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AlN powder: the effect of contamination
Journal of Alloys and Compounds 315 (2001) 276–283
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Mechanical alloying during cryomilling of a 5000 Al alloy /AlN powder: the effect of contamination ¨ b C. Goujon a , P. Goeuriot a , *, P. Delcroix b , G. Le Caer a
´ ´ ´ , Ecole des Mines, 158 Cours Fauriel, 42023 Saint Etienne Cedex 2, France Departement Ceramiques Speciales b LSG2 M, UMR CNRS 7584, Ecole des Mines, Parc de Saurupt, 54042 Nancy Cedex, France Received 19 October 2000; accepted 31 October 2000
Abstract Elemental powders of 80 vol% 5000 Al alloy (3 wt% Mg) and 20 vol% AlN were milled either in liquid nitrogen or at room temperature under argon. The milling of the mixture involved changes in the chemical composition: Fe contamination, oxidation and in-situ nitridation. The effects of milling time and temperature on the contamination of the system were analysed. Oxidation and in-situ ¨ nitridation were characterized by a LECO analyser and thermal desorption measurements, whereas Mossbauer spectroscopy was used to study the mechanical alloying of the Al alloy. The results show that a metastable solid solution of Mg and Fe in Al is formed during cryomilling. The occurrence of mechanical alloying in spite of the very low milling temperature is interpreted as a consequence of the fine crystallite size of the Al matrix. 2001 Elsevier Science B.V. All rights reserved. ¨ spectroscopy Keywords: Alloys; Nanostructures; Mechanical alloying; Mossbauer
1. Introduction Al /AlN composites are a new class of materials which exhibit interesting properties compared with the Al / SiC or Al /Al 2 O 3 composites [1,2]. Fabricated by powder metallurgy [2] or squeeze-casting [3,4] processes, they have a high thermal conductivity, a low density, a high strength and a relatively low coefficient of thermal expansion, which makes them suitable for use in the microelectronics industry. Moreover, previous work has shown that these composites can present superplasticity [5,6]. A very fine microstructure is necessary to obtain good properties in forming. The present study reports the cryomilling of a mixture of 80 vol% 5000 Al alloy (Al–3 wt% Mg, Eckart Poudmet) and 20 vol% AlN (Cernix [7]). The use of liquid nitrogen prevents problems induced by heating during milling and leads more rapidly to a smaller crystallite size [8]. With the Mg being initially in the form of the Al 12 Mg 17 phase, one of the aims was to realize mechanical alloying of the Al alloy during cryomilling. However, the very low tempera-
*Corresponding author.
ture (about 21968C) makes diffusion difficult. One can then ask if the dissolution of Mg in the Al matrix will occur during cryomilling. In previous work [9], mechanical alloying was studied by following the evolution of the Al lattice parameter, which was expected to increase due to the dissolution of Mg in the Al matrix. Actually, the study showed a very limited increase of the Al lattice parameter during the first 10 h of milling, whatever the temperature [room (1258C) or cryogenic (21968C)]. Moreover, in the case of a powder with a fine crystallite size (previously cryomilled for 6.5 h, thus reducing the crystallite size from about 500 to 50 nm), the Al lattice parameter decreased linearly and the rate of decrease was faster when the temperature was higher [10]. One interpretation for this effect was the possibility of the dissolution of Fe originating from the wear of the steel milling tools. This study focused on the effect of contamination during ¨ cryomilling. Mossbauer spectroscopy was used to characterize the local environments of Fe in the cryomilled powder. The main parameters, such as milling time and temperature, were considered. Subsequently, the oxidation and in-situ nitridation, which also involve a change in the chemical composition of the system, were analysed.
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01303-7
C. Goujon et al. / Journal of Alloys and Compounds 315 (2001) 276 – 283
2. Experimental conditions Cryomilling was carried out in an attritor made of 316L steel (0.03% C, 17% Cr, 13% Ni, 3% Mo) and containing 4 kg of 100C6 steel (1% C, 1.5% Cr) balls. The system could operate at different temperatures: at 21968C, in liquid nitrogen (‘‘cryomilling’’), or at room temperature, with argon gas. The rotation speed of the main arm of the attritor and the ball-to-powder weight ratio were optimized respectively at 500 rpm and 40. Stearic acid (3 wt%) was added to the powder mixture as a process control agent. More details of the characteristics of the milling system can be found in Ref. [9]. After 6 h of cryomilling, the Al composites had a spherical shape with a mean diameter of 50 nm. Those of AlN had an anisotropic shape and the apparent crystallite size perpendicular to the [hk0] direction was about 40 nm. Milling at room temperature led to a larger Al crystallite size (.500 nm), whereas no significant effect of temperature on the crystallite size of AlN was observed [9]. Atomic absorption spectrometry was used to determine the contamination of the powders with Fe, Cr and Ni. 57 Fe ¨ Mossbauer spectra were obtained at room temperature in transmission geometry with a spectrometer operated in the conventional constant acceleration mode. A 57 Co source in Rh with a strength of |20 mCi was used. The spectra were analysed both by fitting the positions, amplitudes and full widths at half maximum (FWHM) of Lorentz lines with a conventional least-squares method and by fitting them with ¨ a constrained Hesse–Rubatsch method [11] that extracts the isomer shift distribution from an experimental spectrum, a fit which may equivalently be looked at as a way of removing Lorentzian broadening from a spectrum (Section 7.2 of Refs. [12,13]). For example, when a Lorentzian line with FWHM GM 5 0.22 mm / s is removed from a calibration spectrum of metallic bcc iron, the inner lines of which have FWHM GL 5 0.24 mm / s, a line well approximated by a Gaussian line of FWHM GG 5 0.13 mm / s remains. The latter deconvolution method thus provides a significant and welcome improvement in resolution which facilitates the analyses of spectra. As usual, the 57 Fe
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isomer shifts are given with respect to a-Fe at room temperature. Oxygen and nitrogen analyses were carried out using a LECO TC 436 analyser. Samples were prepared in an argon-filled glovebox to prevent oxidation of the powders before analyses. In order to characterize oxidation and nitridation, thermal desorption measurements up to 6008C were also carried out at a heating rate of 208C / min and the degassed species were analysed by a mass spectrometer. Microstructural investigations were performed using SEM (Jeol JSM 6400) and HREM (Jeol 2010 and Topcon 002B microscopes) equipped with a Kevex EDS nanoanalyser (2 nm resolution). The TEM samples were prepared by mixing the powder with n-butanol and placing it into a 3 mm diameter carbon-coated TEM grid. XRD measurements were performed with a Siemens D5000 high resolution powder diffractometer. Monochromatic Cu Kb X-rays ( l 5 0.13922 nm) were obtained with a secondary focusing monochromator (graphite) and the Al lattice parameter was determined with a leastsquares refinement program.
