Int. Journal of Refractory Metals and Hard Materials 32 (2012) 33–38
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Int. Journal of Refractory Metals and Hard Materials
journal homepage: www.elsevier.com/locate/IJRMHM
Effects of Al2O3 addition on the microstructure and properties of Ni activated sintered
W matrix composites
Ö. Utku Demirkan, Aziz Genç ⁎, M. Lütfi Öveçoğlu
Particulate Materials Laboratories, Department of Metallurgical and Materials Engineering, Istanbul Technical University, Maslak, 34469, Istanbul, Turkey
a r t i c l e
i n f o
Article history:
Received 31 October 2011
Accepted 21 January 2012
Keywords:
Activated sintering
Mechanical alloying
W
Al2O3
a b s t r a c t
In the present investigation, fabrication of high dense (> 97.8%) W matrix composites with increased
microhardness values were investigated. W and W–1 wt.% Ni powders were mechanical alloyed for 18 h
and sintered at 1400 °C for 1 h under Ar, H2 gas flowing conditions in order to investigate the effects of
1 wt.% Ni addition on the densification and properties of W. The effects of Al2O3 particles additions on the
microstructural and physical properties of the sintered W–1 wt.% Ni sample were investigated. A 92.59% relative density value of the sintered W sample increased to 99.47% with the addition of 1 wt.% Ni. Moreover,
despite the observed grain growth, microhardness values significantly increased from 2.81 ± 0.34 GPa to
4.07 ± 0.16 GPa with the addition of 1 wt.% Ni. The relative density values of the sintered W–1 wt.% Ni sample
slightly decreased with increasing Al2O3 additions, a relative density value of 97.81% was measured for the
W–1 wt.% Ni sample reinforced with 2 wt.% Al2O3 particles. As the average grain size of W in the sintered
W–1 wt.% Ni sample decreased from 4.41 ± 1.71 μm to 1.29 ± 0.39 μm with the addition of 2 wt.% Al2O3
particles, the microhardness of the sample increased to 5.98 ± 0.31 GPa.
© 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Tungsten (W) and its alloys are attractive candidate materials for
various elevated temperature structural applications due to their excellent properties such as high melting point, high modulus, high resistance of thermal shock and low coefficient of thermal expansions
(CTE) [1,2]. However, alloying of monolithic W is mandatory for applications which require high strengths at elevated temperatures
since mechanical properties of monolithic W decreases significantly
with increasing temperature [1–3]. In order to overcome improve
the mechanical properties of W, W matrix composites reinforced
TiC, ZrC, HfC, SiC, B4C, La2O3, Y2O3, HfO2, Sm2O3, ThO2 and TiB2 were
developed over the years [2–16]. Oxide dispersion strengthened
tungsten matrix composites and alloys, with their high strength at
elevated temperatures and controlled fracturing mechanism depending on their oxide content, are one of the promising materials for high
temperature applications [10,16,17].
Although W and its alloys exhibit a combination of very unique
properties, fabrication of W is a difficult task due to its high melting
point and low ductility [1]. High sintering temperatures as high as
2500 °C [7] are used in order to get fully dense W fabricated using
conventional, pressureless, sintering techniques which can be
lowered to the range of 1680–1770 °C with the applied mechanical
alloying technique followed by conventional sintering [12,13]. It is
⁎ Corresponding author. Tel.: + 90 2122857347; fax: + 90 2122853427.
E-mail address: [email protected] (A. Genç).
0263-4368/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijrmhm.2012.01.004
very well known that, activated sintering, which involves small
additions of some transition metals such as Pd, Pt, Ni, Co and Fe,
enables major reductions in the sintering temperature of W even
when the conventional sintering techniques are used [18–32],
owing to the enhanced grain boundary diffusions through the nanoscale Ni-rich, quasi-liquid, interfacial layers [29–32].
