X-ray Spectroscopic Analysis of Solid State Reaction during Mechanical Alloying of Molybdenum and Graphite Powder Mixture

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1 Materials Transactions, Vol. 43, No. 9 (2002) pp to 2296 c 2002 The Japan Institute of Metals X-ray Spectroscopic Analysis of Solid State Reaction during Mechanical Alloying of lybdenum and Powder Mixture Kazutoshi Yamada 1, Teruo Takahashi 1, Muneyuki toyama 1 and Hiroshi Nagai 2 1 Hyogo Prefectural Institute of Industrial Research, Kobe , Japan 2 Department of Materials Science and Processing, Graduate School of Engineering, Osaka University, Suita , Japan lybdenum and graphite powders were mechanically alloyed. Carbon K X-ray emission spectra of the mechanically alloyed powders were measured using electron probe microanalyzer (EPMA) in order to investigate the solid state reaction process. In the early stage of the mechanical alloying ( 36 ks), graphite did not react with molybdenum, but particle size of graphite became smaller. In the next stage of mechanical alloying (36 ), graphite react with molybdenum gradually as the time increases. lybdenum carbides were formed on mechanical alloying for. The mechanically alloyed powders for were heat-treated in a vacuum. 33 at%c system heat-treated at was a mixture of and molybdenum, while 50 at%c system heat-treated was a mixture of, mechanically ground graphite and graphite. (Received May 20, 2002; Accepted July 22, 2002) Keywords: carbon x-ray emssion spectrum, chemical state analysis, mechanical alloying, solid state reaction, epma, carbide 1. Introduction It is a matter of general knowledge that phases of molybdenum carbide are C and. The stable phase is at room temperature. 1) Generally sintered hard alloy is WC Co alloy that is high price because W is rare metal and military material. At the end of 1970s, Hara and Asai made an experimental W change to at a sintered hard alloy. 2) lybdenum carbides were useful for particle dispersion strengthened alloy. Therefore, molybdenum carbides were made from elemental and graphite by mechanical alloying (MA). MA, which was invented by Benjamin in ) as a solid state alloying process by ball milling, has aroused a great deal of interest until today because of its high potentiality in developing new materials, such as dispersion strengthened materials, intermetallic compounds and amorphous alloys. Yamada et al. 4 8) tried to analyze the solid state reaction during MA. It is not easy to clarify the solid state reaction process during MA using conventional X-ray diffractometry since the X- ray diffraction patterns of the MA powders are broadened. The characteristic X-ray emission spectra give information on chemical bonding state, such as valence and coordination number of elements. 9) The carbon K X-ray emission spectra are especially sensitive to chemical bonding state. 10) Therefore, the structural state analysis of carbon by an electron probe microanalyzer (EPMA) is carried out in order to investigate the mechanism of solid state reaction between graphite and molybdenum during MA. 2. Experimental Starting materials were commercially available powders of pure (5 µm in average particle size measured by Brunauer, Emmett, Teller method), pure graphite (2.2 µm). Figure 1 shows the scanning electron micrographs of molybdenum and graphite powders. The composition ratio of molybdenum and graphite was 1:1 or 2:1. Mixtures of molybdenum and graphite powders of 20 g total weight were mechanically alloyed up to under argon atmosphere in Spex-8000 attrition ball mill. The powders were heated in a vacuum at 873 for 3.6 ks after MA. The X-ray diffraction patterns of the mechanically alloyed and heated powders were measured with monochromatized Cu Kα X-rays. When incidence angle was 0.3, X-ray penetration depth was 10 nm. 11) The carbon K X-ray emission spectra were measured by an EPMA (Shimadzu: EPMA- 1500) equipped with lead stearate (2d = 10.2 nm) as an analyzer crystal. The electron acceleration potential was 15 kev and the sample current was 0.1 µa. The electron beam diameter was 100 µm and X-ray emission depth was 3 µm. The X-rays were detected at a take-off angle of 52.5 degrees with a gas flow proportional counter. The carbon K X-ray emission spectra were measured between 9.3 and 8.6 nm in wavelength. Measured carbon K X-ray emission spectra were the secondorder reflection. Therefore, wavelength divided by two gave first-order reflection. 3. Results and Discussion at%c system Figure 2 shows the X-ray diffraction patterns of mechanically alloyed powders of 33 at%c system. The X-ray diffraction lines of graphite were disappeared for the powder mechanically alloyed for 36 ks. The X-ray diffraction lines of became gradually broader with the increase of the time up to. The broadened X-ray diffraction pattern of is detected for the powder mechanically alloyed for. Figure 3 shows the carbon K X-ray emission spectra of graphite, and mechanically ground graphite (MGG) for, as reference materials. The MGG is like amorphous graphite by X-ray diffraction pattern and selected area electron diffraction pattern. The carbon K X-ray emission spectrum of C is not shown in Fig. 3 because C powder is not commercially available. Figure 4 shows the carbon K X-ray emission spectra of 33 at%c mechanically alloyed powders. The spectral shape of the powder after MA for 36 ks

