The Effect of Ti Addition on the Properties of Al-B4C Interface: A Microstructural Study

F. Toptan1,a, A. Kilicarslan1,b and I. Kerti1,c 1 Yildiz Technical University, Department of Metallurgical and Materials Engineering, Faculty of

Chemistry & Metallurgy, Davutpasa Campus, Esenler, Istanbul, Turkey a ftoptan@yahoo.com, bayfer.kilicarslan@hotmail.com, cisilkerti@gmail.com

Keywords: Aluminium, Boron carbide, Casting, Composite, Interfaces, Microstructure, Scanning electron microscopy (SEM), Titanium.

Abstract. In the present work, Al-B4C composites were produced by casting route at 850°C and titanium-containing flux was used to overcome the wetting problem between B4C and liquid aluminium metal. The microstructure of matrix/reinforcement interface was investigated using SEM studies with or without Ti added composites. The reaction layer was also characterized with EDS analysis and X-ray mapping. It was found from the microstructural observations by high resolution field emission gun SEM (FEG-SEM) that the wetting issue was effectively solved by the formation of very thin (80-180 nm in thickness) Ti-C and Ti-B reaction layers.

Introduction

Metal matrix composites (MMCs) have been developed to respond the demand for lighter materials with high specific strength, stiffness and wear resistance [1,2]. Aluminium is favoured as matrix material in MMCs because of its low density, easy fabricability and good engineering properties [3]. Particulate reinforced aluminium matrix composites (AMCs) are attractive MMC materials due to their strength, ductility and toughness as well as their ability to be processed by conventional methods. Therefore, AMCs have been applied successfully to structural components, especially in the automotive and aviation industries, and the number of applications is expected to increase with the development of low-cost processing methods [1].

AMCs can be reinforced with various oxides, carbides, nitrides and borides in particulate, whisker or fiber form such as SiC, Al2O3, B4C, TiC, TiB2, MgO, TiO2, AlN, BN and Si3N4 [4-9]. While SiC and Al2O3 are common reinforcing materials in AMCs, limited research has been conducted on B4C reinforced AMCs due to the higher cost of B4C powders [3,10]. B4C is an attractive reinforcement material because of its excellent chemical and thermal stability; most importantly, B4C has lower density (2,52 g/cm3) and higher hardness (Hv =30 GPa) relative to Al2O3 and SiC [10-14].

Al-B4C composites can be processed with low-cost casting routes [5,15,16]. But poor wetting between Al and B4C below 1100 °C means that it is difficult to produce Al–B4C composites by mixing particles into the liquid phase. In order to enhance the wettability of ceramics and improve their incorporation behaviour into Al melts, particles are often heat treated or coated [17]. Apart from wetting, controlling of the interphases occurring at the Al-B4C interface is also important in the production of cast Al-B4C composites. At least nine phases have been reported in the Al-B-C ternary system [18].

It has been reported that for a given liquid metal, transition metal carbides, borides and nitrides are better wetted than covalently and ionically bonded ceramics [19]. Titanium is one of the reactive metals that can be used to increase wettability in Al-B4C system [17,20].

In the present work, Al-B4C composites were processed through a casting route with addition of K2TiF6 flux to form a reaction layer containing TiC and TiB2 at the interface to increase wettability and interface bonding. To observe the improvement at the interface, castings were also made without flux addition.

Materials Science Forum Vols. 636-637 (2010) pp 192-197Online available since 2010/Jan/12 at www.scientific.net© (2010) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.636-637.192

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Experimental Procedure

B4C particles with an average particle size 52 µm were used as a reinforcement and commercially pure aluminium (AA 1070) was used as a matrix material in this study. In order to enhance the wettability of boron carbide powders and improve their incorporation behaviour into aluminium melts, potassium fluorotitanate (K2TiF6) flux was used.

10% (wt) B4C particulate reinforced AMCs were produced using 400 g of commercially available aluminium (AA 1070) charges which were melted in a boron nitride coated graphite crucible utilizing induction furnace for each specimen. Mixture of B4C particles and the same amount of K2TiF6 flux (with 0.2 Ti/B4C ratio) were added into the melt within 4 minutes at 850°C with no stirring and mechanical stirring at 150 and 500 rpm for five minutes. Finally the melt was poured into a metal mould with 780 °C (± 3%) casting temperature. In order to observe the improvement at the interface, composites were also produced with no flux addition. Casting conditions are summarized in Table 1.

Table 1. Casting conditions.

