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TiC–Al2O3 nanocomposite by mechanical alloying and subsequent heat treatment
Ceramics International 42 (2016) 8895–8899
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Synthesis of Al/TiC–Al2O3 nanocomposite by mechanical alloying and subsequent heat treatment M. Zarezadeh Mehrizi n, R. Beygi, Gh. Eisaabadi Department of Materials Science and Engineering, Faculty of Engineering, Arak University, Arak 38156-8-8349, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 3 February 2016 Received in revised form 21 February 2016 Accepted 23 February 2016 Available online 27 February 2016
In this study, Al/TiC–Al2O3 nanocomposite was synthesized via mechanical alloying and heat treatment. Phase development and structural changes were studied by X-ray diffraction technique and fieldemission gun scanning electron microscopy. The results showed that the phase evolutions during mechanical alloying were strongly dependent on milling time until 20 h by formation of Al3Ti and Al2O3. After that, by increasing milling time, no new phases formed. Heat treatment of 60 h milled powder showed that no new phases (especially TiC) were found after annealing at 500 °C. But increasing temperature to 1000 °C caused the Al3Ti and TiO2 peaks disappeared and TiC peaks emerged. These results confirmed that the formation of TiC is not feasible in an Al–TiO2–C system with excess Al by mechanical alloying. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Al/TiC–Al2O3 Mechanical alloying Milling time Heat treatment
1. Introduction Metal matrix composites (MMCs) reinforced with carbides, nitrides, borides or oxides have been regarded as materials that meet various modern engineering needs where a combination of multiple/complex properties is required [1]. These composites combine metallic properties (ductility and toughness) with ceramic characteristics (high-strength and modulus), leading to greater strength in shear and compression and to higher service temperature capabilities [2]. The high specific strength and stiffness, good toughness and wear resistance of Al/TiC–Al2O3 composite have attracted considerable attention in fields of aeronautics, astronautics, automobile industries [3,4]. In some early reports, TiC and Al2O3-reinforced Al composites with improved mechanical properties and wear resistance were manufactured by field-activated combustion synthesis [3], microwave sintering [5] and spark plasma sintering [6]. The improved mechanical properties can be further obtained by decreasing the grain size of both the matrix and the reinforced phase to nanometer scale [7]. The formation of alloys by solid state reaction that occurs during ball milling of elemental powders is called mechanical alloying. This process is characterized by repeated welding and fracturing of powder particles, and microstructural changes during mechanical alloying are influenced by the mechanical behavior, the powder components and process variables [8,9]. Mechanical alloying (MA) is a solid state technique used for fabrication of n
Corresponding author.
http://dx.doi.org/10.1016/j.ceramint.2016.02.144 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
nanocrystalline alloys and intermetallic compounds. MA allows materials scientists to overcome material limitations and to manufacture alloys that are difficult or impossible to be produced by conventional melting and casting techniques. Through this technique, the fine-grained powders can be formed [10–13]. Two kinds of reaction mechanism for MA have been proposed [7,14]: (1) gradual inter-diffusion of elements and formation of new phases and products by prolonged milling time. (2) Sudden formation of products in a short period of milling time and, therefore, occurrence of mechanically alloyed self-sustaining reaction (MSR) (the calculated adiabatic temperature41800 K). WC formation from W and C raw materials and TiC compound from Ti and C are examples of the first and second mechanism, respectively [7]. There have been some studies to synthesize TiC–Al2O3 nanocomposite by mechanical alloying. Razavi and coworkers used impure Ti chips and elemental powders of Al and carbon black, TiC–Al2O3 was formed during the annealing of milled powder [15]. Kumar et al. synthesized the Al2O3–TiC composite from a mixture of graphite, Al2O3 and TiO2 powders by high energy ball milling followed by spark plasma sintering (SPS) process [6]. Hajalilou et al. synthesized TiC–Al2O3 nanocomposite from titanium dioxide (rutile type), aluminum and graphite powders by mechanical alloying [7]. In these studies (TiC–Al2O3 formation), mechanically alloyed self-sustaining reaction have been proposed as reaction mechanism for MA because the calculated adiabatic temperature
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were more than 1800 K. An excess amount of Al in Al–TiO2–C system will decrease the adiabatic temperature below the threshold value (1800 K) and the SHS reaction cannot take place or self-sustain and reaction mechanism will change. For (4 þx)Al–3TiO2–3C system, if the excess Al content, x, is more than 9 mol, it will be impossible to initiate reaction of the system because its adiabatic combustion temperature is below 1800 K. To the authors' knowledge, no articles were found dealing with the investigation of TiC formation mechanism in Al–TiO2–C system with an excess amount of Al by mechanical alloying. In this study, the 14Al–3TiO2–3C system was evaluated by mechanical alloying and heat treatment and effect of excess Al on TiC formation mechanism has been investigated. The phase transformation, microstructure and morphological evolutions of powders during mechanical alloying and heat treatment were also investigated.