2 X-ray Spectroscopic Analysis of Solid State Reaction during Mechanical Alloying 2293 Fig. 1 Scanning electron micrographs of starting materials; (a) molybdenum, (b) graphite. 36 ks Mechanically ground graphite (MGG) Fig. 2 X-ray diffraction patterns of 33C powders mechanically alloyed for 36, 72, 144 and. was similar to that of MGG shown in Fig. 3; suggesting that graphite was only ground and has not yet react with. The spectral shape of the powder after MA for was similar to that of. The spectral shape of the powders mechanically alloyed for and is affected from that of MGG and. It is thought that the spectrum of the powders after MA for and might be the mixture of the spectrum of mechanically ground graphite and. The powders after MA for were heated in a vacuum at for 3.6 ks. Figure 5 shows the X-ray diffraction patterns of mechanically alloyed for and subsequently heated powders. Figure 6 shows carbon K X-ray emission spectra of mechanically alloyed for and subsequently heated powders. The X-ray diffraction pattern changed from broad to sharp, but carbon K X-ray emission spectral shape did not change. It is considered that only the crystal size of became larger by heat-treatment. The X-ray diffraction Fig. 3 Carbon K X-ray emission spectra of graphite, and mechanically ground graphite (MGG). lines of were appeared in the diffraction pattern of heated powder. It is thought that composition of the formed by MA changed to that of 2 X C. Therefore, surplus is considered to precipitate during heat-treatment at%c system Figure 7 shows the X-ray diffraction patterns of mechanically alloyed powders of 50 at%c system. The X-ray diffraction lines of graphite disappeared for the powder mechanically alloyed for 14.4 ks. The X-ray diffraction lines of became gradually broader with the increase of the time up to. The X-ray diffraction pattern of the powder mechanically alloyed for was corresponded to the broadened X-ray diffraction pattern of C. Figure 8 shows the carbon K X-ray emission spectra of 50 at%c mechani-

3 2294 K. Yamada, T. Takahashi, M. toyama and H. Nagai C 0 ks 36 ks 14.4 ks Fig. 7 X-ray diffraction patterns of 50C powders mechanically alloyed for 14.4, 72, 144, and. Fig. 4 Carbon K X-ray emission spectra of 33C powders mechanically alloyed for 0, 36, 72, 144, and. 0 ks 14.4 ks Fig. 5 X-ray diffraction patterns of mechanically alloyed for and subsequently heated 33C powders. Fig. 8 Carbon K X-ray emission spectra of 50C powders mechanically alloyed for 0, 14.4, 72, 144 and. Fig. 6 Carbon K X-ray emission spectra of mechanically alloyed for and subsequently heated 33C powders. cally alloyed powders. The spectral shape of the powder after MA for 14.4 ks was similar to that of MGG; suggesting that graphite was ground and had not reacted with molybdenum. The spectral shape of the powder after MA for differs from those of reference materials shown in Fig. 3. It is obvious from X-ray diffraction pattern that the spectral shape of the mechanically alloyed powder for is that of C. The mechanically alloyed powder for was heated at 873 for 3.6 ks. The X-ray diffraction patterns of powders heated at different temperatures are shown in Fig. 9. The X-ray diffraction patterns differed at each heated temperature. The X-ray diffraction pattern of the powder heated at 873 K was similar to that of the powder. The X-ray diffraction lines of appeared in the X-ray diffraction patterns of the heated powders at 1073 and. Especially, the

