- Email: [email protected]
Synthesis of Ti3AlC2 ceramic by high-energy ball milling of elemental powders of Ti, Al and C
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 871–875
journal homepage: www.elsevier.com/locate/jmatprotec
Synthesis of Ti3 AlC2 ceramic by high-energy ball milling of elemental powders of Ti, Al and C C. Yang a , S.Z. Jin b , B.Y. Liang b , G.J. Liu a , S.S. Jia a,∗ a
Key Laboratory of Automobile Materials of Education and Department of Materials Science and Engineering, Jilin University, Changchun 130022, China b Department of Materials Science and Engineering, Changchun University of Technology 130012, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
Synthesis of the ternary carbide Ti3 AlC2 by high-energy ball milling of elemental Ti, Al and C
Received 7 March 2007
powders with a stoichiometric composition was tentatively investigated. The results show
Received in revised form
that high content Ti3 AlC2 was successfully obtained after ball milling of powder mixture only
27 January 2008
for 3 h. The milled products consist of powder and a coarse granule with 8 mm in diameter,
Accepted 24 February 2008
and both are mainly composed of Ti3 AlC2 with TiC as impurity based on X-ray diffractometer (XRD) and energy-dispersive spectroscopy (EDS) characterization. It is believed that a mechanically induced self-propagating reaction (MSR) was triggered to form Ti3 AlC2 and
Keywords:
TiC during high-energy ball milling process.
High-energy ball milling
© 2008 Elsevier B.V. All rights reserved.
Ti3 AlC2 MSR
1.
Introduction
As an important ceramic material, titanium aluminum carbide (Ti3 AlC2 ) has recently gained increasing attention because it possesses unusual properties combining the merits of both metals and ceramics. It is thermally and electrically conductive, resistant to high-thermal shock, low density (4.25 g/cm3 ) and easily machinable. In addition, it presents a high flexural strength, thermal stability and hightemperature oxidation resistance. In contrast to the normal brittle ceramics, Ti3 AlC2 exhibits some abnormal roomtemperature compressive plasticity (Tzenov and Barsoum, 2000). Such unique properties make this kind of materials possess wide variety of potential applications in high-tech fields, e.g. elements of chemical equipment and abrasionresistant components. For example, it can be used as a high-temperature structural material instead of expensive high-temperature alloys, etc.
∗
Corresponding author. Tel.: +86 431 85095878; fax: +86 431 85095876. E-mail address: [email protected] (S.S. Jia). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.02.056
Due to the above mentioned excellent advantages, considerable efforts have been devoted to the fabrication of Ti3 AlC2 during the past decades. Pietzka and Schuster (1994) synthesized Ti3 AlC2 powders first by sintering of cold-compacted powder mixture of TiAl, Al4 C3 and carbon in pure hydrogen for 20 h. Thence, different technologies have been developed to fabricate Ti3 AlC2 or Ti3 AlC2 -based composites, such as hot isostatic pressing (HIP) (Tzenov and Barsoum, 2000), selfpropagation high-temperature synthesis (SHS) (Lopacinski et al., 2001), combustion synthesis (CS) (Ge et al., 2003), hot pressing (HP) (Wang and Zhou, 2002), and spark plasma sintering (SPS) (Zhou et al., 2003). Although Ti3 AlC2 has been fabricated using the above mentioned methods, rigorous conditions are still necessary, such as high pressure, high temperature and long time. Therefore, it is necessary to find a new technology to synthesize Ti3 AlC2 . High-energy ball milling, also called mechanical alloying, is a powder metallurgy technique that involves the mechan-
872
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 871–875
ical milling of reactants to form product phase. It is one of the most promising technologies for obtaining compounds at room temperature. It has some advantages, including low fabrication cost, simple synthetic method and easy industrialization. Recently, it has been successfully used to synthesize various ceramic compounds or composites, such as nanocrystalline MoS2 (Patankar et al., 1993), TiC/Ti-Al composites (Ye et al., 1997) and Ti3 SiC2 (Li et al., 2002; Li and Zhai, 2005; Jin et al., 2007). So far, researches to synthesize Ti3 AlC2 powder with high-energy ball milling are still few in the literature. In this contribution, the main purpose is to investigate synthesis of Ti3 AlC2 by high-energy ball milling of the elemental Ti, Al and C powders at room temperature. The related alloying process will be described.