The coupled effects of mechanical alloying and Ni activated
sintering on the densification of W matrix composites fabricated via
conventional sintering at 1400 °C for 1 h have been reported previously [14,15]. In the present study, the efficiency of Ni activated
sintering technique applied along with the mechanical alloying and
its effects on the densification and properties of W are investigated
by the addition of 1 wt.% Ni to the W powders MA'd for 18 h followed
by sintering at 1400 °C for 1 h, which compared with the pure W
compacts fabricated with the same procedure. The effects of Al2O3
particles on the microstructural and mechanical properties of Ni activated sintered W matrix composites were evaluated.
2. Experimental procedures
W (Eurotungstene™, 99.9% purity, 4.5 μm average particle size),
Ni (Alfa Aesar™, 99.9% purity, 3–7 μm average particle size) and
Al2O3 (Sulzer Metco™, 99.99% purity, 44 μm average particle size)
powders were used in the present study.
Blended powder batches with a total amount of 20 g powders having the compositions of W, W–1 wt.% Ni, W–1 wt.% Ni–0.5 wt.% Al2O3,
W–1 wt.% Ni–1 wt.% Al2O3, W–1 wt.% Ni–1.5 wt.% Al2O3 and W–1 wt.%
Ni–2 wt.% Al2O3 (hereafter referred to be as the W, W1Ni, W1Ni–0.5
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Ö.U. Demirkan et al. / Int. Journal of Refractory Metals and Hard Materials 32 (2012) 33–38
Al2O3, W1Ni–1Al2O3, W1Ni–1.5Al2O3, W1Ni–2Al2O3 samples, respectively) were mechanically milled/alloyed (MM'd/MA'd) for 18 h using
a Spex™ Duo Mixer/Mill 8000D with a speed of 1200 rpm in a tungsten
carbide (WC)–cobalt (Co) vial with WC–Co balls having a diameter of
6.35 mm (1/4 inch.). Preliminary studies on the effect of MA durations
on the properties of W1Ni samples (not presented here) showed that
longer MA durations than 18 h have no significant effects on relative
density and microhardness values of the sintered samples. Therefore,
MM/MA duration is fixed as 18 h for all samples. Vials were sealed inside a Plaslabs™ glove box under purified Ar gas (99.995% purity) to
prevent oxidation during MA. The ball-to-powder weight ratio (BPR)
was 7:1. The amount of WC contamination in the MM'd/MA'd powders
was determined via measuring the amount of Co by using a Thermo
Scientific™ XL3t Niton X-ray fluorescence (XRF).
MM'd/MA'd powders were compacted in a 10 t capacity APEX™
3010/4 uni-action hydraulic press to obtain cylinder type compacts
with a diameter of 6.5 mm under an uniaxial pressure of 400 MPa.
The compacts were sintered in a Linn™ high temperature hydrogen
furnace at 1400 °C under inert Ar (introduced between 20–650 °C
and 900–1400 °C) and reducing H2 (introduced between 650–900 °C)
gas flowing conditions for 1 h.
Microstructural characterizations of sintered samples were carried
out using a Bruker™ D8 Advance X-ray diffractometer (XRD) (CuKα
radiation) and a Hitachi™ TM-1000 scanning electron microscope
(SEM). EDS analyses are conducted using a JEOL™ JEM 7000F SEM
equipped with an Oxford™ EDS detector. W grain sizes in the
as-sintered surface SEM micrographs were measured by drawing
two perpendicular lines crisscrossing the equiaxed W grains. The
size of each grain is the arithmetic mean of these two measurements
and grain size results are arithmetic means of about 50 equiaxed W
grains. Sintered densities were measured by using the Archimedes'
method. The relative density results are the arithmetic mean of
measurements taken from at least 2 different samples for each composition. Vickers microhardness tests were conducted on sintered
samples using a Shimadzu™ microhardness tester under a load of
100 g for 15 s. Microhardness test result for each sample is the arithmetic mean of 10 successive indentations and standard deviations.
3. Results and discussions
3.1. Ni activated sintering
In order to investigate the effects of Ni addition on the sinterability
of MA'd W powders, pure W and W–1 wt.% Ni powders were respectively MM'd and MA'd for 18 h and sintered at 1400 °C for 1 h.
