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Microstructural evolution during the formation of Ti3AlC2
Materials Science and Engineering B 173 (2010) 126–129
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Microstructural evolution during the formation of Ti3 AlC2 Michiyuki Yoshida ∗ , Yasuhiro Hoshiyama, Junji Ommyoji, Akira Yamaguchi Okayama Ceramics Research Foundation, 1408-15, Nishikatakami, Bizen-shi, Okayama 705-0021, Japan
a r t i c l e
i n f o
Article history: Received 12 June 2009 Received in revised form 18 December 2009 Accepted 4 January 2010 Keywords: Ti3 AlC2 Layered carbide Microstructure Reaction mechanism
a b s t r a c t A layered ternary carbide Ti3 AlC2 was synthesized by pressureless calcining process from the mixture of Ti/Al/2TiC. Almost single phase Ti3 AlC2 was obtained after calcining at 1400 ◦ C for 1 h. The microstructural evolution during the formation of Ti3 AlC2 was examined at the temperature from 900 ◦ C to 1400 ◦ C. Based on the results of X-ray diffractometry (XRD) and energy-dispersive X-ray spectroscopy (EDS), a possible reaction mechanism was proposed to explain the formation of Ti3 AlC2 . Above the melting point of aluminum, liquid Al reacts with titanium to form the intermetallic compound of AlTi. As the temperature is increased to 1400 ◦ C, the intermetallic compound of AlTi reacts with TiC to form Ti2 AlC and then, Ti2 AlC further reacts with TiC to form the final product of Ti3 AlC2 . © 2010 Elsevier B.V. All rights reserved.
1. Introduction Titanium aluminum carbide (Ti3 AlC2 ) is one of the three ternary compounds existing in Ti–Al–C system. Ti3 AlC2 , which belongs to a family of layered ternary compounds, has attracted increasing attention owing to their unique combinative properties of both ceramics and metals [1–6]. Like metals it is thermally and electrically conductive, easy to be machined with conventional tools and resistant to thermal shock. Like ceramics it is light weight, elastically stiff, thermal stability, and retains its strength to high temperature. Since Pietzka and Schuster [7] first reported the synthesis of Ti3 AlC2 by sintering cold-compacted powder mixtures of Ti, TiAl, Al4 C3 , and C at 1300 ◦ C in H2 (g) for 20 h, different starting materials and processes have been attempted to synthesize Ti3 AlC2 [8–13]. Especially, the mixture of elemental Ti, Al and C has been employed for the synthesis of Ti3 AlC2 by many researchers due to the low processing cost [8–10]. Ti3 AlC2 exists in complex ternary systems in which several quite stable binary and other ternary phases coexist [7]. The single phase Ti3 AlC2 is very difficult to synthesize because of its very narrow phase range in Ti–Al–C ternary phase diagram. In recent years, Ge et al. [11] reported that the addition of TiC to starting mixture was beneficial for the formation of the ternary phase Ti3 AlC2 . Peng et al. [12] fabricated high pure Ti3 AlC2 powder by the pressureless calcination from the mixture of Ti/Al/2TiC. However, the detailed reaction mechanism during the synthesis of
∗ Corresponding author at: Department of Materials Science and Technology, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu City 501-1193, Japan. Tel.: +81 58 293 2566; fax: +81 58 293 2794. E-mail address: [email protected] (M. Yoshida). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.01.006
