In Situ Time-resolved Diffraction Coupled With a Therma

7/28/2019 In Situ Time-resolved Diffraction Coupled With a Therma

1/11

IN SITU TIME-RESOLVED DIFFRACTION COUPLED WITH

A THERMAL I.R. CAMERA TO STUDY MECHANICALLY

ACTIVATED SHS REACTION: CASE OF FeAl BINARY

SYSTEM

F. CHARLOT1,2, F. BERNARD1, E. GAFFET2{, D. KLEIN3 and J. C. NIEPCE1

1Lab. de Recherches sur la Re activite des Solides, UMR 5613 CNRS, BP 400, F-21011 Dijon, France,2UPR CNRS A0423 ``Far from Equilibrium Phase Transitions'' Group, F-90010 Belfort Cedex, France

and 3LMIT Po le Universitaire BelfortMontbe liard, BP 427, F-25211 Montbe liard, France

(Received 11 November 1997; accepted 4 October 1998)

AbstractMechanically activated self-propagating high-temperature synthesis (MASHS) provides anattractive practical alternative to the conventional methods of producing intermetallic compounds, such asiron aluminides. This process involves mainly the combination of two steps; the rst step, a mechanical ac-tivation, where pure elemental (Fe + Al) powders were co-milled inside a planetary mill, for a short timeat given frequency and energy shocks and, the second step, a self-propagating high-temperature synthesis(SHS) reaction, which uses the exothermicity of the Fe + Al reaction. Once ignited with an externalsource, these reactions become self-sustained and propagate to completion within seconds. The combustionfront directly leads to the formation of a nanometric FeAl intermetallic with a relative density of 7080%. To understand this self-sustained reaction, an in situ study in real time was investigated on sampleswhich dier by the shock power during milling and the compaction pressure (porosity). When the combus-tion front goes through the sample, the time-resolved X-ray diraction experiment (TRXRD) using syn-chrotron radiation coupled with an infrared thermography allows the in situ study of the phase formationand the temperature evolution during the MASHS process. # 1999 Acta Metallurgica Inc. Published byElsevier Science Ltd. All rights reserved.

1. INTRODUCTION

The iron aluminides Fe3Al and FeAl show promise

for a broad range of applications. They form pro-

tective oxide scales in hostile environments [1]. They

exhibit lower densities and good high-temperature

properties compared to many structural alloys cur-

rently used [2, 3]. However, cold working these ma-

terials causes them to exhibit brittle fracture and

low ductility at room temperature. Their use as en-

gineering materials has been thus far restricted due

to this limitation. Conventional methods of proces-sing iron aluminides, including casting, hot rolling,

and powder metallurgy, have been investigated [4, 5].

An alternative processing method, variously known

as combustion synthesis, reactive sintering, or self-

propagating high-temperature synthesis (SHS) [6],

has also been used to synthesize several aluminides.

The advantages of SHS processing include inexpen-

sive and easily compacted powder starting ma-

terials, low processing temperature, and exibility in

composition and microstructure control.

The formation of the FeAl compound in the Fe

Al system by mechanically activated self-propagat-

ing high-temperature synthesis (MASHS) was inves-

tigated. In fact, MASHS processing associates, on

the one hand, a mechanical activation of a powder

mixture which is carried out inside a planetary ball

mill [7] and, on the other hand, a self-propagating

high-temperature synthesis (SHS) reaction. It has

been noted that the mechanical activation has been

successfully applied to the synthesis of MoSi2,

FeSi2, and WSi2 induced by low-temperature

annealing [810] and this activation has been also

reported to be eective in the case of the solid and/

or liquid phase sintering [11]. Thus, MASHS keeps

the benet of the SHS process to prepare dense ma-

terials and allows with this new preliminary step,

the mechanical activation, to improve the mechan-

ical properties making possible the elaboration of

nanometric bulk materials [12] by decreasing the

SHS ignition temperature.

MASHS is a relatively novel mode of preparing

high-temperature materials via solid-state reactions.

These reactions are often accompanied by the

release of a large amount of heat. Once ignited with

an external heat source, the SHS reactions become

self-sustained and propagate through the sample

within seconds. These processes are characterized

by a fast moving combustion front (1100 mm/s)and a self-generated heat which allows a rapid

increase of the temperature from 1000 to

Acta mater. Vol. 47, No. 2, pp. 619629, 1999# 1999 Acta Metallurgica Inc.