X-ray diffraction (XRD) patterns taken from the sintered W and
W1Ni samples are given in Fig. 1. Fig. 1a is an XRD pattern taken
from the sintered W sample, revealing the presence of diffraction
peaks belonging only to the W phase, which has a b.c.c. Bravais lattice
and Im3− m space group with the lattice parameter of a = 0.316 nm
[33]. The XRD pattern taken from the sintered W1Ni sample
(Fig. 1b) is also very similar to Fig. 1a, where all peaks belong to the
reflections from the W phase.
Fig. 2a–d are the back scattered SEM micrographs taken from the
sintered W and W1Ni samples. As seen in Fig. 2, two different types
of SEM investigations are conducted for each sample, one is the investigation of the surface of the as-sintered sample without no further
sample preparations and the other one is the SEM investigations of
the polished surface after routine metallographic sample preparations. SEM micrographs of the as-sintered surfaces reveal the grain
structure with a mechanism similar to the process called thermal
etching, where grains with different orientations have different thermal expansion coefficients [34]. Fig. 2a shows the SEM micrograph
taken from the as-sintered surface of the W sample revealing a microstructure with some secondary phases and some open pores throughout the microstructure. SEM/EDS investigations confirmed that light
Fig. 1. XRD patterns taken from the sintered (a) W and (b) W1Ni samples.
contrast grains are W with W contents higher than 99.4 wt.% and the
secondary phases are WC particles comprising of 94.75 ± 1.50 wt.% W
and 5.25 ± 1.50 wt.%C, which inherited by the milling media during
MA. Although contaminated WC particles are not quite visible in the
SEM micrograph taken from the polished surface of the sintered W
sample (Fig. 2b), similar microstructural features with Fig. 2a, i.e.
some open pores, can be observed in this micrograph. Fig. 2c is the
back scattered SEM micrograph taken from the as-sintered surface of
the W1Ni sample and reveals the presence of some dark gray grains
unlike Fig. 2a. On the basis of EDS scans, these dark gray grains are
composed of 86.71 ± 8.50 wt.% W, 12.14 ± 2.43 wt.% Ni and 1.15 ±
0.71 wt.% Co. The presence of Co can be explained by the contamination during MA in which WC–Co vials and balls are used as milling
media. XRF measurements are conducted on the MM'd/MA'd powders
in order to reveal the amount of WC contamination during MA by
tracing the amounts of Co in the MA'd powders. XRF measurements
revealed the presence of 0.11 wt.% and 0.08 wt.% Co for the W and
W1Ni powders MM'd/MA'd for 18 h, respectively. By taking into
account that WC–Co vials and balls are comprised WC–6 wt.% Co, the
amount of milling media contamination during MM/MA can be calculated as about 1.8 wt.% for the W powders MM'd for 18 h and as
about 1.36 wt.% for the W1Ni powders MA'd for 18 h. Moreover, on
the basis of EDS analyses, which revealed that dark gray grains contain
about 12 wt.% Ni in their microstructures, it is possible to state that a
homogeneous distribution of Ni could not be obtained after MA for
18 h. However, this un-homogeneous distribution for Ni does not
seem to have detrimental effects on the sinterability of W (see
discussions about the relative density values). Same dark gray regions
can be seen in the micrograph taken from the polished surface of the
sintered W1Ni sample (Fig. 2d). Indications about the effects of the
Ni addition on the densification of W can be interpreted by comparing
the microstructures presented in Fig. 2a–b and Fig. 2c–d, i.e. the SEM
micrographs taken from the sintered W1Ni sample (Fig. 2c–d) reveals
the absence of open pores throughout the microstructure. The average
grain size measurements reveal that the average grain size of 2.56 ±
1.29 μm for the sintered W sample becomes 4.41 ± 1.71 μm for the sintered W1Ni sample. The growth of W grains with the addition of 1 wt.%
Ni can be explained by two different viewpoints: (i) the activation of
the sintering process is based on the enhanced diffusion during sintering [19,22], which increases the growth of W grains during the final
stage of sintering process [28], (ii) consequently, the pores present in
the microstructure of the sintered W sample, which have a pinning effect on the grain growth, are no longer present in the microstructure of
the sintered W1Ni sample.