Ti3 AlC2 from the mixture of Ti/Al/2TiC is unknown. In the present study, the mixture of Ti, Al and TiC was chosen to study the reaction mechanism during the formation of Ti3 AlC2 . The purpose of present study is to elucidate reaction mechanism by studying the microstructure during the formation of Ti3 AlC2 . 2. Experimental procedure TiC (<2 m powder size, Wako Pure Chemical Industries, Ltd., Japan), Ti (<40 m powder size, 99% purity, Mitsuwa Chemicals Co., Ltd., Japan), Al (<40 m powder size, 99% purity, Mitsuwa Chemicals Co., Ltd., Japan) and graphite powders (<5 m powder size, 99% purity, Kojundo Chemical Laboratory Co., Ltd., Japan) were used as starting materials in this study. The starting materials, with stoichiometric molar ratio of Ti/Al/2TiC and 3Ti/Al/2C, were mixed in ethanol by mechanical stirring for 1 h. After drying, cylindrical compacts: 15 mm × 5 mm were prepared under the pressure of 20 MPa, followed by cold-isostatically pressing (CIP) at 100 MPa. Calcination was carried out in the graphite furnace under Ar-atmosphere (Model FVPHP-R-5, Fujidenpa Kogyo Co. Ltd., Osaka, Japan). The heating rate was controlled at 10 ◦ C/min, and calcining temperature was selected in the range of 900–1400 ◦ C and held for 0–60 min. Phase analysis of pulverized samples was performed by XRD (Model RINT2200, Rigaku Co., Tokyo, Japan) with Cu K␣ radiation at 40 kV and 40 mA. For microstructural observation, the synthesized bodies were incorporated into epoxy resin and mechanically polished (1 m diamond finish). The microstructure of carbon coated samples was observed with scanning electron microscope (Model JSM6490, JEOL, Japan), and energy-dispersive X-ray spectroscopy (Model Genesis2000, EDAX, USA).
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Fig. 2. XRD patterns of the Ti/Al/2TiC powder mixture heated to 900–1400 ◦ C for 0 min. Fig. 1. XRD patterns of the samples heated at 1200–1400 ◦ C for 1 h. (a) The mixture of Ti/Al/2TiC and (b) the mixture of 3Ti/Al/2C.
3. Results and discussion 3.1. XRD results of the calcined samples Fig. 1(a) shows the X-ray diffraction profiles of the calcined samples prepared from the mixture of Ti/Al/2TiC (abbreviate as “2TiC”) heated at the temperature from 1200 ◦ C to 1400 ◦ C for 1 h. When 2TiC sample was heated at 1200 ◦ C, the peaks of TiC, Ti2 AlC and Ti3 AlC2 were detected. With increasing temperature, the intensity of TiC decreased, while the intensity of Ti3 AlC2 increased. When 2TiC sample was heated at 1400 ◦ C, the peaks of Ti2 AlC disappeared, and the dominant peaks were Ti3 AlC2 . For a comparison, the XRD profiles of the sample prepared from the mixture of 3Ti/Al/2C (abbreviate as “0TiC”) were shown in Fig. 1(b). When 0TiC sample was heated to 1200 ◦ C for 1 h, the peaks of unreacted graphite, AlTi3 , TiC, Ti3 AlC and Ti2 AlC were detected. With increasing temperature, the relative intensity of Ti3 AlC, TiC, AlTi3 and graphite decreased. When the sample was heated to 1400 ◦ C, the peaks corresponding to Ti3 AlC2 appeared. The dominant peaks at 1400 ◦ C were Ti2 AlC and TiC. A lot of intermediate products (AlTi3 , Ti3 AlC, TiC and Ti2 AlC) were observed during the synthesis of Ti3 AlC2 from the 0TiC powder. In the case of 2TiC, the peaks corresponding to Ti3 AlC2 were observed at lower temperature than those observed
in 0TiC. The relative intensity of Ti3 AlC2 for the 2TiC sample heated at 1400 ◦ C was higher than that for the 0TiC sample. Our results showed that the addition of TiC powder to the starting mixture was beneficial to the formation of Ti3 AlC2 . Fig. 2 shows X-ray diffraction profiles of 2TiC heated to, and then immediately cooled down from 900 ◦ C, 1000 ◦ C, 1100 ◦ C, 1200 ◦ C, 1300 ◦ C and 1400 ◦ C, respectively. The dominant peaks corresponding to unreacted TiC phase were detected at all temperatures. When the sample was heated to 900 ◦ C, the peaks of the intermetallic compounds Al3 Ti, Al2 Ti and AlTi3 were detected. When the temperature was increased to 1100 ◦ C, the intermetallic compound of AlTi appeared. When the sample was heated to 1200 ◦ C, the peaks of Al3 Ti, Al2 Ti and AlTi3 disappeared, and the observed peaks for intermetallic compound were AlTi only. The main peak of Ti2 AlC at about 2 = 40◦ appeared at the temperature of 1200 ◦ C. When heated to 1300 ◦ C, the peaks of Ti3 AlC2 appeared. With increasing the temperature to 1400 ◦ C, the relative intensity of Ti3 AlC2 and Ti2 AlC increased. According to the XRD results, the main reactions during the formation of Ti3 AlC2 can be expressed as follows: Al + Ti = AlTi
