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Synthesis and characterization of NiAl–Al2O3 nanocomposite powder by mechanical alloying
Journal of Alloys and Compounds 477 (2009) 178–181
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and characterization of NiAl–Al2 O3 nanocomposite powder by mechanical alloying S.Z. Anvari, F. Karimzadeh ∗ , M.H. Enayati Department of Materials Engineering, Nanotechnology and Advanced Materials Institute, Isfahan University of Technology (IUT), Isfahan 84156-83111, Iran
a r t i c l e
i n f o
Article history: Received 9 July 2008 Received in revised form 11 October 2008 Accepted 14 October 2008 Available online 2 December 2008 Keywords: Nanocomposite Mechanical alloying Mechanochemical processing NiAl–Al2 O3
a b s t r a c t The synthesis and characterization of NiAl–Al2 O3 nanocomposite powders containing different amount of Al2 O3 (20 and 40 vol.% of Al2 O3 ) was investigated. Al, Ni and NiO subjected to high-energy ball milling in a planetary ball mill. The structural changes of powder particles during mechanical alloying were studied by X-ray diffractometery and scanning electron microscopy. The mechanical alloying of NiO and Al powder mixture corresponding to the stiochiometric reaction 3NiO + 5Al resulted in the formation of NiAl–40 vol.% Al2 O3 after 0.5 h with rapidly mode. The crystallite size of NiAl and Al2 O3 were 35 and 60 nm, respectively. The formation of NiAl–20 vol.% Al2 O3 took place after 30 h of milling of NiO–34 wt% Al–41 wt% Ni powder mixture. For this composition exothermic reaction with high rate did not occur during the milling process. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years, there has been increased activity in research on materials based on NiAl for high temperature applications. The attractive attributes of NiAl leading to this interest include a high melting temperature, low density, high thermal conductivity and low raw materials cost. Two principle limitations of NiAl are poor toughness at room temperature and inadequate strength and creep resistance at elevated temperature [1–4]. It was showed that the synthesis of nanostructured materials has been successful in increasing ductility in some intermetallics [5]. On the other hand, for optimization of room temperature toughness and high temperature strength, microstructural modifications are required. For these reasons it has been recognized that intermetallic matrix composites (IMCs) may be appropriate materials for structural applications, replacing in some cases intermetallics [6]. Novel fabrication technology of nanostructured materials is versatile and includes a wide range of vapor, liquid and solid state processing routes. The solid state route preparation takes place via mechanical alloying (MA) and mechanochemical synthesis [7–9]. Mechanochemical synthesis involves mechanical activation of solid state displacement reactions in a ball mill. Thus, mechanical energy is used to induce chemical reactions. The chemical precur-
∗ Corresponding author. Tel.: +98 311 3915744; fax: +98 311 3912752. E-mail address: karimzadeh [email protected] (F. Karimzadeh). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.10.043
sors typically consist of mixtures of oxides, chlorides and/or metals that react either during milling or during subsequent heat treatment to form a composite powder consisting of the dispersion of ultrafine particles within a matrix [9–12]. The mechanochemical reactions fall into two categories, namely (a) those which occur during the mechanical activation process and there the reaction enthalpy is highly negative (e.g., adiabatic temperature Tad = 1300–1800 K), and (b) those which occur during subsequent thermal treatment and here the reaction enthalpy is only moderate (e.g., Tad < 1300 K). The first type of reaction takes place in two distinct modes, i.e., either combustion reaction or a progressive reaction [13–15]. Researchers have also studied the formation of NiAl–Al2 O3 composites by milling a mixture of Ni, Al, and Al2 O3 powders [3] and NiAl and Al2 O3 powders [16]. NiO and Al have been used for the synthesis of NiAl–Al2 O3 composite via termite reaction [17]. An earlier study demonstrated the synthesis of nanocrystalline NiAl by mechanical alloying of Ni and Al powder mixtures. The mechanical alloying resulted in the gradual formation of nanocrystalline NiAl. It was found that NiAl phase develops by continuous diffusive reaction at Ni/Al layers interfaces [18]. In the present work, we used NiO, Al powders mixture with and without Ni to synthesis NiAl–Al2 O3 composite by MA. The phase transformation in milling process was investigated. The chemical reaction involved here can be expressed as:
3NiO + 5Al → 3NiAl + Al2 O3
(1)
S.Z. Anvari et al. / Journal of Alloys and Compounds 477 (2009) 178–181
179
Table 1 The fundamental and superlattice peaks of the NiAl(B2) phase. Fundamental peaks of NiAl
Superlattice peaks of NiAl
(h k l)
Angle (2)
(h k l)
Angle (2)
110 200 211
44.865 65.325 82.455
100 111 210
31.305 55.425 73.875
to the reaction (1): G◦ 298 = −1298.59 kJ/mol H ◦ 298 = −1311.2 kJ/mol
Fig. 1. XRD traces of NiO–38 wt% Al powder mixtures as-received and after mechanical alloying.
