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α-Al2O3 nanocomposite by mechanical alloying
Journal of Alloys and Compounds 478 (2009) 257–259
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and characterization of TiAl/␣-Al2 O3 nanocomposite by mechanical alloying N. Forouzanmehr ∗ , F. Karimzadeh, M.H. Enayati Department of Materials Engineering, Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e
i n f o
Article history: Received 8 November 2008 Received in revised form 28 November 2008 Accepted 10 December 2008 Available online 25 December 2008 Keywords: Titanium aluminide Nanocomposite Mechanical alloying
a b s t r a c t TiAl/␣-Al2 O3 nanocomposite was synthesized by mechanical alloying of the Al and TiO2 powder mixture. The powder particles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that the mechanochemical reaction between Al and TiO2 gradually took place through intermediate stages during milling, resulted in the formation of disordered fcc-TiAl and Al2 O3 phases. The in situ processing involved two steps in which diffusion of Ti into Al is prominent. Annealing of milled product led to transition of metastable fcc-TiAl into equilibrium ␥-TiAl. The crystallite size of phases in produced nanocomposite powder estimated to be about 50 nm. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Intermetallic matrix composites are attractive materials for elevated temperature structural applications [1,2]. TiAl based composites are a competitive candidate for aerospace use due to favorable properties of matrix material, such as low density, high specific strength and relatively good properties at elevated temperatures [3]. Commonly synthesized systems are TiAl–TiB2 [4], TiAl–Ti5 Si3 [5] and TiAl–Al2 O3 [6]. The TiAl based composite reinforced by Al2 O3 seems especially promising due to the excellent oxidation resistance [7]. Recently, in situ techniques have been utilized to fabricate TiAl–Al2 O3 composites through displacement reaction between Al and TiO2 [6–9]. Reactions taking place in such in situ process are often complicated. There are often intermediate steps between the initial materials and the final products. The knowledge of reaction characteristics in the Al–TiO2 system is of great importance to optimize the processing [10]. Based on this consideration, only a few research works has been published [10,11]. There is a growing interest in the investigation of nanocomposites due to their superior mechanical properties [3]. Mechanical alloying (MA) is a versatile processing technique for the fabrication of novel materials such as nanocomposites [12]. There has been a few investigation on synthesis of TiAl matrix nanocomposite by MA [5,13]. Mechanical milling can be used to induce displacement reactions at much lower temperatures than normally required. This
allows the in situ formation of composites during milling or during subsequent thermal processing of milled powders [8,14]. In this work, TiAl–Al2 O3 nanocomposite was synthesized by MA and mechanochemical reaction between Al and TiO2 . The formation mechanism of nanocomposite and how the phases of final product are developed from the starting materials through the intermediate stages were also investigated. 2. Experimental procedure A mixture of commercial aluminum and TiO2 were used as the starting materials. The approximate mean particle size and chemical purity of initial powders are given in Table 1. Al/TiO2 ratio was determined according to the following nominal reaction:
3TiO2 + 7Al → 3TiAl + 2Al2 O3 The MA was carried out in a planetary ball mill at room temperature under argon atmosphere. The MA experiments were performed using hardened chromium steel vial (125 ml) containing four steel balls with a diameter of 20 nm. The ball-to-powder weight ratio was 10:1 and the rotation speed of vial was 600 rpm. The milling was interrupted at selected times and a small amount of powder was removed for characterizations. X-ray diffraction (XRD) with Cu K␣ radiation ( = 0.1541 nm) was used to follow phase transformation and structural changes of the powders. Transmission electron microscope (TEM) observation was performed using a Philips CM200 electron microscope with a working voltage of 200 kV. TEM specimens were prepared by mixing the powders in a small amount of ethanol and mounted on a copper microgrid. Crystallite size was estimated by TEM observation as well as Williamson–Hall formula [15] using XRD peaks.
