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Study of preparation of TiB2 by TiC in Al melts
MA TE RI A L S CH A R A CT ER IZ A TI O N 6 3 (2 0 1 2) 5 6–6 2
Available online at www.sciencedirect.com
www.elsevier.com/locate/matchar
Study of preparation of TiB2 by TiC in Al melts Haimin Dinga, b , Xiangfa Liub,⁎, Jinfeng Nieb a
Department of Mechanical Engineering, North China Electric Power University, Baoding 071003, PR China Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China b
AR TIC LE D ATA
ABSTR ACT
Article history:
TiB2 particles are prepared by TiC in Al melts and the characteristics of them are studied. It
Received 8 September 2010
is found that TiC particles are unstable when boron exists in Al melts with high
Received in revised form
temperature and will transform to TiB2 and Al4C3. Most of the synthesized TiB2 particles
1 August 2011
are regular hexagonal prisms with submicron size. The diameter of the undersurfaces of
Accepted 17 October 2011
these prisms is ranging from 200 nm to 1 μm and the height is ranging from 100 nm to
Keywords:
method to prepare small and uniform TiB2 particles.
300 nm. It is considered that controlling the transformation from TiC to TiB2 is an effective © 2011 Elsevier Inc. All rights reserved.
X-ray diffraction Composite materials Phase transformation TiB2
1.
Introduction
TiB2 and TiC particles are particularly interesting in industry because of their unique physical and chemical properties, such as high melting points, hardness, elastic modulus and electrical conductivity [1]. They are widely used in many fields, for example, as reinforced phases in metal-matrix composites which are applied in aviation and transportation industries [2, 3]. Furthermore, it is noticed that the composites containing both TiB2 and TiC have an attractive combination of excellent mechanical and electrical properties, and corrosion resistance which are higher than those of single-phase TiB2 or TiC [4, 5]. Therefore, the TiB2–TiC based material has attracted much attention and is considered to be an excellent candidate for application in high performance cutting tools, wear parts, and new type of electrodes [6-9]. For the application of TiC and TiB2 in composites, one crucial research aspect is to control the morphologies and size of the reinforced particles during preparation. Particles with a uniform and small size are desirable for the improved mechanical properties of the composites [10]. For TiB2, in particular, ⁎ Corresponding author. Tel./fax: + 86 531 88395414. E-mail address: [email protected] (X. Liu). 1044-5803/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2011.10.006
controlling of the morphology and size is essential due to its h.c.p. crystal structure. For the time being, TiB2 are commonly prepared by the reaction of KBF4 and K2TiF6 in Al melts. But when TiB2 are formed by this method, much melting slag and poisonous gas will also be created. In addition, TiB2 are often prepared by the Al–B and Al–Ti master alloys. But it is found that the size of the TiB2 particles synthesized by this method is difficult to control [11]. It is noticed that TiC particles can react with borides to form TiB2 under given conditions. So it may be an effective method to prepare TiB2 by TiC, and the morphology of the formed TiB2 should be different from that prepared by other reactants. So, in this study, TiC particles were used as the raw material to prepare TiB2 and the microstructures of the formed TiB2 were investigated.
2.
Experiment Procedures
The Al–Ti–C master alloy was firstly prepared by the melt reaction method. 99.7 wt.% commercial pure Al, 99.50 wt.% Ti powder, 99.85 wt.% graphite powder with the size of about
MA TE RI A L S CH A R A CT ER IZ A TI O N 6 3 (2 0 1 2) 5 6–6 2
57
were firstly mixed and ball milled for 6 h, and then coldpressed into pellets. Subsequently, the pellets were added into the Al melt at 1200 °C to prepare the Al–Ti–C master alloy. Then the prepared Al–3Ti–0.75C master alloy was remelted to 1200 °C, and was added with 0.8% and 0.4% boron respectively, in form of Al–3B master alloy. After holding for 10 min, the melts were poured. The particles were then extracted from the Al–Ti–C and Al–Ti–C–B master alloys using hydrochloric acid, and analyzed by X-ray diffraction (XRD) and a field emission scanning electron microscope (FESEM).