2. Experimental NiO (99.9% purity and particle size of <40 m), Al (99.5% purity and particle size of 30–150 m) and Ni (99.5% purity and particle size of ∼100 m) elemental powders were used as starting materials. Mechanical alloying was carried out in a planetary ball mill equipped with hardened chromium steel container and balls. The milling media consisted of four 20 mm diameter balls confined in a 120 ml volume container. A total of 16 g powder with no process control agent was used in all MA runs. The powders were mixed with starting composition of NiO–38 wt% Al and NiO–34 wt% Al–41 wt% Ni, in order to give the final product NiAl–Al2 O3 composite with about 40 and 20 vol.% contribution of alumina. Structural changes of powder particles were studied by X-ray diffraction (XRD) in a Philips X’PERT MPD diffractometer using filtered Cu K␣ radiation ( = 0.1542 nm). Crystal size and internal strain in the milled samples were calculated from the XRD line broadening using the Williamson–Hall equation [19]. The powder particles morphology was investigated by scanning electron microscopy (SEM) in a Philips XL30 at an acceleration voltage of 30 kV with an energy-dispersive X-ray spectrometer (EDX) attachment. Heat treatment of asmilled powders was conducted in a tube furnace under Ar atmosphere.
During milling in room temperature this reaction can thermodynamically occur due to its negative free energy change. H ◦ 298 for Al–NiO reaction is also negative, indicating that the Al–NiO reaction is exothermic [17]. X-ray diffraction patterns of NiO–38 wt% Al powder mixture after different milling times are shown in Fig. 1. As-received powder mixture consisted of NiO and Al. After 0.5 h of MA the XRD peaks of the raw material disappeared due to the reaction of NiO and Al to give NiAl and Al2 O3 phase. It is worthwhile to notice that this phase transformation was accompanied by an increase of the temperature of the container. This is suggesting that the reaction of NiO and Al takes place highly exothermic. Fig. 2 shows the SEM micrograph of the NiAl–40 vol.% Al2 O3 powder particles after 0.5 h of milling. The fused-shape morphology of powder particles indicates that NiO–Al reaction occurred rapidly. Although the powders are more loosely dispersed in the container during ball milling, the mixing of the reactants on a nanometer scale favors mass transfer and the diffusion path length is considerably reduced. It is believed that a high heat of formation serves as the driving force of the exothermic reaction with high rate during the milling processes. Researches have shown that the adiabatic temperature (Tad ), which is the maximum temperature achieved under adiabatic conditions as a consequence
3. Results and discussion 3.1. Mechanical alloying NiAl–40 vol.% Al2 O3 nanocomposite can be synthesized by a displacement reaction during ball milling of NiO–38 wt% Al according
Fig. 2. SEM micrograph of the NiAl–40 vol.% Al2 O3 powder particles after 0.5 h of milling.
Fig. 3. XRD traces of NiO–34 wt% Al–41 wt% Ni powder mixtures as-received and after different milling times.