2.
Experimental procedure
Ti (average particle size: 80 m, purity >99.36%), Al (average particle size: 50 m, purity >99.6%) and graphite C (average particle size: 20 m, purity >99.0%) powders were used as raw materials. These powders were weighed in a mole ratio according to the stoichiometric composition of Ti3 AlC2 with Ti:Al:C = 3:1:2. The powder mixtures were put in a steel jar, which were then sealed in a glove box under argon protective atmosphere. High-energy ball milling was carried out by swinging, high-energy ball mill with cylindrical jars and grinding balls (12 mm in diameter) made from chromium steel. The rotation speed was set as 600 rpm, and the weight ratio of balls to powders was 5:1. The phase analysis of the products prepared by ball milling at different time was carried out by X-ray diffractometer with Cu K␣ (XRD) (Model Rigaku-D/Max-2500PC, Japan). The morphology and microstructure of the products were analyzed by scanning electron microscopy (SEM) (Model JSM-5600, Japan) equipped with energy-dispersive spectroscopy (EDS). It is very difficult to measure the temperature of the powders directly during ball milling. Therefore, during the ball milling, the mill was stopped at different milling times in order to measure the temperature of jar wall using a thermocouple.
3.
Results and discussion
Fig. 1 shows the XRD patterns of the Ti, Al, and C powder mixtures subjected to ball milling at various times. No new phase was formed after ball milling for 1 h. The diffraction peaks of graphite, Ti and Al broaden while their intensities decrease after 2 h of milling, suggesting that reduction of the average crystallite size, buildup of defects and formation of internal strains. After only 3 h of ball milling, all the peaks of Ti, Al and C disappeared while the peaks of TiC and Ti3 AlC2 phase (characteristic peak: 2 = 9.5◦ ) were observed, and Ti3 AlC2 was the main phase. The final products existed in the form of both powder and coarse granule. By weight, the fraction of Ti3 AlC2 in the synthesized powders was estimated to be about 83 wt.% according to the calibrated relationship between the diffraction intensity ratio of Ti3 AlC2 and TiC (Wang et al., 2005). With increasing milling time, all of the diffraction peaks became increasingly broad in width and weak in intensity. The intensity of Ti3 AlC2 peaks almost disappeared after milling for 10 h,
Fig. 1 – XRD patterns of the powders after different ball milling time.