3. Fig. 1 – XRD patterns of the samples (a) Al–3Ti–0.75C, and (b) Al–Ti–C–0.8B master alloy.
10 μm and 98.00 wt.% pure Al powder, were used to produce the Al–Ti–C master alloy. The weight ratio of Ti:C in the prepared master alloy is 4:1 with the nominal composition of Al–3Ti–0.75C. The Ti powder, graphite and pure Al powder
Results and Discussion
Fig. 1a shows the XRD pattern of the prepared Al–3Ti–0.75C master alloy. It can be seen that the reflections of TiC are obvious, indicating the successful preparation of the Al–Ti–C alloy. The XRD pattern of Al–Ti–C–0.8B master alloy is shown in Fig. 1b. It is found that TiB2 and Al4C3 phases are formed and the reflections of TiC almost disappear, which suggests that TiC particles have reacted with B to form TiB2 and Al4C3.
Fig. 2 – Microstructures of the master alloys (a) Al–3Ti–0.75C, (b) Al–Ti–C–0.8B,(c) EDS analysis of point A, and (d) EDS analysis of point B.
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MA TE RI A L S CH A R A CT ER IZ A TI O N 6 3 (2 0 1 2) 5 6–6 2
The microstructures of the samples before and after the addition of B are shown in Fig. 2. Form Fig. 2a, it can be seen that TiC particles are uniformly distributed in the Al matrix. After the addition of B, two new kinds of phases are generated, as shown in Fig. 2b. The EDS analysis results of points A and B shown in Fig. 2c and d indicate that the bright particle-like phase mainly contains Ti, B and Al, while the gray blocky phase contains C and O apart from Al. In terms of the XRD pattern, it is deduced that the bright particle-like phase is TiB2 and the gray blocky phase is Al4C3. Because Al4C3 can easily react with H2O, the existence of O might have been introduced during the preparation of the specimen through the following chemical reaction: Al4 C 3 þ H 2 O→Al OHÞ3 þ CH 4 ↑
ð1Þ
In order to further examine the morphologies of the TiC and TiB2, they were extracted from the master alloys using hydrochloric acid. Fig.3a and b shows the XRD patterns of the extracted particles. It is found that TiC is the only phase extracted from the Al–Ti–C master alloy. As for particles obtained from the Al–Ti–C–0.8B master alloy, however, TiB2 is the main phase and Al4C3 is not found, as shown in
Fig. 3 – XRD patterns of the extracted samples from (a) Al–3Ti–0.75C, and (b) Al–Ti–C–0.8B master alloy.
Fig. 4 – Morphologies of the prepared TiC and TiB2 (a) TiC in Al–3Ti–0.75C master alloy, and (b) TiB2 formed from TiC.
Fig. 5 – TEM analysis of TiB2 formed from TiC (a) microstructure, and (b) the corresponding diffraction selected area (SAD) pattern of (a),Z. A. = [0001].
MA TE RI A L S CH A R A CT ER IZ A TI O N 6 3 (2 0 1 2) 5 6–6 2
Fig. 3b. It is considered that Al4C3 has absolutely reacted with H2O by reaction (1) during the extracting process, so it can not be found in the Fig. 3b. These results further indicate that most of the TiC particles in the Al–3Ti–0.75C alloy have transformed to TiB2 particles after the addition of 0.8% B. The morphologies of the extracted particles are shown in Fig. 4. As shown in Fig. 4a, the TiC particles in Al–3Ti–0.75C master alloy are polygonal with the size ranging from 0.5 μm to 2 μm. Most of the TiC particles have changed into regular hexagonal prisms with submicron size after the addition of B as shown in Fig. 4b. The diameter of the undersurfaces of these prisms is ranging from 200 nm to 1 μm and the height is ranging from 100 nm to 300 nm. Fig. 5a and b is the TEM observation results of these particles, it is confirmed that the formed hexagonal particles are TiB2. However, after analysis by the EDS illustrated in Fig. 6, carbon element is found in the formed TiB2 particles. It has been reported that B can be doped in TiC particles to form TiCxBy particles due to its small size and similar electronic structure with C [12]. So, it is considered that C can also substitute some B atoms in TiB2 to form TiB(2-x)Cx for the same reason. In order to examine the transformation process, 0.4%B was added into the prepared Al–Ti–C master alloy. In this case, some TiC will transform to TiB2 and others will be remained. The XRD pattern of the Al–Ti–C–0.4B is shown in Fig. 7. It can be seen that the TiB2 and Al4C3 phases appear after the addition of 0.4% B. At the same time, the reflections of TiC can also be found. The microstructures of the extracted particles are shown in Fig. 8. Like the particles in Fig. 4b, the morphology of the formed TiB2 is also regular hexagonal prism with submicron size. The EDS analysis result in Fig. 8b indicates that the TiB2 also contain carbon element. Fig. 8c and d show the morphology
59
Fig. 7 – XRD patterns of the Al–Ti–C–0.4B master alloy.