2. Experimental procedure Fig. 1. X-ray diffraction results of powders milled for different durations.
The Al/TiC–Al2O3 nanocomposite from starting powders of Al ( 50 mm, 99.2%), TiO2 ( 10 mm, 99.5%), and graphite ( 10 mm, 99.9%) was prepared by reaction as:
3TiO2 + 3 C + (4 + x) Al=3TiC+2Al2 O3 + x Al
(1)
The powders were mixed in the desired stoichiometry (Reaction (1)) with excess Al content x ¼10 mol to produce Al/TiC–Al2O3 composite. Mechanical alloying of powder mixture was performed in a planetary ball mill at room temperature. The milling media consisted of balls with different diameters, confined in a 310 mL volume vial. The ball and bowl materials were made of hardened chromium steel. The ball to powder weight ratio was 20:1, and the vial rotation speed was 350 rev min 1. A total amount of 10 g of powder mixture was milled. No process control agent was added to powder mixture. Isothermal annealing of the milled specimens was carried out at 1000 °C for 1 h. The phase changes that occurred during ball milling and heat treatment were investigated by X-ray diffraction analysis using a Philips X'PERT multipurpose X-ray diffractometer (Philips Analytical BV, Almelo, The Netherlands) with Cu Kα radiation (0.15405 nm). The crystallite size and internal strain of powders were estimated using the Williamson–Hall method [16]:
β cos θ =
0.91λ + 2Aε sin θ d
(2)
where θ is the Bragg diffraction angle, d is the crystallite size, ε is the average internal strain, λ is the wavelength of the radiation used, β is the diffraction peak width at half-maximum intensity, and A is the coefficient, which depends on the distribution of strain. The average internal strain can be estimated from the linear slope of β cos θ versus sin θ, and the average crystallite size can be estimated from intersect of this line at sin θ ¼0. The microstructure and morphology of powder particles were analyzed using a field-emission gun scanning electron microscope (MIRA3 TESCAN).
3. Results and discussion 3.1. Phase development Fig. 1 showed XRD patterns of Al, TiO2 and C (graphite) powder mixture as received and after different milling times. As seen, diffraction patterns showed strong Al peaks because Al has a highmass percentage. Also, the intensity of Al and TiO2 diffraction
peaks decreased during MA and their width increased progressively with increasing milling time. With an increase in milling time up to 5 h, the (002) reflections of graphite completely disappeared indicating that graphite particles were heavily deformed and fractured into fine slices. On the other hand, the intensities of graphite peaks declined significantly while no new phase was observed. This drop can be attributed to the mixing of powders at the beginning of ball milling. The brittle behavior of graphite after 5 h of milling made the grain size to decrease significantly. In addition with increasing milling time, the intensity of Al and TiO2 peaks decreased and their width increased as a result of refinement of crystallite size and enhancement of lattice strain. As milling continued to 10 h, the Al peaks slightly shifted towards higher angles. Since graphite has smaller radii than Al, institutional dissolution of graphite into Al lattices decreased the lattice parameter of Al. These changes suggest that a Al(C) solid solution begins to form during this stage of the process. In this pattern, it can see broad Al-based solid solution peaks and a small broad halo. The peak broadening may be due to severe plastic deformation or due to an amorphous component seeming to overlap the most intense peak [11]. Further milling time (20 h), new peaks related to Al2O3 and Al3Ti appeared, suggesting that the reactions in the as-milled powders of 14Al þ3TiO2 þ 3C took place not according to Reaction (1). This result has been reported previously by Kim and coworkers in combustion reaction in Al–TiO2–C system with excess Al [17]. By increasing the milling time from 20 h to 60 h, no structural change was detected except peak broadening due to the crystallite size reduction. In an attempt to activate the un-reacted powders, 60 h milled powders were annealed at different temperatures for 1 h (Fig. 2). As shown in Fig. 2, no new phases (especially TiC) were found after annealing at 500 °C. The width of XRD peaks decreased and their intensity increased due to stress release as well as grain growth. But phase identification after heat treatment at 1000 °C of 60 h milled powder indicates that the Al3Ti and TiO2 peaks disappeared and TiC peaks emerged. It can be seen that TiO2 and Al3Ti completely changed to new phases. According to Fig. 2, after annealing of 60 h milled powder at 1000 °C, the average crystallite size of Al, TiC and Al2O3 were ∼43 nm, ∼28 nm and ∼31 nm, respectively. The mean lattice strains for this sample, determined by the same formula, were 0.506%, 0.554% and 0.405% for Al, TiC and Al2O3, respectively.