(1)
AlTi + TiC = Ti2 AlC
(2)
Ti2 AlC + TiC = Ti3 AlC2
(3)
128
M. Yoshida et al. / Materials Science and Engineering B 173 (2010) 126–129
Fig. 3. Back-scatter electron images (BEI) of Ti/Al/2TiC powder mixture heated to 900–1400 ◦ C for 0 min; also shown are EDS dot maps for elemental Ti and Al taken at the same location as BEI.
The intermediate compound of AlTi reacted with TiC to form Ti2 AlC and then, Ti2 AlC further reacted with TiC to form the final product of Ti3 AlC2 . 3.2. Microstructural evolution during the formation of Ti3 AlC2 Fig. 3 shows back-scatter electron image (BEI) of the 2TiC samples heated to, and then immediately cooled down from 900 ◦ C, 1200 ◦ C, and 1400 ◦ C, respectively; also shown are EDS dot maps for elemental Ti and Al taken at the same location as BEI. The sample heated at 900 ◦ C had a bimodal distribution of the larger grains (d ≈ 50 m) and the smaller grains (d ≤ 10 m). As the temperature was increased to 1400 ◦ C, the larger grains were surrounded by the smaller grains, and the grain growth of the smaller grains occurred. EDS dot maps of the 2TiC sample heated at 900 ◦ C showed that the larger grains with d ≈ 50 m consisted of the inner core of Ti-rich intermetallic compound and the outer layer comprised of Al-rich intermetallic compound. As the temperature was increased to 1200 ◦ C, the Al was uniformly distributed in the larger grains. According to the XRD profile of the sample heated at 1200 ◦ C, the composition of intermetallic compound was AlTi. This indicated that the diffusion of Al atoms from the outer layer to the inner core occurred and the composition of these grains became single phase AlTi. When the temperature was increased to 1400 ◦ C, the Al atoms, which localized in the larger grains below the temperature of 1200 ◦ C, were also detected around the larger grains.
3.3. Reaction mechanism for the synthesis of Ti3 AlC2 from the mixture of Ti/Al/2TiC Based on the X-ray analysis and microstructural/EDS observation, a possible reaction mechanism for the formation of Ti3 AlC2 from the mixture of Ti/Al/2TiC is proposed as shown in Fig. 4. (1) Above the melting point of aluminum (660 ◦ C), a large amount of Al atoms diffuse rapidly and accumulate on the surface of Ti grain, forming Al3 Ti or Al2 Ti outer layer (at 900 ◦ C). (2) The diffusion of Al atoms from the outer layer to the inner core occurs and the composition of intermetallic grains becomes single phase AlTi (1200 ◦ C). (3) As the temperature is increased to 1400 ◦ C, AlTi reacts with TiC, forming Ti2 AlC. Then, Ti2 AlC reacts with TiC to form Ti3 AlC2 . There was the difference between 0TiC and 2TiC in the reaction route during the formation of Ti3 AlC2 (Fig. 1). In 0TiC sample, firstly formed intermediate compound of AlTi3 reacted with graphite to form Ti3 AlC and then, formed Ti3 AlC reacted with graphite to form Ti2 AlC and TiC. The final product of Ti3 AlC2 formed through the reaction between Ti2 AlC and TiC in 0TiC sample. In the case of 2TiC sample, intermediate compound of AlTi reacted with TiC to form Ti2 AlC and then, Ti2 AlC further reacted with TiC to form Ti3 AlC2 . The number of observed intermediate compounds in 2TiC during the synthesis of Ti3 AlC2 was fewer than that observed in 0TiC. The reaction route during the formation of Ti3 AlC2 changed depending on whether the free graphite was contained or not in the starting mixture.