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S.Z. Anvari et al. / Journal of Alloys and Compounds 477 (2009) 178–181
Table 2 Grain size evolution of the synthesized phases during milling. Milling time (h)
d (NiAl) (nm)
d (Al2 O3 ) (nm)
20 30 40 60
30 27 25 24
43 38 20a 15a
a
Scherrer method was used.
of the evolution of heat from the reaction, should be above 1800 K in thermally ignited systems [20]. Thus, the value of Tad can be used as a rough guide to predict the reaction rate during milling process. According to the thermodynamic calculation the value of Tad of reaction (1) is about 3500 ◦ C [17]. Therefore, it is expected that the formation of NiAl occurs with a high rate during ball milling from Al and NiO powders. The fundamental and superlattice peaks of NiAl(B2) are shown in Table 1. The superlattice (1 0 0) reflection of NiAl(B2) appeared on the XRD patterns after 0.5 h of milling suggesting that ordered phase is formed. The average crystallite size and internal strain of powder after milling was determined by Williamson–Hall equation [19]: ˇ cos =
0.9 + 2ε sin d
(2)
where is the Bragg diffraction angle, d is the crystallite size, ε is the average internal strain, is the wavelength of the radiation used, and ˇ is the diffraction peak width at half-maximum intensity. Instrumental peak broadening was corrected by using a coarse grain strain free sample of pure annealed Ni. The shape of both contributions coming from the refinement and strain was assumed Fig. 5. SEM micrograph of NiO–34 wt% Al–41 wt% Ni powder after: (a) 10 h and (b) 30 h milling times.
Gaussian. The average crystallite sizes of NiAl and Al2 O3 after 0.5 h of processing were 35 and 60 nm, respectively. Fig. 3 shows XRD patterns from NiO–34 wt% Al–41 wt% Ni powder mixture as-received and after different milling times. As-
Fig. 4. Cross-sectional microstructure of NiO–34 wt% Al–41 wt% Ni powder particles after: (a) 5 h and (b) 10 h of milling times.
Fig. 6. XRD traces of NiAl–20 vol.% Al2 O3 powders (a) as-milled for 60 h and (b) after subsequent annealing for 4 h at 800 ◦ C.
S.Z. Anvari et al. / Journal of Alloys and Compounds 477 (2009) 178–181
received powder mixture consists of NiO, Al and Ni that mixed according to the following reaction: Ni + 3NiO + 6Al → 4NiAl + Al2 O3
(2)
As seen in Fig. 3 increasing milling time to 10 h led to the disappearance of the Al and NiO peaks, while NiAl and Al2 O3 peaks began to appear. Complete transformation of NiO, Al and Ni to the NiAl and Al2 O3 phase occurred after 30 h of ball milling time. With increasing the milling time to 40 h no further reaction took place and only the XRD peaks broadened. In this system NiAl and Al2 O3 appears gradually. Mechanical alloying can be explained in two mechanisms [21]: (1) Gradual interdiffusion of components and formation of more products by increasing milling time. (2) Suddenly formation of products in a short period of milling time and consequently occurrence of mechanically alloyed selfsustaining reaction (MSR). WC formation from W and C raw materials and TiC system with Ti and C are examples of the first and second mechanism, respectively [22]. Although Al and NiO with stiochiometric composition react during MA in highly exothermic, dilution of reactants increases the ignition temperature and therefore reaction mode is changed to gradual. In the present work, due to the dilution of the Ni phase, the value of Tad of reaction and the reaction velocity can be remarkably reduced. Wu has also reported that dilution of reactants in Ti–C system increases the ignition temperature for explosion reaction [23]. The crystallite size of NiAl and Al2 O3 phases is shown in Table 2. Increasing milling time to 60 h decreases the NiAl and Al2 O3 grain size to 24 and 15 nm, respectively. Fig. 4 shows microstructure of powder milled for 5 and 10 h. An inhomogeneous lamellar structure was developed in the powder particles. The light phase is NiO and the dark phase is mixture of Ni and Al. With further milling, the microstructure became finer. Fig. 5 shows the morphology of powders after 10 and 30 h of milling times. As it can be seen, there is a distribution of very small irregular particles and large agglomerates consisting of very fine particles. The average size of powder particle after 10 h is 5 m, and after 30 h of milling time it became finer as about 1 m (Fig. 5b). 3.2. Heat treatment Thermal stability of NiAl–Al2 O3 composite structure was investigated by annealing of milled powders at 800 ◦ C for 4 h. The XRD patterns of samples as-milled for 60 h and after subsequent anneal-
