Studies on the synthesis of a Mo-30 wt% W alloy by non-conventional approaches

Article abstract of DOI:10.1016/j.jallcom.2010.04.114

In the present study, alternate routes for the synthesis of a single phase Mo30wt%Walloy were pursued to surmount the limitations experienced in conventional technique. The process essentially consists of preparation of active Mo andWpowders by H₂ reduction of the respective oxide intermediates through multiple processing steps and then converting those pure powders into Mo30wt%Walloy by mechanical alloying technique (MA) at RT under Ar atmosphere in a high-energy planetary ball mill. The structural evolution of the alloy from the milled powders at different interval of time was studied by X-ray diffraction (XRD) and phase corresponding to Mo30W alloy was confirmed. The broadening of peaks in XRD pattern was due to crystallite refinement during milling towards the formation of the designated alloy. A high rate of densification for MA powder was achieved during sintering between 900 °C and 1200 °C and density close to theoretical density was attained. The microstructure of sintered alloy exhibited uniform, polyhedral grains with average grain size of about 3μm. The morphological evolution of as-milled powder was studied by Scanning electron microscopy (SEM) which revealed the formation of nano sized crystallites with polyhedral shapes. The crystallites were initially arranged in clusters which later on got distributed uniformly with the progress in milling time. The average crystallite size of MA powder was found to be 7.3nm after 25 h of milling.

Full text of DOI:10.1016/j.jallcom.2010.04.114

Journal of Alloys and Compounds 501 (2010) 211–217
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Studies on the synthesis of a Mo–30 wt% W alloy by non-conventional approaches
S.P. Chakraborty∗, S. Banerjee, Gopal Sanyal, V.S. Bhave, Bhaskar Paul, I.G. Sharma, A.K. Suri
Materials Group, Bhabha Atomic Research Center, Trombay, Mumbai-400085, India
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present study, alternate routes for the synthesis of a single phase Mo–30 wt% W alloy were pursued
to surmount the limitations experienced in conventional technique. The process essentially consists of
preparation of active Mo and W powders by H2 reduction of the respective oxide intermediates through
multiple processing steps and then converting those pure powders into Mo–30 wt% W alloy by mechani-
cal alloying technique (MA) at RT under Ar atmosphere in a high-energy planetary ball mill. The structural
evolution of the alloy from the milled powders at different interval of time was studied by X-ray diffrac-
tion (XRD) and phase corresponding to Mo–30W alloy was confirmed. The broadening of peaks in XRD
pattern was due to crystallite refinement during milling towards the formation of the designated alloy.
A high rate of densification for MA powder was achieved during sintering between 900 ◦C and 1200 ◦C
and density close to theoretical density was attained. The microstructure of sintered alloy exhibited uni-
form, polyhedral grains with average grain size of about 3 ␮m. The morphological evolution of as-milled
powder was studied by Scanning electron microscopy (SEM) which revealed the formation of nano sized
crystallites with polyhedral shapes. The crystallites were initially arranged in clusters which later on got
distributed uniformly with the progress in milling time. The average crystallite size of MA powder was
found to be 7.3 nm after 25 h of milling.
Received 10 March 2010
Accepted 9 April 2010
Available online 24 April 2010
Keywords:
Synthesis
Mechanical alloying
Sintering
High energy milling
Nanostructured
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
solution strengthening. Addition of 30 wt% W in Mo increases the
capability of the alloy to retain strength at a temperature in excess
Molybdenum (Mo) base refractory alloys are characterized
by high strength at elevated temperature, excellent corrosion
resistance against molten metal’s and high melting temperature.