The difference between microstructures is also reflected on the
relative density values, in which a relative density value of 92.59 ±
Ö.U. Demirkan et al. / Int. Journal of Refractory Metals and Hard Materials 32 (2012) 33–38
35
Fig. 2. SEM micrographs taken from the as-sintered and polished surfaces of the sintered W ((a) and (b)) and W1Ni ((c) and (d)) samples.
0.09% for the sintered W sample increased to 99.47 ± 0.40% with the
addition of 1 wt.% Ni (W1Ni sample). This increase in the relative
density value is unambiguously contributed by the positive effects
of Ni addition, Ni activated sintering, on the sinterability of W.
Activated sintering of refractory metals such as W is a very well
documented technique since the Vajek 's first report in 1959 [18],
and over the years many researchers have conducted investigations
using the activated sintering technique [19–32]. In these studies, a
Ni containing precursor (chloride, nitrate, etc.) is used for coating of
W particles with desired Ni amounts, except Kravchik et al. [25]
used a vibrational milling equipment for uniform distribution of
0.5 wt.% Ni in W powders. They [25] reported that the addition of Ni
into W increases the rate of pore reduction by the order of two compared to the pure W compacts, however, no specific relative density
values were presented in their study. Diliberto et al. [35] reported
on the activated sintering of mechanically alloyed refractory metal
powders, they investigated the sintering behavior of Mo–W–Cr alloys
activated with the Pd, Ni and Ru additions [35]. The possibility of
fabricating high dense W compacts with mechanical alloying and Ni
activated sintering were reported by the present authors [14,15],
but there were no sintered compacts from pure W powders enabling
the comprehensible comparison about the effects of the Ni addition in
these studies. In this study, the effects of Ni addition on the sinterability of MM'd W powders have been clearly established. As mentioned
above, SEM micrographs contain some grains with higher Ni contents
indicating inhomogeneous distribution of Ni throughout the W
grains. Nevertheless, the obtained relative density results undoubtedly indicate that this has no detrimental effects on the densification of
W compacts, which can be explained by the critical amount of Ni necessary for the activated sintering of W. In the studies about activated
sintering by Ni-coated W powders, it is reported that a few
monolayer thick Ni coatings are enough for the activated sintering
of W compacts [23,26,27], which is about 0.3–0.4 wt.% Ni depending
on the particle size of starting W powders. Here, it is possible to
state that the amount of Ni used for activated sintering, 1 wt.%, can
tolerate the observed inhomogeneous distribution of Ni. Moreover,
reported relative density value of 92.59 ± 0.09% for the sintered W
sample is quite high by comparing the reported results about conventional sintering of pure W [7] and this higher relative density value of
the MM'd and sintered W can be attributed to the grain refinements
during MM and the presence of Co, which is also known as an activator agent for sintering of W [19,23,24].
Moreover, despite the observed grain growth with the addition of
1 wt.% Ni, microhardness value significantly increases with the addition of 1 wt.% Ni, where a microhardness value of 2.81 ± 0.34 GPa
measured for the sintered W sample increased to 4.07 ± 0.16 GPa
for the sintered W1Ni sample. This increase in the microhardness
values can be correlated with the increased relative density values,
in which the detrimental effects of the porous structure of the
sintered W sample on the microhardness no longer prevailed.
3.2. The effects of Al2O3 addition
Following the fabrication of fully dense W1Ni sample, different
amounts of Al2O3 particles were added as reinforcements to the
W1Ni matrix. This section comprises the experimental results and
relevant discussions about the effects of Al2O3 particles addition on
the properties of the W1Ni sample.
Fig. 3. XRD patterns taken from the sintered W1Ni samples reinforced with different
amounts of Al2O3 particles: (a) W1Ni–0.5Al2O3, (b) W1Ni–1Al2O3, (c) W1Ni–1.5Al2O3
and (d) W1Ni–2Al2O3 samples.