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Fig. 4. A possible reaction mechanism during the formation of Ti3 AlC2 from the mixture of Ti/Al/2TiC.
Barsoum et al. [14] have revealed the strong relationship among the unit cells of Ti2 AlC, Ti3 AlC2 and TiC. In Ti3 AlC2 , aluminum close-packed planes separate two layers of edge-shared Ti6 C octahedral, while in Ti2 AlC one layer of aluminum close-packed plane separates one layer of edge-shared Ti6 C octahedra. If none of these aluminum planes exist, the edge-shared Ti6 C octahedral link together directly to form TiC. In the present study, ternary carbide of Ti2 AlC formed through the reaction between AlTi and TiC without any intermediate products. The addition of TiC, which has strong relationship with ternary carbides in the crystal structure, may promote the formation of ternary carbides of Ti2 AlC and Ti3 AlC2 . 4. Conclusion In this paper, we studied the microstructural evolution during the formation of Ti3 AlC2 from the mixture of Ti/Al/2TiC. Based on the X-ray analysis and microstructural/EDS observation, a possible reaction mechanism was proposed to explain the formation of Ti3 AlC2 . Above the melting temperature of aluminum, a large amount of Al atoms diffuse rapidly to Ti grain, forming the inter-
metallic compound of AlTi. As the temperature is increased to 1400 ◦ C, the intermetallic compound of AlTi reacts with TiC to form Ti2 AlC and then, Ti2 AlC further reacts with TiC to form the final product of Ti3 AlC2 . References [1] M.W. Barsoum, T. El-Raghy, Am. Sci. 89 (2001) 334–343. [2] M.W. Barsoum, H.-I. Yoo, I.K. Polushina, V.Yu. Rud, Y.V. Rud, T. El-Raghy, Phys. Rev. B 62 (2000) 10194–10198. [3] N.V. Tzenov, M.W. Barsoum, J. Am. Ceram. Soc. 83 (2000) 825–832. [4] X.H. Wang, Y.C. Zhou, Corros. Sci. 45 (2003) 891–907. [5] D.B. Lee, S.W. Park, Mater. Sci. Eng. A443 (2006) 147–154. [6] Y.W. Bao, X.H. Wang, H.B. Zhang, Y.C. Zhou, J. Eur. Ceram. Soc. 25 (2005) 3367–3374. [7] M.A. Pietzka, J.C. Schuster, J. Phase Equilib. 15 (1994) 392–400. [8] X.H. Wang, Y.C. Zhou, Acta Mater. 50 (2000) 3141–3149. [9] A. Zhou, C. Wang, Z. Ge, L. Wu, J. Mater. Sci. Lett. 20 (2001) 1971–1973. [10] Y. Khoptiar, I. Gotman, E.Y. Gutmanas, J. Am. Ceram. Soc. 88 (2005) 28–33. [11] Z. Ge, K. Chen, J. Guo, H. Zhou, J.M.F. Ferreira, J. Eur. Ceram. Soc. 23 (2003) 567–574. [12] C. Peng, C.A. Wang, Y. Song, Y. Huang, Mater. Sci. Eng. A428 (2006) 54–58. [13] A. Zhou, C. Wang, Y. Huang, Mater. Sci. Eng. A352 (2003) 333–339. [14] M.W. Barsoum, D. Brodkin, T. El-Raghy, Scr. Mater. 36 (1997) 535–541.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Microstructural evolution during the formation of Ti3 AlC2 Michiyuki Yoshida ∗ , Yasuhiro Hoshiyama, Junji Ommyoji, Akira Yamaguchi Okayama Ceramics Research Foundation, 1408-15, Nishikatakami, Bizen-shi, Okayama 705-0021, Japan
a r t i c l e
i n f o
Article history: Received 12 June 2009 Received in revised form 18 December 2009 Accepted 4 January 2010 Keywords: Ti3 AlC2 Layered carbide Microstructure Reaction mechanism