M.Z. Mehrizi et al. / Ceramics International 42 (2016) 8895–8899
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Fig. 2. XRD patterns of powder mixture before and after annealing.
3.2. Mechanism of TiC formation Previous studies showed that Al2O3–TiC nanocomposite was synthesized by mechanical alloying according to Reaction (1) with x ¼0 (without any Al in final product) which involved two reactions: reduction of TiO2 by Al to form elemental Ti followed by the reaction of Ti with C to form TiC. These two reactions can be represented as a thermite reaction, Reaction (3), and a subsequent synthesis reaction, Reaction (4):
4Al + 3TiO2 = 2Al2 O3 + 3Ti ΔG°298K = − 500.2 kJ/mol, ΔH °298K = − 521.2 kJ/mol
(3)
to 1000 °C, molten Al reacted with remained TiO2 according to Reaction (5) and Al3Ti/ Al2O3 formed. Also, the rest of molten Al dissolved graphite particles and Al3C4 phase was formed (Reaction (6)). Based on Hu et al. proposed mechanism, Al3C4 and Al3Ti reacted together and TiC was formed (Reaction (7)). The following reactions are: During mechanical alloying:
13Al + 3TiO2 → 2Al2 O3 + 3Al3 Ti ΔG°298K = −1022.0 kJ mol−1, ΔH °298K = −960.4 kJ mol−1
(5)
During heat treatment at 1000 °C:
4Al + 3C → Al4 C3 ΔG°298K = −242.5 kJ mol−1, ΔH °298K = −215.7 kJ mol−1
Ti+C = TiC, ΔG°298K 298 = −180 kJ/mol, ΔH °298K = −183.7 kJ/mol
(6)
(4)
Hu et al. [3] studied the formation mechanism of TiC in Al–TiO2 –C system containing an excess amount of Al (in comparison of Reaction (1)) during combustion synthesis. They reported that TiC was not produced by a single reaction as given by Eq. (1), but rather formed as a result of a series of reactions that occurred between the reactions products produced during the intermediate stages. At first, Al tended to react with TiO2 due to its high affinity, forming Al2O3 and Al3Ti, and Al4C3 was produced by the reaction between Al and C. Subsequently, these intermittent reaction products, Al3Ti and Al4C3, reacted with each other to form TiC. The mechanical alloying is a chemical solid state process, and diffusion has an essential role on the formation of different phases in solid state [10,11]. In this study, TiC was not formed during the mechanical alloying. The XRD results confirmed that Reaction (5) has been taken place during ball milling and Al3Ti formed. Heat treatment of 60 h milled powder at 500 °C has no effect and no new phases were formed. By increasing of annealing temperature
3Al3 Ti + Al4 C3 → 3TiC + 13Al ΔG°298K = −125.2 kJ mol−1, ΔH °298K = −116.7 kJ mol−1
(7)
This results showed that TiC was not formed in solid state in Al–TiO2–C system with an excess amount of Al. During mechanical alloying just one of the intermediate reaction products, Al3Ti and Al2O3 were formed. The subsequent heat treatment must be done at temperatures above Al melting point. In this temperatures, molten Al reacted to graphite and Al4C3 intermediate product formed. The reaction between Al3Ti and Al4C3 caused to TiC formation.
4. Morphological changes Fig. 3 showed the morphology of powder particles during ball milling at various durations. As can be seen in Fig. 3a, after 5 h milling, raw material powders in the form of big agglomerates and
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Fig. 3. Cross-sectional SEM micrographs of powder mixture after (a) 5 h, (b) 20 h, (c) 40 h, (d) 60 h of milling times and (e), (f) high-magnification of 60 h milled powder at two different zones.