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Ö.U. Demirkan et al. / Int. Journal of Refractory Metals and Hard Materials 32 (2012) 33–38
Fig. 3 shows the XRD patterns taken from the sintered W1Ni
samples reinforced with different amounts of Al2O3 particles. XRD
pattern of the sintered W1Ni-0.5Al2O3 sample (Fig. 3a) reveals only
the characteristic peaks of the matrix W phase, where no other
diffraction peaks from Ni and Al2O3 phases are observed. Likewise,
matrix W phase is the only phase observed in the XRD patterns
taken from the sintered W1Ni–1Al2O3 (Fig. 3b), W1Ni–1.5Al2O3
(Fig. 3c) and W1Ni–2Al2O3 (Fig. 3d) samples.
Fig. 4 shows back scattered SEM micrographs taken from the
sintered W1Ni samples reinforced with different amounts of Al2O3 particles. As in Fig. 4, two types, as-sintered and polished surfaces, of SEM
investigations are conducted for each sample. Fig. 4a–b shows SEM micrographs taken from the sintered W1Ni–0.5Al2O3 sample revealing
the presence of black particles having diameters of smaller than 1 μm,
which are mainly located at the grain boundaries. SEM/EDS analyses
suggested that these black particles are Al2O3 particles having
compositions of about 55.77 ± 8.85 wt.% W, 14.34 ± 4.92 wt.% Al and
29.89 ± 4 wt.% O. It should be noted here that the presence of high
amounts of W determined via EDS does not necessarily reveals the formation of Al–W–O particles. More likely, the obtained high W amounts
can be attributed to the spread of X-ray beam, which collects data
around the submicron sized Al2O3 particles. As seen in SEM micrographs taken from the sintered W1Ni–1Al2O3 sample (Fig. 4c–d), increasing the Al2O3 amount decreases the grain size of W, which is
also observed in the SEM micrographs taken from the sintered
W1Ni–1.5Al2O3 (Fig. 4e–f) and W1Ni–2Al2O3 (Fig. 2g–h) samples.
SEM micrographs taken from the surfaces of the as-sintered samples
(Fig. 4a, c, e and g) suggest that the clustering of Al2O3 particles take
place with increasing Al2O3 content, however the SEM micrographs
taken from the polished surfaces of the same samples clearly indicate
that clustering of Al2O3 particles doesn't take place for the bulk of the
samples (Fig. 4b, d, f and h). Yet, as the amount of the Al2O3 increases,
Fig. 4. SEM micrographs taken from the sintered W1Ni sample reinforced with different amounts of Al2O3 particles: (a–b) W1Ni–0.5Al2O3, (c–d) W1Ni–1Al2O3, (e–f)
W1Ni–1.5Al2O3 and (g, h) W1Ni–2Al2O3 samples.
Ö.U. Demirkan et al. / Int. Journal of Refractory Metals and Hard Materials 32 (2012) 33–38
the distribution of Al2O3 particles get more homogeneous and their
diameters get smaller due to the increased fracturing mechanism
during MA. Simultaneously, the size of W grains systematically
decreases with the increasing Al2O3 contents. The amount of WC contaminated via measuring the amount of Co suggested that the Co contents between 0.08 wt.% and 0.12 wt.% are present in the W1Ni–Al2O3
powders MA'd for 18 h suggesting the presence of WC–Co contamination between 1.36 wt.% and 2 wt.%, whereas no comparative trend
was observed in the WC–Co contamination with increasing Al2O3
amount.
Fig. 5 shows the relative density evolution of the sintered W1Ni
sample after reinforcing with different amounts of Al2O3 particles
and reveals a slight decrease with increasing Al2O3 content, where
99.47 ± 0.40% relative density value of the sintered W1Ni sample
decreases to 97.81 ± 0.33% for the sintered W1Ni–2Al2O3 sample.
Even after the addition of Al2O3 particles, the high relative density
value of the W1Ni sample is preserved. As seen in the SEM micrographs presented in Fig. 4, no open pores can be observed in the surfaces of the sintered W1Ni–Al2O3 samples. However, as revealed by
the relative density measurements, these samples contain some
pores in their microstructures and it can be interpreted that these
pores are mainly closed pores and their fractions vary between 0.7%
and 2.2%.
