Combustion wave structure during the MoSi2 synthesis by

Mechanically-Activated Self-propagating High-temperature

Synthesis (MASHS): In situ time-resolved investigations

Christophe Gras a,b,*, Eric Gaffet a, Frederic Bernard b

a Nanomaterials Research Group, UMR 5060, CNRS/UTBM, F-90010 Belfort, Franceb Nanostructured Materials: Interface phenomena Group, LRRS, UMR 5613, CNRS, University of Burgundy, BP47870, F-21078 Dijon, France

Received 28 April 2005; received in revised form 18 July 2005; accepted 10 September 2005

Available online 8 November 2005

Abstract

In situ synchrotron time-resolved X-ray diffraction experiments coupled with an infrared imaging camera have been used to reveal the

combustion wave structure during the production of MoSi2 by Mechanically Activated Self-propagating High-temperature Synthesis (MASHS).

The fast combustion front exhibits a form described as an ‘equilibrium structure’ where the chemical reaction is the sole major driving force. In the

MASHS process, oxide-free interfaces between Mo and Si nanocrystallites enhance the reaction MoC2Si/MoSi2. Exhaustive time-resolved

investigations show a possible solid-state process in the first second of the reaction within the combustion front. If preheating is added, the reaction

rate is increased, whereas, the presence of a- and b-MoSi2 phases, produced during the ball milling (i.e. mechanical alloying), cause fluctuations in

combustion rate. Therefore, an increasing of the mechanical pre-treatment duration leads to an unstable wave by acting directly on the thermal and

matter transfer rates.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: A. Silicides, various; B. Phase transformation (crystallographic aspects kinetics and mechanisms) (see also ‘martensitic transformations’, this section);

C. Mechanical alloying and milling; Reaction synthesis

1. Introduction

Due to their attractive high-temperature properties, tran-

sition metal silicides have been the focus of numerous

investigations [1–4]. In particular, MoSi2 has recently received

considerable attention as a material for high-temperature

applications. Its properties provide a desirable combination

of a high melting temperature (2293 K), high Young’s modulus

(440 GPa) [3,5], high oxidation resistance in air [6] or in a

combustion gas environment [7], and a relatively low density

(6.25 g cmK3). However, commercialization of this compound

has been limited because of the low ductility exhibited at room

temperature. Consequently, refining grain size to the nano-

crystalline level (!100 nm) has been suggested as being a way

of improving strength while enhancing ductility and toughness

0966-9795/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.intermet.2005.09.001

* Corresponding author. Address: Nexia Solutions, B168 Harwell Inter-

national Business Didcot, Didcot, Oxford OX11 0QJ, UK. Tel.: C44 870 190

8444; fax: C44 870 190 8398.

E-mail address: christophe.gras@nexiasolutions.com (C. Gras).

[8]. Indeed, nanocrystalline materials (i.e. polycrystalline

metals, alloys and compounds with grain size within the

nanometer range) exhibit unusual and promising physical,

chemical, and mechanical properties [9–11].

However, like many similar high-temperature compounds,

dense MoSi2 is usually prepared by arc melting or by powder

metallurgical methods; the latter involves a multi-step process

of synthesis and subsequent densification through sintering or

hot pressing [12–14]. The gas atomization of pre-sintered rods

and subsequent consolidation via hot isostatic pressing are

employed to industrially manufacture oxidation-resistant Mo-

based silicides intermetallic alloy [15]. Arc melting or powder

metallurgy techniques are often long, costly and lead to a large

grain structure (several hundred micrometers) with the

presence of intergranular silica [13,14].

In the last 10 years, an emerging synthesis route coupling a

short duration high-energy ball milling with a self-sustaining

reaction has been extensively explored [16–21]. The addition

of a mechanical activation step to the SHS process turns out to

be a critical improvement, changing the process parameters

(e.g. wave velocity) and the nature of the end product [22]. The

production of nanostructured aluminides {FeAl [22] and NbAl3[23]} or silicides {MoSi2 [24]} by mechanically activated

Intermetallics 14 (2006) 521–529

www.elsevier.com/locate/intermet

C. Gras et al. / Intermetallics 14 (2006) 521–529522

self-propagating high temperature synthesis (MASHS) has

been reported. Although the basic concepts of the MASHS

method are relatively easy to apply in principle, there remain a

number of basic questions related to the physical and to the

chemical natures of this phenomenon, as well as, some issues

about the phase transformations within the moving combustion

front.