3. Results
3.1. Contamination by the milling tools 3.1.1. Characterization of the contamination The wear of the steel milling tools (attritor and balls) leads to the presence of Fe, Cr and Ni in the cryomilled powders. Fig. 1 presents the evolution of the elemental contents vs. milling time. Contamination occurs during the first 2 h of milling, when the powder is still very abrasive. The contents then reach constant values of about 3.00 wt% for Fe, 0.10 wt% for Cr and 0.04 wt% for Ni. Contamination by Fe is particularly high, probably because of the brittle behaviour of the steel balls in liquid nitrogen. However, the %Cr / %Ni ratio is similar to that in 316L steel (close to 2); the wear could come from both the attritor and the balls. SEM maps of Al and Fe, prepared for the case of a 6 h cryomilled powder, show that the Fe
Fig. 1. Evolution of Fe, Cr and Ni contents during cryomilling.
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from the balls (88% Fe). The central peak with an isomer shift of 20.1160.01 mm / s (indicated by the solid arrow in Fig. 3) characterizes Fe in 316 L stainless steel from the attritor. The hyperfine magnetic fields of the two main magnetic sextets seen in the figure are 33.560.1 and 30.860.1 T, as expected for Fe atoms in 100C6 steel (balls). These two sextets are mainly due to Fe atoms with 0 and 1 Cr atoms as the first and second nearest neighbours, respectively. A weak central doublet (2% Fe) with a mean quadrupole splitting of 0.4860.02 mm / s and a mean isomer shift of 0.1560.02 mm / s is further due to carbides of the 100C6 steel. ¨ Fig. 4 presents the Mossbauer spectra of powders cryomilled for respectively (a) 2 h and (b) 26 h. Both spectra reveal the presence of a supplementary intense line centered at 0.4060.01 mm / s which is arrowed in the figure. Its area becomes larger with milling time. Fig. 4a further shows a broad featureless magnetic contribution which changes the overall background shape when comparing the spectrum of Fig. 4a with that of Fig. 4b. The latter contribution is not associated with a phase found in steel as seen from Fig. 3, but results from an alloy formed between Fe and elements from the milled powders which is not detected by XRD and could be amorphous. We conclude that the contamination from the milling tools
Fig. 2. SEM maps of (a) Al and (b) Fe in a 6 h cryomilled powder.
distribution is homogeneous in the aluminium agglomerates (Fig. 2). ¨ 3.1.2. Mossbauer spectroscopy ¨ Fig. 3 shows the room temperature Mossbauer spectrum of a mixture of powders from the attritor (12% Fe) and
57 ¨ Fig. 3. Room temperature Fe Mossbauer spectrum of a mixture of powders from the milling tools (attritor and balls).
Fig. 4. Room temperature 26 h cryomilled powder.
57
¨ Fe Mossbauer spectra of (a) a 2 h and (b) a
C. Goujon et al. / Journal of Alloys and Compounds 315 (2001) 276 – 283
reacts partly with the milled powders to form alloys or compounds. It is difficult to identify the compound(s) which contribute(s) to the paramagnetic line at 20.40 mm / s solely ¨ from the Mossbauer spectra as Fe-containing carbides (stearic acid is a possible source of carbon) or nitrides (possibly Fe 2 N) may be formed during milling. Further, a non-magnetic solid solution of Fe in Al or an Al–Fe intermetallic compound must also be considered. As none of the previous compounds is identified from X-ray diffraction patterns, it seems reasonable to assume that the main contribution comes from a solid solution of Fe in Al. In any case, it is possible to follow the evolution of the spectral areas due to the broad magnetic contribution, called ‘‘magnetic’’ hereafter, and to the non-magnetic contribution, called ‘‘paramagnetic’’ hereafter, after removal of the known spectral components originating from the unreacted contaminating fragments. Fig. 5 presents the milling time evolution of the Fe fractions in the ‘‘magnetic’’ and ‘‘paramagnetic’’ components deduced from the corresponding spectral areas when we assume that all phases have equal Lamb– ¨ Mossbauer factors. Note that, after the first 2 h of cryomilling, the total amount of Fe does not change (see Fig. 1). At this stage, 16 at% Fe is alloyed to Al: 10 at% in the ‘‘magnetic’’ form and 6 at% in the ‘‘paramagnetic’’ form. After 26 h of cryomilling, the Fe content of the ‘‘paramagnetic’’ component increases linearly to 15 at%, whereas the Fe content of the ‘‘magnetic’’ form decreases to 6 at%. This graph indicates that Al is progressively alloyed with Fe originating from the milling balls to form
Fig. 5. Evolution of the ‘‘paramagnetic’’ and ‘‘magnetic’’ Fe contents vs. milling time in liquid nitrogen.
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an Fe-rich phase and then an Al-rich phase. Considering absolute values, the ‘‘paramagnetic’’ Fe curve has a larger (positive) slope than the ‘‘magnetic’’ Fe curve (negative slope), suggesting that Fe in solution also comes from the attritor.