These alloys are capable of working at much higher temper-
ature than conventional nickel-based super alloys because of
having higher thermal conductivities with appreciably lower ther-
mal gradients and thermal strains. Amongst the various Mo-base
alloys, Mo–30 wt% W alloy has gained its prominence in missile
and aerospace applications for having high melting temperature
(∼2832 ◦C) that can withstand erosion or incipient melting of
rocket nozzles at higher propellant combustion temperatures. The
Mo–30 wt% W alloy is also a promising structural material for high-
temperature nuclear reactor (HTR) applications for its elevated
temperature strength and excellent chemical inertness in presence
of corrosive molten zinc and lead. According to the W–Mo binary
phase diagram as depicted in Fig. 1, it is interesting to note that W
forms continuous solid solution with Mo in full phase field, which
implies that Mo (or W) elements completely dissolve in the remain-
ing bcc W (or Mo)-based phase. As W is added to Mo, the alloy is
gradually strengthened by the mechanism of substitutional solid
of 1700 ◦C. The Mo–30 wt% W alloy is also readily machinable and
capable of forming into a variety of wrought shapes including round
bars, sheets and plates as well as simple assemblies [1,2].
Mo–30 wt% W alloy is commercially prepared by vacuum arc
melt-casting route technique. However, this method is expensive
and highly energy intensive in view of using high temperature and
high vacuum for melting. Moreover, due to large variation in the
melting temperatures of Mo (2610 ◦C) and W (3410 ◦C), the alloy-
ing components do not mix well leading to their segregation. As a
result, the final alloy is obtained as inhomogeneous mass. Repeated
crushing and melting steps are required till a homogeneous alloy is
formed. However, such exercise is time consuming and expensive.
Mechanical alloying is basically a solid-state powder processing
technique where no elaborate energy intensive steps of high tem-
perature and high vacuum are required. Synthesis of the alloy is
carried out by milling the elemental powders in simple experi-
mental set up of different kind of mills, e.g. attrition, planetary,
centrifugal, etc. The elemental powders are repeatedly fractured
and cold welded during the process of milling leading to alloying.
Homogenized alloy or composite metal powders with controlled
and fine microstructure can be produced by this process [3–5].
In the present investigation, alternative approaches were
employed to synthesize Mo–30 wt% W alloy to overcome the dif-
ficulties experienced in conventional melting technique. In this
∗
Corresponding author. Tel.: +91 22 25593931; fax: +91 22 25505151/19613.
E-mail addresses: spc@barc.gov.in, spc.barc@gmail.com (S.P. Chakraborty).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.04.114

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S.P. Chakraborty et al. / Journal of Alloys and Compounds 501 (2010) 211–217
compaction by cold isostatic pressing and sintering. Structural evo-
lution of the as-milled powder was established by X-ray diffraction
(XRD) analysis. The effect of sintering temperature and powder
characteristics on densification behavior was studied. The mor-
phology of the as-milled powder after different hours of milling was
investigated by scanning electron microscope (SEM) observations.
2. Experimental
2.1. Preparation of materials
The starting materials used for the synthesis of Mo–30W alloy are W and Mo
metal powders. These powders were freshly prepared in laboratory by reduction
with H2 of the respective oxides. The as-reduced active metal powders were then
used for mechanical alloying. The reduction process involving conversion of WO2
and MoO2 into respective metal powders was carried out in a resistance heating
furnace with horizontal retort, as shown in Fig. 2. The charge materials were kept in
a clean Mo boat at the centre of the retort. A number of experiments were conducted
on 100 g scale by varying temperature, time and H2 flow rate. The best yield for W
was achieved at a temperature of around of 850 ◦C and for Mo at 700 ◦C within
2 3 h. The flow rate of H2 was maintained approximately at 250–300 l/min. Prior to
sending H2 gas, the inside chamber of the furnace was flushed with Ar till moisture
and air were completely driven out. H2 gas after use was burnt at the exit to convert
into H2O.
Fig. 1. Binary phase diagram of Mo–W system.