It is difficult to investigate this type of reaction by a

conventional method such as quenching without interfering

with the process itself. In the past 20 years, a tremendous

improvement of synchrotron technology has led scientific

instruments to offer new capabilities to study the combustion

reaction in real time [25–28]. The time-resolved X-ray

diffraction (TRXRD) technique has been improved to record

one XRD pattern per 0.033 s during 34 s [28]. This segment of

the work was carried out using the synchrotron facilities at

LURE DCI D 43 (Orsay, Paris) where both temperature (high

speed infrared camera, IR) and real time structural changes

(TRXRD) can be recorded in situ. This work is an attempt to

study in details the MASHS process and define the chemical

reaction(s) responsible for the formation of MoSi2 from the

mechanically activated powder MoC2Si. The structure of the

combustion wave and the evolution of the phase conversion

rate were estimated from the TRXRD data.

2. Experimental procedure

2.1. In situ time resolved experiment

Fig. 1 gives a view and illustrates the schematic

representation of the TRXRD experimental set-up.

A high temperature chamber made of stainless steel [29]

was filled with pure helium gas to prevent oxidation and the

attenuation of the X-ray beam. It contained an alumina sample

holder that was centered at the goniometer axis and an

electrical heater. The compacted sample was initiated by a

graphite resistive heating element located close to the sample

(electrical powerZ1.78 kW). In addition, a tungsten resistive

heating element located under the sample can be used for

preheating the sample.

HeHe

(1)

(6)

(2)(4)

(3)

(5)

(7)

(8)

Fig. 1. General view of TRXRD experiment—XRD and IR coupled apparatus. (

(3) Tungsten heating element and graphite coil, (4) Mylar window, (5) synchrotron

(7) conventional TV camera, (8) sample.

Thermal data were recorded using an infrared imaging

camera (model AVIO HGH TVS 2000 ST). Thermal infrared

(IR) images were continuously recorded and directly

transformed into digital data. Infrared thermography allows

imaging the entire sample surface with a temporal resolution

close to 0.066 s per IR image. The spatial resolution of the

infrared system is 0.8!1.45 mm per pixel. Infrared emissivity

measurements were carried out on reacted and unreacted

samples at 120 8C by Papini [30]. The value of the emissivity

for the Mo–Si system before and after reaction was found to be

within the range of 0.8–0.9, and did not exhibit large variations

between the initial and the final steps. In addition, the

instrumental corrections and the calibration were carried out

between 298 and 773 K on a reacted sample for reducing Mylar

window absorption and low temperature emissivity

fluctuations.

The time-resolved X-ray diffraction (TRXRD) exper-

iments were carried out using the LURE DCI D43

synchrotron beam line (Orsay, France). The XRD wave-

length was fixed at 0.154 nm for all experiments. A pair of

Soller slits was used to minimize the spot size on samples

(i.e. 1.5!1 mm2) in order to avoid an average of different

events that did not occur at the same time. A fast X-ray

detector, designed by Berar et al. [31], was used to record

diffracted X-rays. This detector has an angular aperture of

308 2q and is centered at (38.0G0.5)8 2q. This configuration

allows Si (111), Mo (110), and Si (220) reactant XRD

peaks to be collected as well as a-MoSi2 {(101), (110),

(103) and (112)} after the reaction. The detection system

was programmed to record 1024 XRD patterns of 512

channels at 0.033 s intervals for a total time of 34 s. The

detector was calibrated using an aluminum standard pellet

in the reaction chamber instead of the sample. Optics

adjustments were performed to reach an optimum signal-to-

noise ratio within 78–80 for a pattern recorded in 0.033 s.

The average spatial definition was in the range of 0.098 2q

per channel. Coupling the temperature measurements (IR)

and the structural transformations, the TRXRD experiments

have been performed following the procedure reported

elsewhere [32–34].

1) Fast X-ray detector, (2) reaction chamber with controlled He atmosphere,

radiation, lZ0.154 nm, LURE DCI D43, Orsay, France, (6) infrared camera,

C. Gras et al. / Intermetallics 14 (2006) 521–529 523

2.2. Sample preparation: mechanical activation and

cold compaction

Pure elemental powders (MoC2Si) were co-milled inside a

planetary mill hereafter called the G5 machine [35], which

allows the shock frequency (u) and the shock energy (U) to be

independently controlled. The specific ball-milling conditions

selected for this work were based on our previous experience

[24]. The rotation speeds of the disk (U) and vials (u) were set,

respectively, at 350 (clockwise) and 250 rpm (counter-clock-

wise) for high power and at 250 rpm for both rotation speeds in

the case of low power. In summary, two levels of mechanical

activation were considered in this study: the high (H) and low

(L) power, P, with PZ2.2 and 4.8 W, respectively. The

kinematics studies of the G5 milling machine by Abdellaoui

and Gaffet [35,36] served as basis for this estimation.