Published by Elsevier Science Ltd. All rights reservedPrinted in Great Britain

1359-6454/99 $19.00 + 0.00PII: S1359-6454(98)00368-1

{To whom all correspondence should be addressed.

619


2/11

4000 K [13]. Although the basic concepts of this

method of materials synthesis are relatively easy to

apply in principle, there remain a number of basic

questions concerning the physical and chemicalnature, as well as the dynamics of the phase trans-

formations within the moving combustion front.

Until recently, it has been dicult to investigate

these reactions by conventional techniques due to

the high temperature and to the fast rates of com-

bustion velocity. Indeed, these techniques do not

allow the study of the mechanisms which control

these reactions (i.e. the role of liquid formation, the

eect of the intermediate phases and of the other

parameters which induced a modication of the end

products' microstructure). At the moment, in situ

investigation of structural changes and chemical

dynamics in the combustion area in real time withsynchrotron radiation is possible. Table 1 sums up

the dierent technical improvements which are sup-

plied by the time-resolved X-ray diraction

(TRXRD) technique.

The investigated parameters of such a new pow-

der processing method, combining a short duration

ball-milling step hereafter called ``mechanical acti-

vation'' (leading to nanoscale three-dimensional

polyinterfaces of the elemental components [11])

with a SHS reaction, are as follows: (i) the shock

energy, the shock frequency concerning the mechan-

ical activation part, and, (ii) the ignition tempera-

ture, the combustion temperature and the velocity

of the combustion front concerning the SHS reac-tion. Their inuence on the solid (liquid) state

phase formation kinetic paths were characterized.

TRXRD studies were carried out in order to under-

stand the part played by this mechanical activation

on the diusion paths. The TRXRD system uses a

synchrotron X-ray beam (LURE D43, Orsay), a

fast detection system [17] and an X-ray high-tem-

perature chamber [18,19] to monitor the phase

transformation during the reaction.

Moreover, the SHS reaction was monitored by

an infrared camera allowing direct observation of

the heat release leading to a temperature increase of

about 15002000 K/s. The in situ high-temperatureinvestigations of the SHS reaction of the Fe/Al sys-

tem allows quantitative results to be obtained

during the full reaction, which occurs in a few sec-

onds. Then, the various parameters corresponding

to the distinct stage of the MASHS process are

reported.

2. EXPERIMENTAL CONDITIONS

2.1. Mechanical activation (MA)

Two preparation modes of the rst step leading

to the formation of the Fe/Al mixture were per-

formed to study the ball-milling inuence on the

SHS process:

(i) Mechanically inactivated Fe + Al (hereafter

called B0). This mixture was blended in a turbula

mixer for 4 h.

(ii) Mechanically activated Fe + Al (hereafter

called B1 and B2). Mixtures B1 and B2 were co-

milled using the following ball-milling conditions:

a mixture of pure elemental Al (40 mm) and Fe

(10 mm) powders, corresponding to Fe50Al50 stoi-

chiometry, was sealed in a 45 ml stainless steel

vial with ve stainless steel balls (15 mm in diam-

eter and 14 g in weight) under enclosed air. The

ball to powder ratio in weight was 7/1. The

mechanical activation treatment was carried out

using a specially designed planetary ball mill

(hereafter called G5). This machine was rst used

to determine the parameter phase diagram which

was found to be controlled by the injected mech-

anical power in the case of Ni10Zr7, Ni11Zr9,

Ni3Al compounds [23, 24]. The frequency and the

energy of the shocks occurring during this pro-

cess may be selected independently. The physical

Table 1. Technical improvements of in situ time-resolved X-ray diraction experiments

System Year Reference Experiment

NiAl 1981 Boldyrev et al. [14] detector 88 (2y)0.51 s each scan

TiC, NiTi, AlNi 1990 Wong et al. [15] detector 68 (2y)200 ms per scan, 200 discreet measurements

TaC, Ta2C 1993 Larson et al. [16] detector 2 88 (2y)100 ms per scan

500 discreet measurements + i.r. thermographyAlNiTi 1993 Berar et al. [17] rapid detector 178 (2y)

1996 Javel et al. [18, 19] 30 ms per scan2048 discreet measurements

FeAl 1997 Gaet et al. [20] detector 178 (2y) coupled with i.r. thermography180 ms per scan