fine particles with irregular morphologies and heterogeneous distribution were observed. The formation of fine particles with agglomerates could be attributed to cold welding and plastic deformation of powders. The fine particles agglomerated beyond 20 h of milling due to the occurrence of an exothermic reaction between the raw materials according to Reaction (3) and the formation of Al3Ti and Al2O3, which increased the temperature locally (Fig. 3b). Also, the existence of fine Al and C powders caused these particles to adhere together, and further agglomerates were formed. Increasing milling time to 40 h decreased the size of agglomerates again as a result of work hardening and the increasing fracture rate of powders when compared to the cold welding rate
(Fig. 3c). In fact during MA, between 20 and 40 h, work hardening of powder takes place and fracturing overcame cold welding and the distribution of the powder particles became more homogenous. With increasing milling time from 40 h to 60 h, powder particles adhered together and agglomeration process occurred. The average powder particles size seems to be within 1–10 mm (Fig. 3d). High-resolution SEM images for different powder particles milled after 60 h confirmed that the average particles size of agglomerates is 25 nm (Fig. 3e and f). 5. Conclusion The following conclusions can be made from the present research:
M.Z. Mehrizi et al. / Ceramics International 42 (2016) 8895–8899
Al/TiC–Al2O3 nanocomposite was formed by mechanical alloy
ing and subsequent heat treatment. During ball milling of Al–TiO2–C with excess Al, TiC was not formed. During ball milling, Al3Ti and Al2O3 formed after 20 h and by increasing milling time to 60 h, no new phases were formed. The annealing of 60 h milled powder showed that no new phases (especially TiC) were found after annealing at 500 °C. But increasing temperature to 1000 °C caused the Al3Ti and TiO2 peaks disappeared and TiC peaks emerged.
References [1] S.G. Son, B.H. Lee, J.M. Lee, J.C. Lee, Low-temperature synthesis of (TiC þAl2O3)/ Al alloy composites based on dopant-assisted combustion, J. Alloy. Compd. 649 (2015) 409–416. [2] G. Xiao, Q. Fan, M. Gu, Z. Jin, Microstructural evolution during the combustion synthesis of TiC–Al cermet with larger metallic particles, Mater. Sci. Eng. A 425 (2006) 318–325. [3] Q. Hu, P. Luo, Y. Yan, Microstructures, densification and mechanical properties of TiC–Al2O3–Al composite by field-activated combustion synthesis, Mater. Sci. Eng. A 486 (2008) 215–221. [4] H. Zhu, Y. Jiang, Y. Yao, J. Song, J. Li, Z. Xie, Reaction pathways, activation energies and mechanical properties of hybrid composites synthesized in-situ from Al–TiO2–C powder mixtures, Mater. Chem. Phys. 137 (2012) 532–542. [5] Y. Cheng, S. Sun, H. Hu, Preparation of Al2O3/TiC micro-composite ceramic tool materials by microwave sintering and their microstructure and properties,
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Ceram. Int. 40 (2014) 16761–16766. [6] R. Kumar, A.K. Chaubey, S. Bathula, B.B. Jha, A. Dhar, Synthesis and characterization of Al2O3–TiC nano-composite by spark plasma sintering, Int. J. Refract. Met. Hard Mater. 54 (2016) 304–308. [7] A. Hajalilou, M. Hashim, M. Nahavandi, I. Ismail, Mechanochemical carboaluminothermic reduction of rutile to produce TiC–Al2O3 nanocomposite, Adv. Powder Technol. 25 (2014) 423–429. [8] C. Suryanarayana, Mechanical alloying and milling, Prog. Mater. Sci. 46 (2001) 1–184. [9] C. Suryanarayana, N. Al-Aqeeli, Mechanically alloyed nanocomposites, Prog. Mater. Sci. 58 (2013) 383–502. [10] M. Zarezadeh Mehrizi, M. Shamanian, A. Saidi, Synthesis of CoWSi–WSi2 nanocomposite by mechanical alloying and subsequent heat treatment, Ceram. Int. 40 (2014) 9493–9498. [11] B.S.B. Reddy, K. Das, S. Das, A review on the synthesis of in situ aluminum based composites by thermal, mechanical and mechanical–thermal activation of chemical reactions, J. Mater. Sci. 42 (2007) 9366–9378. [12] C. Koch, Structural nanocrystalline materials: an overview, J. Mater. Sci. 42 (2007) 1403–1414. [13] M.Z. Mehrizi, A. Saidi, M. Shamanian, Production of Fe3Al–TiC nanocomposite by mechanical alloying of FeTi2–Al–C powders, Mater. Sci. Technol. 27 (2011) 1465–1468. [14] C.J. Lu, Z.Q. Li, Structural evolution of the Ti–Si–C system during mechanical alloying, J. Alloy. Compd. 395 (2005) 88–92. [15] M. Razavi, M.R. Rahimipour, R. Mansoori, Synthesis of TiC–Al2O3 nanocomposite powder from impure Ti chips, Al and carbon black by mechanical alloying, J. Alloy. Compd. 450 (2008) 463–467. [16] G.K. Williamson, W.H. Hall, X-ray line broadening from filed aluminium and wolfram, Acta Met. 1 (1953) 22–31. [17] J.-W. Kim, J.-M. Lee, J.-H. Lee, J.-C. Lee, Role of excess Al on the combustion reaction in the Al–TiO2–C system, Met. Mater. Int. 20 (2014) 1151–1156.