4 X-ray Spectroscopic Analysis of Solid State Reaction during Mechanical Alloying 2295 (a) Intensity (arbitary units) 1073 K 873 K : 0 : 10 2 : 8 4 : 6 6 : Fig. 9 X-ray diffraction patterns of mechanically alloyed for and subsequently heated 50C powders. 8 : 2 10 : 0 (b) Mechanically ground graphite (MGG) 1073 K : MGG 0 : 10 2 : 8 4 : 6 6 : 4 8 : K 10 : 0 Fig. 11 Synthesized C K X-ray emission spectra; (a) with graphite, (b) with mechanically ground graphite (MGG) Wavelength, 4.50 /nm 4.55 Fig. 10 Carbon K X-ray emission spectra of mechanically alloyed for and subsequently heated 50C powders. X-ray diffraction pattern of the mechanically alloyed powder for subsequently heated at was corresponded to that of. The carbon K X-ray emission spectra of powders heated at different temperatures are shown in Fig. 10. The carbon K X-ray emission spectra of the powders heated at different temperature are also different from each other. The carbon K X-ray emission spectrum of the mechanically alloyed powder for subsequently heated at was not similar to that of. It is considered that the mechanically alloyed powder for subsequently heated at was composed of a mixture of and the other. Figure 11 show the carbon K X-ray emission spectra of mixture of spectra of (a) and graphite, and of (b) and MGG. To explain measured carbon K X-ray emission shapes, many researchers performed a theoretical spectral shape anal- ysis, using discrete variational Xα molecular orbital calculations ) We contemplated the spectral shapes by comparison method because it is easy and convenient method. Any calculated carbon K X-ray emission spectrum shown in Figs. 11(a) and (b) does not correspond to carbon K X-ray emission spectrum of the mechanically alloyed powders for subsequently heated at. X-ray diffraction lines of graphite were disappeared as seen in Fig. 9. Therefore, it is considered that this powder might be a mixture such as small quantity of graphite and large quantity of MGG with. Figure 12 shows the carbon K X-ray emission spectrum of, MGG, graphite and calculated carbon K X-ray spectrum. The composition ratio of, MGG and graphite was 5:4:1. The calculated carbon K X-ray emission spectrum is similar to that of the mechanically alloyed for subsequently heated at. It was thought that C changed into and carbon that was mixture of large quantity of MGG and small quantity of graphite by heat-treatment.

5 2296 K. Yamada, T. Takahashi, M. toyama and H. Nagai Wavelength, 4.50 /nm Mechanically ground graphite 4.55 : MGG : = 5 : 4 : 1 Fig. 12 Synthesized C K X-ray emission spectra of, graphite and mechanically ground graphite (MGG). 4. Conclusions Carbon K X-ray emission spectra of mechanically alloyed molybdenum and graphite powders were measured by an EPMA. The solid state reaction during the mechanical alloying and heating process were analyzed by both X-ray emission spectra and X-ray diffraction. The results obtained are summarized as follows: (1) In the early stage (such as 14.4 or 36 ks) of the mechanically alloying, graphite did not react with, but the particle size of graphite became finer. (2) The formed by MA was crystallized by heattreatment. (3) The C formed by MA changed into and carbon, which was mixture of large quantity of MGG and small quantity of graphite by heat-treatment. REFERENCES 1) H. Suzuki: Cemented carbides and sintered hard materials, (Maruzen, Japan, 1986) p ) A. Hara and T. Asai: Bulletin of the Japan Institute of Metals 18 (1979) ) J. S. Benjamin: Metal. Trans. 1 (1970) ) K. Yamada, T. Takahashi and M. toyama: J. Japan Inst. Metals 60 (1996) ) K. Yamada, T. Kaneyoshi, T. Takahashi and M. toyama: Adv. X-ray Chem. Anal. Japan 28 (1997) ) K. Yamada, T. Takahashi and M. toyama: Adv. X-ray Chem. Anal. Japan 29 (1998) ) K. Yamada, T. Takahashi and M. toyama: Spec. Chem. Acta B 54 (1999) ) K. Yamada, T. Takahashi and M. toyama: J. Japan Inst. Metals 63 (1999) ) H. Soezima: Electron Probe Microanalysis, (Nikkan Kogyo Shinbun, Japan, 1987) p ) M. toyama and G. Hashizume: J. Spectroscopic. Soc. Japan 29 (1980) ) H. Izumi, F. O. Adurodija, T. Kaneyoshi, T. Ishihara, H. Yoshioka and M. toyama: J. Appl. Phys. 91 (2002) ) H. Adachi, M. Tsukada and C. Satoko: J. Phys. Soc. Jpn. 45 (1978) ) J. Kawai: Phys. Rev. B 47 (1993) ) Y. Muramatsu, S. Hirono, S. Umemura, Y. Ueno, T. Hyashi, M. M. Grush, E. M. Gullikson and R. C. C. Perera: Carbon 39 (2001)