Time-Resolved XRD Experiments for a Fine Description of Mechanisms Induced During Reactive Sintering

The control of Mechanically Activated Field Activated Pressure Assisted Synthesis hereafter called the MAFAPAS process is the main objective to be achieved for producing nanostructure materials with a controlled consolidation level. Consequently, it was essential to develop characterization tools “in situ” such as the Time Resolved X-ray Diffraction (TRXRD), with an X-ray synchrotron beam (H10, LURE Orsay) coupled to an infrared thermography to study simultaneously structural transformations and thermal evolutions. From the 2003 experiments, we took the opportunity to modify the sample-holder in order to reproduce the better synthesis conditions of the MAFAPAS process, but without the consolidation step. The versatility of the setup has been proved and could even be enhanced by the design of new sample holders. In addition, this work clearly shows that this equipment will allow, on the one hand, to make progress of the understanding of MAFAPAS mechanisms and, on the other hand, to adjust reaction parameters (mechanical activation and combustion synthesis) for producing many materials with an expected microstructure.


Introduction
The MAFAPAS process [1][2][3][4][5] is composed of two main steps: (i) the mechanical activation of reactant mixtures and (ii) the simultaneous synthesis and densification of nanophase iron aluminide by field activation.Al + Fe powders were co-milled in an especially designed planetary mill [6,7] in order to obtain nanometric reactants but to avoid formation of any product phase.The mechanically activated powders were first coldcompacted into a cylindrical die using a uniaxial pressure (382 MPa).These were then subjected to high alternating current (1500A; 60Hz) and pressure (106MPa).Under these conditions, a reaction is initiated and completed within a short period of time (2-5min).After 3-min duration, the current is turned off.Temperatures were measured by an optical pyrometer.The maximum temperature recorded on the graphite die external surface reached 1300 K.At the end of this process the samples were allowed to cool before being removed from the graphite die.However, in order to control such a process (i.e. to produce materials with a perfectly controlled degree of densification and microstructure), progress in the understanding of mechanisms involved in these processes is necessary especially when a SHS reaction is initiated by an electric stimulation.
Until recently, it has been difficult to investigate reactive synthesis implying an SHS reaction by conventional techniques due to the high temperatures involved and the fast rates of combustion.Conventional techniques do not permit a study of the mechanisms controlling these reactions, such as the role of liquid formation, the existence and the effect of transitory phases and of other parameters, which may induce changes to the texture, or nature of the end products.Recently, real time in-situ investigations of structural changes and chemical dynamics in the combustion area have been made possible by the use of synchrotron radiation [8][9][10][11][12][13][14][15][16][17][18][19].Many studies were carried out with the help of a synchrotron X-ray beam (for example the French synchrotron facility at LURE DCI Orsay, France), a fast detection system for monitoring the phase transformations and a high temperature reaction chamber.In addition, the thermal evolution of each sample during SHS reactions was determined by means of an embedded thermocouple and an infrared camera.The IR camera records the sample surface temperature, whose variation rates may reach about +1500K/s.In this paper, such equipment should enable, on the one hand, progress of the understanding of MAFAPAS mechanisms and, on the other hand, adjustment of reaction parameters (mechanical activation and combustion synthesis) for producing many materials with an expected microstructure.

II. Specific tools adapted to SHS reactions. II.1. General Description
SHS reactions have been investigated in-situ using Time resolved X-ray Diffraction (TRXRD), with an X-ray synchrotron beam (LURE, Orsay) coupled to an infrared thermography to study simultaneously structural transformations and thermal evolutions.From 1999, the TRXRD experiment has been transferred to LURE, and we took the opportunity to do more than just adapt the existing experiment.
Experimental setups that were used at LURE in 2000 (H10 beamline) are presented on Fig. 1.This device allowed studying of the SHS process by TRXRD coupled to surface

Infrared camera
Fig. 1 General description of TRXRD device used on H10 beamline temperature recording with an infrared camera.The synchrotron X-ray beam hits the sample, which is inside a small chamber with a kapton window featuring a 190° aperture and He atmosphere at ambient pressure.This reactive chamber is located to a 4-circle goniometer.The reaction is started with an igniter made of a graphite or metal foil.A thermocouple and/or an infrared camera then record the temperature, and the combustion is filmed by a video camera.The infrared signal is then analyzed and stored in a special memory bank, before being transferred to a computer.Simultaneously, X-ray patterns are measured by a curved detector having an angular aperture of 120°, and sent to electronic racks integrating the patterns, before being sent to computers.General description of the H10 beam line, which is equipped with a 4-circle goniometer (+2), featuring 0.001° precision and vertical adjustment can be found on the LURE's web site [20].

I.2. X-ray diffraction chamber characteristics
The whole setup presented here, under operating conditions, has a total weight of 1470 grams.The infrared camera can then be placed closer to the sample, for better resolution.It is mainly built in aluminum alloy, except for the insulating parts where PTFE was mainly used, and for near-sample or near-igniter parts, where stainless steel or even graphite were chosen.This reactive chamber is composed of three parts (Fig. 2): (1) the top of the chamber, (2) the sample holder and (3) the goniometer adapter.Recently, the sample holder has been completely redesigned (Fig. 3), to allow fast sample setup, easy adjustment, and energy within the sample.With the same aim of a better understanding of the MAFAPAS process implying SHS reactions, complete control of reaction ignition is possible: the electric power is supplied by an adjustable high intensity current, provided by a 0-250V, 20 A variable transformer followed by a 220 to 12V, 200A regular transformer.Combined with a pre-calibrated telescope or laser pointer, this has allowed us to clearly see the position of the X-Ray beam on the sample, and thus to know precisely which point of the sample has been analyzed by XRD.Therefore, a very good cross-analysis between infrared thermography and TRXRD can be obtained: we know precisely which pixel(s) on thermographs are analyzed by TRXRD and, knowing the location of the goniometer's axis, we can make sure that the analyzed point is correctly selected.