a b s t r a c t A layered ternary carbide Ti3 AlC2 was synthesized by pressureless calcining process from the mixture of Ti/Al/2TiC. Almost single phase Ti3 AlC2 was obtained after calcining at 1400 ◦ C for 1 h. The microstructural evolution during the formation of Ti3 AlC2 was examined at the temperature from 900 ◦ C to 1400 ◦ C. Based on the results of X-ray diffractometry (XRD) and energy-dispersive X-ray spectroscopy (EDS), a possible reaction mechanism was proposed to explain the formation of Ti3 AlC2 . Above the melting point of aluminum, liquid Al reacts with titanium to form the intermetallic compound of AlTi. As the temperature is increased to 1400 ◦ C, the intermetallic compound of AlTi reacts with TiC to form Ti2 AlC and then, Ti2 AlC further reacts with TiC to form the final product of Ti3 AlC2 . © 2010 Elsevier B.V. All rights reserved.
1. Introduction Titanium aluminum carbide (Ti3 AlC2 ) is one of the three ternary compounds existing in Ti–Al–C system. Ti3 AlC2 , which belongs to a family of layered ternary compounds, has attracted increasing attention owing to their unique combinative properties of both ceramics and metals [1–6]. Like metals it is thermally and electrically conductive, easy to be machined with conventional tools and resistant to thermal shock. Like ceramics it is light weight, elastically stiff, thermal stability, and retains its strength to high temperature. Since Pietzka and Schuster [7] first reported the synthesis of Ti3 AlC2 by sintering cold-compacted powder mixtures of Ti, TiAl, Al4 C3 , and C at 1300 ◦ C in H2 (g) for 20 h, different starting materials and processes have been attempted to synthesize Ti3 AlC2 [8–13]. Especially, the mixture of elemental Ti, Al and C has been employed for the synthesis of Ti3 AlC2 by many researchers due to the low processing cost [8–10]. Ti3 AlC2 exists in complex ternary systems in which several quite stable binary and other ternary phases coexist [7]. The single phase Ti3 AlC2 is very difficult to synthesize because of its very narrow phase range in Ti–Al–C ternary phase diagram. In recent years, Ge et al. [11] reported that the addition of TiC to starting mixture was beneficial for the formation of the ternary phase Ti3 AlC2 . Peng et al. [12] fabricated high pure Ti3 AlC2 powder by the pressureless calcination from the mixture of Ti/Al/2TiC. However, the detailed reaction mechanism during the synthesis of
∗ Corresponding author at: Department of Materials Science and Technology, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu City 501-1193, Japan. Tel.: +81 58 293 2566; fax: +81 58 293 2794. E-mail address: [email protected] (M. Yoshida). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.01.006
Ti3 AlC2 from the mixture of Ti/Al/2TiC is unknown. In the present study, the mixture of Ti, Al and TiC was chosen to study the reaction mechanism during the formation of Ti3 AlC2 . The purpose of present study is to elucidate reaction mechanism by studying the microstructure during the formation of Ti3 AlC2 . 2. Experimental procedure TiC (<2 m powder size, Wako Pure Chemical Industries, Ltd., Japan), Ti (<40 m powder size, 99% purity, Mitsuwa Chemicals Co., Ltd., Japan), Al (<40 m powder size, 99% purity, Mitsuwa Chemicals Co., Ltd., Japan) and graphite powders (<5 m powder size, 99% purity, Kojundo Chemical Laboratory Co., Ltd., Japan) were used as starting materials in this study. The starting materials, with stoichiometric molar ratio of Ti/Al/2TiC and 3Ti/Al/2C, were mixed in ethanol by mechanical stirring for 1 h. After drying, cylindrical compacts: 15 mm × 5 mm were prepared under the pressure of 20 MPa, followed by cold-isostatically pressing (CIP) at 100 MPa. Calcination was carried out in the graphite furnace under Ar-atmosphere (Model FVPHP-R-5, Fujidenpa Kogyo