Specimen Molten Metal Stirring Speed [rpm] Flux S1 - K2TiF6 S2 150 K2TiF6 S3 500 K2TiF6 S4 500 -

Metallographic samples sectioned from the cast bars were prepared using diamond grinders and

suspensions. In order to examine the matrix/reinforcement interface, samples were etched with Keller’s reagent; both etched and non-etched samples were investigated. Microstructures were examined under the Leica ICM 1000 optical microscope (OM) and JEOL JSM 7000F field emission gun scanning electron microscope (FEG-SEM) equipped with EDS analysis.

Results and Discussion

The aim of Ti addition in the casting of Al-B4C composites is to form a reaction layer on the interface that contains titanium carbide (TiC) and titanium boride (TiB2). When Ti is added in the form of K2TiF6, K and F contribute to remove the oxide film from the Al surface [5,17]. In order to evidentiate the improvement on the interface bonding, composites were produced with no flux addition (S4).

Comparatively homogeneous particle distribution was seen on low magnification optical microscope (OM) images as shown in Fig. 1. It was seen that particle yield was increased on K2TiF6-added samples, compared to the other sample. Furthermore, on the samples processed with no K2TiF6 addition, it was seen that a lot of particles had been removed from the surface during the grinding and polishing sequence, and the matrix was scratched by those particles.

The first three castings (S1, S2 and S3) were produced with no stirring (or just induction stirring), stir casting and vortex, respectively. Regarding the formation of the reaction layer, no important differences between three routes were found; in other words, the B4C particles were surrounded by a continuous reaction layer with a thickness that varied between 80 to 180 nm. However, vortex casting has the added advantage of the more uniform particle distribution along the cast bars, as mentioned earlier [21]. The formation of the reaction layer can be seen on the Ti elemental maps shown in Fig. 2.

Materials Science Forum Vols. 636-637 193

Fig. 1. Optical microscope images of a) S1, b) S2, c) S3 and d ) S4.

Since the

43

Ti + 41

B4C à 21

TiB2 + 41

TiC (1)

reaction has the lowest Gibbs free energy, ∆G, in our process temperatures, in the Al-Ti-B4C system, the most favourable reaction products are TiB2 and TiC [22]. From that point, in the Al-B4C composite processed with Ti addition, a reaction layer that consists of TiB2 and TiC can be expected on the matrix/reinforcement interface. That reaction layer also acts as a “reaction barrier” on the surface of B4C particulates and limits the interfacial reactions between the B4C and the aluminium matrix responsible for the production of the undesirable interphases [15].

Fig. 2. SEM images of a) S1, c) S2, e) S3 and d, e, f) matching Ti elemental maps (dark regions in

SEM images are boron carbide particles).

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It was found from the microstructural observations by high resolution field emission gun SEM (FEG-SEM) that a very thin (80-180 nm in thickness) reaction layer formed on the B4C surface(Fig. 3 a). The EDS analysis taken from the reaction layer (Fig. 3 b) confirmed that the reaction layer consists of TiB2 and TiC.

Fig. 3. a) SEM image of matrix/reinforcement interface of S1 and b) EDS spectrum taken from “+”

marked point in a. On the other hand, no trace of bonding was observed on the etched matrix/reinforcement

interfaces of the samples processed with no flux addition (Fig. 4 a). However, an effective bonding was observed on the etched microstructure of K2TiF6 added sample (Fig. 4 b).

Fig. 4. SEM images of matrix/reinforcement interfaces of a) S4 and b) S3 etched samples.

Conclusions

Al-B4C composites were processed through a casting route with addition of K2TiF6 flux to form a reaction layer contains TiC and TiB2 at the interface, in order to increase wettability and interface bonding. To observe the improvement at the interface, castings were also made without Ti addition. From this study the following can be concluded:

• Due to the poor wetting of B4C particles by liquid aluminium, an effective bonding could not be formed on the matrix/reinforcement interface in Al-B4C composites produced at relatively lower temperatures like 850°C.

• The wetting issue was effectively solved by the formation of very thin (80-180 nm in thickness) TiC and TiB2 reaction layers with addition of K2TiF6 flux.

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• As to the forming of a reaction layer, no important differences between three routes (no stirring, stir casting and vortex) were found; in other words, the B4C particles were surrounded by a continuous reaction layer between 80 to 180 nm in thickness. However, vortex casting offers the advantage of a more uniform particle distribution along the cast bars.

Acknowledgements

This study was supported by TUBITAK (The Scientific and Technological Research Council of Turkey) under Grant No. 107M338. The authors would also like to thank Istanbul Technical University Metallurgical & Materials Engineering Department for the provision of SEM facilities.