suggesting that the crystallized Ti3 AlC2 phase probably transformed to a nanocrystallite and/or amorphous phase. Fig. 2 shows the SEM micrograph and the maps of EDS of the powder after ball milling for 3 h. From Fig. 2a, it can be seen that the products include the fine powder particles and the agglomerates. The results of elemental EDS (see Fig. 2b–d) also proved that the synthesized final products were composed of Ti3 AlC2 and TiC. Another form of the milled product is a large coarse granule as shown in Fig. 3a. The larger granule with the size of 8 mm were pulverized, and then characterized by XRD shown in Fig. 3b. It can be observed that the granule was composed of Ti3 AlC2 and TiC without other phase diffraction peaks. This rigid and porous coarse granule is very similar to the results of Ti3 SiC2 prepared by mechanical alloying (Li et al., 2002; Li and Zhai, 2005; Jin et al., 2007). Their results indicated that synthesis of Ti3 SiC2 was attributed to a mechanically induced self-propagating reaction (MSR) (Takacs, 2002). It was inferred that formation of the coarse granule in the present experiment suggested occurrence of MSR during 3 h of ball milling. SEM observation on fracture of the coarse granule is shown in Fig. 4. Observation at low magnification in Fig. 4a shows the coarse granule consists of granular grains and layered ones, Further chemical composition characterization using EDS in Fig. 5 indicates that the granular grain is composed of Ti and C elements (see Fig. 5a), indicating that the granular grains are TiC, while the typical layered grain consists of Ti, Al and C with the corresponding atomic ratio of these elements proportional to stoichiometric composition of Ti3 AlC2 (see Fig. 5b). This confirms that the granule is composed of Ti3 AlC2 and TiC, while the smaller TiC grains are surrounded by layered Ti3 AlC2 grains, in agreement with the XRD result given in Fig. 3b. Fig. 4b is an enlarged micrograph of “A” zone marked in Fig. 4a where the average diameters of layered Ti3 AlC2 grains are in the range of 3–5 m. Previous works (Lopacinski et al., 2001; Ge et al., 2003; Wang and Zhou, 2002; Zhou et al., 2003) reported that the formation temperature of Ti3 AlC2 was in the range of
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 871–875
873
Fig. 2 – SEM/EDS results of the powders after ball milling for 3 h: (a) micrograph of the powders, (b) Ti map, (c) Al map and (d) C map.
Fig. 3 – The coarse granule formed during the mechanical alloying: (a) appearance and (b) XRD pattern.
1300–1700 ◦ C by SHS, HP and SPS methods. In this experiment, Ti3 AlC2 grains were successfully obtained by high-energy ball milling, suggesting that the local temperature of the mixed powders was very high during ball milling triggered SHS. It is possible that some powder mixtures in micro zone were melted upon an exothermic reaction, and rapidly solidified to form small bulks as the reaction is finished. Based on the above results and analysis, due to the formation of Ti3 AlC2 phase and TiC phase in a short milling time of 3 h, and a phenomenon of rapid solidification occurred during ball milling, it is impossible that the products were fabricated gradually by diffusion mechanism. This alloying is completed by the interface diffusion of atoms with continuous ball milling (Suryanarayana, 2001). However, in the present work, it is believed that Ti3 AlC2 and TiC were synthesized by mechanically induced self-propagating reaction (MSR) as mentioned above. The occurrence of MSR can be confirmed by the temperature rise of the jar wall. Fig. 6 shows the temperature as a function of milling time. It was noticed that the temperature of the jar wall increased slowly as the milling time extended and then reached 39 ◦ C after milling for 2.5 h, which is mainly because of the colliding and friction between the balls and the jar wall produced mechanical energy, leading to the temperature rise of the jar wall. However, the temperature of the jar abruptly rose to around 76 ◦ C after milling for 3 h and then quickly decreased, suggesting exothermal reactions occurred. The exothermal reactions that occurred in Ti–Al–C system should be related to the formation of Ti3 AlC2 as characterized by XRD (see Fig. 1). Ye et al. (1997) reported that the exothermal reactions in Ti–Al–C system led to an increase in temperature up to 1700 K in local regions during milling. At the same time, the surface temperature of the stainless-steel container reached 70 ◦ C after milling for 210 min.
874
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 871–875
Fig. 4 – Microstructure of the fracture surface of the coarse granule (a) and (b) higher magnification micrograph taken from “A” area, marked in (a).
The alloying process of the MSR is relatively complicated. Takacs (2002) reported that during the high-energy ball milling, the MSR may be ignited after a certain activation time, as milling reduces the particle size, thoroughly mixes the components, and increases the number of chemically active defect sites. Atzmon (1990) suggested that when each particle reached a critical lamellar thickness during ball milling, the MSR is ignited and completed quickly. In the present experiment, the average crystalline size of Ti, Al, and C powders decreased gradually during milling. The MSR doesn’t occur at the beginning of the ball milling stage. The process begins with an activation period, during which the powders undergo the repeated fracture and cold welding process. Viz., the C particles were easier to be pulverized than the Al and Ti particles at the initial stage of ball milling because they were very brittle. As the milling proceeded, some of the pulverized C particles were rolled into the Ti and Al particles. Proper pulverization resulted in the refinement of particle. As the milled powders reached a defined critical size, the diffusion distance decreased and a large surface area formed, which contributed to the combustion reaction occurring. The energy induced by continuous colliding was concentrated in a small volume and finally reached the value for the triggered self-
Fig. 5 – Energy-dispersive spectroscopy analysis results in Fig. 4(a). (a) “1” granular grain and (b) “2” layered grain.