181
ing are shown in Fig. 6. The width of XRD peaks of NiAl and Al2 O3 decreased and their intensity increased after annealing due to the reduction of lattice strain as well as grain growth. The grain size increases from 24 to 40 nm and the lattice strain decreases from 2.9% to 0.2%. Thermal behavior results showed that nanocomposite powder is stable during heating. 4. Conclusions NiAl–Al2 O3 nanocomposite has been successfully synthesized by ball milling of mixture of NiO, Al and Ni powders. It is found that NiAl–40 vol.% Al2 O3 nanocomposite formed with high rate. In contrast for NiAl–20 vol.% Al2 O3 compositions high exothermic reaction did not occur during the milling process, due to the higher Ni content in initial mixture. Increasing Ni concentration increased the start time of NiAl and Al2 O3 formation, and changed the morphology of products. Thermal behavior results showed that nanocomposite powders are stable during heating. References [1] K. Morsi, Mater. Sci. Eng. A 299 (2001) 1–15. [2] S. Milenkovic, R. Caram, J. Mater. Process. Technol. 143–144 (2003) 629– 635. [3] C. Lin, S. Hong, P. Lee, Intermetallics 8 (2000) 1042–1048. [4] S.G. Pyo, P. Nash, N.J. Kim, Scripta Mater. 34 (1996) 1231–1235. [5] L.Z. Zhou, J.T. Guo, G.S. Li, L.Y. Xiong, S.H. Wang, C.G. Li, Mater. Des. 18 (1997) 373–377. [6] C.C. Koch, Mater. Sci. Eng. A 244 (1998) 39–48. [7] F. Karimzadeh, M.H. Enayati, M. Tavoosi, Mater. Sci. Eng. A 486 (2008) 45–48. [8] T. Mousavi, F. Karimzadeh, M.H. Abbasi, Mater. Sci. Eng. A 487 (2008) 46–51. [9] S.C. Tjong, H. Chen, Mater. Sci. Eng. R 45 (2004) 1–88. [10] M. Khodaei, M.H. Enayati, F. Karimzadeh, J. Mater. Sci. 43 (2008) 132–138. [11] P.M. Botta, R.C. Mercader, E.F. Aglietti, J.M. Porto Lopez, Scripta Mater. 48 (2003) 1093–1098. [12] C. Suryanarayana, E. Ivanov, V.V. Boldyrev, Mater. Sci. Eng. A 304–306 (2001) 151–158. [13] M. Zakeri, R. Yazdani-Rad, M.H. Enayati, M.R. Rahimipoor, Mater. Sci. Eng. A 430 (2006) 185–188. [14] C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1–184. [15] B.S.B. Reddy, K. Das, S.K. Pabi, S. Das, Mater. Sci. Eng. A 445–446 (2007) 341–346. [16] T.R. Smith, K.S. Vecchio, Nanostruct. Mater. 5 (1995) 11–23. [17] H.X. Zhu, R. Abbaschian, Mater. Sci. Eng. A 282 (2002) 1–7. [18] M.H. Enayati, F. Karimzadeh, S.Z. Anvari, J. Mater. Process. Technol. 200 (2008) 312–315. [19] K. Williamson, W.H. Hall, Acta Metall. 1 (1953) 22–31. [20] J. Li, F. Li, K. Hu, J. Mater. Process. Technol. 147 (2004) 236–240. [21] C.J. Lu, Z.Q. Li, J. Alloys Compd. 395 (2005) 88–92. [22] N.J. Calos, J.S. Forrester, G.B. Schaffer, J. Solid State Chem. 158 (2001) 268–278. [23] N.Q. Wu, Mater. Sci. Eng. A 257 (1998) 357–360.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and characterization of NiAl–Al2 O3 nanocomposite powder by mechanical alloying S.Z. Anvari, F. Karimzadeh ∗ , M.H. Enayati Department of Materials Engineering, Nanotechnology and Advanced Materials Institute, Isfahan University of Technology (IUT), Isfahan 84156-83111, Iran
a r t i c l e
i n f o
Article history: Received 9 July 2008 Received in revised form 11 October 2008 Accepted 14 October 2008 Available online 2 December 2008 Keywords: Nanocomposite Mechanical alloying Mechanochemical processing NiAl–Al2 O3