References

[1] Tokaji, K., Effect of stress ratio on fatigue behaviour in SiC particulate-reinforced aluminium alloy composite, Fatigue Fract. Eng. Mater. Struct. 28, 539–545

[2] Lopez, V.H., Scoles, A., Kennedy, A.R., The thermal stability of TiC particles in an Al7wt.%Si alloy, Mater. Sci. & Eng. A356 (2003) 316-325

[3] Khan, K.B., Kutty, T.R.G., Surappa, M.K., Hot hardness and indentation creep study on Al–5% Mg alloy matrix–B4C particle reinforced composites, Mater. Sci. & Eng. A 427 (2006) 76–82

[4] Lee, K.B., Ahn, J.P., Kwon, H., Characteristics of AA6061/BN composite fabricated by pressureless infiltration technique, Metall. Mater. Trans. A 32A (2001) (4), 1007–1018

[5] Kerti, I., Toptan, F., Microstructural variations in cast B4C-reinforced aluminium matrix composites (AMCs), Mater. Lett. 62 (2008) 1215–1218

[6] Ipek, R., Adhesive wear behaviour of B4C and SiC reinforced 4147 Al matrix composites (Al/B4C–Al/SiC), J. Mater. Procng. Technol. 162–163 (2005) 71–75

[7] Bedir, F., Characteristic properties of Al–Cu–SiCp and Al–Cu–B4Cp composites produced by hot pressing method under nitrogen atmosphere, Mater. & Design 28 (2007) 1238–1244

[8] Kalkanli, A., Yılmaz, S., Synthesis and characterization of aluminum alloy 7075 reinforced with silicon carbide particulates, Mater. & Design 29 (2008) 775–780

[9] Kerti, I. , Production of TiC reinforced-aluminum composites with the addition of elemental carbon, Mater. Lett. 59 (2005) 3795–3800

[10] Zhang, H., Ramesh, K. T., Chin, E.S.C., High Strain Rate Response of Aluminium 6092/B4C Composites, Mater. Sci. and Eng., A 384 (2004) 26-34

[11] Aizenshtein, M., Froumin, N., Shapiro-Tsoref, E., Dariel, M.P., Frage, N., Wetting and interface phenomena in the B4C/(Cu–B–Si) system, Scripta Mater. 53 (2005) 1231–1235

[12] Jung, J., Kang, S., Advances in Manufacturing Boron Carbide–Aluminum Composites, J. Am. Ceram. Soc., 87 [1] 47–54 (2004)

[13] Zhu, X., Dong, H., Lu, K., Coating different thickness nickel–boron nanolayers onto boron carbide particles, Surf. & Coat. Technol. 202 (2008) 2927–2934

[14] Shrestha, N.K., Kawai, M., Saji, T., Co-deposition of B4C particles and nickel under the influence of a redox-active surfactant and anti-wear property of the coatings, Surf. & Coat. Technol. 200 (2005) 2414– 2419

196 Advanced Materials Forum V

[15] Zhang, Z., Chen, X.-G., Charette, A., Particle distribution and interfacial reactions of Al–7%Si–10%B4C die casting composite, J. Mater. Sci. (2007) 42:7354–7362

[16] Shorowordi, K.M., Laoui, T., Haseeb, A. S. M. A., Celis, J. P., Froyen, L., Microstructure and interface characteristics of B4C, SiC and Al2O3 reinforced Al matrix composites: a comparative study, J. Mater. Procng. Technol. 142 (2003) 738–743

[17] Kennedy, A.R., Brampton, B., The Reactive Wetting and Incorporation of B4C Particles into Molten Aluminium, Scripta Mater., 44 (2001)1077-1082

[18] Halverson, D.C., Pyzik, A.J., Aksay, I.A., Snowden, W.E., Processing of Boron Carbide-Aluminum Composites, J. Am. Ceram. Soc., 72 [5] 775-80 (1989)

[19] Kennedy, A.R., Karantzalis, A.E., The incorporation of ceramic particles in molten aluminium and the relationship to contact angle data, Mater. Sci. & Eng. A264 (1999) 122–129

[20] D.C. Halverson, A.J. Pyzik, I.A. Aksay, Boron carbide aluminum and boron carbide-reactive metal cermets, US Patent no. 4,605,440, 1986

[21] Toptan, F., Kilicarslan, A., Karaaslan, A., Cigdem, M., Kerti, I., Optimization of particle addition conditions in production of Al-B4C composites by casting route, 14th Int. Metall. & Mater. Cong., October 16th-18th, 2008 Istanbul (In Turkish)