Fig. 6 shows the evolution of microhardness values and average W
grain sizes with increasing Al2O3 contents. As seen in Fig. 6, microhardness of the sintered W1Ni sample increases with increasing
Al2O3 contents, where a microhardness value of 4.07 ± 0.16 GPa for
the sintered W1Ni sample increases to 5.98 ± 0.31 GPa for the
sintered W1Ni–2Al2O3 sample. With the addition of 0.5 wt.% Al2O3,
the microhardness value slightly increases to 4.24 ± 0.12 GPa, however, it significantly increases and becomes 5.33 ± 0.21 GPa with the addition of 1 wt.% Al2O3 particles. The evolution of average W grain sizes
is also presented in Fig. 6, as observed in the SEM micrographs in
Fig. 4, the size of W grains decrease with increasing Al2O3 contents.
A mean grain size of 4.41 ± 1.71 μm of W grains for the sintered
W1Ni sample (Fig. 2c–d) decreases to 1.29 ± 0.39 μm for the sintered
W1Ni–2Al2O3 sample (Fig. 4g–h). Moreover, as the distribution of
Al2O3 particles becomes more homogeneous with increasing
contents, the standard deviation of the average grain sizes decreases.
Fig. 6 clearly shows that the evolution of microhardness values and
grain sizes are in great consistency with each other, in which the
decreases in the grain size are accompanied with increases in microhardness. A significant increase in the microhardness value of the sintered W1Ni–1Al2O3 sample can also be correlated with its
microstructure, in which the size of W grains declines to 1.99 ±
0.88 μm.
Fig. 5. Relative density evolution vs. Al2O3 content for the sintered samples.
37
Fig. 6. Evolution of microhardness values and average W grain size with the addition of
Al2O3 particles.
4. Summary and conclusions
In the first instance, the effects of addition of 1 wt.% Ni on the
densification of W powders, namely Ni activated sintering of W, are
investigated. W and W–1 wt.% Ni powders were respectively MM'd
and MA'd for 18 h and sintered at 1400 °C for 1 h. Along with the increased the relative density values, a significant increase in the microhardness values is observed with the addition of 1 wt.% Ni, revealing
the coupled effects of mechanical alloying and Ni activated sintering
for the fabrication of W compacts at a lower sintering temperature
of 1400 °C using conventional sintering routes. Following the fabrication of fully dense W1Ni samples, different amounts (0.5, 1, 1.5 and
2 wt.%) of Al2O3 particles are used for dispersion strengtheners for
the sintered W1Ni sample. Although the relative density of the
W1Ni sample slightly decreased with the increasing Al2O3 addition,
microhardness values are systematically increased. Homogeneously
distributed dispersoids throughout the W matrix with smaller grain
sizes are achieved by increasing the Al2O3 addition.
On the basis of the results of the present investigations, the
following conclusions can be drawn:
1) XRD patterns of the sintered samples revealed the presence of
only W phase, no other peaks from the Ni and Al2O3 phases are
observed.
2) SEM micrographs of the sintered W sample revealed the existence
of some pores in its microstructure, whereas no open pores were
observed in the microstructure of W1Ni sample. SEM micrographs
taken from the samples containing different amounts of Al2O3
particles revealed that these particles are mainly located at W
grain boundaries and distribution of Al2O3 particles in the matrix
became more homogeneous and their size became smaller with
increasing content.
3) Average size of W grains increased with the addition of 1 wt.% Ni
and increasing amount of Al2O3 decreased the size of W grains.
Grain size of 2.56 ± 1.29 μm for the sintered W sample became
4.41 ± 1.71 μm for the sintered W1Ni sample, which decreased
to 3.23 ± 1.35 μm, 1.99 ± 0.88 μm, 1.44 ± 0.45 μm and 1.29 ±
0.39 μm for the W1Ni–0.5Al2O3, W1Ni–1Al2O3, W1Ni–1.5Al2O3
and W1Ni–2Al2O3 samples, respectively.