3.1.3. Calculation of the Fe and Mg contents dissolved in the Al matrix Considering the measures of Fe content dissolved in the Al matrix (corresponding to the ‘‘paramagnetic’’ fraction ¨ obtained from Mossbauer spectra) and of the total Fe content (measured by atomic absorption spectrometry), one can calculate the expected value of the lattice parameter of Al, if the Vegard law holds for the considered alloy. Moreover, XRD analyses lead to the real value of the lattice parameter of Al. By comparing these two values, it is possible to estimate the Mg content in solid solution in Al and to see if the dissolution of Mg has actually occurred during cryomilling. However, we note that the calculation overestimates the contents of Fe and Mg dissolved in the Al matrix, because the ‘‘paramagnetic’’ components can also be partly associated with Fe in other phases such as carbides, nitrides or Al–Fe intermetallics. 3.1.4. Influence of milling time on the dissolution of Fe and Mg in the Al matrix Fig. 6 presents the evolution of the Fe and Mg contents dissolved in the Al matrix during cryomilling according to the previous calculation. The evolution of the lattice parameter of Al is also shown. The content of dissolved Fe increases linearly up to 1.5 wt% after 26 h of cryomilling. The Mg content also increases during the first 6 h of milling but remains constant at about 0.8 wt% during the following hours. Therefore, the formation of a metastable solid solution of
Fig. 6. Evolution of the Al lattice parameter (from XRD), Fe content ¨ (from Mossbauer spectroscopy) and Mg content (calculated from the Vegard law) dissolved in the Al matrix vs. milling time in liquid nitrogen.
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Mg and Fe in Al during cryomilling is found to be possible.
3.1.5. Influence of temperature Fig. 7 shows the total content of Fe in the powder (wt%), the percentage of dissolved Fe compared with the total Fe (%) and the total amount of dissolved elements (Mg1Fe) (wt%). Millings in liquid nitrogen and at room temperature (with argon gas) were studied and two kinds of powders, with fine (50 nm) or coarse (500 nm) crystallite size, were compared [9]. In the case of the coarse crystallite size powders, the total content of Fe and the percentage of dissolved Fe are higher when the milling is performed at room temperature. The calculation leads to a larger content of dissolved Mg and, finally, the total amount of dissolved elements (Mg1 Fe) is 3.5 times higher when powders are milled at room temperature. For the powders with a fine crystallite size, the total Fe content is similar for the two milling temperatures. The
percentage of dissolved Fe (compared with the total Fe) is higher in powders milled at room temperature, but the calculation leads to a smaller percentage of dissolved Mg. In conclusion, the total weight of dissolved elements (Mg1Fe) does not depend on the milling temperature. The last observation is a comparison between powders milled at room temperature with different crystallite sizes (represented in black or white in Fig. 7). The total amount of dissolved elements (Mg1Fe) is higher in the case of a powder with a coarse crystallite size, but it is related to a total Fe content two times larger. If the percentages of dissolved Fe are compared, it is much larger in the case of a powder with a fine crystallite size. This suggests that Fe diffusion increases when the crystallite size is finer.
3.2. Oxidation With the oxidation of milled aluminium powders being very easy, it is interesting to see whether or not the use of liquid nitrogen as milling medium limits this oxidation. The initial oxygen content of the mixture was about 1.3 wt%: 0.3 wt% comes from the 5000 Al alloy powder, 0.7 wt% comes from the AlN powder and 0.3 wt% comes from stearic acid.
3.2.1. Influence of milling time and temperature Fig. 8 presents the evolution of the oxygen content vs. milling time in liquid nitrogen. In the first 2 h of cryomilling, the oxygen content is quadrupled. It then increases linearly at a slower rate during the following hours. To study the influence of temperature, the oxygen contents of powders milled 10 h either in liquid nitrogen or at room temperature were compared (Table 1). It appears that cryomilling leads to oxidation of the powders almost two times larger than milling at room temperature.
Fig. 7. Total Fe content (wt%), percentage of dissolved Fe compared with total Fe content (%) and amount of dissolved elements (Mg1Fe) (wt%) at room or cryogenic milling temperature, in the case of powders with fine or coarse crystallite size.
Fig. 8. Evolution of the oxygen content in the cryomilled powders vs. milling time.
C. Goujon et al. / Journal of Alloys and Compounds 315 (2001) 276 – 283 Table 1 Oxygen content (wt%) of powders milled for 10 h at cryogenic (21968C) or room temperature (1258C) Milling time (h)
21968C (wt%)
1258C (wt%)
0 10
1.3 7.3
1.3 4.7
3.2.2. Nature of the oxides Two kinds of oxides are observed in the cryomilled powders: 1. H 2 O thermal desorption measurements performed on unmilled and 6 h cryomilled powders show the presence of two peaks for the cryomilled powder only (Fig. 9). The first peak occurs at about 1508C and is probably
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due to the degassing of adsorbed H 2 O. The second peak occurs at about 2408C and may be related to the hydrolysis of the Al and AlN phases [14,15]. 2. HREM observation of a 6 h milled powder shows the presence of a 3 nm thick layer on the surface of the dense Al aggregates (Fig. 10). Moreover, Fig. 11 presents EDS analyses of O, Al and Mg vs. distance from the surface of the Al aggregates. The increase of the oxygen content close to the surface suggests that the layer is amorphous alumina.
3.3. In-situ nitridation The evolution of the nitrogen content in the powder during cryomilling is shown in Fig. 12 (the initial content for a mixture with 20 vol% AlN is 7.2 wt%). It increases linearly to about 9.5 wt% after 26 h of cryomilling.
Fig. 9. Thermal desorption rate of H 2 O in unmilled and 6 h cryomilled powders.
Fig. 10. Three nanometer thick layer of amorphous alumina observed by HREM on the surface of dense aggregates of Al after 6 h of cryomilling.
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Fig. 11. EDS analyses of O, Al and Mg from the surface to the center of Al aggregates.
This in-situ nitridation suggests the formation of aluminium oxynitrides [16] or AlN nanoparticles [17,18]. However, measurements of nitrogen thermal desorption exhibits, in the case of a cryomilled powder only, a nitrogen desorption which starts at 3008C and has its highest rate at about 4408C (Fig. 13). The existence of this peak is probably related to the decomposition of a metastable nitride. Fe 2 N, the decomposition temperature of which is 4008C [19], could be formed during cryomilling. This hypothesis is supported by the fact that, in a powder with a smaller Fe content, no thermal desorption of nitrogen is observed in spite of a higher nitridation (1.1 wt% instead of 0.2 wt%, therefore five times higher). This suggests that, in this case, a stable phase such as AlON or AlN is formed. By assuming that all the Fe is in the Fe 2 N form, one can evaluate the nitrogen content in stable and metastable nitrides. For example, in the 26 h cryomilled powder, the calculation leads to 7.4% N in Fe 2 N and 16% N in stable nitrides formed during cryomilling. However, this last value is underestimated because Fe is not only present in the Fe 2 N form but also in solid solution or as an intermetallic Al–Fe phase.