2048 discreet measurementsFeAl, FeSi2 1997 Bernard et al. [21] detector 178 (2y) coupled with i.r. thermography

180 ms per scan2048 discreet measurements

FeAl, MoSi2 1997 Charlot et al. [22] detector 308 (2y) coupled with i.r. thermography50 ms per scan2048 discreet measurements

CHARLOT et al.: IN SITU TIME-RESOLVED DIFFRACTION620


3/11

parameters of the G5 machine were as follows:

the vials are xed onto a rotating disk (rotation

speed O) and rotate in the opposite direction to

the disk with a speed of o. The O and o werechecked and controlled by an ultrasonic tach-

ometer. The selected O/o values are expressed in

r.p.m. The duration of the milling process is

denoted Dt. So, milling conditions are character-

ized by three important parameters O,o ,Dt.

Two milling conditions were selected, one of

them G5(150/200/4 h) so-called B1 corresponding

to the friction mode and the other G5(150/50/

4 h) so-called B2 to a direct shock mode. The

duration of the milling process was determined

from a kinetic study performed on milled pow-

ders taken at dierent times (i.e. 1, 2, 3, 4, 6,

12 h). Finally, from XRD analysis, the milling

duration was xed at 4 h in order to avoid the

formation of some intermetallic fractions during

the MASHS processing steps but to enable the

formation of the chemical gradient at a nanos-

cale (layered structure of Fe and Al).

2.2. Post-mortem SHS experiments

SHS processes are those which involve exo-

thermic reactions and which are able to self-

propagate, or even self-accelerate. As the

reaction Fe AlAFeAl is exothermic

(DH 51 kJamol [25]; 32 kJ/mol [26]), the com-bustion of Fe and Al starts at the hottest point of

the system and, if the thermal conditions are favor-

able, propagates through all the sample. A homo-

geneous sample is generally the result of a

homogeneous reaction in each combustion zone

and a stable value of combustion velocity. The sim-

plest way of ignition is local heating of a powder

compact when the enthalpy of reaction is su-

ciently exothermic to produce a self-sustained reac-

tion.After extraction from the milling vials, the

mechanically activated particles were introduced

into a cylindrical tempered steel die (f 13 mm)

and were cold compacted using a uniaxial pressure

system. In this case, samples were made at several

distinct pressures from 200 MPa to 2 GPa. A geo-

metric measurement of the green density was car-

ried out in order to study the inuence of the initial

green density on the MASHS reactions.

In this part, cylindrical samples were put on a

pre-heated sample holder at a xed temperature T

controlled by a thermistor probe in the range of

3504508C, the reaction is ignited after a short

delay, simultaneously an infrared camera studies

the thermal evolution on the sample surface. This

temperature T will be assimilated to the ignition

temperature. To exhibit the contribution of the

ball-milling and compaction pressure conditions on

the MASHS process in the Fe/Al system the ig-

nition temperature and the nature of the end pro-

ducts were chosen. The latter is carried out by X-

ray diraction analysis using a D5000 Siemens high

resolution powder diractometer.

2.3. In situ time-resolved experiments

A specic device described in Fig. 1 was es-

pecially used to study simultaneously the structural

evolution using a time-resolved X-ray diraction

(TRXRD) system and the surface temperature eld

using an infrared camera.

Fig. 1. Diagram of the in situ time-resolved instrument.

CHARLOT et al.: IN SITU TIME-RESOLVED DIFFRACTION 621


4/11

2.3.1. Time-resolved X-ray diraction (TRXRD).

The high-temperature chamber which was pre-

viously developed by Gachon and co-

workers [18, 19] is built in stainless steel with a vac-uum technology. It supports the alumina sample

holder which is centered at the goniometer axis and

supports an electrical heater. The compacted

samples were ignited at one end by this heating

tungsten resistor located on the sample holder.

During this experiment, a rectangular geometry of

the sample (20 10 mm) having a plane surface was

imposed to lead the XRD investigations. In this

case, only two compaction pressures were chosen,

200 and 600 MPa, respectively.