Co. Ltd., Osaka, Japan). The heating rate was controlled at 10 ◦ C/min, and calcining temperature was selected in the range of 900–1400 ◦ C and held for 0–60 min. Phase analysis of pulverized samples was performed by XRD (Model RINT2200, Rigaku Co., Tokyo, Japan) with Cu K␣ radiation at 40 kV and 40 mA. For microstructural observation, the synthesized bodies were incorporated into epoxy resin and mechanically polished (1 m diamond finish). The microstructure of carbon coated samples was observed with scanning electron microscope (Model JSM6490, JEOL, Japan), and energy-dispersive X-ray spectroscopy (Model Genesis2000, EDAX, USA).
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Fig. 2. XRD patterns of the Ti/Al/2TiC powder mixture heated to 900–1400 ◦ C for 0 min. Fig. 1. XRD patterns of the samples heated at 1200–1400 ◦ C for 1 h. (a) The mixture of Ti/Al/2TiC and (b) the mixture of 3Ti/Al/2C.
3. Results and discussion 3.1. XRD results of the calcined samples Fig. 1(a) shows the X-ray diffraction profiles of the calcined samples prepared from the mixture of Ti/Al/2TiC (abbreviate as “2TiC”) heated at the temperature from 1200 ◦ C to 1400 ◦ C for 1 h. When 2TiC sample was heated at 1200 ◦ C, the peaks of TiC, Ti2 AlC and Ti3 AlC2 were detected. With increasing temperature, the intensity of TiC decreased, while the intensity of Ti3 AlC2 increased. When 2TiC sample was heated at 1400 ◦ C, the peaks of Ti2 AlC disappeared, and the dominant peaks were Ti3 AlC2 . For a comparison, the XRD profiles of the sample prepared from the mixture of 3Ti/Al/2C (abbreviate as “0TiC”) were shown in Fig. 1(b). When 0TiC sample was heated to 1200 ◦ C for 1 h, the peaks of unreacted graphite, AlTi3 , TiC, Ti3 AlC and Ti2 AlC were detected. With increasing temperature, the relative intensity of Ti3 AlC, TiC, AlTi3 and graphite decreased. When the sample was heated to 1400 ◦ C, the peaks corresponding to Ti3 AlC2 appeared. The dominant peaks at 1400 ◦ C were Ti2 AlC and TiC. A lot of intermediate products (AlTi3 , Ti3 AlC, TiC and Ti2 AlC) were observed during the synthesis of Ti3 AlC2 from the 0TiC powder. In the case of 2TiC, the peaks corresponding to Ti3 AlC2 were observed at lower temperature than those observed
in 0TiC. The relative intensity of Ti3 AlC2 for the 2TiC sample heated at 1400 ◦ C was higher than that for the 0TiC sample. Our results showed that the addition of TiC powder to the starting mixture was beneficial to the formation of Ti3 AlC2 . Fig. 2 shows X-ray diffraction profiles of 2TiC heated to, and then immediately cooled down from 900 ◦ C, 1000 ◦ C, 1100 ◦ C, 1200 ◦ C, 1300 ◦ C and 1400 ◦ C, respectively. The dominant peaks corresponding to unreacted TiC phase were detected at all temperatures. When the sample was heated to 900 ◦ C, the peaks of the intermetallic compounds Al3 Ti, Al2 Ti and AlTi3 were detected. When the temperature was increased to 1100 ◦ C, the intermetallic compound of AlTi appeared. When the sample was heated to 1200 ◦ C, the peaks of Al3 Ti, Al2 Ti and AlTi3 disappeared, and the observed peaks for intermetallic compound were AlTi only. The main peak of Ti2 AlC at about 2 = 40◦ appeared at the temperature of 1200 ◦ C. When heated to 1300 ◦ C, the peaks of Ti3 AlC2 appeared. With increasing the temperature to 1400 ◦ C, the relative intensity of Ti3 AlC2 and Ti2 AlC increased. According to the XRD results, the main reactions during the formation of Ti3 AlC2 can be expressed as follows: Al + Ti = AlTi