Fig. 12. Evolution of the nitrogen content in the cryomilled powders vs. milling time.
Fig. 13. Thermal desorption rate of nitrogen in unmilled and 6 h cryomilled powders.
4. Discussion In previous work [9], it was shown that the cryomilling of an Al–Mg–AlN mixture comprises two main stages: the first is characterized by a sharp decrease of the crystallite size of Al and AlN and the second corresponds to a chemical homogenization with no further significant variations of the crystallite size. Here, Fe contamination of the powders during cryomilling is also characterized by two main stages. Note that the duration of each stage depends on the energetic conditions of milling. During cryomilling, the high power of impacts allows the alloying of Al and Fe. Fe-rich solid solution, ¨ characterized by a ‘‘magnetic’’ Mossbauer spectrum, is mainly present in the first 2 h of cryomilling. During this first stage, Mg starts to dissolve in the Al matrix (the exact content cannot be determined), but the initial Al 12 Mg 17 phase still exists, as shown in previous work [9]. In the second stage of milling (chemical homogenization), Al continues dissolving in the Fe-rich solid solution leading progressively to an Al-rich solid solution, characterized by ¨ a ‘‘paramagnetic’’ Mossbauer spectrum (with an intense peak at 0.40 mm / s). During this last stage, the dissolution of Mg occurs until 6 to 8 h of cryomilling and then stops. In the case of coarse crystallite size powders, the total amount of dissolved elements (Mg1Fe) is 3.5 times higher under room temperature conditions than at 21968C. However, in the case of fine crystallite size powders, the total amount of dissolved elements (Mg1Fe) does not depend on the temperature. These observations are in accordance with the diffusion laws: to increase diffusion, a higher temperature and / or shorter diffusion paths are necessary. In the case of Al, cryomilling leads to a very fine crystallite size (down to 50
C. Goujon et al. / Journal of Alloys and Compounds 315 (2001) 276 – 283
nm) which allows some dissolution in spite of the very low temperature. However, the milling temperature remains the predominant parameter. Indeed, milling at room temperature makes mechanical alloying easier, although the crystallite size is much larger. With respect to the oxidation of the powders, the effect of cryomilling is not clear in the literature. According to Huang et al. [20] the use of liquid nitrogen limits oxidation, whereas according to Aikin et al. [17] cryomilling makes oxidation easier. The observations of this work support the fact that oxidation mainly occurs after cryomilling, probably during the reheating of the mill. Indeed, the very low temperature makes the adsorption of H 2 O molecules easier (as the effect of a liquid nitrogen trap on a vacuum pump). During cryomilling, H 2 O is in a solid state and oxidation is not possible. But during reheating, H 2 O becomes liquid and oxidation is expected to occur. Moreover, the presence of oxygen in the center of the Al aggregates suggests that in-situ oxidation also takes place. We can also assume that oxidation of Mg occurs, probably leading to the formation of amorphous phases, with compositions similar to MgO or MgAl 2 O 4 .
5. Conclusions The conclusions of this investigation can be summarized as follows: 1. The wear of the steel milling tools leads to contamination of the cryomilled powders with Fe, Cr and Ni. This contamination essentially occurs during the first 2 h of cryomilling. 2. The formation of a metastable solid solution of Fe and ¨ Mg in Al is clearly established by Mossbauer spectroscopy. Even if temperature is the most important parameter in the occurrence of mechanical alloying, the very fine crystallite size obtained by cryomilling allows some dissolution of these elements in the Al matrix. 3. Oxidation is not negligible and mainly occurs after cryomilling, during the reheating of the mill. 4. In-situ nitridation takes place during cryomilling at a constant rate. The chemical composition of the nitrides depends on the amount of Fe contamination, but stable nitrides (AlN or AlON) were formed during cryomilling.
Acknowledgements The authors thank F. Bernard and J.C. Niepce from the ´ ´ des Solides’’ at Dijon for Research Laboratory ‘‘Reactivite
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´ the XRD measurements, M. Chedru, J. Vicens and J.L. Chermant from LERMAT at Caen for the SEM and HREM observations, and P. Verdier and Y. Laurent from ´ the Laboratory ‘‘Verres et Ceramiques’’ at Rennes for the use of the LECO analyser.