This reaction chamber can be used with various

atmospheres, in our case, all the Fe + Al reactions

were performed in a helium atmosphere to avoidmaterial oxidation and to minimize air attenuation

of the incident and diracted X-ray beams. The

chamber includes a 1808 mylar window to allow

access of incident and diracted X-ray beams and

of i.r. beams. This apparatus was designed to

accept a synchrotron beam at an angle of about

308 on a combustion sample. During our exper-

iments, a position sensitive detecting system

designed by Berar et al. [17] for rapid acquisition

was used. Its beryllium window has an aperture of

1206 mm2. It is located at a distance of 400 mm

from the diracting center and in these conditions,

it has an equivalent angular aperture of 178 in 2y.The structural evolution of the X-ray diraction

experiment was performed at LUREDCI (Orsay,

France) on the beamline D43 (Head: J. Doucet)

using a focused beam monochromator

(l = 0.099 nm). The detector was centered at 248

to collect simultaneously the Al(111) and

Fe(110) + Al(200) diraction peaks at the starting

point of the reaction, as well as the major dirac-

tion peaks of FeAl(110) products. Two distinct

congurations to record XRD patterns can be

chosen: (i) one favoring a short acquisition time to

follow kinetics reactions, which is characterized by

the collection of 2048 XRD patterns having aspatial extension on 256 channels and an acqui-

sition time of 30 ms and (ii) one favoring a long

acquisition time to dene the initial and nal

states which is characterized by the collection of

one XRD pattern with an acquisition time of 30 s.

In addition, from this apparatus, it is possible to

record simultaneously all thermal and structural

events during 1 min.

Standard aluminum powder, using Al(111) and

Al(200) peaks, was used to calibrate each diracto-

gram in the angular range dened above.

Nevertheless, taking into account the high random

component of the XRD background, it was necess-ary to sum at least ve elemental scans to eectively

reveal the diraction peak.

2.3.2. Time-resolved infrared thermography. The

sample temperature was simultaneously recorded

with the TRXRD measurements using an imaginginfrared camera (AGEMA THERMOVISION

470). This apparatus is equipped with a lens (208)

having a eld of view of 13 13 cm2 and each

pixel of one infrared picture corresponds to

11 mm2. Thermal infrared images were continu-

ously recorded with a video system using the

Broadcast Standard for U-matic magnetoscope.

Emissivity measurements which were performed by

Klein with this camera, show a change of the

sample emissivity between initial and nal states.

Nevertheless, in order to obtain a real temperature

from i.r. signals, only one value of the emissivity

must be used with this camera. So an averagevalue which is determined from previous measure-

ments was used and was equal to 0.8. The mylar

Fig. 2. (a) Spatial coupling. The white rectangle representsthe irradiated area in an infrared image. One pixel rep-resents about 1 mm2 (case of B1 200 MPa sample). x andy labels represent the number of lines which dene a pixelon this picture. (b) Temporal coupling. Determination ofthe beginning of X-ray acquisition on the i.r. image. Theincandescent lamp is switched on image 47

(t = 02200 ms).



5/11

window of the chamber was transparent in the

range of wavelength where the i.r. captor was sen-

sible (25 mm).

2.3.3. Spatio-temporal synchronization. Neverthe-less, it is essential to perform a spatio-temporal syn-

chronization of X-ray diraction and infrared data.

2.3.3.1. View Of The Irradiated Area On The

Sample: SPATIAL COUPLING

With a uorescent paper representing the sample

size, the X-ray spot was revealed and recorded by a

visible camera. After the analysis of this image, the

coordinates of the irradiated area were dened

knowing the pixel size of the numerical infrared

image. This area must be now rened on the infra-

red image [Fig. 2(a)].

2.3.3.2. Determination Of The Acquisition Starting

Time (T0): TEMPORAL COUPLING

Temporal coupling is essential to correlate the X-

ray diraction acquisition (recorded in steps of

30 ms) to i.r. images (recorded in steps of 200 ms).

Nevertheless, it is essential to set the beginning of

X-ray acquisition on the corresponding i.r. image

number. This was performed with an incandescent

lamp located in the camera eld, which was

switched on when the diraction data collection

was activated [Fig. 2(b)]. These various operations

were performed to clearly dene the temperature

map within the irradiated area in real time as the

reaction front propagates through the sample.

