 mol% WB2) is possible at the
eutectic temperature (2230°C). The solubility limit decreases
markedly as the temperature is lowered, declining to 8 mol% WB2
at 1500°C. On the other side, the TiB2 content in the (W,Ti)B2
phase is relatively low (ϳ1–3 mol%) and changes little with
temperature. When annealed at temperatures below the solvus line,
TiB2–WB2 solid solutions decompose to two phases; titanium-rich
(Ti,W)B2 and tungsten-rich (W,Ti)B2. This process and the mor-
phology of the resulting precipitate give rise to improved fracture
toughness.6
WB2 were synthesized with a stoichiometric ratio (Ti ؉
1
W)/B ؍
⁄2; however, samples with excess boron were also
made to counter the loss of boron by evaporation. The
dependence of the lattice constants of the resulting solid
solutions on composition was determined. The “a” parameter
decreased only slightly with an increase in the WB2 content,
whereas the “c” parameter exhibited a significant decrease
over the range 40–60 mol% WB2. Solid-solution powders
formed by the IFACS method were subsequently sintered in a
spark plasma sintering (SPS) apparatus. After 10 min at
1800°C, the samples densified to relative density 86%. XRD
analysis showed the presence of only the solid-solution phase.
I. Introduction
NTEREST in transition-metal diborides as a class of materials has
I
been motivated by such attractive properties as a refractory
nature, high hardness and electrical and thermal conductivities,
and good corrosion resistance.1 Among these borides, TiB2 has
received considerable attention as a monolithic ceramic or as a
second phase in composites. As an example of the latter, as an
addition to SiC and B4C ceramics, TiB2 is thought to improve their
mechanical properties for such applications as wear-resistant
parts.2,3 However, densification of transition-metal diborides
through sintering is difficult because the dominant mechanism in
material transport is evaporation–condensation with no net shrink-
age.4 Another aspect of the transition-metal diborides, one that is
of interest to this work, is the tendency to form extensive solid
solutions at high temperatures. The significant aspect of this
thermodynamic property is seen from the subsequent decomposi-
tion to two limited solubility phases at lower temperatures. In some
systems, e.g., the titanium diboride–tungsten diboride (TiB2–WB2)
binary, the annealing of the solutions results in the precipitation
II. Experimental Materials and Methods
Powders of 99.5% pure titanium, with an average particle size
of ϳ22 ␮m (Sumitomo Sitix, Inc., Amagasaki, Japan), 99.9% pure
tungsten, with an average particle size of ϳ8 ␮m (Kojundo
Chemical Laboratory, Inc., Sakaido, Japan), and 99% pure crys-
talline boron, with a particle size Ͻ45 ␮m (Kojundo Chemical
Laboratory, Inc.), were used in this work. These were weighed out
in several mole ratios and dry-mixed in an automatic agate mortar
for an hour. The composition of the solid solutions to be synthe-
sized was changed according to the molar ratio of TiB2 and WB2
in a stoichiometric (Ti,W)B2, i.e., maintaining a ratio of (Ti ϩ
J. Petrovic—contributing editor
Manuscript No. 187351. Received December 4, 2001; approved December 3,
2002.
Supported by a grant based on High-Tech Research Center Program for Private
Universities from the Japan Ministry of Education, Culture, Sport, Science, and
Technology (MO) and by the U.S. Army Research Office, ARO (ZAM).
*Member, American Ceramic Society.
W)/B ϭ 1⁄
. In some samples, excess boron was added such that the
2
706

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TiB2-WB2 Solid Solutions Formed by Induction-Field-Activated Combustion Synthesis
707
form of activated synthesis have been provided in previous
publications.12,13 The reaction temperature was measured by
W-5Re/W-26Re thermocouples (Nilaco, Inc., Tokyo, Japan) in-
serted into the sample through the small hole of carbon sheet. The
voltage outputs from the thermocouples were monitored using an
oscillographic recorder (Model RDM-200A, Kyowa, Inc., Tokyo,
Japan).
The TiB2–WB2 solid solutions produced, which were in powder
form, were then crushed in a silicon nitride ball mill (with both the
jar and balls made of this nitride) until an average particle size of
2 ␮m was obtained. The powders were then sintered in an SPS
apparatus16 at the following conditions: 1800°C, 50 MPa, 10 min.
The inside diameter of the carbon die used in the SPS was 20 mm.
The products were analyzed by X-ray powder diffraction (Model
RAD-C system, Rigaku, Tokyo, Japan) using CuK␣ radiation. For
phase identification, the scanning rate was set at 2° minϪ1 from
20°–80°. For lattice constant determinations, a step-scan of ⌬2␪ ϭ
0.02° and a collection time of 2 s were used with five peaks of the
sample and an internal standard.
