NORMAS RELEVANTES DENTRO DE LA VIGENCIA DE LA CONSTITUCIÓN DE 1991

We will not here describe fully the many possibilities of SHS to synthesize MAX phases, as this topic is by itself the subject of another chapter. However, we will briefly present some results, obtained in conditions similar to the ones where aluminothermic reactions are involved.

Samples were prepared from commercial powders of aluminum, titanium and graphite, which were carefully weighed to obtain the compositions of the required stoichiometry. Once weighed, the powders were thoroughly mixed using a Turbula® mixer, co-milled using a Fritsh® Pulverisette 6 planetary mill at 200RPM for 2h with a Ball-to-Powder Ratio (BPR) of 10:1 (i.e. the weight of the balls is 10 times the weight of the powders) and were then pressed. The resulting samples were then set in the reaction chamber, described in References [77, 78]. Ignition of the powder mixtures was then carried out by a graphite plate heated up to a temperature of about 2000 K by a high intensity electrical current (up to 12 V – 200 A), and the reaction was then analyzed by infrared thermography using an AVIO® TVS 2000 ST IR– camera.

X-Ray Diffraction (XRD) analysis has been performed using an XRG-3000 diffractometer from INEL®. This diffractometer works in the asymmetric Bragg-Brentano geometry, with a curved detector with a 90° aperture; with 50 cm curvature radius and 8192 digital channels, this detector provides a precision close to 0.01°. The radiation wavelength used is the Cu-k1 radiation, 1.54056 Ǻ, monochromated by a germanium monocrystal; out of

the two slits placed just before the sample, the one parallel to the linear beam is opened for only 13 µm in order to eliminate the Cu-k2 radiation.

3.1. Influence of the Cooling Rate

In order to have comparative and regular densities, the samples were isostatically pressed at a pressure chosen at 80 MPa. Because aluminum is a very soft metal this pressure is sufficient to get a sample with mechanical properties good enough to manipulate it.

Due to an inhomogeneity in the temperature field and/or in the emissivity at the surface of the sample during reaction, we suspected that the cooling rate could influence strongly the reaction kinetics. A detailed analysis by x-ray diffraction, presented in Reference [79], made us believe that the purity of the final product could be improved by reducing the maximum temperature and/or by reducing the time the sample spent at high temperature, which naturally brought us to try to increase the cooling rate of our sample.

With such an aim, it may seem paradoxical to use mechanical activation, as it is known that such a treatment has a tendency to increase the maximal temperature. However, the increased stability of the reaction front allowed us to increase the heat losses beyond the quenching threshold, i.e. beyond the point where a self-propagating reaction could not propagate in unmilled powders.

Three samples with different diameters of 10, 14 and 18 mm were then pressed; they were maintained using thin tungsten wires, making radiation and natural convection the dominant heat loss phenomena, and heat conduction between the sample and the sample holder could be neglected.

Figure 1. Apparent temperature of 3 samples during cooling after SHS from Ti-Al-C witha 2:1:1 stoichiometry. The gray levels are the physical precision of the IR camera; the estimated temperature is determined by setting the camera emissivity parameter to 1.

Figure 2. Diffraction patterns of the three samples. The relative intensity of the TiC peaks increase with the samples diameter.

Figure 1 presents the cooling rate of these three samples, as seen at their surface by the infrared camera. It should be here noted that, because the emissivity parameter of the camera was set arbitrary to 1, and is necessarily smaller, the temperature of the sample is always underestimated by the camera. However, the emissivity of our samples is not well defined, as it must evolve during the reaction, due to temperature, physical state and chemical composition. Considering for example a sample with an initial temperature of 840°C, the cooling down to a temperature of 500°C would take 10s for the =10mm diameter sample, 15s for the =14mm diameter sample and 25s for the =18mm diameter sample. Beside this, the propagation of the reaction front is very similar for all samples, considering the maximal temperature and the heating rate.

The resulting compositions of these samples as determined by X-ray diffraction are presented in Figure 2, on which it can clearly be seen that a decrease in the samples diameter yields immediately a decrease into the TiC impurity. Estimating the TiC content from the comparative area of the main peak of each phase yields a respective TiC content of 6.2% TiC for the f=10 mm sample; 13.7% for f=14mm; and 17,1% for f=18mm. Although this is not meant to be an accurate measure of the TiC content, it proves very clearly the key role of the cooling rate in the impurity content.

From these numbers, it would seem that a sample with a diameter of about 5 to 7 mm would present a pure Ti2AlC phase. Unfortunately, the method cannot be applied for so small

a sample, for we didn‘t succeed in retrieving an unbroken sample after isostatic pressing with a diameter smaller than 10 mm. Nevertheless, the same method could be applied with other geometries, e.g. powder beds.

3.2. Influence of the Stoichiometry

In order to improve our control on the temperature history of the samples, we decided to change the stoichiometry of the initial powder mixture. As the final composition of the samples observed in the previous section showed the presence of TiC, we therefore chose to increase slightly the aluminum content to improve the chances of the formation of a ternary compound. The reaction equation was then:

2Ti + (1+n/2) Al + C  Ti2Al1+n/2C (9)

with n = 0  3.

As the mechanical behavior of the powder mixture must evolve with the composition, we may expect an increase of the density with the aluminum content if we keep the same compacting pressure, this metal being the softest of the three elements. To avoid this, we chose to uniaxially press the sample, fixing not the compaction pressure but the sample final volume to obtain a relative density of 60% of the theoretical value (which naturally changes with the composition).

Figure 3 presents the evolution of phase composition after reaction as a function of the initial aluminum content. Surprisingly, the Ti2AlC content decreases sharply even for n=1,

dropping from 83 to 49%; however, during the same time, the TiC amount also drops from 17 to 4.5%, which is exactly why we developed this dilution method. The rest of the sample is

made of Ti3AlC2, which is also a MAX phase and therefore may be considered as a positive

result, the total amount of MAX phases being 95.5%. Nevertheless, this is quite surprising, since this later MAX phase has a theoretical Al content smaller than Ti2AlC. From this, it

may be deduced that the measured composition by X-ray diffraction may be interesting as long as crystal structures are concerned, but should be taken cautiously, as evidently the stoichiometry of the observed compounds are far from being the theoretical ones.

Figure 3. Phase compositions of the samples after SHS as a function of the initial composition of the powder mixture (equation 9). Percentage of each phase is determined by the relative area of its main peak.

If the content in aluminum is further increase (n=2), the TiC amount in our samples further decreases to a value of 1.8%. In the mean time, Ti2AlC completely disappears from

the sample composition, and, if Ti3AlC2 increases, it only reaches a value of 88%, and the

total amount of MAX phases in our sample is thus decreasing: indeed, a new impurity appears in the sample, TiAl3. Going then from n=2 to n=3 does not improve the situation: TiC

increases back, and TiAl3 reaches a value close to 25%.

Figure 4 summarizes the phase evolution of the system, as a function of Al content in the initial mixture. An optimal value for n may be found between 1 and 2, where the appearance of TiAl3 is not yet detectable, and where the TiC amount reaches a minimum value. As for

MAX phases compositions, they would be in favor of Ti3AlC2, but may still contain some

Ti2AlC.