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Synthesis of Al/TiC–Al2O3 nanocomposite by mechanical alloying and subsequent heat treatment M. Zarezadeh Mehrizi n, R. Beygi, Gh. Eisaabadi Department of Materials Science and Engineering, Faculty of Engineering, Arak University, Arak 38156-8-8349, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 3 February 2016 Received in revised form 21 February 2016 Accepted 23 February 2016 Available online 27 February 2016
In this study, Al/TiC–Al2O3 nanocomposite was synthesized via mechanical alloying and heat treatment. Phase development and structural changes were studied by X-ray diffraction technique and fieldemission gun scanning electron microscopy. The results showed that the phase evolutions during mechanical alloying were strongly dependent on milling time until 20 h by formation of Al3Ti and Al2O3. After that, by increasing milling time, no new phases formed. Heat treatment of 60 h milled powder showed that no new phases (especially TiC) were found after annealing at 500 °C. But increasing temperature to 1000 °C caused the Al3Ti and TiO2 peaks disappeared and TiC peaks emerged. These results confirmed that the formation of TiC is not feasible in an Al–TiO2–C system with excess Al by mechanical alloying. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Al/TiC–Al2O3 Mechanical alloying Milling time Heat treatment
1. Introduction Metal matrix composites (MMCs) reinforced with carbides, nitrides, borides or oxides have been regarded as materials that meet various modern engineering needs where a combination of multiple/complex properties is required [1]. These composites combine metallic properties (ductility and toughness) with ceramic characteristics (high-strength and modulus), leading to greater strength in shear and compression and to higher service temperature capabilities [2]. The high specific strength and stiffness, good toughness and wear resistance of Al/TiC–Al2O3 composite have attracted considerable attention in fields of aeronautics, astronautics, automobile industries [3,4]. In some early reports, TiC and Al2O3-reinforced Al composites with improved mechanical properties and wear resistance were manufactured by field-activated combustion synthesis [3], microwave sintering [5] and spark plasma sintering [6]. The improved mechanical properties can be further obtained by decreasing the grain size of both the matrix and the reinforced phase to nanometer scale [7]. The formation of alloys by solid state reaction that occurs during ball milling of elemental powders is called mechanical alloying. This process is characterized by repeated welding and fracturing of powder particles, and microstructural changes during mechanical alloying are influenced by the mechanical behavior, the powder components and process variables [8,9]. Mechanical alloying (MA) is a solid state technique used for fabrication of n
Corresponding author.
http://dx.doi.org/10.1016/j.ceramint.2016.02.144 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
nanocrystalline alloys and intermetallic compounds. MA allows materials scientists to overcome material limitations and to manufacture alloys that are difficult or impossible to be produced by conventional melting and casting techniques. Through this technique, the fine-grained powders can be formed [10–13]. Two kinds of reaction mechanism for MA have been proposed [7,14]: (1) gradual inter-diffusion of elements and formation of new phases and products by prolonged milling time. (2) Sudden formation of products in a short period of milling time and, therefore, occurrence of mechanically alloyed self-sustaining reaction (MSR) (the calculated adiabatic temperature41800 K). WC formation from W and C raw materials and TiC compound from Ti and C are examples of the first and second mechanism, respectively [7]. There have been some studies to synthesize TiC–Al2O3 nanocomposite by mechanical alloying. Razavi and coworkers used impure Ti chips and elemental powders of Al and carbon black, TiC–Al2O3 was formed during the annealing of milled powder [15]. Kumar et al. synthesized the Al2O3–TiC composite from a mixture of graphite, Al2O3 and TiO2 powders by high energy ball milling followed by spark plasma sintering (SPS) process [6]. Hajalilou et al. synthesized TiC–Al2O3 nanocomposite from titanium dioxide (rutile type), aluminum and graphite powders by mechanical alloying [7]. In these studies (TiC–Al2O3 formation), mechanically alloyed self-sustaining reaction have been proposed as reaction mechanism for MA because the calculated adiabatic temperature
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were more than 1800 K. An excess amount of Al in Al–TiO2–C system will decrease the adiabatic temperature below the threshold value (1800 K) and the SHS reaction cannot take place or self-sustain and reaction mechanism will change. For (4 þx)Al–3TiO2–3C system, if the excess Al content, x, is more than 9 mol, it will be impossible to initiate reaction of the system because its adiabatic combustion temperature is below 1800 K. To the authors' knowledge, no articles were found dealing with the investigation of TiC formation mechanism in Al–TiO2–C system with an excess amount of Al by mechanical alloying. In this study, the 14Al–3TiO2–3C system was evaluated by mechanical alloying and heat treatment and effect of excess Al on TiC formation mechanism has been investigated. The phase transformation, microstructure and morphological evolutions of powders during mechanical alloying and heat treatment were also investigated.