Fig. 1. Binary phase diagram of the TiB2–WB2 system.6
III. Results and Discussion
overall stoichiometry was (Ti,W)B2ϩx or (Ti ϩ W)/B ϭ 1/(2 ϩ x).
The molar ratio, expressed here in mol% WB2, was varied
nominally from 40–60 mol% WB2.
Figure 3 shows temperature profiles during the synthesis of a
sample (50 mol% WB2 nominal composition) by conventional CS
(curve “a”) and by IFACS (curve “b”). The maximum combustion
temperatures obtained with these methods are ϳ2250° and
2060°C, respectively. In addition to the difference in peak tem-
perature, the two experiments also differed in the cooling rate. The
IFACS samples cooled at a much slower rate, as can be seen from
the figure. Figure 4 shows XRD patterns of products of 50 mol%
WB2 samples fabricated by (a) CS and by (b) IFACS. The product
obtained by CS contains the solid solution (Ti,W)B2 as the main
phase and smaller amounts of WB2 and WB. In contrast, the
product obtained by IFACS contained only the solid solution
(Ti,W)B2. The shoulders on the (001) and (101) peaks (near 2␪
values of 28° and 45°, respectively) in the latter pattern are
attributed to nonuniformity in the solid solution caused by the loss
of boron, as will be discussed later. The difference in the results of
these two experimental approaches suggests a contribution to the
kinetics of dissolution by the field (current) other than thermal
(Joule heating). Such possible contributions have been discussed in
recent work and have been partially identified with the influence of
the current on mass transport, including electromigration.17,18
Phase formation in multilayered systems was found to increase
dramatically in the presence of a current.
For conventional CS, cylindrical compacts (ϳ16 mm in diam-
eter and 30 mm in length) were formed in a stainless-steel die with
double-acting rams by uniaxial pressing at 3.0 MPa. The green
density of these was ϳ60%. Each compact was placed in a porous
crucible inside the combustion chamber and then ignited by
passing a current through a carbon ribbon. The majority of the
experiments were conducted by the IFACS method. For this, the
mixed powders were placed inside a cylindrical carbon sheet, ϳ40
mm in diameter and 5 mm long, as shown in Fig. 2(a). The packing
density of each powder compact was ϳ50%. Each sample was
placed in commercial casting sand inside a silicon nitride crucible
with a 70 mm inside diameter, 140 mm outside diameter, and 70
mm depth. This crucible was put in the IFACS apparatus, as shown
in Fig. 2(b). The sample was ignited by passing an induction
current through the carbon sheet under the induction conditions of
85 V, 170 A, 70 kHz, and a typical time of 120 s. Details of this
Temperature profiles during synthesis by the IFACS method
(with the same settings) for samples with compositions of 40, 50,
and 60 mol% WB2 and for a 50 mol% WB2 nonstoichiometric
samples (Ti,W)B2ϩx with x ϭ 0.3 are shown in Fig. 5. The
temperature on all samples increased gradually at the beginning up
to an “ignition” temperature of ϳ1000°C. It then increased rapidly
Fig. 2. Schematic diagrams of (a) sample cell and (b) induction field-
Fig. 3. Temperature profiles during the synthesis of stoichiometric
activated combustion synthesis apparatus.
(Ti,W)B2 with 50 mol% WB2: by (a) CS and (b) IFACS methods.

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Journal of the American Ceramic Society—Shibuya et al.
Vol. 85, No. 4
Fig. 4. XRD patterns of products prepared by: (a) CS and (b) IFACS for
stoichiometric (Ti,W)B2 with 50 mol% WB2.
Fig. 6. XRD patterns of product prepared by IFACS: (a) stoichiometric
with 60 mol% WB2, (b) stoichiometric with 50 mol% WB2, (c) nonstoi-
chiometric sample (Ti,W)B2.3, and (d) stoichiometric sample with 40
mol% WB2.
to a maximum value then decreased slowly. The maximum
combustion temperatures for the 40, 50, and 60 mol % WB2
(Ti-2B/W-2B ϭ 60/40, 50/50, and 40/60) were ϳ2050°, 2060°,
and 1800°C, respectively. The maximum temperature in the high
WB2 content samples (60 mol% of WB2) was, expectedly, lower
because of the difference in the formation enthalpy of TiB2 and
WB2, 293 and 146 kJ⅐molϪ1, respectively.