References [1] S.W. Lai, D.D.L. Chung, J. Mater. Sci. 29 (1994) 6181. [2] C. Troadec, P. Goeuriot, P. Verdier, Y. Laurent, J. Vicens, G. Boitier, J.L. Chermant, B.L. Mordike, J. Eur. Ceram. Soc. 17 (1997) 1867. ´ ´ [3] M. Chedru, Elaboration et caracterisation de composites a` matrice ´ ´ par des particules de nitrure metallique d’aluminium renforcee d’aluminium, Doctoral Thesis, University of Caen, France, 1999, 207 pp. [4] R. Couturier, D. Ducret, P. Merle, J.P. Disson, P. Joubert, J. Eur. Ceram. Soc. 17 (1997) 1861. [5] T. Imai, G. L’Esperance, B.D. Hong, Scripta Metall. Mater. 31 (1994) 321. [6] T. Imai, S. Kojima, G. L’Esperance, Scripta Mater. 35 (1996) 1199. [7] M. Passaret, J. Lostec, J.P. Bertot, Y. Laurent, P. L’Haridon, J. ´ ´ de synthese ` a` basse temperature ´ Guyader, Procede de poudres de nitrures, en particulier de nitrure d’aluminium, Patent F 91.11801, 1991. [8] K. Yamada, C.C. Koch, J. Mater. Res. 8 (1993) 1317. ´ [9] C. Goujon, P. Goeuriot, M. Chedru, J. Vicens, J.L. Chermant, F. Bernard, J.C. Niepce, P. Verdier, Y. Laurent, Powder Technol. 105 (1999) 328. ´ [10] C. Goujon, Elaboration par cryobroyage et metallurgie des poudres ´ par de nanocomposites a` matrice d’alliage d’aluminium renforcee des particules de nitrure d’aluminium, Doctoral Thesis, Ecole ´ Nationale Superieure des Mines de St Etienne and Institut National Polytechnique de Grenoble, France, 2000, 264 pp. ¨ J.M. Dubois, J. Phys. E: Sci. Instr. 12 (1979) 1083. [11] G. Le Caer, ¨ R.A. Brand, J. Phys. Condens. Matter 10 (1998) 10715. [12] G. Le Caer, ¨ C.K. Huang, B.V. Rat[13] A. Hightower, P. Delcroix, G. Le Caer, nakumar, C.C. Ahn, B. Fultz, J. Electrochem. Soc. 147 (2000) 1. [14] A.I. Litvintsev, L.A. Arbuzova, Poroshk. Metall. 49 (1967) 1. [15] L.A. Arbuzova, L.L. Kunin, V.A. Danilkin, Hydrogen Met. Congr. Int. 1972 (1) (1973) 222. [16] M.J. Luton, C.S. Jayanth, M.M. Disko, S. Matras, J. Vallone, Mater. Res. Soc. Symp. Proc. 132 (1989) 79. [17] B.J.M. Aikin, R.M. Dickerson, D.T. Jayne, Scripta Metall. Mater. 30 (1994) 119. [18] R.J. Perez, B. Huang, E.J. Lavernia, Nanostruct. Mater. 7 (1996) 565. ´ [19] P. Pascal (Ed.), Nouveau Traite´ de Chimie Minerale, Vol. Tome XVII, Masson et Cie, 1967, pp. 798–809. [20] B. Huang, J. Vallone, M.J. Luton, Nanostruct. Mater. 5 (1995) 631.
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Mechanical alloying during cryomilling of a 5000 Al alloy /AlN powder: the effect of contamination ¨ b C. Goujon a , P. Goeuriot a , *, P. Delcroix b , G. Le Caer a
´ ´ ´ , Ecole des Mines, 158 Cours Fauriel, 42023 Saint Etienne Cedex 2, France Departement Ceramiques Speciales b LSG2 M, UMR CNRS 7584, Ecole des Mines, Parc de Saurupt, 54042 Nancy Cedex, France Received 19 October 2000; accepted 31 October 2000
Abstract Elemental powders of 80 vol% 5000 Al alloy (3 wt% Mg) and 20 vol% AlN were milled either in liquid nitrogen or at room temperature under argon. The milling of the mixture involved changes in the chemical composition: Fe contamination, oxidation and in-situ nitridation. The effects of milling time and temperature on the contamination of the system were analysed. Oxidation and in-situ ¨ nitridation were characterized by a LECO analyser and thermal desorption measurements, whereas Mossbauer spectroscopy was used to study the mechanical alloying of the Al alloy. The results show that a metastable solid solution of Mg and Fe in Al is formed during cryomilling. The occurrence of mechanical alloying in spite of the very low milling temperature is interpreted as a consequence of the fine crystallite size of the Al matrix. 2001 Elsevier Science B.V. All rights reserved. ¨ spectroscopy Keywords: Alloys; Nanostructures; Mechanical alloying; Mossbauer
1. Introduction Al /AlN composites are a new class of materials which exhibit interesting properties compared with the Al / SiC or Al /Al 2 O 3 composites [1,2]. Fabricated by powder metallurgy [2] or squeeze-casting [3,4] processes, they have a high thermal conductivity, a low density, a high strength and a relatively low coefficient of thermal expansion, which makes them suitable for use in the microelectronics industry. Moreover, previous work has shown that these composites can present superplasticity [5,6]. A very fine microstructure is necessary to obtain good properties in forming. The present study reports the cryomilling of a mixture of 80 vol% 5000 Al alloy (Al–3 wt% Mg, Eckart Poudmet) and 20 vol% AlN (Cernix [7]). The use of liquid nitrogen prevents problems induced by heating during milling and leads more rapidly to a smaller crystallite size [8]. With the Mg being initially in the form of the Al 12 Mg 17 phase, one of the aims was to realize mechanical alloying of the Al alloy during cryomilling. However, the very low tempera-
*Corresponding author.
ture (about 21968C) makes diffusion difficult. One can then ask if the dissolution of Mg in the Al matrix will occur during cryomilling. In previous work [9], mechanical alloying was studied by following the evolution of the Al lattice parameter, which was expected to increase due to the dissolution of Mg in the Al matrix. Actually, the study showed a very limited increase of the Al lattice parameter during the first 10 h of milling, whatever the temperature [room (1258C) or cryogenic (21968C)]. Moreover, in the case of a powder with a fine crystallite size (previously cryomilled for 6.5 h, thus reducing the crystallite size from about 500 to 50 nm), the Al lattice parameter decreased linearly and the rate of decrease was faster when the temperature was higher [10]. One interpretation for this effect was the possibility of the dissolution of Fe originating from the wear of the steel milling tools. This study focused on the effect of contamination during ¨ cryomilling. Mossbauer spectroscopy was used to characterize the local environments of Fe in the cryomilled powder. The main parameters, such as milling time and temperature, were considered. Subsequently, the oxidation and in-situ nitridation, which also involve a change in the chemical composition of the system, were analysed.