The selected ball milling durations were short enough to

avoid the formation of a large volume MoSi2 compound during

the milling but sufficient to produce a mixture between

particles of reactants at a nanometre scale. The charge ratio

CR (ball to powder mass ratio) was 7/1. In Table 1, calculated

values for the shock energy, the frequency and the shock power

used to activate 10 g of a stoichiometric MoC2Si elemental

powder mixture are reported for each specific ball-milling

condition. In addition to the pre-milled powders, a reference

(i.e. no mechanical activation) composed of a stoichiometric

mixture of elemental powders treated for 3 h in a turbulaw mill

was prepared.

As-milled powders were pressed into stainless steel

containers using a uniaxial load of 300 MPa applied during

60 s. The brittle compacted samples measured 13!8!4 mm

and had densities within the range of 55–60%.

3. Results and discussion

3.1. The mechanically activated state: as-milled powder

characterization

Preliminary work [24,28] was carried out to determine the

microstructure of the as-milled MoC2Si powders, their

reactivity under SHS-like conditions, and the characteristics

of the end product. Based on TEM observations [24], XRD

profile analysis [24,28], and laser granulometry [24], the

structure of the mechanically activated powders is seen as

aggregates {0.2–200 mm, Fig. 2(a)} composed of Mo and Si

nanocrystallites. Consequently, the mechanical activation

leads to the formation of a large contact area between

reactants. Conventional interpretative methods (i.e. XRD

peak profile analysis and TEM observations) showed that

Table 1

Milling conditions and calculated characteristics determined from references

[35,36]

Dt (h) Shock energy

(J ShockK1)

Shock

frequency (Hz)

Shock

power (W)

Low energy 1–3–4–6 0.08–0.09 28.5 2.2

High energy 1–2–3 0.16–0.17 33.0 4.8

the Mo and Si particles exhibit average crystallite sizes of 50

and 30 nm, respectively. As a consequence, repeated fracture

and welding during the short duration ball milling create

polyinterfaces at a nanometer scale and destroy the oxide

surface layers on Si and Mo.

Chemical EDXS analyses showed that iron contamination

due to the milling tools did not exceed 0.7 at.% of iron after 6 h

of ball milling. In addition, some a and b-MoSi2 were observed

in the as-milled powder. According to Liu et al. [37], formation

of the high temperature phase (b-MoSi2) is kinetically

enhanced during the ball-milling process. Fig. 2(b) shows the

amount of a and b-MoSi21 as-produced versus the ball milling

duration. The maximum amount of (aCb-MoSi2) after 3 h

of mechanical activation is 6.5 vol% (3.8 wt%) for the high-

energy milling condition.

3.2. In situ time resolved synchrotron investigations

Fig. 3 shows a complete set of data extracted from the

coupled IR and TRXRD experiments performed on a low

energy mechanically activated sample (i.e. 2.2 W for 4 h). The

depicted TRXRD patterns (Fig. 3(a)) are taken during the time

interval corresponding to the abrupt temperature increase

(Fig. 3(b)). Expectedly, the XRD patterns reveal that the abrupt

increase of temperature is due to the formation of a-MoSi2, in

less than 0.3 s. Since the onset of the abrupt increase in

temperature is at a ‘relatively’ low temperature, it can be

considered that the initiation of the reaction may have involved

a solid-state process (between Mo and Si). However, this does

not exclude the presence of molten silicon during the sharp

increase in temperature. In a similar way, the melting of Si

could also be enhanced by the nature of the reactive particles

(size 30 nm and less); indeed, the melting temperature may be

decreased in correlation with grain size [38].

After 30 s, no change in the XRD patterns is observed. In

addition, the a-MoSi2 produced by MASHS did not exhibit

any strong preferential orientation: the intensity ratios

calculated from the time-resolved XRD patterns have been

found to be in good agreement with those reported in the

JCPDS file (no. 41-0612).