2. Experimental procedure Fig. 1. X-ray diffraction results of powders milled for different durations.
The Al/TiC–Al2O3 nanocomposite from starting powders of Al ( 50 mm, 99.2%), TiO2 ( 10 mm, 99.5%), and graphite ( 10 mm, 99.9%) was prepared by reaction as:
3TiO2 + 3 C + (4 + x) Al=3TiC+2Al2 O3 + x Al
(1)
The powders were mixed in the desired stoichiometry (Reaction (1)) with excess Al content x ¼10 mol to produce Al/TiC–Al2O3 composite. Mechanical alloying of powder mixture was performed in a planetary ball mill at room temperature. The milling media consisted of balls with different diameters, confined in a 310 mL volume vial. The ball and bowl materials were made of hardened chromium steel. The ball to powder weight ratio was 20:1, and the vial rotation speed was 350 rev min 1. A total amount of 10 g of powder mixture was milled. No process control agent was added to powder mixture. Isothermal annealing of the milled specimens was carried out at 1000 °C for 1 h. The phase changes that occurred during ball milling and heat treatment were investigated by X-ray diffraction analysis using a Philips X'PERT multipurpose X-ray diffractometer (Philips Analytical BV, Almelo, The Netherlands) with Cu Kα radiation (0.15405 nm). The crystallite size and internal strain of powders were estimated using the Williamson–Hall method [16]:
β cos θ =
0.91λ + 2Aε sin θ d
(2)
where θ is the Bragg diffraction angle, d is the crystallite size, ε is the average internal strain, λ is the wavelength of the radiation used, β is the diffraction peak width at half-maximum intensity, and A is the coefficient, which depends on the distribution of strain. The average internal strain can be estimated from the linear slope of β cos θ versus sin θ, and the average crystallite size can be estimated from intersect of this line at sin θ ¼0. The microstructure and morphology of powder particles were analyzed using a field-emission gun scanning electron microscope (MIRA3 TESCAN).
3. Results and discussion 3.1. Phase development Fig. 1 showed XRD patterns of Al, TiO2 and C (graphite) powder mixture as received and after different milling times. As seen, diffraction patterns showed strong Al peaks because Al has a highmass percentage. Also, the intensity of Al and TiO2 diffraction
peaks decreased during MA and their width increased progressively with increasing milling time. With an increase in milling time up to 5 h, the (002) reflections of graphite completely disappeared indicating that graphite particles were heavily deformed and fractured into fine slices. On the other hand, the intensities of graphite peaks declined significantly while no new phase was observed. This drop can be attributed to the mixing of powders at the beginning of ball milling. The brittle behavior of graphite after 5 h of milling made the grain size to decrease significantly. In addition with increasing milling time, the intensity of Al and TiO2 peaks decreased and their width increased as a result of refinement of crystallite size and enhancement of lattice strain. As milling continued to 10 h, the Al peaks slightly shifted towards higher angles. Since graphite has smaller radii than Al, institutional dissolution of graphite into Al lattices decreased the lattice parameter of Al. These changes suggest that a Al(C) solid solution begins to form during this stage of the process. In this pattern, it can see broad Al-based solid solution peaks and a small broad halo. The peak broadening may be due to severe plastic deformation or due to an amorphous component seeming to overlap the most intense peak [11]. Further milling time (20 h), new peaks related to Al2O3 and Al3Ti appeared, suggesting that the reactions in the as-milled powders of 14Al þ3TiO2 þ 3C took place not according to Reaction (1). This result has been reported previously by Kim and coworkers in combustion reaction in Al–TiO2–C system with excess Al [17]. By increasing the milling time from 20 h to 60 h, no structural change was detected except peak broadening due to the crystallite size reduction. In an attempt to activate the un-reacted powders, 60 h milled powders were annealed at different temperatures for 1 h (Fig. 2). As shown in Fig. 2, no new phases (especially TiC) were found after annealing at 500 °C. The width of XRD peaks decreased and their intensity increased due to stress release as well as grain growth. But phase identification after heat treatment at 1000 °C of 60 h milled powder indicates that the Al3Ti and TiO2 peaks disappeared and TiC peaks emerged. It can be seen that TiO2 and Al3Ti completely changed to new phases. According to Fig. 2, after annealing of 60 h milled powder at 1000 °C, the average crystallite size of Al, TiC and Al2O3 were ∼43 nm, ∼28 nm and ∼31 nm, respectively. The mean lattice strains for this sample, determined by the same formula, were 0.506%, 0.554% and 0.405% for Al, TiC and Al2O3, respectively.