propagating reaction, the combustion reaction was ignited. The MSR was accompanied by the release of a large amount of heat, which not only made Ti, Al and C form Ti3 AlC2 , but also raised the temperature in the micro zone at a high level resulting in “sintering” small part of powders to coarse granule.
Fig. 6 – Temperature of the jar wall as a function of milling time.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 871–875
The practical MSR may be a very complicated process, and the reacting time is only several seconds. The overall reactions for the formation of products from elemental powders may be summarized as follows: Ti + C → TiC 3Ti + Al + 2C → Ti3 AlC2 It can be seen that both the listed reactions do not balance for a stoichiometric initial composition of reactants. It is possible that remaining Al existed in the synthesized products. However, the diffraction peaks of the remaining Al from the XRD pattern of the powders after milling for 3 h were not found. There are many defects (dislocations, vacancies and increased number of grain boundaries) in the compounds formed by ball milling. Therefore, it was inferred that the remaining Al may mainly dissolve into the lattice of the synthesized products. On the other hand, part of the remaining Al probably transformed to amorphous phase. Ti3 AlC2 with higher yield was prepared by high-energy ball milling. However, formation path of Ti3 AlC2 during this process remains unclear, and further work is required. In addition, high-purity Ti3 AlC2 powders probably may be obtained through adjusting the mole ratio of element powders or removing TiC by heating the powders under lower temperature, and the related work is now in progress in our group.
4.
Conclusion
In this work, Ti3 AlC2 could be synthesized using 3Ti/Al/2C powder mixture as the starting material by high-energy ball milling. The Ti3 AlC2 powders with about 83 wt.% were obtained. The formation of the final product is attributed to the occurrence of a mechanically induced self-propagating reaction during ball milling process.
Acknowledgements This work was supported by Research Foundation for the Doctoral Program of Higher Education under Grant n. 20060183058 and Scientific and Technological Development Projects of Jilin Province in China under Grant n. 20070511.
875
references
Atzmon, M., 1990. In situ thermal observation of explosive compound-formation reaction during mechanical alloying. Phys. Rev. Lett. 64, 487–490. Ge, Z.B., Chen, K.X., Guo, J.M., Zhou, H.P., Ferreira, J.M.F., 2003. Combustion synthesis of ternary carbide Ti3 AlC2 in Ti–Al–C system. J. Eur. Ceram. Soc. 23, 567–574. Jin, S.Z., Liang, B.Y., Li, J.F., Ren, L.Q., 2007. Effect of Al addition on phase purity of Ti3 Si(Al)C2 synthesized by mechanical alloying. J. Mater. Process. Technol. 182, 445–449. Li, J.F., Matsuki, T., Watanabe, R., 2002. Mechanical-alloying-assisted synthesis of Ti3 SiC2 powder. J. Am. Ceram. Soc. 85 (4), 1004–1006. Li, S.B., Zhai, H.X., 2005. Synthesis and reaction mechanism of Ti3 SiC2 by mechanical alloying of elemental Ti, Si, and C powders. J. Am. Ceram. Soc. 88 (8), 2092–2098. Lopacinski, M., Puszynski, J., Lis, J., 2001. Synthesis of ternary titanium aluminum carbides using self-propagating high-temperature synthesis technique. J. Am. Ceram. Soc. 84 (12), 3051–3053. Patankar, S.N., Xiao, S.Q., Lewandowski, J.J., Heuer, A.H., 1993. The mechanism of mechanical alloying of MoS2 . J. Mater. Res. 8, 1311–1316. Pietzka, M.A., Schuster, J.C., 1994. Summary of constitutional data on the aluminium–carbon–titanium-system. J. Phase Equilib. 