XRD patterns of the products of these samples (Ti-2B/W-2B,
40/60, 50/50, 60/40) and Ti-2.3B/W-2.3B, 50/50) are shown in
Fig. 6. In all but one case, the product contained only the solid
solution. For the sample with the 60 mol% WB2, the XRD results
show the presence of an additional phase, WB2. As discussed
above, for this composition, the maximum temperature was
1800°C, which is in the two-phase region, Fig. 1.
As was indicated above, in the case of stoichiometric (x ϭ 0.0)
samples with 50 mol% WB2, the (001) and (101) diffraction peaks
of the product exhibit shoulders (near 2␪ values of 28° and of 45°
in pattern “b” of Fig. 6). This feature is taken as indication of
nonuniformity in the composition of the solid solution, as has been
observed in previous investigations on the AlN–SiC solid solu-
tion,19,20 and is attributed in this case to the loss of boron. Loss of
boron during the heating of a tungsten boride has been report-
ed.21,22 From XRD analysis of evaporation condensates the loss
of boron was identified with the formation of B2O3 by a reaction
with oxygen impurities in the starting powders.22 To counteract
this, excess boron was added to the samples in the reported
investigations.
(Ti,W)B2.3 with 50 mol% WB2. The XRD patterns of the product
of such samples did not exhibit the shoulders in the diffraction
peaks of (001) and, (101) as seen in Fig. 6(c), implying a more
homogeneous solid solution. Further evidence for this is based on
measurements of the full width at half maximum (FWHM) of the
(101) peaks obtained from samples with and without excess boron.
The corresponding values are 0.1657 and 0.2230. Similar obser-
vations on the relationship between FWHM and homogeneity of
the AlN–SiC solid solution have been reported before.19 The
addition of excess boron (corresponding to an excess of 15 at.%)
did not decrease the combustion temperature nor did it signifi-
cantly alter the cooling rate of the samples, as can be seen from
Fig. 5. These results are in disagreement with previously published
observations by Telle et al.6 They reported that the presence of 5
at.% excess boron decreases the region of solid solution such that
at compositions Ͼ20 mol% of WB2, no solid solutions formed
under equilibrium condition.
The matter of the loss of boron that was observed in the 50
mol% WB2 samples is now considered for other compositions. For
a sample with a 40 mol% WB2 composition, the shoulders on the
(001) and (101) peaks are absent, as can be seen in Fig. 6(d).
Because this sample did not contain excess boron, the apparent
lack of loss of boron is of interest. Tungsten diboride is stable over
a relatively wide stoichiometric region, WB2Ϫx with 0.03 Յ x Յ
0.20.21 Because the stability region of TiB2 is considerably
narrower than that of WB2, we expect the deficiency of boron in
the solid solutions to be influenced largely by the content of the
latter. Thus, the sample with higher WB2 content is more likely to
show the effect of boron depletion, as was observed in this work.
In samples with lower WB2 content, the deficiency of boron is
relatively small and the XRD pattern does not exhibit the shoul-
ders, as seen in the pattern for the 40 mol% WB2 sample, Fig. 6(d).
This is further supported by the results obtained on samples
prepared by normal CS, Fig. 4(a). In this case, the reaction to form
the solid solution was not complete, as seen by the presence of
WB2 and WB phases in the product. Although the reactant mixture
contained the equivalent of 50 mol% WB2, the resulting solid
solution did not exhibit the shoulders because, per force, it
contained less WB2 than the intended amount.
To confirm the assumption that the shoulders on the peaks are
associated with the loss of boron, experiments were conducted in
which excess boron was added to provide a stoichiometry of
The dependence of the lattice parameters of the synthesized
solid solutions on composition (i.e., Ti/W ratio expressed as mol%
WB2) is shown in Fig. 7 for the “a” and “c” parameters for the
samples. Because samples with 40 mol% WB2 did not show
evidence for loss of boron, excess boron was added only to
samples with compositions Ͼ50 mol% WB2. The straight line in
Fig. 5. Temperature profiles during the IFACS synthesis of various
compositions in the Ti-W-B system.

April 2003
TiB2-WB2 Solid Solutions Formed by Induction-Field-Activated Combustion Synthesis
709
Fig. 8. XRD pattern of a 50 mol% of WB2.3 sample after sintering in the
SPS.