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01303-7
C. Goujon et al. / Journal of Alloys and Compounds 315 (2001) 276 – 283
2. Experimental conditions Cryomilling was carried out in an attritor made of 316L steel (0.03% C, 17% Cr, 13% Ni, 3% Mo) and containing 4 kg of 100C6 steel (1% C, 1.5% Cr) balls. The system could operate at different temperatures: at 21968C, in liquid nitrogen (‘‘cryomilling’’), or at room temperature, with argon gas. The rotation speed of the main arm of the attritor and the ball-to-powder weight ratio were optimized respectively at 500 rpm and 40. Stearic acid (3 wt%) was added to the powder mixture as a process control agent. More details of the characteristics of the milling system can be found in Ref. [9]. After 6 h of cryomilling, the Al composites had a spherical shape with a mean diameter of 50 nm. Those of AlN had an anisotropic shape and the apparent crystallite size perpendicular to the [hk0] direction was about 40 nm. Milling at room temperature led to a larger Al crystallite size (.500 nm), whereas no significant effect of temperature on the crystallite size of AlN was observed [9]. Atomic absorption spectrometry was used to determine the contamination of the powders with Fe, Cr and Ni. 57 Fe ¨ Mossbauer spectra were obtained at room temperature in transmission geometry with a spectrometer operated in the conventional constant acceleration mode. A 57 Co source in Rh with a strength of |20 mCi was used. The spectra were analysed both by fitting the positions, amplitudes and full widths at half maximum (FWHM) of Lorentz lines with a conventional least-squares method and by fitting them with ¨ a constrained Hesse–Rubatsch method [11] that extracts the isomer shift distribution from an experimental spectrum, a fit which may equivalently be looked at as a way of removing Lorentzian broadening from a spectrum (Section 7.2 of Refs. [12,13]). For example, when a Lorentzian line with FWHM GM 5 0.22 mm / s is removed from a calibration spectrum of metallic bcc iron, the inner lines of which have FWHM GL 5 0.24 mm / s, a line well approximated by a Gaussian line of FWHM GG 5 0.13 mm / s remains. The latter deconvolution method thus provides a significant and welcome improvement in resolution which facilitates the analyses of spectra. As usual, the 57 Fe
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isomer shifts are given with respect to a-Fe at room temperature. Oxygen and nitrogen analyses were carried out using a LECO TC 436 analyser. Samples were prepared in an argon-filled glovebox to prevent oxidation of the powders before analyses. In order to characterize oxidation and nitridation, thermal desorption measurements up to 6008C were also carried out at a heating rate of 208C / min and the degassed species were analysed by a mass spectrometer. Microstructural investigations were performed using SEM (Jeol JSM 6400) and HREM (Jeol 2010 and Topcon 002B microscopes) equipped with a Kevex EDS nanoanalyser (2 nm resolution). The TEM samples were prepared by mixing the powder with n-butanol and placing it into a 3 mm diameter carbon-coated TEM grid. XRD measurements were performed with a Siemens D5000 high resolution powder diffractometer. Monochromatic Cu Kb X-rays ( l 5 0.13922 nm) were obtained with a secondary focusing monochromator (graphite) and the Al lattice parameter was determined with a leastsquares refinement program.
3. Results
3.1. Contamination by the milling tools 3.1.1. Characterization of the contamination The wear of the steel milling tools (attritor and balls) leads to the presence of Fe, Cr and Ni in the cryomilled powders. Fig. 1 presents the evolution of the elemental contents vs. milling time. Contamination occurs during the first 2 h of milling, when the powder is still very abrasive. The contents then reach constant values of about 3.00 wt% for Fe, 0.10 wt% for Cr and 0.04 wt% for Ni. Contamination by Fe is particularly high, probably because of the brittle behaviour of the steel balls in liquid nitrogen. However, the %Cr / %Ni ratio is similar to that in 316L steel (close to 2); the wear could come from both the attritor and the balls. SEM maps of Al and Fe, prepared for the case of a 6 h cryomilled powder, show that the Fe
Fig. 1. Evolution of Fe, Cr and Ni contents during cryomilling.
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from the balls (88% Fe). The central peak with an isomer shift of 20.1160.01 mm / s (indicated by the solid arrow in Fig. 3) characterizes Fe in 316 L stainless steel from the attritor. The hyperfine magnetic fields of the two main magnetic sextets seen in the figure are 33.560.1 and 30.860.1 T, as expected for Fe atoms in 100C6 steel (balls). These two sextets are mainly due to Fe atoms with 0 and 1 Cr atoms as the first and second nearest neighbours, respectively. A weak central doublet (2% Fe) with a mean quadrupole splitting of 0.4860.02 mm / s and a mean isomer shift of 0.1560.02 mm / s is further due to carbides of the 100C6 steel. ¨ Fig. 4 presents the Mossbauer spectra of powders cryomilled for respectively (a) 2 h and (b) 26 h. Both spectra reveal the presence of a supplementary intense line centered at 0.4060.01 mm / s which is arrowed in the figure. Its area becomes larger with milling time. Fig. 4a further shows a broad featureless magnetic contribution which changes the overall background shape when comparing the spectrum of Fig. 4a with that of Fig. 4b. The latter contribution is not associated with a phase found in steel as seen from Fig. 3, but results from an alloy formed between Fe and elements from the milled powders which is not detected by XRD and could be amorphous. We conclude that the contamination from the milling tools
Fig. 2. SEM maps of (a) Al and (b) Fe in a 6 h cryomilled powder.
distribution is homogeneous in the aluminium agglomerates (Fig. 2). ¨ 3.1.2. Mossbauer spectroscopy ¨ Fig. 3 shows the room temperature Mossbauer spectrum of a mixture of powders from the attritor (12% Fe) and
57 ¨ Fig. 3. Room temperature Fe Mossbauer spectrum of a mixture of powders from the milling tools (attritor and balls).
Fig. 4. Room temperature 26 h cryomilled powder.
57
¨ Fe Mossbauer spectra of (a) a 2 h and (b) a
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reacts partly with the milled powders to form alloys or compounds. It is difficult to identify the compound(s) which contribute(s) to the paramagnetic line at 20.40 mm / s solely ¨ from the Mossbauer spectra as Fe-containing carbides (stearic acid is a possible source of carbon) or nitrides (possibly Fe 2 N) may be formed during milling. Further, a non-magnetic solid solution of Fe in Al or an Al–Fe intermetallic compound must also be considered. As none of the previous compounds is identified from X-ray diffraction patterns, it seems reasonable to assume that the main contribution comes from a solid solution of Fe in Al. In any case, it is possible to follow the evolution of the spectral areas due to the broad magnetic contribution, called ‘‘magnetic’’ hereafter, and to the non-magnetic contribution, called ‘‘paramagnetic’’ hereafter, after removal of the known spectral components originating from the unreacted contaminating fragments. Fig. 5 presents the milling time evolution of the Fe fractions in the ‘‘magnetic’’ and ‘‘paramagnetic’’ components deduced from the corresponding spectral areas when we assume that all phases have equal Lamb– ¨ Mossbauer factors. Note that, after the first 2 h of cryomilling, the total amount of Fe does not change (see Fig. 1). At this stage, 16 at% Fe is alloyed to Al: 10 at% in the ‘‘magnetic’’ form and 6 at% in the ‘‘paramagnetic’’ form. After 26 h of cryomilling, the Fe content of the ‘‘paramagnetic’’ component increases linearly to 15 at%, whereas the Fe content of the ‘‘magnetic’’ form decreases to 6 at%. This graph indicates that Al is progressively alloyed with Fe originating from the milling balls to form
Fig. 5. Evolution of the ‘‘paramagnetic’’ and ‘‘magnetic’’ Fe contents vs. milling time in liquid nitrogen.