(1)
AlTi + TiC = Ti2 AlC
(2)
Ti2 AlC + TiC = Ti3 AlC2
(3)
128
M. Yoshida et al. / Materials Science and Engineering B 173 (2010) 126–129
Fig. 3. Back-scatter electron images (BEI) of Ti/Al/2TiC powder mixture heated to 900–1400 ◦ C for 0 min; also shown are EDS dot maps for elemental Ti and Al taken at the same location as BEI.
The intermediate compound of AlTi reacted with TiC to form Ti2 AlC and then, Ti2 AlC further reacted with TiC to form the final product of Ti3 AlC2 . 3.2. Microstructural evolution during the formation of Ti3 AlC2 Fig. 3 shows back-scatter electron image (BEI) of the 2TiC samples heated to, and then immediately cooled down from 900 ◦ C, 1200 ◦ C, and 1400 ◦ C, respectively; also shown are EDS dot maps for elemental Ti and Al taken at the same location as BEI. The sample heated at 900 ◦ C had a bimodal distribution of the larger grains (d ≈ 50 m) and the smaller grains (d ≤ 10 m). As the temperature was increased to 1400 ◦ C, the larger grains were surrounded by the smaller grains, and the grain growth of the smaller grains occurred. EDS dot maps of the 2TiC sample heated at 900 ◦ C showed that the larger grains with d ≈ 50 m consisted of the inner core of Ti-rich intermetallic compound and the outer layer comprised of Al-rich intermetallic compound. As the temperature was increased to 1200 ◦ C, the Al was uniformly distributed in the larger grains. According to the XRD profile of the sample heated at 1200 ◦ C, the composition of intermetallic compound was AlTi. This indicated that the diffusion of Al atoms from the outer layer to the inner core occurred and the composition of these grains became single phase AlTi. When the temperature was increased to 1400 ◦ C, the Al atoms, which localized in the larger grains below the temperature of 1200 ◦ C, were also detected around the larger grains.
3.3. Reaction mechanism for the synthesis of Ti3 AlC2 from the mixture of Ti/Al/2TiC Based on the X-ray analysis and microstructural/EDS observation, a possible reaction mechanism for the formation of Ti3 AlC2 from the mixture of Ti/Al/2TiC is proposed as shown in Fig. 4. (1) Above the melting point of aluminum (660 ◦ C), a large amount of Al atoms diffuse rapidly and accumulate on the surface of Ti grain, forming Al3 Ti or Al2 Ti outer layer (at 900 ◦ C). (2) The diffusion of Al atoms from the outer layer to the inner core occurs and the composition of intermetallic grains becomes single phase AlTi (1200 ◦ C). (3) As the temperature is increased to 1400 ◦ C, AlTi reacts with TiC, forming Ti2 AlC. Then, Ti2 AlC reacts with TiC to form Ti3 AlC2 . There was the difference between 0TiC and 2TiC in the reaction route during the formation of Ti3 AlC2 (Fig. 1). In 0TiC sample, firstly formed intermediate compound of AlTi3 reacted with graphite to form Ti3 AlC and then, formed Ti3 AlC reacted with graphite to form Ti2 AlC and TiC. The final product of Ti3 AlC2 formed through the reaction between Ti2 AlC and TiC in 0TiC sample. In the case of 2TiC sample, intermediate compound of AlTi reacted with TiC to form Ti2 AlC and then, Ti2 AlC further reacted with TiC to form Ti3 AlC2 . The number of observed intermediate compounds in 2TiC during the synthesis of Ti3 AlC2 was fewer than that observed in 0TiC. The reaction route during the formation of Ti3 AlC2 changed depending on whether the free graphite was contained or not in the starting mixture.