3. Results and discussion ∗ Corresponding author. Tel.: +98 6113331387; fax: +98 2133319901. E-mail addresses: [email protected], n f [email protected] (N. Forouzanmehr). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.047
TiAl/47.6 wt%Al2 O3 nanocomposite can be synthesized by a displacement reaction during ball milling of Al with 56 wt%TiO2
258
N. Forouzanmehr et al. / Journal of Alloys and Compounds 478 (2009) 257–259
Table 1 The approximate mean particle size and chemical purity of initial powders.
Purity (%) Particle size (m)
Al
TiO2
99.7 50–70
99 0.2
according to the following reaction: 3TiO2 + 7Al → 3TiAl + 2Al2 O3
Table 2 2 (◦ ) values with related reflection of disorder TiAl obtained after MA for 60 h and of ␥-TiAl in JCPDS files. Structure
2 (◦ )
Reflection
Disordered TiAl
fcc
(2 0 0)
44.945
␥-TiAla
L10
(0 0 2) (2 0 0)
44.4 45.6
a
In JCPDS files (Ref. no. 05-0678).
(1)
G◦ 298 = −212, 600 J/mol This reaction is thermodynamically feasible at room temperature due to its negative free energy change. The occurrence of the reaction at ambient temperatures is limited by kinetic considerations. MA can provide the means to substantially increase the reaction kinetics of the reduction reaction [14]. Fig. 1(a) shows the XRD patterns of powders mechanically alloyed for different milling times. The diffraction pattern of initial powder mixture shows expected peaks of Al and TiO2 . With increasing milling time, the intensity of Al and TiO2 peaks decreased and their width increased due to a decrease in crystallite size and enhancement of lattice strain. After 20 h of milling the XRD pattern indicates disappearance of TiO2 peaks and the shift of Al peaks toward higher angles. Displacement of (1 1 1) reflection of Al is shown in Fig. 1(b). This implies that during milling the reaction between Al and TiO2 takes place. The Ti reduced from TiO2 dissolves into Al, forming Al(Ti) solid solution. The formation of fcc-Al(Ti) solid solution during MA has been previously reported [16]. The enhanced solid solubility might be due to the high dislocation density produced during MA [14]. The peaks of Al2 O3 phase formed by the reaction cannot clearly be identified in the XRD pattern. This may be due to very small size of Al2 O3 formed during milling. After 60 h of milling the XRD pattern indicates the formation of a new phase. The diffraction peaks of this new phase could be corresponding to disordered TiAl phase with fcc structure. The position of the (2 0 0) reflection of this fcc structure is nearly in the middle of the (2 0 0) and (0 0 2) reflections of the tetragonal TiAl structure (Table 2). Therefore, it is obvious to explain it by the formation of a disordered fcc-TiAl phase.
Fig. 2. The widely accepted Ti–Al phase diagram [11].
Fig. 3. X-ray diffraction patterns of Al–TiO2 mixture milled for 60 h followed by annealing at different temperatures for 30 min.
From the widely accepted Ti–Al phase diagram, Fig. 2, it can be seen that Al has an extended solubility in Ti, while Ti has a limited solubility in Al on high temperature. If the diffusion of Al into hcpTi is dominant process, hcp-based phase, e.g. Ti3 Al, maybe formed primarily, as it was shown in our previous study [17]. Otherwise fcc-based phase such as ␥-TiAl or fcc-TiAl, is prior if Ti into Al is dominant process. The formation of fcc-TiAl from Al–TiO2 mixture, through the diffusion of Ti into liquid Al at high heating rate, has been reported [11]. Fan et al. suggested that at high heating rate the melted aluminum has no time to spread when the reaction takes place and Table 3 The crystallite size and the internal strain of ␥-TiAl and ␣-Al2 O3 produced by MA for 60 h and subsequent annealing to 900 ◦ C for 30 min.
Fig. 1. (a) X-ray diffraction patterns of Al–TiO2 mixture milled for different times and (b) displacement of Al (1 1 1) XRD peak during milling followed by the formation of new phase.