a b s t r a c t The synthesis and characterization of NiAl–Al2 O3 nanocomposite powders containing different amount of Al2 O3 (20 and 40 vol.% of Al2 O3 ) was investigated. Al, Ni and NiO subjected to high-energy ball milling in a planetary ball mill. The structural changes of powder particles during mechanical alloying were studied by X-ray diffractometery and scanning electron microscopy. The mechanical alloying of NiO and Al powder mixture corresponding to the stiochiometric reaction 3NiO + 5Al resulted in the formation of NiAl–40 vol.% Al2 O3 after 0.5 h with rapidly mode. The crystallite size of NiAl and Al2 O3 were 35 and 60 nm, respectively. The formation of NiAl–20 vol.% Al2 O3 took place after 30 h of milling of NiO–34 wt% Al–41 wt% Ni powder mixture. For this composition exothermic reaction with high rate did not occur during the milling process. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years, there has been increased activity in research on materials based on NiAl for high temperature applications. The attractive attributes of NiAl leading to this interest include a high melting temperature, low density, high thermal conductivity and low raw materials cost. Two principle limitations of NiAl are poor toughness at room temperature and inadequate strength and creep resistance at elevated temperature [1–4]. It was showed that the synthesis of nanostructured materials has been successful in increasing ductility in some intermetallics [5]. On the other hand, for optimization of room temperature toughness and high temperature strength, microstructural modifications are required. For these reasons it has been recognized that intermetallic matrix composites (IMCs) may be appropriate materials for structural applications, replacing in some cases intermetallics [6]. Novel fabrication technology of nanostructured materials is versatile and includes a wide range of vapor, liquid and solid state processing routes. The solid state route preparation takes place via mechanical alloying (MA) and mechanochemical synthesis [7–9]. Mechanochemical synthesis involves mechanical activation of solid state displacement reactions in a ball mill. Thus, mechanical energy is used to induce chemical reactions. The chemical precur-
∗ Corresponding author. Tel.: +98 311 3915744; fax: +98 311 3912752. E-mail address: karimzadeh [email protected] (F. Karimzadeh). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.10.043
sors typically consist of mixtures of oxides, chlorides and/or metals that react either during milling or during subsequent heat treatment to form a composite powder consisting of the dispersion of ultrafine particles within a matrix [9–12]. The mechanochemical reactions fall into two categories, namely (a) those which occur during the mechanical activation process and there the reaction enthalpy is highly negative (e.g., adiabatic temperature Tad = 1300–1800 K), and (b) those which occur during subsequent thermal treatment and here the reaction enthalpy is only moderate (e.g., Tad < 1300 K). The first type of reaction takes place in two distinct modes, i.e., either combustion reaction or a progressive reaction [13–15]. Researchers have also studied the formation of NiAl–Al2 O3 composites by milling a mixture of Ni, Al, and Al2 O3 powders [3] and NiAl and Al2 O3 powders [16]. NiO and Al have been used for the synthesis of NiAl–Al2 O3 composite via termite reaction [17]. An earlier study demonstrated the synthesis of nanocrystalline NiAl by mechanical alloying of Ni and Al powder mixtures. The mechanical alloying resulted in the gradual formation of nanocrystalline NiAl. It was found that NiAl phase develops by continuous diffusive reaction at Ni/Al layers interfaces [18]. In the present work, we used NiO, Al powders mixture with and without Ni to synthesis NiAl–Al2 O3 composite by MA. The phase transformation in milling process was investigated. The chemical reaction involved here can be expressed as:
3NiO + 5Al → 3NiAl + Al2 O3
(1)
S.Z. Anvari et al. / Journal of Alloys and Compounds 477 (2009) 178–181
179
Table 1 The fundamental and superlattice peaks of the NiAl(B2) phase. Fundamental peaks of NiAl
Superlattice peaks of NiAl
(h k l)
Angle (2)
(h k l)
Angle (2)
110 200 211
44.865 65.325 82.455
100 111 210
31.305 55.425 73.875
to the reaction (1): G◦ 298 = −1298.59 kJ/mol H ◦ 298 = −1311.2 kJ/mol
Fig. 1. XRD traces of NiO–38 wt% Al powder mixtures as-received and after mechanical alloying.