Time resolved experiments, clearly highlight that the

mechanical activation step added to the combustion reaction

does not modify the reaction paths; no intermediate phase was

observed using a temporal resolution of 0.033 s. a-MoSi2formation by MASHS follows the Khaikin–Merzhanov [39]

model of so-called ‘equilibrium structure’; consequently, the

chemical reaction is likely to control the combustion front

velocity.

Because of the reasonably good quality of the TRXRD

patterns, the software DIFFRAC AT supplied by Siemens can

be used to analyze each diffractogram. In Fig. 4, the time

dependence of the FWHM (full width at half maximum) for

the Mo (110) and a-MoSi2 (103) TRXRD peaks is reported.

The instrumental contribution to the broadening of the TRXRD

1 Volume fraction calculated from XRD patterns.

Time (hour)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

0 1 2 3 4 5 6

R α

and

β M

oSi 2

(%

)

High Energy

Low energy

x 40 000

(a) (b)

Fig. 2. Mechanical activation. (a) SEM—secondary electron image of some aggregates after mechanical activation. (b) XRD volume fraction of each mechanically

induced phase (%). The volume fraction for each major phase was obtained from the following formula: Rx Zax=P

i ai. (xZa-MoSi2, b-MoSi2, Si, Fe, and iron

oxide) where ax is the area of the principal peak.

25 30 35 40 45 50 55

SiMo

TRXRD(a)

(b)

300500700900

11001300150017001900

4 5 6 7 8 9

coupledIR

2 theta - 0.152 nm

cou

nts

5.907 s

Si

α-MoSi2 α-MoSi2

α-MoSi2α-MoSi2

10

temps (s)

T IR

(K

)

T=900 K

T=1200 K

T=1420 K

T=1620 K

5.643 s

Fig. 3. Synchrotron time-resolved experiments—after 4 h of a low energy

mechanical activation. (a) TRXRD (lZ0.154 nm)—temporal resolution

0.033 s. (b) Infrared thermogram of the irradiated zone (temporal resolution

0.066 s).

C. Gras et al. / Intermetallics 14 (2006) 521–529524

peaks is accounted for the FWHM measured using an Al

reference pellet in the same diffraction conditions (i.e. with

same acquisition duration and same angular domain).

When the first XRD peaks of the a-MoSi2 appear, the

FWHM of Mo XRD peaks increased from 0.5 to 0.78 2q,

whereas the FWHM of MoSi2 XRD peaks roughly decreased

from 0.68 2q to a value closer to the instrumental contribution.

As a consequence, a reduction of the Mo crystallite size

probably slightly compensated by the release of the micro-

distorsions (stresses, structural defects, etc.) inside the Mo

grains occurs when the reaction takes place. In the mean time,

the a-MoSi2 crystallites formed inside the combustion wave

exhibit a rapid growth as indicated by the fast increase of

MoSi2 FWHM. Not long after the transformation MoSi2FWHM reaches an asymptotic value that remains stable 30 s

after reaction completion.

3.3. Description of the combustion wave: chemical

and thermal fronts

As the intensity of Si XRD peaks is very weak (XRD signal

absorption and presence of amorphous phase in the as-milled

powder), a determination of the evolution of the Si XRD

volume fraction is not possible. An alternative approach is to

plot the ratio RXZImax(t)/Imax (to or tf) versus the time (t).

Where Imax is the maximum intensity of the XRD peaks for the

phase X (XZMo or a-MoSi2), to and tf represent the beginning

(switched on) and the end (switched off) of the TRXRD

recording, respectively. to and tf were used for reactants (Mo)

and for products (a-MoSi2), respectively. Thus, the function

Ra-MoSi2 will give a direct representation of the phase

conversion rate inside the combustion wave. The functions

RMo and Ra-MoSi2 extracted from TRXRD results presented in

Fig. 3 are shown in Fig. 5. Also included in this figure is the

temperature profile (open circles) of the irradiated zone.

A first estimation of the detection limit has been established

during a series of tests involving an increasing amount of

a-MoSi2 added to the reactant mixture. Some TRXRD patterns

have been recorded in the same XRD conditions and the

minimum amount of detectable product is within 8–10 vol%.

In addition, some oscillations of the value of Ra-MoSi2 at the end

of the reaction can be the consequence of diffraction condition

fluctuations observed; indeed, the volume changes resulting

from either molar volume reduction during the reaction

{DVZK40.6% [40,41]} or thermal expansion (XRD peak

shifts versus the temperature).