␥-TiAl Al2 O3
Crystallite size (nm)
Internal strain (%)
58 50
0.65 0.42
N. Forouzanmehr et al. / Journal of Alloys and Compounds 478 (2009) 257–259
259
Fig. 4. TEM micrographs with related selected-area diffraction pattern of TiAl/Al2 O3 composite powder: (a) BF and (b) DF.
the liquid aluminum exhibits similar character to solid in reaction. In this study, through the solid state reaction, diffusion of Ti into Al is dominant process, resulting in the formation of fcc-TiAl. The in situ processing involves two steps, which are as follows: Step 1: 3TiO2 + 4Al → 3[Ti] + 2Al2 O3 ; Step 2: 3[Ti] + 3Al → 3fcc-TiAl. At the first step, Ti reduced from TiO2 dissolves into Al due to predominance of diffusion of Ti into Al and the Al(Ti) solid solution forms. With increasing milling time, the reaction kinetics increases, results in the occurrence of the second step and the formation of fcc-TiAl. As can be seen TiO2 is reduced by Al through diffusion-controlled mechanism. It has been demonstrated that the value of adiabatic temperature Tad can be used as a rough guide to the existence of combustion in milling process. Tad should be above 1527 ◦ C in thermally ignited systems [18]. The value of Tad of reaction (1) is about 1460 ◦ C [19]. Therefore, it seems that the explosion reaction is suppressed during milling and the mode of the reaction is gradual. Fig. 3 shows the XRD patterns of Al–TiO2 powder mixture milled for 60 h followed by annealing at different temperatures for 30 min. In the XRD pattern of the powder after heating up to 700 ◦ C the XRD peaks corresponding to disorder fcc-TiAl and ␣-Al2 O3 phases are still detectable that show no change in the structure. With further increase in the temperature to 900 ◦ C, the XRD peaks corresponding to ␥-TiAl appear in the XRD pattern, showing the transformation of metastable fcc-TiAl to equilibrium ␥-TiAl. It can be seen splitting in some diffraction peaks, such as (2 0 0)fcc → (0 0 2)(2 0 0)L10 and (2 2 0)fcc → (2 0 2)(2 2 0)L10 . The splitting in the peaks is resulted in the tetragonal distortion during transformation of disorder fcc-TiAl into ordered L10 phase. Table 3 shows the crystallite size and the internal strain of the products, estimated from broadening of XRD peaks using the Williamson–Hall formula: 0.9 ˇ cos = 2ε sin + D where ˇ is the full width at half maximum of the XRD peaks, is diffraction angles, ε is the internal strain, is the wave length of X-ray and D is the mean grain size. Comparing this result with that obtained in our earlier work [17] for nanocrystalline TiAl, it is observed that the nanocrystalline TiAl in the composite as well as
monolithic have approximately the same grain size. Thus the Al2 O3 phase had little effect on crystallite size of TiAl. Fig. 4 shows TEM micrographs and related selected-area diffraction pattern (SADP) of the powder milled for 60 h followed by annealing at 900 ◦ C. Fig. 4(a) displays the bright field (BF) micrograph and Fig. 4(b) shows dark field (DF) micrograph indicating nano-sized phases with less than 50 nm. The SADP, Fig. 4(c), also indicates fine crystalline size as well as the absence of preferred orientation. As can be seen relatively good agreement exists regarding the grain size of the phases estimated from TEM observation and that from XRD analysis using Williamson–Hall method. 4. Conclusion This study investigated synthesis and characterization of ␥TiAl/␣-Al2 O3 nanocomposite by MA and subsequent annealing. The titanium oxide is found to be gradually reduced by Al during MA through intermediate stages. The Al(Ti) solid solution formed at the early stage of milling. Diffusion of Ti into Al is found to be dominant process. Following the milling, fcc-TiAl/␣-Al2 O3 composite obtained. During annealing, the metastable fcc-TiAl transformed into ␥-TiAl. The final product was ␥-TiAl matrix nanocomposite reinforced by Al2 O3 particles with grain size of about 50 nm. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