The as-reduced powders of Mo and W with a purity of around 99.9% and average
size of 5 and 10 ␮m, respectively were mechanically alloyed in a planetary ball mill
for a period of 25 h to synthesize Mo–W alloy of composition Mo–30 wt% W. The Mo
and W powders were thoroughly blended in their desired ratio and a total charge of
100 g of Mo + W powder mixture corresponding to the composition of Mo–30 wt%
W was filled in the milling chamber of the high energy ball mill. The chamber was
made up of stainless steel bowl, internally lined with hard layer of tungsten carbide
The bowl after filling the charge, was air tightened with ‘O’ ring fitted lid and was
then purged with high purity Ar gas during milling operation. Subsequently, milling
was performed with ball to powder weight ratio of 10:1 using 10 mm dia hard balls
of W carbide. The rotation speed was set at 250 rpm. Milling time intervals were set
to 5, 10, 15 and 25 h, respectively. The experimental parameters used in performing
the mechanical alloying were described in Table 1.
context, high pure Mo and W powders, the starting components
for the preparation of Mo–30 wt% W alloy, were freshly prepared
in the laboratory from their respective oxide intermediates by H2
reduction following multiple processing steps. Since these pow-
ders are highly susceptible to oxidation, and their respective oxide
intermediates are also highly unstable and sensitive to different
temperatures, hence, achieving these elements in their pure state
is a very difficult task. Therefore, elaborate experimental campaign
was carried out in order to establish an optimized experimental
condition to maximize the powder yield. The freshly prepared pow-
ders were then used in place of commercially available expensive
powders of Mo and W with higher oxygen content to synthe-
size the alloy. The designated alloy was then synthesized from
as-reduced Mo, W powders at room temperature by employing
mechanical alloying technique in a planetary ball milling system
under Ar atmosphere. The mechanically alloyed powder was sub-
sequently consolidated by powder metallurgical technique such as
2.2. Characterization
Milled powders were taken at regular intervals of time for structural analysis.
Structural analysis of the as milled powders was carried out by X-ray diffraction
technique using Cu K␣ radiation (ꢀ = 1.54 Å) at a tube rating of 40 kV and 30 mA in a
commercial powder X-ray diffractometer (Seifert 3003 T/T). The extent of alloying
was determined qualitatively by monitoring the relative intensities of the Mo, W
and MoW peaks. As-milled powders were subsequently cold pressed into columned
Fig. 2. Horizontal heating furnace used for the preparation of Mo, W powders.

S.P. Chakraborty et al. / Journal of Alloys and Compounds 501 (2010) 211–217
213
Table 1
Planetary ball milling parameters.
Parameters
Value
Mill capacity
1 l (250 ml for each bowl)
Container material
Speed of rotation
Ball diameter
304 stainless steel with W carbide lining
300 rpm
8 mm
Charge to ball ratio (w/o)
Total charge
1:10
200 g
Atmosphere
Toluene
green compacts having 6 mm dia and 10 mm length with the application of pressure
of 300 MPa. The shrinkage profile of the sample was studied by carrying out sintering
in a vertical thermal mass analyzer (TMA) [Setsys Evolution TMA 1600, SETARAM]
instrument with no load conditions. The sample was heated at a heating rate of
10 ◦C per minute up to a set temperature of 1300 ◦C and isothermally held for 1 h at
that temperature. The powder processing technique such as cold isostatic pressing
(CIP) was applied to study the consolidation behavior of the mechanically alloyed
powders and parent powder mixture. Prior to CIP, the as-milled and parent mixture
powders were tightly filled inside flexible silicon rubber moulds and then encased in
surgical gloves. The encased moulds containing the samples were subsequently put
inside the cold isostatic press (CIP). Samples having 10 mm dia and 15 mm length
were made in the CIP using water pressure of 3000 bar. Sintering of the samples
were carried out in the horizontal furnace as described in Fig. 2 under gas mixture
of Ar + H2 at the temperature range of 600–1500 ◦C.
Green densities of the samples were measured using the geometrical dimen-
sions. Density after sintering was measured both by Archimedes principle and
geometrically. The hardness of the sintered specimen was measured by a micro
hardness tester. The morphology of the as-milled powder and sintered compacts
were observed in a scanning electron microscope.
3. Results and discussion
Fig. 3. Equilibrium curves for the reduction of W oxides by H2.