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Fig. 4. A possible reaction mechanism during the formation of Ti3 AlC2 from the mixture of Ti/Al/2TiC.
Barsoum et al. [14] have revealed the strong relationship among the unit cells of Ti2 AlC, Ti3 AlC2 and TiC. In Ti3 AlC2 , aluminum close-packed planes separate two layers of edge-shared Ti6 C octahedral, while in Ti2 AlC one layer of aluminum close-packed plane separates one layer of edge-shared Ti6 C octahedra. If none of these aluminum planes exist, the edge-shared Ti6 C octahedral link together directly to form TiC. In the present study, ternary carbide of Ti2 AlC formed through the reaction between AlTi and TiC without any intermediate products. The addition of TiC, which has strong relationship with ternary carbides in the crystal structure, may promote the formation of ternary carbides of Ti2 AlC and Ti3 AlC2 . 4. Conclusion In this paper, we studied the microstructural evolution during the formation of Ti3 AlC2 from the mixture of Ti/Al/2TiC. Based on the X-ray analysis and microstructural/EDS observation, a possible reaction mechanism was proposed to explain the formation of Ti3 AlC2 . Above the melting temperature of aluminum, a large amount of Al atoms diffuse rapidly to Ti grain, forming the inter-
metallic compound of AlTi. As the temperature is increased to 1400 ◦ C, the intermetallic compound of AlTi reacts with TiC to form Ti2 AlC and then, Ti2 AlC further reacts with TiC to form the final product of Ti3 AlC2 . References [1] M.W. Barsoum, T. El-Raghy, Am. Sci. 89 (2001) 334–343. [2] M.W. Barsoum, H.-I. Yoo, I.K. Polushina, V.Yu. Rud, Y.V. Rud, T. El-Raghy, Phys. Rev. B 62 (2000) 10194–10198. [3] N.V. Tzenov, M.W. Barsoum, J. Am. Ceram. Soc. 83 (2000) 825–832. [4] X.H. Wang, Y.C. Zhou, Corros. Sci. 45 (2003) 891–907. [5] D.B. Lee, S.W. Park, Mater. Sci. Eng. A443 (2006) 147–154. [6] Y.W. Bao, X.H. Wang, H.B. Zhang, Y.C. Zhou, J. Eur. Ceram. Soc. 25 (2005) 3367–3374. [7] M.A. Pietzka, J.C. Schuster, J. Phase Equilib. 15 (1994) 392–400. [8] X.H. Wang, Y.C. Zhou, Acta Mater. 50 (2000) 3141–3149. [9] A. Zhou, C. Wang, Z. Ge, L. Wu, J. Mater. Sci. Lett. 20 (2001) 1971–1973. [10] Y. Khoptiar, I. Gotman, E.Y. Gutmanas, J. Am. Ceram. Soc. 88 (2005) 28–33. [11] Z. Ge, K. Chen, J. Guo, H. Zhou, J.M.F. Ferreira, J. Eur. Ceram. Soc. 23 (2003) 567–574. [12] C. Peng, C.A. Wang, Y. Song, Y. Huang, Mater. Sci. Eng. A428 (2006) 54–58. [13] A. Zhou, C. Wang, Y. Huang, Mater. Sci. Eng. A352 (2003) 333–339. [14] M.W. Barsoum, D. Brodkin, T. El-Raghy, Scr. Mater. 36 (1997) 535–541.