Fig. 7. Effects of the addition of tungsten (expressed as mol% WB2) on
the lattice parameters of TiB2 (a) “a” parameter, (b) “c” parameter. Excess
boron was added in samples with Ͼ50 mol% of WB2.
both parts of Fig. 7 represents the ideal rule of mixtures based on
the two lattice parameters for TiB2 and WB2. It should be noted
that the products obtained from reactants containing excess boron
in the compositions of 60 mol% WB2.3 and 55 mol% WB2.3 were
not complete solid solutions, but contained residual WB2. The
amount of the residual WB2 was estimated from a calibration
curve in which the intensity of the WB2 primary peak was plotted
against the amount of intentionally added WB2 to samples of solid
solutions. From this calibration, the WB2 content in the solid
solution for the products obtained from the reactants containing 60
mol% WB2.3 and 55 mol% WB2.3 was estimated as 56.8 mol%
WB2 and 53.6 mol% WB2. The data for these samples are plotted
in Fig. 7 using the corrected concentration values.
Figure 7 shows that the “a” parameter decreased only slightly
with an increase in the content of WB2, but the “c” parameter
decreased significantly and was in agreement with the rule of
mixtures (solid line). This observation is consistent with the nature
of the structure of TiB2. Changes in the “a” parameter are dictated
by the strong covalent bonds in the boron layer and are only
slightly influenced by the surrounding metal atoms. On the other
hand, the substitution of another metal on the metal sublattice is
expected to influence the “c” parameter significantly.7
As indicated in the Experimental Procedure section, the
products of IFACS are in the form of agglomerated powders,
and thus a second step is needed to form high-density products.
This was done by a subsequent sintering by the SPS method,
which was done on TiB2–WB2 solid solutions of samples
containing 50 mol% of WB2 with 15 at.% excess boron.
Because interest in these solid solutions focuses on the micro-
structure resulting from a subsequent decomposition, we chose
an SPS temperature, 1800°C, such that the solid solutions are
thermodynamically unstable. This temperature was reached in
ϳ9 min and was maintained for ϳ10 min. Rapid shrinkage took
place during the heat up period, at ϳ1300°C over a period of
ϳ1.5 min. Following sintering, XRD analysis showed the
presence of the only solid-solution phase, Fig. 8. According to
the phase diagram, Fig. 1, the temperature at which the SPS
experiments were made is ϳ300°C lower than the correspond-
ing solvus value, and thus the decomposition of the solid
solution is thermodynamically favored. Two possible explana-
tions can be provided for this observation: the real temperature
of the sample is higher than the surface temperature of the
carbon die, as measured by an optical pyrometer, or that the
time during sintering was too short for decomposition to the two
phases. From previous observations, we believe that a differ-
ence of 300°C between the surface and center of the sample is
unlikely, especially because no combustion reaction is taking
place. Our interpretation, therefore, focuses on the kinetic
aspects of the decomposition process. This interpretation is in
agreement with previous reports. Mitra and Telle6 reported that
the decomposition of these solid solutions is kinetically hin-
dered, requiring a long time. It was this observation that led to
the addition of chromium that was reported to enhance the rate
of the decomposition.6,7 In the more recent work by Kaga et
al.,8 solid solutions containing chromium showed little decom-
position when annealed at 1500°C for 7 h and required 14 h at
this temperature to result in complete decomposition.
The density of the sintered solid solutions was 7.70 g⅐cmϪ3
,
which is based on the nature of the product, corresponds to 86% of
the theoretical value, as calculated from the lattice constants
determined by the XRD measurement. Higher densities (as high as
94%) were obtained in the recent study using the SPS method,8 but
in that case the consolidation occurred simultaneously with the
reaction to form the solid solution from elemental reactants, and
the applied pressure was higher (64 MPa).
IV. Conclusion
Using elemental reactants, solid solutions of titanium dibo-
ride–tungsten diboride (TiB2–WB2) were synthesized within
120 s by the method of induction-field-activated combustion
synthesis (IFACS). Conventional methods of synthesis require
long periods (up to 8 h) and high temperatures (2000°C). Loss
of boron is believed to take place and to result in nonuniform
products in samples containing 50 mol% and larger amounts of
WB2. The lack of this observation in samples containing lesser
amounts of WB2 is explained by the tendency of WB2 to form
substoichiometric phases, WB2Ϫx. Experiments on high WB2
content were performed with excess boron, equivalent to
(Ti,W)B2.3. The products from these experiments showed better
uniformity. Measurements of the lattice parameter of the solid
solutions as a function of composition were made for WB2
content ranging from 40–60 mol%. With an increase in the
WB2 content, the “a” parameter decreased slightly but the “c”
parameter exhibited a significant decrease. Subsequent to the
formation of the solid solutions, the powders were sintered in a
spark plasma sintering apparatus, resulting in a relative density
of ϳ86% for the product obtained from the reactants containing
of 50 mol% of WB2 with 0.3 mol of excess boron. During this
process (1800°C, 10 min), the solid solutions did not decom-
pose to the equilibrium phases.
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