4) A relative density value of 92.59% for the sintered W sample increases to 99.47% for the sintered W1Ni sample, whereas the relative density values slightly decreased with increasing amount
Al2O3. The relative density values of the samples containing
Al2O3 particles varied between 97.81% and 99.26%.
5) The microhardness of the W sample significantly increased with the
addition of 1 wt.% Ni and increasing amount of Al2O3 particles
further increased the microhardness values. 2.81 ± 0.34 GPa
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Ö.U. Demirkan et al. / Int. Journal of Refractory Metals and Hard Materials 32 (2012) 33–38
microhardness value of the sintered W sample increased to 4.07 ±
0.16 GPa with the addition of 1 wt.% Ni and W1Ni–2Al2O3 sample
had a microhardness value of 5.98 ± 0.31 GPa.
Overall, based on the relative density and microhardness values of
the sintered samples, it is possible to state that activated sintering
technique applied along with the mechanical alloying treatment is a
very promising route to fabricate fully dense W products at a relatively
low sintering temperature of 1400 °C. Using Al2O3 particles as
reinforcements is also an efficient way to increase the properties of
W matrix composites with no detrimental effects on relative densities.
Acknowledgements
The authors would like to express their gratitude to State Planning
Organization (DPT) for funding the project entitled “Advanced Technologies in Engineering” with the project number 2001K120750 out
of which the main infrastructure of the Particulate Materials Laboratories was founded. The present study is one of the outcomes of a
research project entitled “Development of Tungsten based Hybrid
Composites via Activated Sintering and Mechanical Alloying using
High Energy Milling at Cryogenic and Ambient Conditions and Related
Characterization Investigations” funded by the Scientific and Research
Council of Turkey (TÜBİTAK) with the project number 110M130.
References
[1] Lassner E, Schubert WD. Tungsten: properties, chemistry, technology of the
element, alloys and chemical compounds. New York: Kluwer Academic; 1999.
[2] Song GM, Wang YJ, Zhou Y. The mechanical and thermophysical properties of
ZrC/W composites at elevated temperature. Mater Sci Eng A 2002;334:223–32.
[3] Song GM, Wang YJ, Zhou Y. Thermomechanical properties of TiC particlereinforced tungsten composites for high temperature applications. Int J Refract
Met 2003;21:1–12.
[4] Kitsunai Y, Kurishita H, Kayano H, Hiraoka Y, Igarashi T, Takida T. Microstructure
and impact properties of ultra-fine grained tungsten alloys dispersed with TiC. J
Nucl Mater 1999;271–272:423–8.
[5] King GW, Sell HG. The effect of thoria on the elevated tensile properties of recrystallized high-purity tungsten. Trans AIME 1965;233:1104–13.
[6] Chen LC. Dilatometric analysis of sintering of tungsten and tungsten with ceria
and hafnia dispersions. Refract Met Hard Mater 1993–1994;12:41–51.
[7] Kim Y, Hong MH, Lee SH, Kim EP, Lee S, Noh JW. The effect of yttrium oxide on the
sintering behavior and hardness of tungsten. Met Mater Int 2006;12:245–8.
[8] Chen Y, Wu YC, Yu FW, Chen JL. Microstructure and mechanical properties of
tungsten composites co-strengthened by dispersed TiC and La2O3 particles. Int J
Refract Met 2008;26:525–9.
[9] Rea KE, Viswanathan V, Kruize A, De Hosson JTM, O'Dell S, McKechnie T, et al.
Structure and property evaluation of a vacuum plasma sprayed nanostructured
tungsten–hafnium carbide bulk composite. Mater Sci Eng A 2008;477:350–7.
[10] Kim Y, Lee KH, Kim EP, Cheong DI, Hong SH. Fabrication of high temperature
oxides dispersion strengthened tungsten composites by spark plasma sintering
process. Int J Refract Met 2009;27:842–6.
[11] Monge MA, Auger MA, Leguey T, Ortega Y, Bolzoni L, Gordo E, Pareja R. Characterization of novel W alloys produced by HIP. J Nucl Mater 2009;386–388:613–7.
[12] Ünal N, Öveçoğlu ML. Effects of wet and dry milling conditions on properties
mechanically alloyed and sintered W–C and W–B4C–C composites. Powder Metall
2009;52:254–65.