The temperature and phase conversion rate profiles in

Fig. 5 show that Si remains solid (measured temperature is

lower than the Si melting temperature) during the first part

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

5.35 5.4 5.45 5.5 5.55 5.6 5.65

time (s)

FW

HM

(°2

θ) Mo(110)

MoSi2(103)

Mo detected No Mo left

Instrumental FWHM Al(200)

Fig. 4. Full width at half maximum (FWHM) evolution during the passage of

the combustion wave—TRXRD peaks of Mo (110) and a-MoSi2 (103).

Mechanical activation: duration 4 h—low energy mechanical activation.

C. Gras et al. / Intermetallics 14 (2006) 521–529 525

of the temperature raise but also during the first stage of the

reactant consumption. Thus, the formation of the bulk of

a-MoSi2 by MASHS can involve, in the early stage of the

reaction, a solid-state diffusion process; the latter is

certainly enhanced by the nanometric nature of the

reactants, the intimate mixing between reactants and by

the larger number of defects introduced during ball-milling.

As reported by Doland et al. [42], deposition of

molybdenum thin film (5–10 nm thick) onto amorphous

silicon subtract leads to the formation of molybdenum

disilicide after annealing at a temperature between 300 and

400 8C. As a result, the polyinterfaces created at a

nanometer scale would promote low temperature solid-

state diffusion.

However, the melting of one of the reactants has usually

been observed as the responsible step of ignition reaction in

several combustion processes involving intermetallic com-

pounds [43]. The mechanical activation can reduce in many

cases the ignition temperature to values well below the melting

point of both reactants suggesting that solid–solid reactions can

occur [24,43,44]. Consequently, the solid–solid interactions,

enhanced by the mechanical activation (i.e. by reducing the

grain size, introducing many defects, etc.) might have produced

sufficient heat to promote the initial stage of the SHS reaction,

400

600

800

1000

1200

1400

1600

1800

2000

5.4 5.5 5.6 5.7 5.8 5.9 6 6.1

time (s)

T (

K)

–0.1

0.1

0.3

0.5

0.7

0.9

1.1

RM

o -

R M

oS

i 2

T R Mo R MoSi2

Fig. 5. Structure of the combustion wave. R(X)—TRXRD {R(X)ZImax(t)

/Imax(to or tf)) versus the time (t). Imax is the maximum intensity of the

diffraction peak for the phase X (XZMo, Si or a-MoSi2); to, the starting point

of the TRXRD recording, was used for the reactants and tf is the end of the

recording session in the case of a-MoSi2}. Infrared temperature (TIR)

evolution of the surface for the X-ray irradiated area.

but once the reaction is initiated, the much faster solid–liquid

interactions take over and become the main driving force.

Nevertheless, it is also possible that the melting of the

nanocrystalline Si takes place at a lower temperature [45]

due to an enhancement of surface effect [38].

TRXRD experiments carried out on SHS reactions

performed on micrometric powders have shown that, under

the same conditions, the reaction is difficult to initiate and it

does not reach completion. In addition, the phase conversion

rate is slower and the conversion at any given level is

associated with higher temperatures, as has been shown by

Wang et al. [46]. For example, it was reported that for a

conversion rate approaching 0.5, the reaction temperature is

within the range 1600–1700 K. In many cases, the repeated

fracturing and welding process that occurs during the milling

initiates a permanent exchange of matter between particles,

ensuring a better mixing of the powders and the destruction of

the oxide surface layers. The ball milling effects on both grain

sizes and residual stresses were reported to modify the phase

transformation kinetics in the final self-sustaining synthesis.

In general, the mechanically activated combustion reaction

exhibits a substantial increase of the reaction front velocity (by

a factor of 3), an increase of the maximum thermal heating rate

and a decrease of the ignition temperature during the SHS

process [24,26].

The current observations are also in agreement with those

reported by Park et al. [42] who investigated the volume

ignition of mechanically activated MoC2Si pellets. Their

works support the solid-state reaction assumption for which the

ignition temperature is lower when crystallite size decreases.

As indicated above, the occurrence of the melting of

nanometric reactants (more likely Si) is not totally excluded,

the result, however, shows that a solid-state reaction can play a

significant role in the first part of the ignition process when

reactant are mechanically activated. The TRXRD results alone

cannot confirm that the solid-state reaction take place in the

early stage of the MASHS reaction. Unfortunately, the current

temporal resolution limits the investigation in the early stage of

the wave formation. Nonetheless, the comparison with

localized thermal information opens new consideration

regarding solid-state reaction in the ignition step.