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an Fe-rich phase and then an Al-rich phase. Considering absolute values, the ‘‘paramagnetic’’ Fe curve has a larger (positive) slope than the ‘‘magnetic’’ Fe curve (negative slope), suggesting that Fe in solution also comes from the attritor.
3.1.3. Calculation of the Fe and Mg contents dissolved in the Al matrix Considering the measures of Fe content dissolved in the Al matrix (corresponding to the ‘‘paramagnetic’’ fraction ¨ obtained from Mossbauer spectra) and of the total Fe content (measured by atomic absorption spectrometry), one can calculate the expected value of the lattice parameter of Al, if the Vegard law holds for the considered alloy. Moreover, XRD analyses lead to the real value of the lattice parameter of Al. By comparing these two values, it is possible to estimate the Mg content in solid solution in Al and to see if the dissolution of Mg has actually occurred during cryomilling. However, we note that the calculation overestimates the contents of Fe and Mg dissolved in the Al matrix, because the ‘‘paramagnetic’’ components can also be partly associated with Fe in other phases such as carbides, nitrides or Al–Fe intermetallics. 3.1.4. Influence of milling time on the dissolution of Fe and Mg in the Al matrix Fig. 6 presents the evolution of the Fe and Mg contents dissolved in the Al matrix during cryomilling according to the previous calculation. The evolution of the lattice parameter of Al is also shown. The content of dissolved Fe increases linearly up to 1.5 wt% after 26 h of cryomilling. The Mg content also increases during the first 6 h of milling but remains constant at about 0.8 wt% during the following hours. Therefore, the formation of a metastable solid solution of
Fig. 6. Evolution of the Al lattice parameter (from XRD), Fe content ¨ (from Mossbauer spectroscopy) and Mg content (calculated from the Vegard law) dissolved in the Al matrix vs. milling time in liquid nitrogen.
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Mg and Fe in Al during cryomilling is found to be possible.
3.1.5. Influence of temperature Fig. 7 shows the total content of Fe in the powder (wt%), the percentage of dissolved Fe compared with the total Fe (%) and the total amount of dissolved elements (Mg1Fe) (wt%). Millings in liquid nitrogen and at room temperature (with argon gas) were studied and two kinds of powders, with fine (50 nm) or coarse (500 nm) crystallite size, were compared [9]. In the case of the coarse crystallite size powders, the total content of Fe and the percentage of dissolved Fe are higher when the milling is performed at room temperature. The calculation leads to a larger content of dissolved Mg and, finally, the total amount of dissolved elements (Mg1 Fe) is 3.5 times higher when powders are milled at room temperature. For the powders with a fine crystallite size, the total Fe content is similar for the two milling temperatures. The
percentage of dissolved Fe (compared with the total Fe) is higher in powders milled at room temperature, but the calculation leads to a smaller percentage of dissolved Mg. In conclusion, the total weight of dissolved elements (Mg1Fe) does not depend on the milling temperature. The last observation is a comparison between powders milled at room temperature with different crystallite sizes (represented in black or white in Fig. 7). The total amount of dissolved elements (Mg1Fe) is higher in the case of a powder with a coarse crystallite size, but it is related to a total Fe content two times larger. If the percentages of dissolved Fe are compared, it is much larger in the case of a powder with a fine crystallite size. This suggests that Fe diffusion increases when the crystallite size is finer.
3.2. Oxidation With the oxidation of milled aluminium powders being very easy, it is interesting to see whether or not the use of liquid nitrogen as milling medium limits this oxidation. The initial oxygen content of the mixture was about 1.3 wt%: 0.3 wt% comes from the 5000 Al alloy powder, 0.7 wt% comes from the AlN powder and 0.3 wt% comes from stearic acid.
3.2.1. Influence of milling time and temperature Fig. 8 presents the evolution of the oxygen content vs. milling time in liquid nitrogen. In the first 2 h of cryomilling, the oxygen content is quadrupled. It then increases linearly at a slower rate during the following hours. To study the influence of temperature, the oxygen contents of powders milled 10 h either in liquid nitrogen or at room temperature were compared (Table 1). It appears that cryomilling leads to oxidation of the powders almost two times larger than milling at room temperature.
Fig. 7. Total Fe content (wt%), percentage of dissolved Fe compared with total Fe content (%) and amount of dissolved elements (Mg1Fe) (wt%) at room or cryogenic milling temperature, in the case of powders with fine or coarse crystallite size.
Fig. 8. Evolution of the oxygen content in the cryomilled powders vs. milling time.
C. Goujon et al. / Journal of Alloys and Compounds 315 (2001) 276 – 283 Table 1 Oxygen content (wt%) of powders milled for 10 h at cryogenic (21968C) or room temperature (1258C) Milling time (h)
21968C (wt%)
1258C (wt%)
0 10
1.3 7.3
1.3 4.7
3.2.2. Nature of the oxides Two kinds of oxides are observed in the cryomilled powders: 1. H 2 O thermal desorption measurements performed on unmilled and 6 h cryomilled powders show the presence of two peaks for the cryomilled powder only (Fig. 9). The first peak occurs at about 1508C and is probably
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due to the degassing of adsorbed H 2 O. The second peak occurs at about 2408C and may be related to the hydrolysis of the Al and AlN phases [14,15]. 2. HREM observation of a 6 h milled powder shows the presence of a 3 nm thick layer on the surface of the dense Al aggregates (Fig. 10). Moreover, Fig. 11 presents EDS analyses of O, Al and Mg vs. distance from the surface of the Al aggregates. The increase of the oxygen content close to the surface suggests that the layer is amorphous alumina.