and the EDS analysis result of residual TiC particles. It is found that the morphology of the particles tends to be spheroidal. It is deduced that because of the weak bonding energy in the edges of the polyhedral TiC, the reaction between TiC and B will firstly occurs at these places which results in the change of the morphology from polyhedron to sphere. The results mentioned above indicate that TiB2 is much more stable than TiC in Al melts. It is considered that the relative stability of TiC and TiB2 is mainly due to their different crystal structures and electron configurations. It is known that TiB2 has a hexagonal AlB2-type structure. The B atoms fill the trigonal prisms formed by Ti atoms and each B atom has three boron neighbors in a trigonal planar arrangement, forming a two-dimensional honeycomb network. The additional electron of B will interact with Ti to form π bond. TiC crystallizes with a NaCl-type structure and C atoms are located in the octahedron interspaces formed by Ti. Masataka Mizuno et al. [13] systemically studied the chemical bonding in titanium-metalloid compounds. For TiC and TiB2, the strength of both bonds between Ti and C in TiC and Ti and B in TiB2 is very strong. But there are also covalent bonds between B and B in TiB2, so it is calculated that the total strength of covalent bonds in TiB2 is much higher than that in TiC. Therefore, TiB2 will be much more stable than TiC under the same condition. It is known that there is a reversible reaction in the Al–Ti–C melt, as shown in reaction (2). TiC ⇔ Ti þ C
ð2Þ
where Ti and C stand for Ti and C solutes in the melt. When the Al–Ti–C melt is holding at a given temperature for a period of time, the reaction will be in equilibrium. After the addition of B, it will react with Ti to form TiB2. As a result, the equilibrium of reaction (2) will be broken and more TiC will be decomposed to Ti and C. And then Ti will continue to react with B, while C will react with Al to form Al4C3. So the sequential overall reactions in the melts should be:
AlB2 → AlðlÞ þ B TiC → Ti þ C Fig. 6 – The EDS result of the prepared TiB2 (a) microstructure, and (b) composition of TiB2.
Ti þ B → TiB2 C þ AlðlÞ → Al4 C 3
60
MA TE RI A L S CH A R A CT ER IZ A TI O N 6 3 (2 0 1 2) 5 6–6 2
Fig. 8 – Microstructure and EDS analysis of the TiB2 and TiC in the Al–Ti–C–0.4B master alloy (a) and (b) the prepared TiB2, (c) and (d) the residual TiC.
The overall reaction is: 3TiC þ 3AlB2 þ AlðlÞ → 3TiB2 þ Al4 C3
ð3Þ
The thermodynamics condition of the reaction between TiC and AlB2 to formTiB2 is calculated. According to the chemical thermal data in reference [14], the Gibbs free energy of the reaction (3) at 1400 k is: ΔG1400K ¼ 3 ΔG1400K ðTiB2 Þ þ ΔG1400K ðAl4 C3 Þ 3 ΔG1400K ðTiCÞ−3 ΔG1400K ðAlB2 Þ−ΔG1400K ðAlÞ ¼ 3 ð−429:51Þ þ ð−494:90Þ−3 ð−262:28Þ−3 ð−166:85Þ−ð−74:41ÞkJ=mol ¼ −412:31 kJ=mol; The Gibbs free energy is negative, which suggests that the transformation of TiC to TiB2 in the high temperature Al melt is thermally favorable. So TiB2 can be easily prepared by TiC in Al melts.
It has been suggested that TiC and TiB2 could establish coherency between their most close packed lattice planes which are (111) of TiC and (0001) of TiB2 [15]. The arrangement of Ti in both of them is almost the same, as shown in Fig. 9. The degree of disregistry between the substrate phase and the crystalline phase can be measured by the following Turnbull– Vonnegut equation: δ¼
j as ac j 100% ac
ð4Þ
where as and ac is the interatomic distance of substrate plane and crystalline plane without deformation, respectively. When the lattice parameter of TiC is 4.327 Ǻ and the lattice parameter of TiB2 is a = 3.028 Ǻ, c = 3.228 Ǻ, the lattice disregistry of (111) of TiC and (0001) of TiB2 is: a −a 3:028−3:060 100% ¼ 1:057% Δδ ¼ TiB2 TiC 100% ¼ 3:028 aTiB2
Fig. 9 – The atoms arrangement on the TiC (111) and TiB2 (0001) planes (a) TiC(111) (b) TiB2(0001). The big white balls denote Ti atoms, small blue ones represent C atoms and the pink atoms are B.