C.C. Koch, Mater. Sci. Eng. A 244 (1998) 39–48. C.M. Ward-Close, R. Minor, P.J. Doorbar, Intermetallics 4 (1996) 217–229. K.P. Rao, Y.J. Du, Mater. Sci. Eng. A 277 (2000) 46–56. C.L. Yeh, S.H. Su, J. Alloy. Compd. 407 (2006) 150–156. O.N. Senkov, M. Cavusoglu, F.H. Froes, Mater. Sci. Eng. A 300 (2001) 85–93. Z.W. Li, W. Gao, D.L. Zhang, Z.H. Cai, Corros. Sci. 46 (2004) 1997–2007. N. Travitzky, I. Gotman, N. Claussen, Mater. Lett. 57 (2003) 3422–3426. Z.H. Cai, D.H. Zhang, Mater. Sci. Eng. A 419 (2006) 310–317. D.H. Zhang, Z.H. Cai, M. Newby, Mater. Technol. 18 (2003) 94–98. C.F. Feng, L. Froyen, Composites A 31 (2000) 385–390. R.H. Fan, B. Liu, J.D. Zhang, J.Q. Bi, Y.Sh. Yin, Mater. Chem. Phys. 91 (2005) 140–145. D.L. Zhang, Prog. Mater. Sci. 49 (2004) 537–560. K.Y. Wang, J.G. Wang, G.L. Chen, J. Mater. Res. 10 (1995) 1247–1255. C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1–184. G.K. Williamson, W.H. Hall, Acta Metall. 1 (1953) 22–31. C.E. Wen, J. Mater. Sci. 35 (2000) 2099–2105. N. Forouzanmehr, F. Karimzadeh, M.H. Enayati, J. Alloys Compd., in press, available online. J. Li, F. Li, K. Hu, J. Mater. Process. Technol. 147 (2004) 236–240. D. Horvitz, I. Gotman, E.Y. Gutmanas, N. Claussen, J. Eur. Ceram. Soc. 22 (2002) 947–954.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and characterization of TiAl/␣-Al2 O3 nanocomposite by mechanical alloying N. Forouzanmehr ∗ , F. Karimzadeh, M.H. Enayati Department of Materials Engineering, Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e
i n f o
Article history: Received 8 November 2008 Received in revised form 28 November 2008 Accepted 10 December 2008 Available online 25 December 2008 Keywords: Titanium aluminide Nanocomposite Mechanical alloying
a b s t r a c t TiAl/␣-Al2 O3 nanocomposite was synthesized by mechanical alloying of the Al and TiO2 powder mixture. The powder particles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that the mechanochemical reaction between Al and TiO2 gradually took place through intermediate stages during milling, resulted in the formation of disordered fcc-TiAl and Al2 O3 phases. The in situ processing involved two steps in which diffusion of Ti into Al is prominent. Annealing of milled product led to transition of metastable fcc-TiAl into equilibrium ␥-TiAl. The crystallite size of phases in produced nanocomposite powder estimated to be about 50 nm. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Intermetallic matrix composites are attractive materials for elevated temperature structural applications [1,2]. TiAl based composites are a competitive candidate for aerospace use due to favorable properties of matrix material, such as low density, high specific strength and relatively good properties at elevated temperatures [3]. Commonly synthesized systems are TiAl–TiB2 [4], TiAl–Ti5 Si3 [5] and TiAl–Al2 O3 [6]. The TiAl based composite reinforced by Al2 O3 seems especially promising due to the excellent oxidation resistance [7]. Recently, in situ techniques have been utilized to fabricate TiAl–Al2 O3 composites through displacement reaction between Al and TiO2 [6–9]. Reactions taking place in such in situ process are often complicated. There are often intermediate steps between the initial materials and the final products. The knowledge of reaction characteristics in the Al–TiO2 system is of great importance to optimize the processing [10]. Based on this consideration, only a few research works has been published [10,11]. There is a growing interest in the investigation of nanocomposites due to their superior mechanical properties [3]. Mechanical alloying (MA) is a versatile processing technique for the fabrication of novel materials such as nanocomposites [12]. There has been a few investigation on synthesis of TiAl matrix nanocomposite by MA [5,13]. Mechanical milling can be used to induce displacement reactions at much lower temperatures than normally required. This