3.4. Comparison SHS–MASHS

3.4.1. Case of the ignition zone

In order to compare both processes (i.e. SHS and MASHS),

experiments were carried out under the same conditions; each

test was preceded by a short preheating at 573 K for 20 s to

ensure ignition. Fig. 6 shows two typical SHS and MASHS

temperature profiles recorded in the vicinity of the heated side

where the reaction should start (the so-called ignition zone) but

sufficiently far to avoid the parasitic effect of heating source.

It is clear from Fig. 6 that: (1) ignition is achieved in a much

shorter time for the MASHS samples and (2) the ignition

average temperature is slightly lower than the one recorded for

the SHS reaction. The quantitative results are presented in

Table 2. Here, to is the starting time for the IR recording system

300

500

700

900

1100

1300

1500

1700

1900

2100

2300

0 2 4 6 8 10

T IR

(K

)

SHS

MASHS

tig MASHS

tig SHSto d= 7 +/- 1 mm

Time (s)

Fig. 6. Evolution of the surface temperature inside the ignition zone:

thermograms of the ignition zone for the SHS and the MASHS processes—

ignition time (tig) and ignition temperature (Tig) for milled powders with

different levels of mechanical activation. (d is the distance between the graphite

resistor and the edge of the sample in millimeters.)

Table 2

Ignition delay (tig–to) and ignition temperature for different mechanical

activations under preheating (573 K, 20 s) conditions

Mechanical pretreatment Duration (h) tig–to (s) Tig (K)

None (classical SHS) 4.4 1475

Low energy 3 3.6 1362

Low energy 4 3.7 1406

Low energy 6 3.5 1326

High energy 1 3.3 1380

High energy 2 3.4 1399

High energy 3 2.3a nd

nd: not determined.a Crude estimation: ignition point difficult to determine.

C. Gras et al. / Intermetallics 14 (2006) 521–529526

and tig is the time where a sudden temperature increase has

been recorded. The latter defines the ignition time and Tig is the

temperature measured at this particular moment.

Although the temperature profile shape is similar for the

SHS and MASHS processes, the time to ignition is significantly

shorter (by 20–25%) for the MASHS samples. The same trend

is also observed for the ignition temperature: Tig(MASHS) is

100G20 K lower than Tig(SHS). These results are in

agreement with earlier observations on the Mo–Si [24,28]

and Fe–Al [22,34] systems. The same behavior has been

reported in the reaction involving mechanically activated SiO2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

6.8 7.2 7.6 8 8.4 8.8

R X

R Mo

R Si

R MoSi2

SHS

Time (s)

(a)

Fig. 7. TRXRD—R(X) for XZMo, Si or a-MoS

and Al [47], for field activated synthesis of dense MoSi2 [48]

or during the shock-wave synthesis of MoSi2 starting from

ball-milled mixtures [49].

3.4.2. During the propagation of the combustion front

Once a combustion wave has been initiated, it propagated in

a steady-state mode. In previous investigations [24,28], it was

shown that the MASHS wave velocity was three times higher

than that measured in SHS experiments. However, no direct

measurements have been made to extract useful information

about the chemical reaction itself. Fig. 7 shows the evolution of

RX with XZMo and a-MoSi2 for the SHS and MASHS

processes (these reactions were initiated after a preheating

at 573 K).

In every case, mechanically activated powders react faster

(see Fig. 7). Complete conversion of the reactants takes place

in less than 0.066 s for the MASHS process, whereas it needs

more than 0.33 s when no activation is applied. Considering the

reaction path MoCSi/MoSi2, V(Mo)ZdRMo/dt, where

V(Mo) the evolution velocity of RMo can be used as a crude

estimation of reaction conversion rate inside the combustion

front. Based on a series of TRXRD experiments, V(Mo)MASHS

is always 2–3 times higher than V(Mo)SHS. This result is also

valid for V(a-MoSi2).

3.5. Influence of the preheating and as-milled powder

composition

Table 3 shows the thermal data recorded during a complete

set of experiments where the effects of both mechanical

activation parameters (energy and duration) and the preheating

(subscript a: 573 K during 20 s, subscript b: no preheating)

were investigated. The maximum temperature (Tc) and the

average velocity of the combustion front (U, estimated from

the accumulation of IR data restricted to a line perpendicular to

the combustion wave [28]) are sorted using both the

mechanical activation duration and the mechanical activation

energy level (i.e. high and low energy) as main variables. In

addition, the values measured for the conventional SHS

process are also reported (i.e. data quoted as Turbula).