MA TE RI A L S CH A R A CT ER IZ A TI O N 6 3 (2 0 1 2) 5 6–6 2
61
Fig. 10 – Morphologies of the prepared TiC and TiB2 (a) TiC in the Al–3Ti–0.75C master alloy; (b), (c) and (d) TiB2 formed from TiC.
The result indicates that TiC (111) and TiB2 (0001) is coherent interfaces. Due to this relationship, the nucleation and growth of TiB2 formed by TiC will be inevitably influenced by the TiC particles in the melt. Since the (111) of TiC is the most closest packed plane, it tends to be exposed in the prepared TiC particles according to the crystal growth law. Fig. 10a shows the microstructures of TiC in the Al–Ti–C master alloy, it can be found that the (111) planes are exposed in the prepared TiC. Therefore, (111) plane of TiC can be played as the nucleation substrates during the formation of TiB2, which leads to the great increase of nucleation rate of TiB2. In addition, it can be seen some small particles emerged which are labeled by the arrow ①, ② and ③ as shown in Fig. 10b. It indicates that, besides nucleating on the (111) of TiC, some TiB2 also nucleate on the formed TiB2 surfaces. In these nucleation ways, numerous TiB2 nuclei are formed in Al melts, and ultimately grow into small TiB2 particles. From the microstructure of the prepared TiB2 particles as shown in Fig. 10b and c, plenty of flat growth steps can be found which show the characteristic of two-dimensional nucleation and layered growth. Because the TiB2 particles will firstly nucleate on the (111) of TiC with the (0001) plane as the base, >directions. It is known that further growth will be in <10 10 the growth of TiB2 at <10 10> directions is easier than that at <0001 >directions. Therefore, once a two-dimensional nucleus is generated, it prefers to sweep the whole layer before a new nucleus is formed on its surface. Under this condition, TiB2 can steadily grow into perfect hexagonal prism as shown in Fig. 10d. So it is considered that utilizing the transformation
of TiC is a good method to prepare small and uniform TiB2 particles.
4.
Conclusion
The TiB2 particles are prepared by the transformation of TiC in Al melts. It is found that TiC particles are unstable when B exists in Al melt, and will transform to TiB2 and Al4C3. Most of TiB2 formed by TiC are regular hexagonal prisms with submicron size. The diameter of the undersurfaces of these prisms is ranging from 200 nm to 1 μm and the height is ranging from 100 nm to 300 nm. It is considered that controlling the transformation from TiC toTiB2 is an effective method to prepare small and uniform TiB2 particles.
Acknowledgments This work was supported by a grant from National Nature Science Fund of China (No. 51071097) and a grant from the Fundamental Research Funds for the Central Universities (No. 11QG67).
REFERENCES [1] Bhaumik SK, Divakar C, Singh AK, Upadhyaya GS. Synthesis and sintering of TiB2 and TiB2–TiC composite under high pressure. Mater Sci Eng A 2000;279:275–81.
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[2] Zarrinfar N, Kennedy AR, Shipway PH. Reaction synthesis of Cu–TiCx master-alloys for the production of copper-based composites. Scripta Mater 2004;50:949–52. [3] Mukhopadhyay A, Raju GB, Bikramjit B, Suri AK. Correlation between phase evolution, mechanical properties and instrumented indentation response of TiB2-based ceramics. J Eur Ceram Soc 2009;29:505–16. [4] Wen G, Li SB, Zhang BS, Guo ZX. Reaction synthesis of TiB2–TiC composites with enhanced toughness. Acta Mater 2001;49:1463–70. [5] Yeh CL, Chen YL. Combustion synthesis of TiC–TiB2 composites. J Alloy Compd 2008;463:373–7. [6] Zhang X, Zhu C, Qu W, He XD, Kvanin VL. Self-propagating high temperature combustion synthesis of TiC/TiB2 ceramic–matrix composites. Composit Sci Technol 2002;62:2037–41. [7] Kanetake K, Kobashi M. Innovative processing of porous and cellular materials by chemical reaction. Scripta Mater 2006;54: 521–5. [8] Shen P, Zou B, Jin S, Jiang QC. Reaction mechanism in self-propagating high temperature synthesis of TiC-TiB2/Al composites from an Al–Ti–B4C system. Mater Sci Eng A 2007;454–455:300–9.