M.Z. Mehrizi et al. / Ceramics International 42 (2016) 8895–8899
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Fig. 2. XRD patterns of powder mixture before and after annealing.
3.2. Mechanism of TiC formation Previous studies showed that Al2O3–TiC nanocomposite was synthesized by mechanical alloying according to Reaction (1) with x ¼0 (without any Al in final product) which involved two reactions: reduction of TiO2 by Al to form elemental Ti followed by the reaction of Ti with C to form TiC. These two reactions can be represented as a thermite reaction, Reaction (3), and a subsequent synthesis reaction, Reaction (4):
4Al + 3TiO2 = 2Al2 O3 + 3Ti ΔG°298K = − 500.2 kJ/mol, ΔH °298K = − 521.2 kJ/mol
(3)
to 1000 °C, molten Al reacted with remained TiO2 according to Reaction (5) and Al3Ti/ Al2O3 formed. Also, the rest of molten Al dissolved graphite particles and Al3C4 phase was formed (Reaction (6)). Based on Hu et al. proposed mechanism, Al3C4 and Al3Ti reacted together and TiC was formed (Reaction (7)). The following reactions are: During mechanical alloying:
13Al + 3TiO2 → 2Al2 O3 + 3Al3 Ti ΔG°298K = −1022.0 kJ mol−1, ΔH °298K = −960.4 kJ mol−1
(5)
During heat treatment at 1000 °C:
4Al + 3C → Al4 C3 ΔG°298K = −242.5 kJ mol−1, ΔH °298K = −215.7 kJ mol−1
Ti+C = TiC, ΔG°298K 298 = −180 kJ/mol, ΔH °298K = −183.7 kJ/mol
(6)
(4)
Hu et al. [3] studied the formation mechanism of TiC in Al–TiO2 –C system containing an excess amount of Al (in comparison of Reaction (1)) during combustion synthesis. They reported that TiC was not produced by a single reaction as given by Eq. (1), but rather formed as a result of a series of reactions that occurred between the reactions products produced during the intermediate stages. At first, Al tended to react with TiO2 due to its high affinity, forming Al2O3 and Al3Ti, and Al4C3 was produced by the reaction between Al and C. Subsequently, these intermittent reaction products, Al3Ti and Al4C3, reacted with each other to form TiC. The mechanical alloying is a chemical solid state process, and diffusion has an essential role on the formation of different phases in solid state [10,11]. In this study, TiC was not formed during the mechanical alloying. The XRD results confirmed that Reaction (5) has been taken place during ball milling and Al3Ti formed. Heat treatment of 60 h milled powder at 500 °C has no effect and no new phases were formed. By increasing of annealing temperature
3Al3 Ti + Al4 C3 → 3TiC + 13Al ΔG°298K = −125.2 kJ mol−1, ΔH °298K = −116.7 kJ mol−1
(7)
This results showed that TiC was not formed in solid state in Al–TiO2–C system with an excess amount of Al. During mechanical alloying just one of the intermediate reaction products, Al3Ti and Al2O3 were formed. The subsequent heat treatment must be done at temperatures above Al melting point. In this temperatures, molten Al reacted to graphite and Al4C3 intermediate product formed. The reaction between Al3Ti and Al4C3 caused to TiC formation.
4. Morphological changes Fig. 3 showed the morphology of powder particles during ball milling at various durations. As can be seen in Fig. 3a, after 5 h milling, raw material powders in the form of big agglomerates and
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Fig. 3. Cross-sectional SEM micrographs of powder mixture after (a) 5 h, (b) 20 h, (c) 40 h, (d) 60 h of milling times and (e), (f) high-magnification of 60 h milled powder at two different zones.