15 (4), 392–400. Suryanarayana, C., 2001. Mechanical alloying and milling. Prog. Mater. Sci. 46, 1–184. Takacs, L., 2002. Self-sustaining reactions induced by ball milling. Prog. Mater. Sci. 47, 355–414. Tzenov, N.V., Barsoum, M.W., 2000. Synthesis and characterization of Ti3 AlC2 . J. Am. Ceram. Soc. 83 (4), 825–832. Wang, C.A., Zhou, A.G., et al., 2005. Quantitative phase analysis in the Ti–Al–C ternary system by X-ray diffraction. Powder Diffr. 20 (3), 218–223. Wang, X.H., Zhou, Y.C., 2002. Microstructure and properties of Ti3 AlC2 prepared by the solid-liquid reaction synthesis and simultaneous in-situ hot pressing process. Acta Mater. 50, 3141–3149. Ye, L.L., Liu, Z.G., Li, S.D., Quan, M.X., Hu, Z.Q., 1997. Thermochemistry of combustion reaction in Al–Ti–C system during mechanical alloying. J. Mater. Res. 12 (3), 616–618. Zhou, A.G., Wang, C.A., Huang, Y., 2003. Synthesis and mechanical properties of Ti3 AlC2 by spark plasma sintering. J. Mater. Sci. 38, 3111–3115.
journal homepage: www.elsevier.com/locate/jmatprotec
Synthesis of Ti3 AlC2 ceramic by high-energy ball milling of elemental powders of Ti, Al and C C. Yang a , S.Z. Jin b , B.Y. Liang b , G.J. Liu a , S.S. Jia a,∗ a
Key Laboratory of Automobile Materials of Education and Department of Materials Science and Engineering, Jilin University, Changchun 130022, China b Department of Materials Science and Engineering, Changchun University of Technology 130012, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
Synthesis of the ternary carbide Ti3 AlC2 by high-energy ball milling of elemental Ti, Al and C
Received 7 March 2007
powders with a stoichiometric composition was tentatively investigated. The results show
Received in revised form
that high content Ti3 AlC2 was successfully obtained after ball milling of powder mixture only
27 January 2008
for 3 h. The milled products consist of powder and a coarse granule with 8 mm in diameter,
Accepted 24 February 2008
and both are mainly composed of Ti3 AlC2 with TiC as impurity based on X-ray diffractometer (XRD) and energy-dispersive spectroscopy (EDS) characterization. It is believed that a mechanically induced self-propagating reaction (MSR) was triggered to form Ti3 AlC2 and
Keywords:
TiC during high-energy ball milling process.
High-energy ball milling
© 2008 Elsevier B.V. All rights reserved.
Ti3 AlC2 MSR
1.
Introduction
As an important ceramic material, titanium aluminum carbide (Ti3 AlC2 ) has recently gained increasing attention because it possesses unusual properties combining the merits of both metals and ceramics. It is thermally and electrically conductive, resistant to high-thermal shock, low density (4.25 g/cm3 ) and easily machinable. In addition, it presents a high flexural strength, thermal stability and hightemperature oxidation resistance. In contrast to the normal brittle ceramics, Ti3 AlC2 exhibits some abnormal roomtemperature compressive plasticity (Tzenov and Barsoum, 2000). Such unique properties make this kind of materials possess wide variety of potential applications in high-tech fields, e.g. elements of chemical equipment and abrasionresistant components. For example, it can be used as a high-temperature structural material instead of expensive high-temperature alloys, etc.