III. Time-Resolved recording and displaying XRD and IR data
A detailed description of the results obtained will be presented elsewhere [16], but as an example an SHS reaction Fe + Al FeAl reaction will be used.Small programs have been written in order to check the results of both infrared thermography and TRXRD measurements.
TRXRD results are presented in Fig. 4. In order to keep a good peak definition, we deliberately chose to alter the delay line in order to measure only 80°.This figure represents the low-resolution results, as a succession of 77 patterns of 2 seconds each.The boxed area corresponds to the time zone analyzed by the second computer.
The memory of this acquisition card is limited to 1 Mb, and here we chose a record of 1024 channels, 512 time steps, with 2 bytes for each data.The time step is taken at 30 ms, and therefore the 512 patterns correspond roughly to 8 patterns on the first system.As an example of the advantages of such a system, we should note that the low-resolution system can be started before the reaction, and be stopped only after the cooling down.As a result, we can clearly see the peak shifts due to thermal expansion, but also post combustion (re-) crystallization, with the appearance of a sup-structure, which can be identified through the Fig. 4 Maximun intensity of XRD peaks (reactants and products) of each phase versus the time inside the irradiated area.low angle peak.Such a result would have been impossible to obtain with the previous setup.On the other hand, it is possible to measure the time between aluminum melting (disappearance of one peak), iron phase transformation and start of the reaction only with the high resolution system.As previously mentioned, high resolution patterns were measured before and after the reaction.

IV. Treatment of TRXRD Data IV.1. Infrared Data
Thermal data were recorded by an infrared imaging camera (AVIO TVS 2000ST).This apparatus is equipped with a lens exhibiting a field of view of 9.5 cm x 6.25 cm and each pixel of one infrared picture corresponds to an area of 0.79 mm x 0.79 mm.The infrared thermography is able to give a two dimensional representation of the thermal evolution and can be coupled to the structural evolutions.Fig. 5 shows the temperature evolution inside the X-ray irradiated area, which has been monitored from the IR camera.X=0 corresponds to thermal evolution on one point located to the center of this irradiated area.These temperature evolutions show clearly the presence of the SHS reaction when an alternative current runs through the sample.

IV.2. XRD diffraction data
Diffraction patterns were collected and stored at constant time intervals from the initiation of the reaction through completion for a total collection time of 30s.A typical experiment might consist of 1024 patterns collected at 30ms each, as shown in Fig. 6, in the case of the Fe/Al system.The curve detector, which exhibits an angular aperture of 80° 2θ However, XRD peaks of Fe 2 Al 5 seemed to appear before the melting of Al corresponding, certainly to the solid-state reaction between Al(s) and Fe(s).Then, as the combustion front wave propagates inside the irradiated area, the FeAl intermetallic phase appears.Simultaneously, a transitory peak appears and disappears.This latter, in comparison with the previous work, corresponds to γFe [15].Moreover the Fe 2 Al 5 phase disappears at the end of the reaction before the cooling down.Consequently, the Fe 2 Al 5 phase formation seems to be enhanced when a current stimulation is used for igniting the SHS reaction whereas this phase isn't observed when the SHS reaction is initiated to one extremity of the sample [15,16].One can therefore conclude with certainly that a full elucidation of the reaction path from elemental powders to FeAl intermetallics via FAPASA techniques requires complementary experimentation with TRXRD such as a study of the intensity, the position and the broadening of TRXRD peaks versus the temperature evolution.This approach will be essential for determining with a good accuracy the role of each processing parameter.In addition, further improvements of the quality of diffractograms would be achieved by the spatial resolution of detectors.

V. Conclusion
This paper shows how it is possible to analyze a solid-solid or solid-liquid reaction by TRXRD coupled with IR imaging (time resolution 20 ms).A new experimental setup for TRXRD analysis combined with thermography of highly exothermic reactions has been designed.Preliminary results obtained using this setup seem very encouraging.The versatility of the setup has been proved and could even be enhanced by the design of new sample holders, thus expanding its area of use at low cost.In addition, this work clearly shows that this equipment will allow, on the one hand, to make progress of the understanding of the SHS reaction and, on the other hand, to adjust reaction parameters (mechanical activation and SHS ones) for producing many materials with an expected microstructure.

Fig. 2
Fig. 2 Description of the reactive chamber.

Fig. 3
Fig. 3 Description of sample holder allowing a electric current within the sample.

Fig. 5
Fig. 5 Thermal profile monitored by the IR Camera within the irradiated area.x=0 corresponds to the central point located in the middle of the X-Ray spot.

Fig. 6
Fig. 6 Structural evolution inside the irradiated area.The right part which is a 2D representation corresponds to a zoom of the rectangle in dashed line.It consists of 1024 XRD patterns collected at 30ms each/