fine particles with irregular morphologies and heterogeneous distribution were observed. The formation of fine particles with agglomerates could be attributed to cold welding and plastic deformation of powders. The fine particles agglomerated beyond 20 h of milling due to the occurrence of an exothermic reaction between the raw materials according to Reaction (3) and the formation of Al3Ti and Al2O3, which increased the temperature locally (Fig. 3b). Also, the existence of fine Al and C powders caused these particles to adhere together, and further agglomerates were formed. Increasing milling time to 40 h decreased the size of agglomerates again as a result of work hardening and the increasing fracture rate of powders when compared to the cold welding rate
(Fig. 3c). In fact during MA, between 20 and 40 h, work hardening of powder takes place and fracturing overcame cold welding and the distribution of the powder particles became more homogenous. With increasing milling time from 40 h to 60 h, powder particles adhered together and agglomeration process occurred. The average powder particles size seems to be within 1–10 mm (Fig. 3d). High-resolution SEM images for different powder particles milled after 60 h confirmed that the average particles size of agglomerates is 25 nm (Fig. 3e and f). 5. Conclusion The following conclusions can be made from the present research:
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Al/TiC–Al2O3 nanocomposite was formed by mechanical alloy
ing and subsequent heat treatment. During ball milling of Al–TiO2–C with excess Al, TiC was not formed. During ball milling, Al3Ti and Al2O3 formed after 20 h and by increasing milling time to 60 h, no new phases were formed. The annealing of 60 h milled powder showed that no new phases (especially TiC) were found after annealing at 500 °C. But increasing temperature to 1000 °C caused the Al3Ti and TiO2 peaks disappeared and TiC peaks emerged.
References [1] S.G. Son, B.H. Lee, J.M. Lee, J.C. Lee, Low-temperature synthesis of (TiC þAl2O3)/ Al alloy composites based on dopant-assisted combustion, J. Alloy. Compd. 649 (2015) 409–416. [2] G. Xiao, Q. Fan, M. Gu, Z. Jin, Microstructural evolution during the combustion synthesis of TiC–Al cermet with larger metallic particles, Mater. Sci. Eng. A 425 (2006) 318–325. [3] Q. Hu, P. Luo, Y. Yan, Microstructures, densification and mechanical properties of TiC–Al2O3–Al composite by field-activated combustion synthesis, Mater. Sci. Eng. A 486 (2008) 215–221. [4] H. Zhu, Y. Jiang, Y. Yao, J. Song, J. Li, Z. Xie, Reaction pathways, activation energies and mechanical properties of hybrid composites synthesized in-situ from Al–TiO2–C powder mixtures, Mater. Chem. Phys. 137 (2012) 532–542. [5] Y. Cheng, S. Sun, H. Hu, Preparation of Al2O3/TiC micro-composite ceramic tool materials by microwave sintering and their microstructure and properties,
8899
Ceram. Int. 40 (2014) 16761–16766. [6] R. Kumar, A.K. Chaubey, S. Bathula, B.B. Jha, A. Dhar, Synthesis and characterization of Al2O3–TiC nano-composite by spark plasma sintering, Int. J. Refract. Met. Hard Mater. 54 (2016) 304–308. [7] A. Hajalilou, M. Hashim, M. Nahavandi, I. Ismail, Mechanochemical carboaluminothermic reduction of rutile to produce TiC–Al2O3 nanocomposite, Adv. Powder Technol. 25 (2014) 423–429. [8] C. Suryanarayana, Mechanical alloying and milling, Prog. Mater. Sci. 46 (2001) 1–184. [9] C. Suryanarayana, N. Al-Aqeeli, Mechanically alloyed nanocomposites, Prog. Mater. Sci. 58 (2013) 383–502. [10] M. Zarezadeh Mehrizi, M. Shamanian, A. Saidi, Synthesis of CoWSi–WSi2 nanocomposite by mechanical alloying and subsequent heat treatment, Ceram. Int. 40 (2014) 9493–9498. [11] B.S.B. Reddy, K. Das, S. Das, A review on the synthesis of in situ aluminum based composites by thermal, mechanical and mechanical–thermal activation of chemical reactions, J. Mater. Sci. 42 (2007) 9366–9378. [12] C. Koch, Structural nanocrystalline materials: an overview, J. Mater. Sci. 42 (2007) 1403–1414. [13] M.Z. Mehrizi, A. Saidi, M. Shamanian, Production of Fe3Al–TiC nanocomposite by mechanical alloying of FeTi2–Al–C powders, Mater. Sci. Technol. 27 (2011) 1465–1468. [14] C.J. Lu, Z.Q. Li, Structural evolution of the Ti–Si–C system during mechanical alloying, J. Alloy. Compd. 395 (2005) 88–92. [15] M. Razavi, M.R. Rahimipour, R. Mansoori, Synthesis of TiC–Al2O3 nanocomposite powder from impure Ti chips, Al and carbon black by mechanical alloying, J. Alloy. Compd. 450 (2008) 463–467. [16] G.K. Williamson, W.H. Hall, X-ray line broadening from filed aluminium and wolfram, Acta Met. 1 (1953) 22–31. [17] J.-W. Kim, J.-M. Lee, J.-H. Lee, J.-C. Lee, Role of excess Al on the combustion reaction in the Al–TiO2–C system, Met. Mater. Int. 20 (2014) 1151–1156.