3.1. Preparation of W powder
3.2. Preparation of Mo powder
The starting materials for the preparation of W in general are
ammonium paratungstate (APT), tungstic acid or tungsten blue. In
the present study, tungstic acid (H2WO4) was chosen as the starting
material because ultimately chemically defined oxide, WO2 can be
obtained from it for the preparation of W powder by subsequent H2
reduction. WO2 being a stable oxide, unwanted formation of other
suboxides can be avoided during H2 reduction. Hence, as-received
tungstic acid was thermally decomposed initially to prepare yellow
oxide, tungstic anhydride (WO3) by heating it in air at 250 ◦C for
3 h. WO3 was then converted into the brown oxide, WO2 by H2
reduction at 720 ◦C. The brown oxide was finally mixed with a fresh
charge of yellow oxide, WO3 for W metal production at 860 ◦C. The
overall reduction process of tungstic anhydride (WO3) to W by H2
can be summarized as a three-stage process. These stages can be
differentiated by following set of chemical equations. The operating
conditions were judiciously adjusted finally to attain W powder as
per the following reactions:
As-received high-grade molybdenite (MoS2) concentrate pow-
der containing 50–53% Mo value was used as a source for MoO2.
The concentrate was roasted in air to convert it into MoO3 as per
the following equation:
MoS2 + 3.5O2 = MoO3 + 2SO2
(IV)
The roasting temperature was not allowed to exceed 650 ◦C,
else the MoO3 will be melted due to local overheating and high
volatility. A dynamic flow of air was maintained during roasting in
order to avoid any localized overheating and also to convert any
sulphorous residue present in the molybdenite concentrate into
SO2 as the roasting temperature was kept low. The vapor pres-
sure of MoO3 varied significantly with temperature as is evident
from Table 2. The reduction of Mo trioxide to Mo metal involved
three-stage reduction as indicated below
2MoO3 + H2 = Mo2O5 + H2O
Mo2O5 + H2 = 2MoO2 + H2O
MoO2 + 2H2 = Mo + H2O
(V)
(VI)
2WO3 + H2 ⇔ W2O5 + H2O
W2O5 + 3H2 ⇔ 2WO2 + H2O
WO2 + 2H2 ⇔ W + 2H2O
(I)
(II)
(VII)
Since, MoO3 might partially melt at 500–600 ◦C and the reac-
tion is easy to carry out, hence to avoid such occurrence, the first
step was carried out at 450 ◦C. The second step was carried out at
1000 ◦C because this reaction requires more elevated temperature
and lower water vapor contents in the gases. The MoO3 was then
reduced by H2 to prepare MoO2 [6,7].
(III)
As these reactions are reversible in nature, each stage of the
above reactions therefore was guided by the equilibrium constant
as given below:
PH
O
2
KP
=
PH
2
Table 2
Variation of the vapour pressure of molybdic acid, MoO3 with temperature.
where PH is the equilibrium partial pressure of water vapor; PH
O
2
2
the equilibrium partial pressure of H2. The respective equilibrium
curves are shown in Fig. 3. It is observed that every next stage
requires a higher temperature and greater H2 content in the gas
mixture. Accordingly, the experiments were conducted.
Temperature (◦C)
650
800
900
1000
1100
1150
P (mm Hg)
0.05
10.1
53.9
198.3
476.2
760.0

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Fig. 4. XRD pattern of thermally decomposed tungstic acid to obtain WO2.
Fig. 7. XRD pattern of H2 reduced MoO2 to obtain Mo powder.
demonstrated the structural evolutions of as-milled powder in the
form of XRD patterns at different time intervals. Formation of Mo-W
phase was affirmed after 5 h of milling by the appearance of Bragg
reflections corresponding to (2 0 0), (1 1 0) and (2 1 1) crystal planes.
As the milling progressed, the initially sharp diffraction peaks were
getting broadened and reduced in intensity because of increase in
atomic level strain due to crystallite size refinement and structural
disorder accumulation processes that occurred during alloying of
Mo and W. These features originated from the repeated fracturing
and alloying of the particles. XRD patterns further showed that ele-
mental Mo and W powders gradually transformed into Mo–30 wt%
W alloy at the expense of Mo and W [8]. The crystallite size of the
as-milled powders at different duration of time was measured from
the corresponding XRD plots by using Scherrer equation,
Fig. 5. XRD pattern of H2 reduced WO2 to obtain W powder.