2. Experimental NiO (99.9% purity and particle size of <40 m), Al (99.5% purity and particle size of 30–150 m) and Ni (99.5% purity and particle size of ∼100 m) elemental powders were used as starting materials. Mechanical alloying was carried out in a planetary ball mill equipped with hardened chromium steel container and balls. The milling media consisted of four 20 mm diameter balls confined in a 120 ml volume container. A total of 16 g powder with no process control agent was used in all MA runs. The powders were mixed with starting composition of NiO–38 wt% Al and NiO–34 wt% Al–41 wt% Ni, in order to give the final product NiAl–Al2 O3 composite with about 40 and 20 vol.% contribution of alumina. Structural changes of powder particles were studied by X-ray diffraction (XRD) in a Philips X’PERT MPD diffractometer using filtered Cu K␣ radiation ( = 0.1542 nm). Crystal size and internal strain in the milled samples were calculated from the XRD line broadening using the Williamson–Hall equation [19]. The powder particles morphology was investigated by scanning electron microscopy (SEM) in a Philips XL30 at an acceleration voltage of 30 kV with an energy-dispersive X-ray spectrometer (EDX) attachment. Heat treatment of asmilled powders was conducted in a tube furnace under Ar atmosphere.
During milling in room temperature this reaction can thermodynamically occur due to its negative free energy change. H ◦ 298 for Al–NiO reaction is also negative, indicating that the Al–NiO reaction is exothermic [17]. X-ray diffraction patterns of NiO–38 wt% Al powder mixture after different milling times are shown in Fig. 1. As-received powder mixture consisted of NiO and Al. After 0.5 h of MA the XRD peaks of the raw material disappeared due to the reaction of NiO and Al to give NiAl and Al2 O3 phase. It is worthwhile to notice that this phase transformation was accompanied by an increase of the temperature of the container. This is suggesting that the reaction of NiO and Al takes place highly exothermic. Fig. 2 shows the SEM micrograph of the NiAl–40 vol.% Al2 O3 powder particles after 0.5 h of milling. The fused-shape morphology of powder particles indicates that NiO–Al reaction occurred rapidly. Although the powders are more loosely dispersed in the container during ball milling, the mixing of the reactants on a nanometer scale favors mass transfer and the diffusion path length is considerably reduced. It is believed that a high heat of formation serves as the driving force of the exothermic reaction with high rate during the milling processes. Researches have shown that the adiabatic temperature (Tad ), which is the maximum temperature achieved under adiabatic conditions as a consequence
3. Results and discussion 3.1. Mechanical alloying NiAl–40 vol.% Al2 O3 nanocomposite can be synthesized by a displacement reaction during ball milling of NiO–38 wt% Al according
Fig. 2. SEM micrograph of the NiAl–40 vol.% Al2 O3 powder particles after 0.5 h of milling.
Fig. 3. XRD traces of NiO–34 wt% Al–41 wt% Ni powder mixtures as-received and after different milling times.