∗
Corresponding author. Tel.: +86 431 85095878; fax: +86 431 85095876. E-mail address: [email protected] (S.S. Jia). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.02.056
Due to the above mentioned excellent advantages, considerable efforts have been devoted to the fabrication of Ti3 AlC2 during the past decades. Pietzka and Schuster (1994) synthesized Ti3 AlC2 powders first by sintering of cold-compacted powder mixture of TiAl, Al4 C3 and carbon in pure hydrogen for 20 h. Thence, different technologies have been developed to fabricate Ti3 AlC2 or Ti3 AlC2 -based composites, such as hot isostatic pressing (HIP) (Tzenov and Barsoum, 2000), selfpropagation high-temperature synthesis (SHS) (Lopacinski et al., 2001), combustion synthesis (CS) (Ge et al., 2003), hot pressing (HP) (Wang and Zhou, 2002), and spark plasma sintering (SPS) (Zhou et al., 2003). Although Ti3 AlC2 has been fabricated using the above mentioned methods, rigorous conditions are still necessary, such as high pressure, high temperature and long time. Therefore, it is necessary to find a new technology to synthesize Ti3 AlC2 . High-energy ball milling, also called mechanical alloying, is a powder metallurgy technique that involves the mechan-
872
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 871–875
ical milling of reactants to form product phase. It is one of the most promising technologies for obtaining compounds at room temperature. It has some advantages, including low fabrication cost, simple synthetic method and easy industrialization. Recently, it has been successfully used to synthesize various ceramic compounds or composites, such as nanocrystalline MoS2 (Patankar et al., 1993), TiC/Ti-Al composites (Ye et al., 1997) and Ti3 SiC2 (Li et al., 2002; Li and Zhai, 2005; Jin et al., 2007). So far, researches to synthesize Ti3 AlC2 powder with high-energy ball milling are still few in the literature. In this contribution, the main purpose is to investigate synthesis of Ti3 AlC2 by high-energy ball milling of the elemental Ti, Al and C powders at room temperature. The related alloying process will be described.
2.
Experimental procedure
Ti (average particle size: 80 m, purity >99.36%), Al (average particle size: 50 m, purity >99.6%) and graphite C (average particle size: 20 m, purity >99.0%) powders were used as raw materials. These powders were weighed in a mole ratio according to the stoichiometric composition of Ti3 AlC2 with Ti:Al:C = 3:1:2. The powder mixtures were put in a steel jar, which were then sealed in a glove box under argon protective atmosphere. High-energy ball milling was carried out by swinging, high-energy ball mill with cylindrical jars and grinding balls (12 mm in diameter) made from chromium steel. The rotation speed was set as 600 rpm, and the weight ratio of balls to powders was 5:1. The phase analysis of the products prepared by ball milling at different time was carried out by X-ray diffractometer with Cu K␣ (XRD) (Model Rigaku-D/Max-2500PC, Japan). The morphology and microstructure of the products were analyzed by scanning electron microscopy (SEM) (Model JSM-5600, Japan) equipped with energy-dispersive spectroscopy (EDS). It is very difficult to measure the temperature of the powders directly during ball milling. Therefore, during the ball milling, the mill was stopped at different milling times in order to measure the temperature of jar wall using a thermocouple.
3.
Results and discussion
Fig. 1 shows the XRD patterns of the Ti, Al, and C powder mixtures subjected to ball milling at various times. No new phase was formed after ball milling for 1 h. The diffraction peaks of graphite, Ti and Al broaden while their intensities decrease after 2 h of milling, suggesting that reduction of the average crystallite size, buildup of defects and formation of internal strains. After only 3 h of ball milling, all the peaks of Ti, Al and C disappeared while the peaks of TiC and Ti3 AlC2 phase (characteristic peak: 2 = 9.5◦ ) were observed, and Ti3 AlC2 was the main phase. The final products existed in the form of both powder and coarse granule. By weight, the fraction of Ti3 AlC2 in the synthesized powders was estimated to be about 83 wt.% according to the calibrated relationship between the diffraction intensity ratio of Ti3 AlC2 and TiC (Wang et al., 2005). With increasing milling time, all of the diffraction peaks became increasingly broad in width and weak in intensity. The intensity of Ti3 AlC2 peaks almost disappeared after milling for 10 h,
Fig. 1 – XRD patterns of the powders after different ball milling time.