Kꢀ
ꢁ =
3.3. XRD analysis
ˇ cos ꢂ
The XRD plot at Figs. 4 and 5 confirmed the formation of WO2
and W in the product after thermal decomposition of tungstic acid.
Similarly, Figs. 6 and 7 confirmed the formation of MoO2 and Mo
as per the chemical reactions in equation nos. (VI) and (VII). Fig. 8
where ꢁ is the mean crystallite dimension, K (=0.94) is the shape
factor, ꢀ is the X-ray wavelength, typically 1.54 Å, ˇ is the line
broadening at half the maximum intensity in radians, and ꢂ is the
Fig. 6. XRD pattern of H2 reduced MoO3 to obtain MoO2.
Fig. 8. XRD spectra of the powder milled in Ar at various milling times.

S.P. Chakraborty et al. / Journal of Alloys and Compounds 501 (2010) 211–217
215
Table 3
The calculation for determining crystallite size for all duration of milling.
Hours of milling
5
10
15
20
25
ꢂ (◦)
40.31
0.72
12.287
40.45
1.23
7.196
40.31
1.1
8.042
40.43
1.19
7.437
40.45
1.22
7.255
ˇ1/2 (◦)
ꢁ (nm)
Bragg angle [9,10]. The average crystallite size after 25 h of milling
was determined to be around 7.3 nm. Table 3 shows the calculation
for determining ꢁ for all duration of milling.
3.4. Milling studies
The particle size distribution of as-reduced Mo and W pow-
ders was depicted in Fig. 9 which indicated that the particle size
of starting Mo and W powders was available in the range of around
20–30 ␮m. It is clearly seen that with the progress of milling, the
initial crystallite size of Mo–30W alloy rapidly decreased during
MA process and got converted into nanocrystalline powders after
a span of 25 h of milling. The variation of crystallite size with the
progress of milling was presented in Fig. 10. This plot was consti-
tuted using Scherrer equation based on the line broadening data
of XRD analysis of as-milled powders. It can be seen from the plot
that the reduction in size of the milled powder progressed linearly
with time. Under the influence of repeated fracturing by mechan-
Fig. 11. Effect of sintering on shrinkage rate and profile as a function of time.
ical alloying coupled with hard and brittle nature of Mo and W
powders, the transformation of micron size particles into nano size
range was rapid and easy. The average crystallite size after 25 h of
milling was evaluated to be 7.3 nm as described above.
3.5. Compaction and sintering
The shrinkage behavior of hydraulically pressed MA powder was
evaluated by conducting an isothermal sintering study at 1300 ◦C
for duration of 1 h as depicted in Fig. 11. The profile has demon-
strated the occurrence of an appreciable shrinkage during sintering
equivalent to around 13.5 wt% indicating good sinteribility of
the MA powder. In general, refractory metals and alloys possess
high melting temperature and brittle characteristics. Consequently,
these materials respond to slow sintering kinetics. Hence, conven-
tionally, in order to accelerate the sintering process, the sintering
temperature of refractory metal and alloys is kept very high, in
the order of 1800–2000 ◦C with longer duration in comparison to
super-alloys and other variety of high strength materials. However,
prolonged duration of soaking at high sintering temperature may
induce oxidation and significant grain growth [11]. In the present
case, Mo–30W alloy processed by MA is already in a high state of
energy with higher surface area which is favorable attributes for
sintering. In the present investigation, a combination of mechanical
alloying and isostatic pressing were applied to the samples prior to
sintering. Obviously, this strategy influenced positively on the rate
of densification and sintering temperature as described below.