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S.Z. Anvari et al. / Journal of Alloys and Compounds 477 (2009) 178–181
Table 2 Grain size evolution of the synthesized phases during milling. Milling time (h)
d (NiAl) (nm)
d (Al2 O3 ) (nm)
20 30 40 60
30 27 25 24
43 38 20a 15a
a
Scherrer method was used.
of the evolution of heat from the reaction, should be above 1800 K in thermally ignited systems [20]. Thus, the value of Tad can be used as a rough guide to predict the reaction rate during milling process. According to the thermodynamic calculation the value of Tad of reaction (1) is about 3500 ◦ C [17]. Therefore, it is expected that the formation of NiAl occurs with a high rate during ball milling from Al and NiO powders. The fundamental and superlattice peaks of NiAl(B2) are shown in Table 1. The superlattice (1 0 0) reflection of NiAl(B2) appeared on the XRD patterns after 0.5 h of milling suggesting that ordered phase is formed. The average crystallite size and internal strain of powder after milling was determined by Williamson–Hall equation [19]: ˇ cos =
0.9 + 2ε sin d
(2)
where is the Bragg diffraction angle, d is the crystallite size, ε is the average internal strain, is the wavelength of the radiation used, and ˇ is the diffraction peak width at half-maximum intensity. Instrumental peak broadening was corrected by using a coarse grain strain free sample of pure annealed Ni. The shape of both contributions coming from the refinement and strain was assumed Fig. 5. SEM micrograph of NiO–34 wt% Al–41 wt% Ni powder after: (a) 10 h and (b) 30 h milling times.
Gaussian. The average crystallite sizes of NiAl and Al2 O3 after 0.5 h of processing were 35 and 60 nm, respectively. Fig. 3 shows XRD patterns from NiO–34 wt% Al–41 wt% Ni powder mixture as-received and after different milling times. As-
Fig. 4. Cross-sectional microstructure of NiO–34 wt% Al–41 wt% Ni powder particles after: (a) 5 h and (b) 10 h of milling times.
Fig. 6. XRD traces of NiAl–20 vol.% Al2 O3 powders (a) as-milled for 60 h and (b) after subsequent annealing for 4 h at 800 ◦ C.
S.Z. Anvari et al. / Journal of Alloys and Compounds 477 (2009) 178–181
received powder mixture consists of NiO, Al and Ni that mixed according to the following reaction: Ni + 3NiO + 6Al → 4NiAl + Al2 O3
(2)
As seen in Fig. 3 increasing milling time to 10 h led to the disappearance of the Al and NiO peaks, while NiAl and Al2 O3 peaks began to appear. Complete transformation of NiO, Al and Ni to the NiAl and Al2 O3 phase occurred after 30 h of ball milling time. With increasing the milling time to 40 h no further reaction took place and only the XRD peaks broadened. In this system NiAl and Al2 O3 appears gradually. Mechanical alloying can be explained in two mechanisms [21]: (1) Gradual interdiffusion of components and formation of more products by increasing milling time. (2) Suddenly formation of products in a short period of milling time and consequently occurrence of mechanically alloyed selfsustaining reaction (MSR). WC formation from W and C raw materials and TiC system with Ti and C are examples of the first and second mechanism, respectively [22]. Although Al and NiO with stiochiometric composition react during MA in highly exothermic, dilution of reactants increases the ignition temperature and therefore reaction mode is changed to gradual. In the present work, due to the dilution of the Ni phase, the value of Tad of reaction and the reaction velocity can be remarkably reduced. Wu has also reported that dilution of reactants in Ti–C system increases the ignition temperature for explosion reaction [23]. The crystallite size of NiAl and Al2 O3 phases