allows the in situ formation of composites during milling or during subsequent thermal processing of milled powders [8,14]. In this work, TiAl–Al2 O3 nanocomposite was synthesized by MA and mechanochemical reaction between Al and TiO2 . The formation mechanism of nanocomposite and how the phases of final product are developed from the starting materials through the intermediate stages were also investigated. 2. Experimental procedure A mixture of commercial aluminum and TiO2 were used as the starting materials. The approximate mean particle size and chemical purity of initial powders are given in Table 1. Al/TiO2 ratio was determined according to the following nominal reaction:
3TiO2 + 7Al → 3TiAl + 2Al2 O3 The MA was carried out in a planetary ball mill at room temperature under argon atmosphere. The MA experiments were performed using hardened chromium steel vial (125 ml) containing four steel balls with a diameter of 20 nm. The ball-to-powder weight ratio was 10:1 and the rotation speed of vial was 600 rpm. The milling was interrupted at selected times and a small amount of powder was removed for characterizations. X-ray diffraction (XRD) with Cu K␣ radiation ( = 0.1541 nm) was used to follow phase transformation and structural changes of the powders. Transmission electron microscope (TEM) observation was performed using a Philips CM200 electron microscope with a working voltage of 200 kV. TEM specimens were prepared by mixing the powders in a small amount of ethanol and mounted on a copper microgrid. Crystallite size was estimated by TEM observation as well as Williamson–Hall formula [15] using XRD peaks.
3. Results and discussion ∗ Corresponding author. Tel.: +98 6113331387; fax: +98 2133319901. E-mail addresses: [email protected], n f [email protected] (N. Forouzanmehr). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.047
TiAl/47.6 wt%Al2 O3 nanocomposite can be synthesized by a displacement reaction during ball milling of Al with 56 wt%TiO2
258
N. Forouzanmehr et al. / Journal of Alloys and Compounds 478 (2009) 257–259
Table 1 The approximate mean particle size and chemical purity of initial powders.
Purity (%) Particle size (m)
Al
TiO2
99.7 50–70
99 0.2
according to the following reaction: 3TiO2 + 7Al → 3TiAl + 2Al2 O3
Table 2 2 (◦ ) values with related reflection of disorder TiAl obtained after MA for 60 h and of ␥-TiAl in JCPDS files. Structure
2 (◦ )
Reflection
Disordered TiAl
fcc
(2 0 0)
44.945
␥-TiAla
L10
(0 0 2) (2 0 0)
44.4 45.6
a
In JCPDS files (Ref. no. 05-0678).
(1)
G◦ 298 = −212, 600 J/mol This reaction is thermodynamically feasible at room temperature due to its negative free energy change. The occurrence of the reaction at ambient temperatures is limited by kinetic considerations. MA can provide the means to substantially increase the reaction kinetics of the reduction reaction [14]. Fig. 1(a) shows the XRD patterns of powders mechanically alloyed for different milling times. The diffraction pattern of initial powder mixture shows expected peaks of Al and TiO2 . With increasing milling time, the intensity of Al and TiO2 peaks decreased and their width increased due to a decrease in crystallite size and enhancement of lattice strain. After 20 h of milling the XRD pattern indicates disappearance of TiO2 peaks and the shift of Al peaks toward higher angles. Displacement of (1 1 1) reflection of Al is shown in Fig. 1(b). This implies that during milling the reaction between Al and TiO2 takes place. The Ti reduced from TiO2 dissolves into Al, forming Al(Ti) solid solution. The formation of fcc-Al(Ti) solid solution during MA has been previously reported [16]. The enhanced solid solubility might be due to the high dislocation density produced during MA [14]. The peaks of Al2 O3 phase formed by the reaction cannot clearly be identified in the XRD pattern. This may be due to very small size of Al2 O3 formed during milling. After 60 h of milling the XRD pattern indicates the formation of a new phase. The diffraction peaks of this new phase could be corresponding to disordered TiAl phase with fcc structure. The position of the (2 0 0) reflection of this fcc structure is nearly in the middle of the (2 0 0) and (0 0 2) reflections of the tetragonal TiAl structure (Table 2). Therefore, it is obvious to explain it by the formation of a disordered fcc-TiAl phase.