3. EXPERIMENTAL RESULTS

3.1. Post-mortem MASHS results

Using the SHS procedure described in Section

2.2, for each ball-milling condition [B1 (150/200/

4 h), B2 (150/50/4 h) and B0] and each compaction

pressure, Table 2 was established. In Table 2, the

XRD analyses were performed to determine which

kinds of intermetallic phases were formed after the

MASHS process. In the case where only the FeAl

compound was observed (except some iron oxides),

the size of the crystallites (size of a region over

which the diraction is coherent) was determinedfrom HalderWagner plots [12] using the XRD line

prole analysis [27].

Table 2 shows that the friction mode (B1) is

more eective to activate the MASHS process in

comparison with the direct shock mode (B2).

Indeed, such a mode (B1) was shown to lower the

ignition temperature. This result can be explained

by an increase of the polyinterface number and sur-

face fracture: i.e. the number of contact surfaces is

increased. In addition, it seems that the ignition

temperature depends on the porosity. Indeed, for

example in the case of the B1 condition at 3508C,

only the sample having 20% of porosity has

Table 2. Post-mortem MASHS results obtained for each ball-milling condition (B0: turbula mixer 4 h; B1: O = 150 r.p.m.,o = 200 r.p.m., Dt = 4 h; B2: O = 150 r.p.m., o = 50 r.p.m., Dt = 4 h) vs the compaction pressure (200, 600 and 2000 MPa) and the

sample holder temperature (350, 400 and 4508C). For each case: the nature of the end products and the size of the FeAl crystallites(denoted S) determined by XRD investigations are listed. In addition, the value of the porosity before the reaction and the relative den-

sity after the reaction are indicated too

B0 (without mechanical activation)3508C 4008C 4508C

Each pressure No reaction

B1 (150/200/4 h)Compactionpressure

Greenporosity

3508C 4008C 4508C Maximumof relative

density afterreaction

200 MPa 20% FeAl, a-Fe2O3,S= 34 nm

FeAl, a-Fe2O3,S= 31 nm

FeAl,a-Fe2O3, g-Fe2O3,

S= 32 nm

70%

600 MPa 10% Fe, Al FeAl, a-Fe2O3,S= 34 nm

80%

2000 MPa


6/11

reacted. This observation will be discussed later

(Section 4.2).

Finally, it has been shown that the MASHS pro-

cess is eective in producing a nanocrystalline Fe

Al intermetallic compound starting from an initial

equimolar FeAl (2934 nm) with an average rela-

tive density of 80%.

In the time-resolved X-ray diraction studies,

only samples prepared using the friction mode (B1)

and pressed with a charge of 200 and 600 MPa will

be compared with the mechanically unactivated

samples (B0, 200 MPa).

3.2. In situ time-resolved results

Typical examples of such an analysis coupling the

structural and the temperature evolution are listed

in Fig. 3. In this gure, we can notice that the rst

XRD patterns recorded just before the ignition of

the reaction show that the intensity of the Al(111)

peak increases as a function of time. This obser-

vation can be due to a reorganization of the alumi-

num phase in the temperature range close to 4508C.

Then, as the combustion wave propagates inside the

irradiated area, the FeAl intermetallic phase grows

up. In the case of a B1 sample, at 23.7 s when the

combustion wave goes through the diracted area(whose temperature is about 7508508C), the inten-

sity of the Fe(110) + Al(200) peaks decreases and

the major FeAl peak (110) appears. At the end of

this SHS reaction, only the FeAl phase was detected

by the XRD experiment.

The ignition and the propagation of the combus-

tion front induced by the SHS reaction were

recorded using an infrared camera. From these in-

frared data, dierent thermal parameters can be

deduced as shown in Fig. 4 in which the origin of

the time is the beginning of the X-ray collection.

The dierent parts of this gure are as follows: (i)

Fig. 4(a) represents the location of the irradiated

area on the sample which allows a thermal analysis

to be performed along a vertical line. (ii) Fig. 4(b)

shows the shape and the velocity of the combustion

front (12 mm/s) on this line vs time. (iii) Fig. 4(c)

represents the temperature evolution at one point of

this line, the intersection of two slopes leads to the

determination of the ignition temperature

(Tign=4008C), the combustion temperature

(Tcomb=9308C) is the highest temperature reached

in the combustion front. (iv) Fig. 4(d) is the deriva-

tive plot of the temperature prole, an increasing

temperature rate is dened.

In order to exhibit MASHS characteristics in

comparison with classical SHS characteristics, par-

ameters such as the SHS ignition temperature, thevelocity of the combustion front and the increasing

temperature rate were chosen.