is shown in Table 2. Increasing milling time to 60 h decreases the NiAl and Al2 O3 grain size to 24 and 15 nm, respectively. Fig. 4 shows microstructure of powder milled for 5 and 10 h. An inhomogeneous lamellar structure was developed in the powder particles. The light phase is NiO and the dark phase is mixture of Ni and Al. With further milling, the microstructure became finer. Fig. 5 shows the morphology of powders after 10 and 30 h of milling times. As it can be seen, there is a distribution of very small irregular particles and large agglomerates consisting of very fine particles. The average size of powder particle after 10 h is 5 m, and after 30 h of milling time it became finer as about 1 m (Fig. 5b). 3.2. Heat treatment Thermal stability of NiAl–Al2 O3 composite structure was investigated by annealing of milled powders at 800 ◦ C for 4 h. The XRD patterns of samples as-milled for 60 h and after subsequent anneal-
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ing are shown in Fig. 6. The width of XRD peaks of NiAl and Al2 O3 decreased and their intensity increased after annealing due to the reduction of lattice strain as well as grain growth. The grain size increases from 24 to 40 nm and the lattice strain decreases from 2.9% to 0.2%. Thermal behavior results showed that nanocomposite powder is stable during heating. 4. Conclusions NiAl–Al2 O3 nanocomposite has been successfully synthesized by ball milling of mixture of NiO, Al and Ni powders. It is found that NiAl–40 vol.% Al2 O3 nanocomposite formed with high rate. In contrast for NiAl–20 vol.% Al2 O3 compositions high exothermic reaction did not occur during the milling process, due to the higher Ni content in initial mixture. Increasing Ni concentration increased the start time of NiAl and Al2 O3 formation, and changed the morphology of products. Thermal behavior results showed that nanocomposite powders are stable during heating. References [1] K. Morsi, Mater. Sci. Eng. A 299 (2001) 1–15. [2] S. Milenkovic, R. Caram, J. Mater. Process. Technol. 143–144 (2003) 629– 635. [3] C. Lin, S. Hong, P. Lee, Intermetallics 8 (2000) 1042–1048. [4] S.G. Pyo, P. Nash, N.J. Kim, Scripta Mater. 34 (1996) 1231–1235. [5] L.Z. Zhou, J.T. Guo, G.S. Li, L.Y. Xiong, S.H. Wang, C.G. Li, Mater. Des. 18 (1997) 373–377. [6] C.C. Koch, Mater. Sci. Eng. A 244 (1998) 39–48. [7] F. Karimzadeh, M.H. Enayati, M. Tavoosi, Mater. Sci. Eng. A 486 (2008) 45–48. [8] T. Mousavi, F. Karimzadeh, M.H. Abbasi, Mater. Sci. Eng. A 487 (2008) 46–51. [9] S.C. Tjong, H. Chen, Mater. Sci. Eng. R 45 (2004) 1–88. [10] M. Khodaei, M.H. Enayati, F. Karimzadeh, J. Mater. Sci. 43 (2008) 132–138. [11] P.M. Botta, R.C. Mercader, E.F. Aglietti, J.M. Porto Lopez, Scripta Mater. 48 (2003) 1093–1098. [12] C. Suryanarayana, E. Ivanov, V.V. Boldyrev, Mater. Sci. Eng. A 304–306 (2001) 151–158. [13] M. Zakeri, R. Yazdani-Rad, M.H. Enayati, M.R. Rahimipoor, Mater. Sci. Eng. A 430 (2006) 185–188. [14] C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1–184. [15] B.S.B. Reddy, K. Das, S.K. Pabi, S. Das, Mater. Sci. Eng. A 445–446 (2007) 341–346. [16] T.R. Smith, K.S. Vecchio, Nanostruct. Mater. 5 (1995) 11–23. [17] H.X. Zhu, R. Abbaschian, Mater. Sci. Eng. A 282 (2002) 1–7. [18] M.H. Enayati, F. Karimzadeh, S.Z. Anvari, J. Mater. Process. Technol. 200 (2008) 312–315. [19] K. Williamson, W.H. Hall, Acta Metall. 1 (1953) 22–31. [20] J. Li, F. Li, K. Hu, J. Mater. Process. Technol. 147 (2004) 236–240. [21] C.J. Lu, Z.Q. Li, J. Alloys Compd. 395 (2005) 88–92. [22] N.J. Calos, J.S. Forrester, G.B. Schaffer, J. Solid State Chem. 158 (2001) 268–278. [23] N.Q. Wu, Mater. Sci. Eng. A 257 (1998) 357–360.