Fig. 2. The widely accepted Ti–Al phase diagram [11].
Fig. 3. X-ray diffraction patterns of Al–TiO2 mixture milled for 60 h followed by annealing at different temperatures for 30 min.
From the widely accepted Ti–Al phase diagram, Fig. 2, it can be seen that Al has an extended solubility in Ti, while Ti has a limited solubility in Al on high temperature. If the diffusion of Al into hcpTi is dominant process, hcp-based phase, e.g. Ti3 Al, maybe formed primarily, as it was shown in our previous study [17]. Otherwise fcc-based phase such as ␥-TiAl or fcc-TiAl, is prior if Ti into Al is dominant process. The formation of fcc-TiAl from Al–TiO2 mixture, through the diffusion of Ti into liquid Al at high heating rate, has been reported [11]. Fan et al. suggested that at high heating rate the melted aluminum has no time to spread when the reaction takes place and Table 3 The crystallite size and the internal strain of ␥-TiAl and ␣-Al2 O3 produced by MA for 60 h and subsequent annealing to 900 ◦ C for 30 min.
Fig. 1. (a) X-ray diffraction patterns of Al–TiO2 mixture milled for different times and (b) displacement of Al (1 1 1) XRD peak during milling followed by the formation of new phase.
␥-TiAl Al2 O3
Crystallite size (nm)
Internal strain (%)
58 50
0.65 0.42
N. Forouzanmehr et al. / Journal of Alloys and Compounds 478 (2009) 257–259
259
Fig. 4. TEM micrographs with related selected-area diffraction pattern of TiAl/Al2 O3 composite powder: (a) BF and (b) DF.
the liquid aluminum exhibits similar character to solid in reaction. In this study, through the solid state reaction, diffusion of Ti into Al is dominant process, resulting in the formation of fcc-TiAl. The in situ processing involves two steps, which are as follows: Step 1: 3TiO2 + 4Al → 3[Ti] + 2Al2 O3 ; Step 2: 3[Ti] + 3Al → 3fcc-TiAl. At the first step, Ti reduced from TiO2 dissolves into Al due to predominance of diffusion of Ti into Al and the Al(Ti) solid solution forms. With increasing milling time, the reaction kinetics increases, results in the occurrence of the second step and the formation of fcc-TiAl. As can be seen TiO2 is reduced by Al through diffusion-controlled mechanism. It has been demonstrated that the value of adiabatic temperature Tad can be used as a rough guide to the existence of combustion in milling process. Tad should be above 1527 ◦ C in thermally ignited systems [18]. The value of Tad of reaction (1) is about 1460 ◦ C [19]. Therefore, it seems that the explosion reaction is suppressed during milling and the mode of the reaction is gradual. Fig. 3 shows the XRD patterns of Al–TiO2 powder mixture milled for 60 h followed by annealing at different temperatures for 30 min. In the XRD pattern of the powder after heating up to 700 ◦ C the XRD peaks corresponding to disorder fcc-TiAl and ␣-Al2 O3 phases are still detectable that show no change in the structure. With further increase in the temperature to 900 ◦ C, the XRD peaks corresponding to ␥-TiAl appear in the XRD pattern, showing the transformation of metastable fcc-TiAl to equilibrium ␥-TiAl. It can be seen splitting in some diffraction peaks, such as (2 0 0)fcc → (0 0 2)(2 0 0)L10 and (2 2 0)fcc → (2 0 2)(2 2 0)L10 . The splitting in