[9] Liang YH, Wang HY, Yang YF, Zhao RY, Jiang QC. Effect of Cu content on the reaction behaviors of self-propagating high-temperature synthesis in Cu–Ti–B4C system. J Alloy Compd 2008;462:113–8. [10] Matin MA, Lu L, Gupta M. Investigation of the reactions between boron and titanium compounds with magnesium. Scripta Mater 2001;45:479–86. [11] Emamy M, Mahta M, Rasizadeh J. Formation of TiB2 particles during dissolution of TiAl3 in Al–TiB2 metal matrix composite using an in situ technique. Composit Sci Technol 2006;66: 1063–6. [12] Nie JF, Liu XF, Ma XG. Influence of trace boron on the morphology of titanium carbide in an Al–Ti–C–B master alloy. J Alloy Compd 2010(491):113–7. [13] Mizuno Masataka, Tanaka Isao, Adachi Hirohiko. Chemical bonding in titanium-metalloid compounds. Phy Rev B 1999;59:15033–47. [14] Liang YJ, Che YC. Thermal data on inorganic compounds. Northeast University of China; 1993 (in Chinese). [15] Vallauri D, Atias Adrian IC, Chrysanthou A. TiC–TiB2 composites: A review of phase relationships, processing and properties. J Eur Ceram Soc 2008;28:1697–713.
Available online at www.sciencedirect.com
www.elsevier.com/locate/matchar
Study of preparation of TiB2 by TiC in Al melts Haimin Dinga, b , Xiangfa Liub,⁎, Jinfeng Nieb a
Department of Mechanical Engineering, North China Electric Power University, Baoding 071003, PR China Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China b
AR TIC LE D ATA
ABSTR ACT
Article history:
TiB2 particles are prepared by TiC in Al melts and the characteristics of them are studied. It
Received 8 September 2010
is found that TiC particles are unstable when boron exists in Al melts with high
Received in revised form
temperature and will transform to TiB2 and Al4C3. Most of the synthesized TiB2 particles
1 August 2011
are regular hexagonal prisms with submicron size. The diameter of the undersurfaces of
Accepted 17 October 2011
these prisms is ranging from 200 nm to 1 μm and the height is ranging from 100 nm to
Keywords:
method to prepare small and uniform TiB2 particles.
300 nm. It is considered that controlling the transformation from TiC to TiB2 is an effective © 2011 Elsevier Inc. All rights reserved.
X-ray diffraction Composite materials Phase transformation TiB2
1.
Introduction
TiB2 and TiC particles are particularly interesting in industry because of their unique physical and chemical properties, such as high melting points, hardness, elastic modulus and electrical conductivity [1]. They are widely used in many fields, for example, as reinforced phases in metal-matrix composites which are applied in aviation and transportation industries [2, 3]. Furthermore, it is noticed that the composites containing both TiB2 and TiC have an attractive combination of excellent mechanical and electrical properties, and corrosion resistance which are higher than those of single-phase TiB2 or TiC [4, 5]. Therefore, the TiB2–TiC based material has attracted much attention and is considered to be an excellent candidate for application in high performance cutting tools, wear parts, and new type of electrodes [6-9]. For the application of TiC and TiB2 in composites, one crucial research aspect is to control the morphologies and size of the reinforced particles during preparation. Particles with a uniform and small size are desirable for the improved mechanical properties of the composites [10]. For TiB2, in particular, ⁎ Corresponding author. Tel./fax: + 86 531 88395414. E-mail address: [email protected] (X. Liu). 1044-5803/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2011.10.006
controlling of the morphology and size is essential due to its h.c.p. crystal structure. For the time being, TiB2 are commonly prepared by the reaction of KBF4 and K2TiF6 in Al melts. But when TiB2 are formed by this method, much melting slag and poisonous gas will also be created. In addition, TiB2 are often prepared by the Al–B and Al–Ti master alloys. But it is found that the size of the TiB2 particles synthesized by this method is difficult to control [11]. It is noticed that TiC particles can react with borides to form TiB2 under given conditions. So it may be an effective method to prepare TiB2 by TiC, and the morphology of the formed TiB2 should be different from that prepared by other reactants. So, in this study, TiC particles were used as the raw material to prepare TiB2 and the microstructures of the formed TiB2 were investigated.