3.3. In-situ nitridation The evolution of the nitrogen content in the powder during cryomilling is shown in Fig. 12 (the initial content for a mixture with 20 vol% AlN is 7.2 wt%). It increases linearly to about 9.5 wt% after 26 h of cryomilling.
Fig. 9. Thermal desorption rate of H 2 O in unmilled and 6 h cryomilled powders.
Fig. 10. Three nanometer thick layer of amorphous alumina observed by HREM on the surface of dense aggregates of Al after 6 h of cryomilling.
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Fig. 11. EDS analyses of O, Al and Mg from the surface to the center of Al aggregates.
This in-situ nitridation suggests the formation of aluminium oxynitrides [16] or AlN nanoparticles [17,18]. However, measurements of nitrogen thermal desorption exhibits, in the case of a cryomilled powder only, a nitrogen desorption which starts at 3008C and has its highest rate at about 4408C (Fig. 13). The existence of this peak is probably related to the decomposition of a metastable nitride. Fe 2 N, the decomposition temperature of which is 4008C [19], could be formed during cryomilling. This hypothesis is supported by the fact that, in a powder with a smaller Fe content, no thermal desorption of nitrogen is observed in spite of a higher nitridation (1.1 wt% instead of 0.2 wt%, therefore five times higher). This suggests that, in this case, a stable phase such as AlON or AlN is formed. By assuming that all the Fe is in the Fe 2 N form, one can evaluate the nitrogen content in stable and metastable nitrides. For example, in the 26 h cryomilled powder, the calculation leads to 7.4% N in Fe 2 N and 16% N in stable nitrides formed during cryomilling. However, this last value is underestimated because Fe is not only present in the Fe 2 N form but also in solid solution or as an intermetallic Al–Fe phase.
Fig. 12. Evolution of the nitrogen content in the cryomilled powders vs. milling time.
Fig. 13. Thermal desorption rate of nitrogen in unmilled and 6 h cryomilled powders.
4. Discussion In previous work [9], it was shown that the cryomilling of an Al–Mg–AlN mixture comprises two main stages: the first is characterized by a sharp decrease of the crystallite size of Al and AlN and the second corresponds to a chemical homogenization with no further significant variations of the crystallite size. Here, Fe contamination of the powders during cryomilling is also characterized by two main stages. Note that the duration of each stage depends on the energetic conditions of milling. During cryomilling, the high power of impacts allows the alloying of Al and Fe. Fe-rich solid solution, ¨ characterized by a ‘‘magnetic’’ Mossbauer spectrum, is mainly present in the first 2 h of cryomilling. During this first stage, Mg starts to dissolve in the Al matrix (the exact content cannot be determined), but the initial Al 12 Mg 17 phase still exists, as shown in previous work [9]. In the second stage of milling (chemical homogenization), Al continues dissolving in the Fe-rich solid solution leading progressively to an Al-rich solid solution, characterized by ¨ a ‘‘paramagnetic’’ Mossbauer spectrum (with an intense peak at 0.40 mm / s). During this last stage, the dissolution of Mg occurs until 6 to 8 h of cryomilling and then stops. In the case of coarse crystallite size powders, the total amount of dissolved elements (Mg1Fe) is 3.5 times higher under room temperature conditions than at 21968C. However, in the case of fine crystallite size powders, the total amount of dissolved elements (Mg1Fe) does not depend on the temperature. These observations are in accordance with the diffusion laws: to increase diffusion, a higher temperature and / or shorter diffusion paths are necessary. In the case of Al, cryomilling leads to a very fine crystallite size (down to 50
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nm) which allows some dissolution in spite of the very low temperature. However, the milling temperature remains the predominant parameter. Indeed, milling at room temperature makes mechanical alloying easier, although the crystallite size is much larger. With respect to the oxidation of the powders, the effect of cryomilling is not clear in the literature. According to Huang et al. [20] the use of liquid nitrogen limits oxidation, whereas according to Aikin et al. [17] cryomilling makes oxidation easier. The observations of this work support the fact that oxidation mainly occurs after cryomilling, probably during the reheating of the mill. Indeed, the very low temperature makes the adsorption of H 2 O molecules easier (as the effect of a liquid nitrogen trap on a vacuum pump). During cryomilling, H 2 O is in a solid state and oxidation is not possible. But during reheating, H 2 O becomes liquid and oxidation is expected to occur. Moreover, the presence of oxygen in the center of the Al aggregates suggests that in-situ oxidation also takes place. We can also assume that oxidation of Mg occurs, probably leading to the formation of amorphous phases, with compositions similar to MgO or MgAl 2 O 4 .
5. Conclusions The conclusions of this investigation can be summarized as follows: 1. The wear of the steel milling tools leads to contamination of the cryomilled powders with Fe, Cr and Ni. This contamination essentially occurs during the first 2 h of cryomilling. 2. The formation of a metastable solid solution of Fe and ¨ Mg in Al is clearly established by Mossbauer spectroscopy. Even if temperature is the most important parameter in the occurrence of mechanical alloying, the very fine crystallite size obtained by cryomilling allows some dissolution of these elements in the Al matrix. 3. Oxidation is not negligible and mainly occurs after cryomilling, during the reheating of the mill. 4. In-situ nitridation takes place during cryomilling at a constant rate. The chemical composition of the nitrides depends on the amount of Fe contamination, but stable nitrides (AlN or AlON) were formed during cryomilling.
Acknowledgements The authors thank F. Bernard and J.C. Niepce from the ´ ´ des Solides’’ at Dijon for Research Laboratory ‘‘Reactivite
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´ the XRD measurements, M. Chedru, J. Vicens and J.L. Chermant from LERMAT at Caen for the SEM and HREM observations, and P. Verdier and Y. Laurent from ´ the Laboratory ‘‘Verres et Ceramiques’’ at Rennes for the use of the LECO analyser.
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