suggesting that the crystallized Ti3 AlC2 phase probably transformed to a nanocrystallite and/or amorphous phase. Fig. 2 shows the SEM micrograph and the maps of EDS of the powder after ball milling for 3 h. From Fig. 2a, it can be seen that the products include the fine powder particles and the agglomerates. The results of elemental EDS (see Fig. 2b–d) also proved that the synthesized final products were composed of Ti3 AlC2 and TiC. Another form of the milled product is a large coarse granule as shown in Fig. 3a. The larger granule with the size of 8 mm were pulverized, and then characterized by XRD shown in Fig. 3b. It can be observed that the granule was composed of Ti3 AlC2 and TiC without other phase diffraction peaks. This rigid and porous coarse granule is very similar to the results of Ti3 SiC2 prepared by mechanical alloying (Li et al., 2002; Li and Zhai, 2005; Jin et al., 2007). Their results indicated that synthesis of Ti3 SiC2 was attributed to a mechanically induced self-propagating reaction (MSR) (Takacs, 2002). It was inferred that formation of the coarse granule in the present experiment suggested occurrence of MSR during 3 h of ball milling. SEM observation on fracture of the coarse granule is shown in Fig. 4. Observation at low magnification in Fig. 4a shows the coarse granule consists of granular grains and layered ones, Further chemical composition characterization using EDS in Fig. 5 indicates that the granular grain is composed of Ti and C elements (see Fig. 5a), indicating that the granular grains are TiC, while the typical layered grain consists of Ti, Al and C with the corresponding atomic ratio of these elements proportional to stoichiometric composition of Ti3 AlC2 (see Fig. 5b). This confirms that the granule is composed of Ti3 AlC2 and TiC, while the smaller TiC grains are surrounded by layered Ti3 AlC2 grains, in agreement with the XRD result given in Fig. 3b. Fig. 4b is an enlarged micrograph of “A” zone marked in Fig. 4a where the average diameters of layered Ti3 AlC2 grains are in the range of 3–5 m. Previous works (Lopacinski et al., 2001; Ge et al., 2003; Wang and Zhou, 2002; Zhou et al., 2003) reported that the formation temperature of Ti3 AlC2 was in the range of
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 871–875
873
Fig. 2 – SEM/EDS results of the powders after ball milling for 3 h: (a) micrograph of the powders, (b) Ti map, (c) Al map and (d) C map.
Fig. 3 – The coarse granule formed during the mechanical alloying: (a) appearance and (b) XRD pattern.
1300–1700 ◦ C by SHS, HP and SPS methods. In this experiment, Ti3 AlC2 grains were successfully obtained by high-energy ball milling, suggesting that the local temperature of the mixed powders was very high during ball milling triggered SHS. It is possible that some powder mixtures in micro zone were melted upon an exothermic reaction, and rapidly solidified to form small bulks as the reaction is finished. Based on the above results and analysis, due to the formation of Ti3 AlC2 phase and TiC phase in a short milling time of 3 h, and a phenomenon of rapid solidification occurred during ball milling, it is impossible that the products were fabricated gradually by diffusion mechanism. This alloying is completed by the interface diffusion of atoms with continuous ball milling (Suryanarayana, 2001). However, in the present work, it is believed that Ti3 AlC2 and TiC were synthesized by mechanically induced self-propagating reaction (MSR) as mentioned above. The occurrence of MSR can be confirmed by the temperature rise of the jar wall. Fig. 6 shows the temperature as a function of milling time. It was noticed that the temperature of the jar wall increased slowly as the milling time extended and then reached 39 ◦ C after milling for 2.5 h, which is mainly because of the colliding and friction between the balls and the jar wall produced mechanical energy, leading to the temperature rise of the jar wall. However, the temperature of the jar abruptly rose to around 76 ◦ C after milling for 3 h and then quickly decreased, suggesting exothermal reactions occurred. The exothermal reactions that occurred in Ti–Al–C system should be related to the formation of Ti3 AlC2 as characterized by XRD (see Fig. 1). Ye et al. (1997) reported that the exothermal reactions in Ti–Al–C system led to an increase in temperature up to 1700 K in local regions during milling. At the same time, the surface temperature of the stainless-steel container reached 70 ◦ C after milling for 210 min.