2.
Experiment Procedures
The Al–Ti–C master alloy was firstly prepared by the melt reaction method. 99.7 wt.% commercial pure Al, 99.50 wt.% Ti powder, 99.85 wt.% graphite powder with the size of about
MA TE RI A L S CH A R A CT ER IZ A TI O N 6 3 (2 0 1 2) 5 6–6 2
57
were firstly mixed and ball milled for 6 h, and then coldpressed into pellets. Subsequently, the pellets were added into the Al melt at 1200 °C to prepare the Al–Ti–C master alloy. Then the prepared Al–3Ti–0.75C master alloy was remelted to 1200 °C, and was added with 0.8% and 0.4% boron respectively, in form of Al–3B master alloy. After holding for 10 min, the melts were poured. The particles were then extracted from the Al–Ti–C and Al–Ti–C–B master alloys using hydrochloric acid, and analyzed by X-ray diffraction (XRD) and a field emission scanning electron microscope (FESEM).
3. Fig. 1 – XRD patterns of the samples (a) Al–3Ti–0.75C, and (b) Al–Ti–C–0.8B master alloy.
10 μm and 98.00 wt.% pure Al powder, were used to produce the Al–Ti–C master alloy. The weight ratio of Ti:C in the prepared master alloy is 4:1 with the nominal composition of Al–3Ti–0.75C. The Ti powder, graphite and pure Al powder
Results and Discussion
Fig. 1a shows the XRD pattern of the prepared Al–3Ti–0.75C master alloy. It can be seen that the reflections of TiC are obvious, indicating the successful preparation of the Al–Ti–C alloy. The XRD pattern of Al–Ti–C–0.8B master alloy is shown in Fig. 1b. It is found that TiB2 and Al4C3 phases are formed and the reflections of TiC almost disappear, which suggests that TiC particles have reacted with B to form TiB2 and Al4C3.
Fig. 2 – Microstructures of the master alloys (a) Al–3Ti–0.75C, (b) Al–Ti–C–0.8B,(c) EDS analysis of point A, and (d) EDS analysis of point B.
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MA TE RI A L S CH A R A CT ER IZ A TI O N 6 3 (2 0 1 2) 5 6–6 2
The microstructures of the samples before and after the addition of B are shown in Fig. 2. Form Fig. 2a, it can be seen that TiC particles are uniformly distributed in the Al matrix. After the addition of B, two new kinds of phases are generated, as shown in Fig. 2b. The EDS analysis results of points A and B shown in Fig. 2c and d indicate that the bright particle-like phase mainly contains Ti, B and Al, while the gray blocky phase contains C and O apart from Al. In terms of the XRD pattern, it is deduced that the bright particle-like phase is TiB2 and the gray blocky phase is Al4C3. Because Al4C3 can easily react with H2O, the existence of O might have been introduced during the preparation of the specimen through the following chemical reaction: Al4 C 3 þ H 2 O→Al OHÞ3 þ CH 4 ↑
ð1Þ
In order to further examine the morphologies of the TiC and TiB2, they were extracted from the master alloys using hydrochloric acid. Fig.3a and b shows the XRD patterns of the extracted particles. It is found that TiC is the only phase extracted from the Al–Ti–C master alloy. As for particles obtained from the Al–Ti–C–0.8B master alloy, however, TiB2 is the main phase and Al4C3 is not found, as shown in
Fig. 3 – XRD patterns of the extracted samples from (a) Al–3Ti–0.75C, and (b) Al–Ti–C–0.8B master alloy.
Fig. 4 – Morphologies of the prepared TiC and TiB2 (a) TiC in Al–3Ti–0.75C master alloy, and (b) TiB2 formed from TiC.
Fig. 5 – TEM analysis of TiB2 formed from TiC (a) microstructure, and (b) the corresponding diffraction selected area (SAD) pattern of (a),Z. A. = [0001].