Fig. 3. Structural evolution and temperature prole of a mechanically activated (B1, 200 MPa) sample:(a) Fe[110] + Al[200]; (b) Al[111]; (c) FeAl[110].



7/11

Fig.

4.

Thermalanalysisona(B1,

200MP

a)sample:(a)locationoftheareairradiated;(b)visualizationofthecombustionfront

onthewholesample;(c)evolutionofthetem-

peratureduringthetimeontheirradiated

area(t=

0correspondstothebeginningoftheX-raydatacollection);(d)increasingte

mperatureratevstimeontheirradiatedarea.



8/11

4. DISCUSSION

The reproducibility of these experiments has been

shown by using the X-ray synchrotron beam over a

period of four days. The short duration of one ex-

periment (close to 1 h) enables measurements on 20

samples per day. Consequently, during four days,

80 samples can be studied.

4.1. Eect of the mechanical activation

It is clearly shown in Table 3(a), for a compac-

tion at 200 MPa, the mechanical activation, intro-

duced during the MASHS milling step noted B1

(friction mode), signicantly decreases the ignition

temperature (more than 1008C) and, signicantlyincreases the front velocity (2.512 mm/s) in com-

parison with the classical SHS process represented

by sample B0 (without mechanical activation).

Moreover, the analysis of the heat release by the

SHS reaction, shows that this heat induces a tem-

perature increase of 16008C/s for sample B1 and

only 4808C/s for sample B0. These observations can

be due to the release of the mechanical energy

stored during the co-milling inside of grain powders

and/or the decrease of the diusion paths because

of the existence of the chemical gradient at a nanos-

cale.

4.2. Eect of the compaction pressure

In Table 3(b), in the case of the mechanical acti-

vation sample (B1), it is shown that a low pressure

compaction sample exhibits an ignition temperature

lower than that corresponding to a high compacted

pressure green sample. A low pressure (200 MPa)

decreases the SHS ignition temperature (about

508C), increases the front velocity (7.512 mm/s) in

comparison with a sample higher pressure

(600 MPa), for the same co-milling mode.

Moreover, the analysis of the heat release by the

SHS reaction, shows that this heat induces a tem-

perature increase of 16008C/s for the sample com-

pacted with 200 MPa and only 8008C/s for thesample compacted with 600 MPa. This fact can be

associated with a porosity dierence which was

found to be equal to 37 and 15% for 200 and600 MPa pressures, respectively. The dierence of

porosity between these samples and those used for

the post-mortem SHS experiments, for the same

compaction pressure, is due to the dierence of the

sample geometry (rectangular instead of cylindri-

cal). These observations are in agreement with the

conclusion of Munir [28] about the part played by

the porosity on the thermal parameters of self-sus-

tained reactions. This observed dierence may be

assumed to be due to a modication of the thermal

conductivity in the whole sample: a higher porosity,

a lower thermal conductivity. Thus, a larger ther-

mal gradient has to be taken into account in the

case of the low pressure compacted sample.Moreover, the eect of the porosity on the velocity

(v) is similar to observations on aluminide interme-

tallics. Previously, it was shown [29, 30] that the

curve representing the velocity evolution vs the por-

osity presents a maximum which is located at 40%

of porosity for Al50Ni50 and Ni50Ti50 compounds

and 35% of porosity for the Al50Co50 phase.

4.3. Future improvements on in situ time-resolved ex-

periments

Many improvements are necessary, such as a new

rapid detector with a larger angular aperture in

order to conrm or to invalidate the presence or

absence of transitory phases during this process.

Recently, two main developments were performed

on the TRXRD device at LURE: (i) the use of a

new rapid detector with an angular aperture of 308

2y which can couple with the old rapid detector

with an angular aperture of 178 2y, thus allowing

an X-ray diraction analysis on an angular domain

of 478 2y. (ii) The use of a parallel slit system which

reduces the irradiated area in the propagation direc-

tion of the combustion front. These improvements

have enabled the selection of a more useful wave-

length (0.183 nm instead of 0.099 nm) allowing the

expansion of the diraction patterns, to easilyidentify phases which can be observed during the

propagation of the front. In this new conguration,

Table 3. (a) Comparison of the thermal parameters deduced from i.r. data for a mechanically activated sample (B1: 150/200/4 h) and fora mechanically unactivated sample (B0: turbula) in the case of 200 MPa compaction pressure. (b) Comparison of the thermal parameters

deduced from i.r. data for two mechanically activated samples (B1: 150/200/4 h) pressed at 200 and 600 MPa, respectively