874
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 871–875
Fig. 4 – Microstructure of the fracture surface of the coarse granule (a) and (b) higher magnification micrograph taken from “A” area, marked in (a).
The alloying process of the MSR is relatively complicated. Takacs (2002) reported that during the high-energy ball milling, the MSR may be ignited after a certain activation time, as milling reduces the particle size, thoroughly mixes the components, and increases the number of chemically active defect sites. Atzmon (1990) suggested that when each particle reached a critical lamellar thickness during ball milling, the MSR is ignited and completed quickly. In the present experiment, the average crystalline size of Ti, Al, and C powders decreased gradually during milling. The MSR doesn’t occur at the beginning of the ball milling stage. The process begins with an activation period, during which the powders undergo the repeated fracture and cold welding process. Viz., the C particles were easier to be pulverized than the Al and Ti particles at the initial stage of ball milling because they were very brittle. As the milling proceeded, some of the pulverized C particles were rolled into the Ti and Al particles. Proper pulverization resulted in the refinement of particle. As the milled powders reached a defined critical size, the diffusion distance decreased and a large surface area formed, which contributed to the combustion reaction occurring. The energy induced by continuous colliding was concentrated in a small volume and finally reached the value for the triggered self-
Fig. 5 – Energy-dispersive spectroscopy analysis results in Fig. 4(a). (a) “1” granular grain and (b) “2” layered grain.
propagating reaction, the combustion reaction was ignited. The MSR was accompanied by the release of a large amount of heat, which not only made Ti, Al and C form Ti3 AlC2 , but also raised the temperature in the micro zone at a high level resulting in “sintering” small part of powders to coarse granule.
Fig. 6 – Temperature of the jar wall as a function of milling time.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 871–875
The practical MSR may be a very complicated process, and the reacting time is only several seconds. The overall reactions for the formation of products from elemental powders may be summarized as follows: Ti + C → TiC 3Ti + Al + 2C → Ti3 AlC2 It can be seen that both the listed reactions do not balance for a stoichiometric initial composition of reactants. It is possible that remaining Al existed in the synthesized products. However, the diffraction peaks of the remaining Al from the XRD pattern of the powders after milling for 3 h were not found. There are many defects (dislocations, vacancies and increased number of grain boundaries) in the compounds formed by ball milling. Therefore, it was inferred that the remaining Al may mainly dissolve into the lattice of the synthesized products. On the other hand, part of the remaining Al probably transformed to amorphous phase. Ti3 AlC2 with higher yield was prepared by high-energy ball milling. However, formation path of Ti3 AlC2 during this process remains unclear, and further work is required. In addition, high-purity Ti3 AlC2 powders probably may be obtained through adjusting the mole ratio of element powders or removing TiC by heating the powders under lower temperature, and the related work is now in progress in our group.
4.
Conclusion
In this work, Ti3 AlC2 could be synthesized using 3Ti/Al/2C powder mixture as the starting material by high-energy ball milling. The Ti3 AlC2 powders with about 83 wt.% were obtained. The formation of the final product is attributed to the occurrence of a mechanically induced self-propagating reaction during ball milling process.
Acknowledgements This work was supported by Research Foundation for the Doctoral Program of Higher Education under Grant n. 20060183058 and Scientific and Technological Development Projects of Jilin Province in China under Grant n. 20070511.
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