the peaks is resulted in the tetragonal distortion during transformation of disorder fcc-TiAl into ordered L10 phase. Table 3 shows the crystallite size and the internal strain of the products, estimated from broadening of XRD peaks using the Williamson–Hall formula: 0.9 ˇ cos = 2ε sin + D where ˇ is the full width at half maximum of the XRD peaks, is diffraction angles, ε is the internal strain, is the wave length of X-ray and D is the mean grain size. Comparing this result with that obtained in our earlier work [17] for nanocrystalline TiAl, it is observed that the nanocrystalline TiAl in the composite as well as
monolithic have approximately the same grain size. Thus the Al2 O3 phase had little effect on crystallite size of TiAl. Fig. 4 shows TEM micrographs and related selected-area diffraction pattern (SADP) of the powder milled for 60 h followed by annealing at 900 ◦ C. Fig. 4(a) displays the bright field (BF) micrograph and Fig. 4(b) shows dark field (DF) micrograph indicating nano-sized phases with less than 50 nm. The SADP, Fig. 4(c), also indicates fine crystalline size as well as the absence of preferred orientation. As can be seen relatively good agreement exists regarding the grain size of the phases estimated from TEM observation and that from XRD analysis using Williamson–Hall method. 4. Conclusion This study investigated synthesis and characterization of ␥TiAl/␣-Al2 O3 nanocomposite by MA and subsequent annealing. The titanium oxide is found to be gradually reduced by Al during MA through intermediate stages. The Al(Ti) solid solution formed at the early stage of milling. Diffusion of Ti into Al is found to be dominant process. Following the milling, fcc-TiAl/␣-Al2 O3 composite obtained. During annealing, the metastable fcc-TiAl transformed into ␥-TiAl. The final product was ␥-TiAl matrix nanocomposite reinforced by Al2 O3 particles with grain size of about 50 nm. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
C.C. Koch, Mater. Sci. Eng. A 244 (1998) 39–48. C.M. Ward-Close, R. Minor, P.J. Doorbar, Intermetallics 4 (1996) 217–229. K.P. Rao, Y.J. Du, Mater. Sci. Eng. A 277 (2000) 46–56. C.L. Yeh, S.H. Su, J. Alloy. Compd. 407 (2006) 150–156. O.N. Senkov, M. Cavusoglu, F.H. Froes, Mater. Sci. Eng. A 300 (2001) 85–93. Z.W. Li, W. Gao, D.L. Zhang, Z.H. Cai, Corros. Sci. 46 (2004) 1997–2007. N. Travitzky, I. Gotman, N. Claussen, Mater. Lett. 57 (2003) 3422–3426. Z.H. Cai, D.H. Zhang, Mater. Sci. Eng. A 419 (2006) 310–317. D.H. Zhang, Z.H. Cai, M. Newby, Mater. Technol. 18 (2003) 94–98. C.F. Feng, L. Froyen, Composites A 31 (2000) 385–390. R.H. Fan, B. Liu, J.D. Zhang, J.Q. Bi, Y.Sh. Yin, Mater. Chem. Phys. 91 (2005) 140–145. D.L. Zhang, Prog. Mater. Sci. 49 (2004) 537–560. K.Y. Wang, J.G. Wang, G.L. Chen, J. Mater. Res. 10 (1995) 1247–1255. C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1–184. G.K. Williamson, W.H. Hall, Acta Metall. 1 (1953) 22–31. C.E. Wen, J. Mater. Sci. 35 (2000) 2099–2105. N. Forouzanmehr, F. Karimzadeh, M.H. Enayati, J. Alloys Compd., in press, available online. J. Li, F. Li, K. Hu, J. Mater. Process. Technol. 147 (2004) 236–240. D. Horvitz, I. Gotman, E.Y. Gutmanas, N. Claussen, J. Eur. Ceram. Soc. 22 (2002) 947–954.