MA TE RI A L S CH A R A CT ER IZ A TI O N 6 3 (2 0 1 2) 5 6–6 2
Fig. 3b. It is considered that Al4C3 has absolutely reacted with H2O by reaction (1) during the extracting process, so it can not be found in the Fig. 3b. These results further indicate that most of the TiC particles in the Al–3Ti–0.75C alloy have transformed to TiB2 particles after the addition of 0.8% B. The morphologies of the extracted particles are shown in Fig. 4. As shown in Fig. 4a, the TiC particles in Al–3Ti–0.75C master alloy are polygonal with the size ranging from 0.5 μm to 2 μm. Most of the TiC particles have changed into regular hexagonal prisms with submicron size after the addition of B as shown in Fig. 4b. The diameter of the undersurfaces of these prisms is ranging from 200 nm to 1 μm and the height is ranging from 100 nm to 300 nm. Fig. 5a and b is the TEM observation results of these particles, it is confirmed that the formed hexagonal particles are TiB2. However, after analysis by the EDS illustrated in Fig. 6, carbon element is found in the formed TiB2 particles. It has been reported that B can be doped in TiC particles to form TiCxBy particles due to its small size and similar electronic structure with C [12]. So, it is considered that C can also substitute some B atoms in TiB2 to form TiB(2-x)Cx for the same reason. In order to examine the transformation process, 0.4%B was added into the prepared Al–Ti–C master alloy. In this case, some TiC will transform to TiB2 and others will be remained. The XRD pattern of the Al–Ti–C–0.4B is shown in Fig. 7. It can be seen that the TiB2 and Al4C3 phases appear after the addition of 0.4% B. At the same time, the reflections of TiC can also be found. The microstructures of the extracted particles are shown in Fig. 8. Like the particles in Fig. 4b, the morphology of the formed TiB2 is also regular hexagonal prism with submicron size. The EDS analysis result in Fig. 8b indicates that the TiB2 also contain carbon element. Fig. 8c and d show the morphology
59
Fig. 7 – XRD patterns of the Al–Ti–C–0.4B master alloy.
and the EDS analysis result of residual TiC particles. It is found that the morphology of the particles tends to be spheroidal. It is deduced that because of the weak bonding energy in the edges of the polyhedral TiC, the reaction between TiC and B will firstly occurs at these places which results in the change of the morphology from polyhedron to sphere. The results mentioned above indicate that TiB2 is much more stable than TiC in Al melts. It is considered that the relative stability of TiC and TiB2 is mainly due to their different crystal structures and electron configurations. It is known that TiB2 has a hexagonal AlB2-type structure. The B atoms fill the trigonal prisms formed by Ti atoms and each B atom has three boron neighbors in a trigonal planar arrangement, forming a two-dimensional honeycomb network. The additional electron of B will interact with Ti to form π bond. TiC crystallizes with a NaCl-type structure and C atoms are located in the octahedron interspaces formed by Ti. Masataka Mizuno et al. [13] systemically studied the chemical bonding in titanium-metalloid compounds. For TiC and TiB2, the strength of both bonds between Ti and C in TiC and Ti and B in TiB2 is very strong. But there are also covalent bonds between B and B in TiB2, so it is calculated that the total strength of covalent bonds in TiB2 is much higher than that in TiC. Therefore, TiB2 will be much more stable than TiC under the same condition. It is known that there is a reversible reaction in the Al–Ti–C melt, as shown in reaction (2). TiC ⇔ Ti þ C
ð2Þ
where Ti and C stand for Ti and C solutes in the melt. When the Al–Ti–C melt is holding at a given temperature for a period of time, the reaction will be in equilibrium. After the addition of B, it will react with Ti to form TiB2. As a result, the equilibrium of reaction (2) will be broken and more TiC will be decomposed to Ti and C. And then Ti will continue to react with B, while C will react with Al to form Al4C3. So the sequential overall reactions in the melts should be:
AlB2 → AlðlÞ þ B TiC → Ti þ C Fig. 6 – The EDS result of the prepared TiB2 (a) microstructure, and (b) composition of TiB2.
Ti þ B → TiB2 C þ AlðlÞ → Al4 C 3
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MA TE RI A L S CH A R A CT ER IZ A TI O N 6 3 (2 0 1 2) 5 6–6 2
Fig. 8 – Microstructure and EDS analysis of the TiB2 and TiC in the Al–Ti–C–0.4B master alloy (a) and (b) the prepared TiB2, (c) and (d) the residual TiC.