The green densities of cold isostatically compacted samples pre-
pared from the parent powder mixture and mechanically alloyed
powder were 76% and 64% respectively of the theoretical density
(12 g cc−1). Although the MA powder was having much smaller par-
ticle size (10 nm) as compared to the parent powder (5–10 ␮m),
however, it produced lower green density as the particles were
hard and brittle. Fig. 12 represents the densification behavior of
both types of samples during sintering between 600 and 1500 ◦C
for 6 h in reducing atmosphere. Because of the lower particle size,
the rate of densification of the MA powder compact was much
higher in the intermediate temperature range (900–1200 ◦C). This
could be attributed to the availability of higher specific surface
area and faster inter grain boundary diffusion in this temperature
range. The sintering kinetics was much slow beyond this temper-
ature for both the cases. A maximum sintering density of 99% TD
was achieved for MA powder at the highest sintering temperature
of 1500 ◦C. The average knoop hardness value measured at 0.98 N
(100 g) load for sintered pellet was 400 HK as compared to 270 HK
for arc consolidated alloy. Higher hardness of Mo–30W alloy was
Fig. 9. Particle size distribution of Mo, W and Mo–30W powders.
Fig. 10. Variation of crystallite size during milling as a function of time.

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Fig. 14. SEM micrograph showing powder morphology after 10 h of milling.
Fig. 12. Effect of sintering on shrinkage rate and profile as a function of time.
due to crystallite refinement, lattice distortion and formation of
fine-grained microstructure during mechanical alloying. This high
hardness value was quite comparable with other high strength Mo
based alloys [12].
3.6. SEM characterization of milled and sintered powder
The SEM images corresponding to the MA powder at 5, 10, 15, 20
and 25 h of milling time were presented in Figs. 13–17, respectively.
According to SEM micrograph at Fig. 13, particles of Mo and W
together have formed several large clustering consisting of smaller
micro-welded particles. Cold welding was the predominant mech-
anism at this stage. As the milling time progressed to 10 h, work
hardening led to the fracturing of the particles and the particle
size was reduced proportionately with decrease in the overall vol-
ume of the particulate mass. This was demonstrated in Fig. 14. The
phenomenon of welding and fracturing with simultaneous alloy-
ing continued as the milling progressed to 15 h. This has resulted
in further reduction in size of the particles along with the total vol-
ume as can be seen from Fig. 15. As the milling time was increased
to 20 h, the alloying was found to be the predominant stage. Due
to alloying, the alloyed particles were dissociated from the clus-
ter and rearranged themselves in a new fashion. This was depicted
in Fig. 16. After 25 h of milling, it was observed that the particles
again formed cluster of finer crystalline grains as demonstrated in
Fig. 15. SEM micrograph showing powder morphology after 15 h of milling.
Fig. 17. The average crystallite size was determined to be around
10 nm which was in close approximation with the value evaluated
by using Scherrer equation.
The SEM micrograph of sintered Mo–30 wt% W alloy in Fig. 18
revealed formation of uniform granular structure with polyhe-
dral shapes and well defined grain boundaries. The grains were
observed to be in close association with each other with hardly any
pores seen indicating good sintering. The average grain size was
3 ␮m. Excellent solid-state diffusion of Mo and W into each other
was responsible for nearly pore free structure [13].
Fig. 13. SEM micrograph showing powder morphology after 5 h of milling.
Fig. 16. SEM micrograph showing powder morphology after 20 h of milling.

S.P. Chakraborty et al. / Journal of Alloys and Compounds 501 (2010) 211–217
217
4. Conclusion
Present investigation demonstrates technical feasibility of syn-
thesis of a Mo–30 wt% W alloy from freshly prepared active Mo
and W powders by mechanical alloying technique. Nanostructured
alloy powders of average crystallite size of ∼7.3 nm with polyhe-
dral shape were achieved after 25 h of milling in Ar. The structural
analysis of the milled powder by XRD confirmed the formation
of Mo–30 wt% W solid solution alloy phase via crystallite refine-
ment. SEM micrographs revealed the morphological details related
to the mechanism of alloy formation. Faster sintering kinetics for
MA powder was observed between 900 and 1200 ◦C. SEM study of
sintered samples revealed the presence of a granular structure with
good solid-state bonding.
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Fig. 18. SEM micrograph of sintered Mo–30 wt% W alloy.