(a)Samplecompacted at200 MPa

Ignitiontemperature

(8C)

Combustiontemperature

(8C)

Combustionvelocity(mm/s)

Increasingtemperature rate

(8C/s)

B1 400 930 12 1600B0 480 900 2.5 480

(b)Sample B1 Ignition

temperature(8C)

Combustiontemperature

(8C)

Combustionvelocity(mm/s)

Increasingtemperature rate

(8C/s)

200 MPa 400 930 12 1600600 MPa 470 900 7.5 800



9/11

Fig.

5.

ObservationofatransitoryphaseduringtheFeAlelaborationbyMASHS.



10/11

we have observed transitory phases during the FeAl

elaboration by the MASHS process as shown in

Fig. 5. This latter observation shows the interest in

using TRXRD experiments coupled with an i.r.camera in order to obtain the diusion paths which

are used to form these materials. The next steps of

this work will be to determine the nature of this

transitory phase and better understand the contri-

bution of the mechanical activation and the com-

paction pressure on the reactivity of this system by

thermal model development.

5. CONCLUSION

This paper clearly shows the inuence of the

mechanical activation and the compaction pressure

on the SHS process. Starting from a mixture of el-

emental powders, the rst MASHS milling step is

basically a solid state mixing process due to chaotic

processes (fracture and welding) which leads to the

formation of micrometer-sized powders which con-

tain nanoscale three-dimensional polyinterfaces

between the elemental Fe and Al components. In

the mechanical activation studies, the solid state

reaction which forms the target phases occurs only

during the subsequent SHS step. The in uence of

milling conditions on grain sizes and residual stres-

ses were reported to modify the phase transform-

ation kinetics by the nal SHS process. The

mechanical activation allows an ignition tempera-ture decrease and an increase of the combustion

front velocity over the classical SHS process. Using

this particular planetary mill, it is possible to select

a friction mode or a direct shock mode to activate

powders during this initial step of the MASHS pro-

cess. Our experiments demonstrate that the friction

mode is more ecient in decreasing the ignition

temperature than the direct shock mode. We con-

clude that mechanical activation depends on the

mode of transfer of mechanical energy to ground

powders. It thus constitutes a very exible way of

inuencing phase transformations. Experimental

results call for more rened models to understand

such a process in more detail.

The compaction pressure (or more precisely the

porosity value) enables to decrease the ignition tem-

perature and increase the front combustion velocity.

This MASHS process produces nanometric bulk

materials, but the weakness of the relative density

(from 70 to 80%) requires some future development

to obtain fully dense bulk materials.

This paper clearly demonstrates the usefulness of

using a monochromatic high energy synchrotron X-

ray beam (LUREOrsay, France) coupled with a

thermal i.r. camera to study in situ the mechanically

activated self-propagating high-temperature syn-

thesis (MASHS) reactions. Indeed, the time-resolved XRD coupled to an infrared camera pro-

vides information on the structural phase trans-

formations and direct observations of the heat

release during this process. It has been found that

the mechanical activation of the initial powders has

been eective in accelerating the ignition of theSHS intermetallic phase in the FeAl system.

MASHS appears a promising technique in compari-

son with the classical SHS reaction.

AcknowledgementsM. Bessiere (LURECNRS), M.Gailhanou (LURECNRS), M. Gramond, J. F. Berar(ESRF CNRS), J. Doucet (LURECNRS), J. C. Gachon(CNRSUniv. Nancy), Ch. Gras, J. F. Javel and B.Zeghmati are gratefully acknowledged for their valuablehelp during the experiments. V. Mathae is especiallyacknowledged for his assistance and his availability for thethermal analysis. One of the authors (F.C.) gratefullyacknowledges the nancial support of the RegionalCouncil of Bourgogne (France) and the General Council

of Territoire de Belfort (France).