0.0

0.2

0.4

0.6

0.8

1.0

1.2

2.8 3 3.2 3.4 3.6 3.8 4 4.2

R X

R Mo

R MoSi2

MASHS

Time (s)

(b)

i2. (a) SHS reaction. (b) MASHS reaction.

Table 3

Maximum temperature (Tc) and combustion wave velocity (U) vs the mechanical activation parameters (ball milling energy level and duration of the pre-treatment).

Tc(a) and U(a) refer to a preheating at 573 K during 20 s, Tc(b) and U(b) no preheating and Turbula is the level 0 of the mechanical activation and is attributed to the

SHS process

Mechanical activation type Preheating (573 K, 20 s) No preheating

Duration (h) Tc(a) (K) Ua (mm sK1) Tc(b) (K) Ub (mm sK1)

Turbula 1973 14 No reaction

Low energy (PZ2.2 W) 3 2120 44 1878 27

Low energy (PZ2.2 W) 4 2054 42 1833 22

Low energy (PZ2.2 W) 6 2060 44 1729 25

High energy (PZ4.8 W) 1 2055 41 1900 26

High energy (PZ4.8 W) 2 1975 36 1609 7

High energy (PZ4.8 W) 3 1810 34 No reaction

5

10

15

20

25

30

U (

mm

s–1)

(a) Steady state propagation

C. Gras et al. / Intermetallics 14 (2006) 521–529 527

The longer the mechanical activation, the slower is the

combustion wave; in parallel, the maximum combustion

temperature decreases. Preheating has been found to not

influence this general trend, but Tc falls faster with ball milling

duration when no preheating is applied. In all cases, Tc(a) and

U(a) are higher than the values recorded without any

preheating. Therefore, the MASHS process responds exactly

like the SHS process when an additional amount of thermal

energy is brought to the system. Preheating does not modify the

reactant crystallite size before the reaction; indeed, no variation

of the FWHM of Mo XRD peaks before the combustion has

been observed whatever mechanical activation powers.

If data on Table 3 and Fig. 8 are carefully analyzed, Tc and U

decrease as the amount of a- and b-MoSi2 in the as-milled

powders increases. This trend is emphasized when the shock

power is nearly doubled. From Fig. 8, it is clear that Tc

undergoes a linear decrease with the percentage of mechani-

cally induced phases. This result is true whatever the

preheating. The ‘linear form’ can be predicted if a crude

calculation in adiabatic conditions is performed for the reaction

(1Kx)MoC2(1Kx)SiCxMoSi2/MoSi2. However, the slope

does not match the experimental one even if the preheating is

considered in the adiabatic calculation. At this point, it is worth

noting that the composition estimation via XRD volume

fraction technique as well as a large loss of heat during the SHS

process might have biased the results. Nevertheless, the linear

character is kept and should be considered as a good indicator;

1500

1600

1700

1800

1900

2000

2100

2200

0 2 4 6 8 10

T (

K)

Tc(b)

Tad(298K)

Tc(a)

Fig. 8. Maximum temperature (Tc) versus the combined weight percentage of

a- and b-MoSi2 produced during the ball-milling pre-treatment—estimation of

the adiabatic temperature (Tad) when a certain amount of end-product is present

before the combustion reaction.

indeed, the amount of mechanically induced phases is a key

parameter to understand the evolution of the main parameters

traditionally used to describe the combustion wave and its

existence domain. The compositional threshold for combustion

propagation depends on the preheating step. The extra thermal

energy brought by the preheating (573 K during 20 s) ensures

the initiation of the self-sustaining combustion reaction

whatever is the mechanical activation.

In summary, adding mechanically induced product (i.e.

MoSi2 phases) in the reactants mixture can act as a thermal sink

that interferes with the heat exchange process during the

propagation of the combustion front. High-energy mechanical

activation performed during 3 h (where 6.5 vol%–3.8 wt% of

mechanically alloyed phases are present) does not lead to any

self-propagating combustion wave. As was pointed out, this

problem can be overcome by heating the sample to 573 K prior

to ignition.

In the case of high-energy mechanical activation, Table 3

reveals a drop of Tc and U after 2 h of pre-treatment (before

reaching the condition of no-ignition). TRXRD was used to

05.2 5.4 5.6 5.8 6 6.2

0

5

10

15

20

25

30

10 10.2 10.4 10.6 10.8 11 11.2 11.4 11.6

U (

mm

s–1)

(b) unstable propagation

Time (s)

Time (s)

Fig. 9. Stability of the wave velocity during the combustion reaction.