[13] Coşkun S, Öveçoğlu ML, Özkal B, Tanoğlu M. Characterization investigations
during mechanical alloying and sintering of W-20 vol.% SiC composites. J Alloys
Comp 2010;492:576–84.
[14] Genç A, Coşkun S, Öveçoğlu ML. Fabrication and properties of mechanically
alloyed and Ni activated sintered W matrix composites reinforced with Y2O3
and TiB2 particles. Mater Charact 2010;61:740–8.
[15] Genç A, Coşkun S, Öveçoğlu ML. Microstructural characterizations of Ni activated
sintered W–2 wt.% TiC composites produced via mechanical alloying. J Alloys
Comp 2010;497:80–9.
[16] Mabuchi M, Okamoto K, Satio N, et al. Deformation behavior and strengthening
mechanisms at intermediate temperatures in W–La2O3. Mater Sci Eng A
1997;237:241–9.
[17] Ryu HJ, Hong SH. Fabrication and properties of mechanically alloyed oxidedispersed tungsten heavy alloys. Mater Sci Eng A 2003;363:179–84.
[18] Vacek J. On influencing the sintering behavior of tungsten. Planseeberiche fur
Pulvermetallurgie 1959;7:6–17.
[19] Hayden HW, Brophy JH. The activated sintering of tungsten with group VIII
elements. J Electrochem Soc 1963;110:805–10.
[20] Toth IJ, Lockington NA. The kinetics of metallic activation sintering of tungsten. J
Less-Common Met 1967;12:353–65.
[21] Samsonov GV, Yakovlev VI. Activated sintering of tungsten with nickel additions.
Poroshkovaya Metallurgiya (In Russian) 1967;8:10–6.
[22] Panichkina VV. Activated sintering of tungsten with small additions of nickel.
Poroshkovaya Metallurgiya (In Russian) 1967;2:1–5.
[23] German RM, Munir ZA. Enhanced low-temperature sintering of tungsten. Metall
Trans A 1976;7A:1873–7.
[24] Samsonov GV, Yakovlev VI. Activation of the sintering of tungsten by the irongroup metals. Poroshkovaya Metallurgiya (In Russian) 1969;10:32–8.
[25] Kravchik AE, Ordanyan SS, Neshpor VS, Savelev GA. Effect of defective structure
on the mechanism of activated sintering of tungsten powders. Poroshkovaya
Metallurgiya (In Russian) 1981;6(222):37–41.
[26] German RM, Munir ZA. Activated sintering of refractory metals by transition
metal additions. Rev Powder Metall Phys Ceram 1982;2:9–43.
[27] Li C, German RM. The properties of tungsten processed by chemically activated
sintering. Metall Trans A 1983;14A:2031–41.
[28] Corti CW. The role of the platinium metals in the activated sintering of refractory
metals. Platinum Met Rev 1986;30:184–95.
[29] Luo J, Gupta VK, Yoon DH, Meyer HM. Segregation-induced grain boundary
premelting in nickel-doped tungsten. Appl Phys Lett 2005;87:231902.
[30] Gupta VK, Yoon DH, Meyer HM, Luo J. Thin intergranular films and solid-state
activated sintering in nickel-doped tungsten. Acta Mater 2007;55:3131–42.
[31] Luo J, Shi X. Grain boundary disordering in binary alloys. Appl Phys Lett 2008;92:
101901.
[32] Luo J. Liquid-like interface complexion: from activated sintering to grain boundary diagrams. Curr Opin Solid State Mater Sci 2008;12:81–8.
[33] Powder Diffraction Files: Card No. 04–0806, Database Edition, The International
Centre for Diffraction Data (ICDD)
[34] Handbook of metal etchants. In: Walker P, Tarn WH, editors. New York: CRC Press
LLC; 1991.
[35] Diliberto S, Kessler O, Rapin C, Steinmetz P, Berthod P. Development of chromia
forming Mo–W–Cr alloys: synthesis and characterization. J Mater Sci 2002;37:
3277–84.