REFERENCES

1. Takasugi, T., Masahashi, N. and Izumi, N. A., Actametall., 1987, 35, 381.

2. Wang, K., Nishikata, A., Shinoda, D. and Suzuki, T.,Z. Metallk., 1990, 81, 581.

3. Watchel, E. and Bayer, E., Z. Metallk., 1984, 75, 61.4. Sikka, V. K., Proc. of the Fifth Annual Conference on

Fossil Energy Materials, Oak Ridge, Tennessee, 1991,p. 97.

5. Knibloe, J. R., Wright, R. N. and Sikka, V. K., inAdvances in Powder Metallurgy, compiled by E. R.Andreotti and P. J. McGeehan. Metal PowderIndustries Federation, Princeton, New Jersey, 1990, p.219.

6. Merzhanov, A. G. and Borovinskaya, I. P., Dolk.Akad. Nauk. SSSR, 1972, 204, 366.

7. Gaet, E. and Yous, L., Mater. Sci. Forum, 1992,383, 88.

8. Malhouroux-Gaet, N. and Gaet, E., J. AlloysComp., 1993, 198, 143.

9. Gaet, E. and Malhouroux-Gaet, N., J. AlloysComp., 1994, 205, 27.

10. Gaet, E., Malhouroux-Gaet, N., Abdellaoui, M.and Malchere, A., Revue Metall./CIT-Sci. GenieMateriaux, 1994, May, 757.

11. Chausse, C., Nardou, F. and Gaet, E., Mater. Sci.Forum, 1995, 179181, 391.

12. Charlot, F., Gaet, E., Bernard, F., Zeghmati, B. and

Niepce, J. C., Mater. Sci. Engng A, in press.13. Munir, Z. A., Ceram. Bull., 1988, 67(2), 342.14. Boldyrev, V. V., Aleksandrov, V. V., Korchagin, M.

A., Tolochko, B. P., Gusenko, S. N., Sokolov, A. S.,Sheromov, M. A. and Lyakhov, N. Z., Comb. Explo.Shock Waves, 1981, 259(5), 722.

15. Wong, J., Larson, E. M., Holt, J. B., Waide, P. A.,Rupp, B. and Frahm, R., Science, 1990, 249, 1406.

16. Larson, E. M., Wong, J., Holt, J. B., Waide, P. A.,Nutt, G., Rupp, B. and Terminello, L. J., J. Mater.Res., 1993, 8(7), 1533.

17. Berar, J. F., Lemmonier, M., Bartol, F., Grammond,M. and Chevreul, J., Nucl. Inst. Meth. Phys. Res.,1993, B82, 146.

18. Javel, J. F., Dirand, M., Nazzik, F. Z. and Gachon, J.C., J. Physique IV, 1996, 6(C2), 229.

19. Javel, J. F., Dirand, M., Kuntz, J. J., Nazzik, F. Z.

and Gachon, J. C., J. Alloys Comp., 1997, 247, 72.20. Gaet, E., Charlot, F., Klein, D., Bernard, F. and

Niepce, J. C., Mater. Sci. Forum, 1998, 269272, 379.



11/11

21. Bernard, F., Charlot, F., Gaet, E. and Niepce, J. C.,Int. J. SHS, 1998, 7(2), 233.

22. Charlot, F., Gras, C., Gramond, M., Gaet, E.,Bernard, F. and Niepce, J. C., J. Physique IV, 1998, 8,

5.23. Gaet, E. and Yous, L., Mater. Sci. Forum, 1992,

8890, 51.24. Gaet, E., Mater. Sci. Engng A, 1991, 135, 291.25. Rabin, B. H. and Wright, R. N., Metall. Trans. A,

1991, 22A, 277.26. Yi, H. C. and Moore, J. J., J. Mater. Sci., 1992, 27,

6797.

27. Langford, J. I., in Accuracy in Powder Diraction II,

NIST Spec. Publ. 846, ed. E. Prince and J. K. Stalick,

1992, pp. 110126.

28. Munir, Z. A., J. Mater. Synth. Proc., 1993, 1(6), 387.29. Naiborodenko, Y. S. and Itin, V. I., Comb. Explo.

Shock Waves USSR, 1975, 11, 293.

30. Bratchikov, A. D., Merhanov, A. G., Itim, V. I.,

Khachin, V. N., Dudarev, E. F., Gyunter, V. E.,

Maslov, V. M. and Chernov, D. B., Soviet Pow.

Metall. Met. Ceram., 1980, 205(1), 5.


Documents

In Situ Time-resolved Diffraction Coupled With a Therma