The overall reaction is: 3TiC þ 3AlB2 þ AlðlÞ → 3TiB2 þ Al4 C3
ð3Þ
The thermodynamics condition of the reaction between TiC and AlB2 to formTiB2 is calculated. According to the chemical thermal data in reference [14], the Gibbs free energy of the reaction (3) at 1400 k is: ΔG1400K ¼ 3 ΔG1400K ðTiB2 Þ þ ΔG1400K ðAl4 C3 Þ 3 ΔG1400K ðTiCÞ−3 ΔG1400K ðAlB2 Þ−ΔG1400K ðAlÞ ¼ 3 ð−429:51Þ þ ð−494:90Þ−3 ð−262:28Þ−3 ð−166:85Þ−ð−74:41ÞkJ=mol ¼ −412:31 kJ=mol; The Gibbs free energy is negative, which suggests that the transformation of TiC to TiB2 in the high temperature Al melt is thermally favorable. So TiB2 can be easily prepared by TiC in Al melts.
It has been suggested that TiC and TiB2 could establish coherency between their most close packed lattice planes which are (111) of TiC and (0001) of TiB2 [15]. The arrangement of Ti in both of them is almost the same, as shown in Fig. 9. The degree of disregistry between the substrate phase and the crystalline phase can be measured by the following Turnbull– Vonnegut equation: δ¼
j as ac j 100% ac
ð4Þ
where as and ac is the interatomic distance of substrate plane and crystalline plane without deformation, respectively. When the lattice parameter of TiC is 4.327 Ǻ and the lattice parameter of TiB2 is a = 3.028 Ǻ, c = 3.228 Ǻ, the lattice disregistry of (111) of TiC and (0001) of TiB2 is: a −a 3:028−3:060 100% ¼ 1:057% Δδ ¼ TiB2 TiC 100% ¼ 3:028 aTiB2
Fig. 9 – The atoms arrangement on the TiC (111) and TiB2 (0001) planes (a) TiC(111) (b) TiB2(0001). The big white balls denote Ti atoms, small blue ones represent C atoms and the pink atoms are B.
MA TE RI A L S CH A R A CT ER IZ A TI O N 6 3 (2 0 1 2) 5 6–6 2
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Fig. 10 – Morphologies of the prepared TiC and TiB2 (a) TiC in the Al–3Ti–0.75C master alloy; (b), (c) and (d) TiB2 formed from TiC.
The result indicates that TiC (111) and TiB2 (0001) is coherent interfaces. Due to this relationship, the nucleation and growth of TiB2 formed by TiC will be inevitably influenced by the TiC particles in the melt. Since the (111) of TiC is the most closest packed plane, it tends to be exposed in the prepared TiC particles according to the crystal growth law. Fig. 10a shows the microstructures of TiC in the Al–Ti–C master alloy, it can be found that the (111) planes are exposed in the prepared TiC. Therefore, (111) plane of TiC can be played as the nucleation substrates during the formation of TiB2, which leads to the great increase of nucleation rate of TiB2. In addition, it can be seen some small particles emerged which are labeled by the arrow ①, ② and ③ as shown in Fig. 10b. It indicates that, besides nucleating on the (111) of TiC, some TiB2 also nucleate on the formed TiB2 surfaces. In these nucleation ways, numerous TiB2 nuclei are formed in Al melts, and ultimately grow into small TiB2 particles. From the microstructure of the prepared TiB2 particles as shown in Fig. 10b and c, plenty of flat growth steps can be found which show the characteristic of two-dimensional nucleation and layered growth. Because the TiB2 particles will firstly nucleate on the (111) of TiC with the (0001) plane as the base, >directions. It is known that further growth will be in <10 10 the growth of TiB2 at <10 10> directions is easier than that at <0001 >directions. Therefore, once a two-dimensional nucleus is generated, it prefers to sweep the whole layer before a new nucleus is formed on its surface. Under this condition, TiB2 can steadily grow into perfect hexagonal prism as shown in Fig. 10d. So it is considered that utilizing the transformation
of TiC is a good method to prepare small and uniform TiB2 particles.
4.
Conclusion
The TiB2 particles are prepared by the transformation of TiC in Al melts. It is found that TiC particles are unstable when B exists in Al melt, and will transform to TiB2 and Al4C3. Most of TiB2 formed by TiC are regular hexagonal prisms with submicron size. The diameter of the undersurfaces of these prisms is ranging from 200 nm to 1 μm and the height is ranging from 100 nm to 300 nm. It is considered that controlling the transformation from TiC toTiB2 is an effective method to prepare small and uniform TiB2 particles.
Acknowledgments This work was supported by a grant from National Nature Science Fund of China (No. 51071097) and a grant from the Fundamental Research Funds for the Central Universities (No. 11QG67).
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