(a) Low energy/4 h, (b) high energy/2 h.

400

600

800

1000

1200

1400

1600

1800

10.6 10.8 11 11.2 11.4 11.6

T IR

(K

)

0.00

0.20

0.40

0.60

0.80

1.00

1.20

RX

(X

=Mo

et

Mo

Si 2

)

RMo

RMoSi2T IR

Time (s)

Fig. 10. Coupled TRXRD and infrared thermography—instability of the

reaction for high energy/2 h condition.

C. Gras et al. / Intermetallics 14 (2006) 521–529528

investigate why Tc and U were seriously affected when the

mechanical activation energy is nearly doubled.

Compared with the low energy activated reactants

(Fig. 9(a)), the combustion front velocity (Fig. 9(b)) was not

constant in time and space. During propagation, significant

fluctuations with no distinctive pattern were observed.

Probably, the temporal resolution was well under the one

required to measure the fluctuation periodicity. Nevertheless,

macroscopic traces of the combustion front oscillation have

been observed on the sample surface after reaction. As a

consequence, the combustion wave accelerates and decelerates

all the time, reaching at one point a velocity of zero (marked

with an arrow—Fig. 9(b)). In the time interval of 11.0–11.2 s,

RMo and Ra-MoSi2 undergo a short plateau, the temperature rise

shows a corresponding discontinuity (see Fig. 10) and the wave

velocity goes from a finite value to 0 and back again.

Those results reinforce the idea that the chemical reaction is

the driver of the MASHS reaction. Nonetheless, if any product

is already formed during the ball-milling pre-treatment, the

chance of affecting the propagation is large, mainly for two

reasons: (i) a part of the potential energy for the reaction has

been used to produced a- and b-MoSi2 in the mill; (ii) the end-

products change the thermal conductivity ahead of the

combustion front and modify the propagation.

4. Conclusions

The novel IRCTRXRD technique has been applied to study

the structural and thermal evolution during the production of

a-MoSi2 by mechanically activated self-propagating high-

temperature synthesis (MASHS). Time-resolved X-ray diffrac-

tion (TRXRD) coupled to infrared thermography (IR) can be

regarded as a powerful tool fully capable of pertinent in situ

investigations during the combustion front propagation.

The study of the MASHS combustion wave demonstrates

that the chemical reaction controls the front velocity. The

combustion wave structure determined in this work matches

the one defined by the macrokinetic studies reported by

Merzhanov et al. [50].

The special nanostructure obtained after mechanical

activation influences the reaction responsible for the self-

sustained combustion. The TRXRD experiments indicate that

a solid-state reaction in the early stage of the combustion wave

is possible. Indeed, the polyinterfaces created during the

continuous fracture and welding process during ball milling are

probably favorable to a fast solid-state diffusion where

reactants are closely intermixed. The same behavior has been

reported in the Fe–Si system [51]. In addition, the solid–solid

interactions, enhanced by the mechanical activation, can

produce sufficient heat to promote the initial stage of the

SHS reaction. Once the reaction is initiated, the much faster

solid–liquid interactions take over.

The incubation time and the ignition temperature are shorter

and lower than those recorded in the SHS process. The

mechanical activation acts on the energy barrier to ease the

initiation of a stable combustion wave.

The MASHS process responds as the SHS does when

subjected to a preheating. The extra amount of energy engaged

in the heat balance improved the reaction conversion rate and

accelerated the combustion front.

The stability of the combustion wave is mainly influenced

by the presence of mechanically induced phases such as a- and

b-MoSi2. A direct correlation has been found between the

combustion wave stability and the amount of mechanically

induced phases (a- and b-MoSi2) in the reactants mixture.

Indeed, high-energy mechanical activation degrades very

quickly (O2 h of treatment) the stability of the combustion

wave. The TRXRD result has shown that pauses in the

chemical reaction are responsible for the thermal fluctuations

recorded during the process.

Acknowledgements

The authors would like to thank D. Vrel (LIMHP CNRS,

University Paris XIII), J.C. Gachon and O. Held (LTM,

University of Nancy), M. Bessiere, J. Doucet and M. Gailhanou

(LURE, Orsay) for their valuable help during time-resolved

X-ray diffraction experiments.

Ch. Gras is grateful to the ‘Ministere de l’Enseignement

Superieur et de la Recherche’ for financial support.

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