[22] Shen, P., Zou, B., Jin, S., Jiang, Q., Reaction mechanism in self-propagating high temperature synthesis of TiC-TiB2/Al composites from an Al-Ti-B4C system, Mater. Sci. & Eng. A 454–455 (2007) 300–309

Materials Science Forum Vols. 636-637 197

Advanced Materials Forum V 10.4028/www.scientific.net/MSF.636-637 The Effect of Ti Addition on the Properties of Al-B4C Interface: A Microstructural Study 10.4028/www.scientific.net/MSF.636-637.192

DOI References

[2] Lopez, V.H., Scoles, A., Kennedy, A.R., The thermal stability of TiC particles in an Al7wt.%Si alloy,

Mater. Sci. & Eng. A356 (2003) 316-325

doi:10.1016/S0921-5093(03)00143-6 [3] Khan, K.B., Kutty, T.R.G., Surappa, M.K., Hot hardness and indentation creep study on Al–5% Mg alloy

matrix–B4C particle reinforced composites, Mater. Sci. & Eng. A 427 (2006) 76–82

doi:10.1016/j.msea.2006.04.015 [4] Lee, K.B., Ahn, J.P., Kwon, H., Characteristics of AA6061/BN composite fabricated by pressureless

infiltration technique, Metall. Mater. Trans. A 32A (2001) (4), 1007–1018

doi:10.1007/s11661-001-0358-5 [5] Kerti, I., Toptan, F., Microstructural variations in cast B4C-reinforced aluminium matrix composites

(AMCs), Mater. Lett. 62 (2008) 1215–1218

doi:10.1016/j.matlet.2007.08.015 [6] Ipek, R., Adhesive wear behaviour of B4C and SiC reinforced 4147 Al matrix composites

(Al/B4C–Al/SiC), J. Mater. Procng. Technol. 162–163 (2005) 71–75

doi:10.1016/j.jmatprotec.2005.02.207 [7] Bedir, F., Characteristic properties of Al–Cu–SiCp and Al–Cu–B4Cp composites produced by hot

pressing method under nitrogen atmosphere, Mater. & Design 28 (2007) 1238–1244

doi:10.1016/j.matdes.2006.01.003 [8] Kalkanli, A., Yılmaz, S., Synthesis and characterization of aluminum alloy 7075 reinforced with silicon

carbide particulates, Mater. & Design 29 (2008) 775–780

doi:10.1016/j.matdes.2007.01.007 [9] Kerti, I., Production of TiC reinforced-aluminum composites with the addition of elemental carbon,

Mater. Lett. 59 (2005) 3795–3800

doi:10.1016/j.matlet.2005.06.032 [10] Zhang, H., Ramesh, K. T., Chin, E.S.C., High Strain Rate Response of Aluminium 6092/B4C

Composites, Mater. Sci. and Eng., A 384 (2004) 26-34

doi:10.1016/j.msea.2004.05.027 [11] Aizenshtein, M., Froumin, N., Shapiro-Tsoref, E., Dariel, M.P., Frage, N., Wetting and interface

phenomena in the B4C/(Cu–B–Si) system, Scripta Mater. 53 (2005) 1231–1235

doi:10.1016/j.scriptamat.2005.08.006 [12] Jung, J., Kang, S., Advances in Manufacturing Boron Carbide–Aluminum Composites, J. Am. Ceram.

Soc., 87 [1] 47–54 (2004)

doi:10.1111/j.1551-2916.2004.00047.x [13] Zhu, X., Dong, H., Lu, K., Coating different thickness nickel–boron nanolayers onto boron carbide

particles, Surf. & Coat. Technol. 202 (2008) 2927–2934

doi:10.1016/j.surfcoat.2007.10.021

[14] Shrestha, N.K., Kawai, M., Saji, T., Co-deposition of B4C particles and nickel under the influence of a

redox-active surfactant and anti-wear property of the coatings, Surf. & Coat. Technol. 200 (2005) 2414– 2419

doi:10.1016/j.surfcoat.2004.08.192 [15] Zhang, Z., Chen, X.-G., Charette, A., Particle distribution and interfacial reactions of Al–7%Si–10%B4C

die casting composite, J. Mater. Sci. (2007) 42:7354–7362

doi:10.1007/s10853-007-1554-5 [16] Shorowordi, K.M., Laoui, T., Haseeb, A. S. M. A., Celis, J. P., Froyen, L., Microstructure and interface

characteristics of B4C, SiC and Al2O3 reinforced Al matrix composites: a comparative study, J. Mater.

Procng. Technol. 142 (2003) 738–743

doi:10.1016/S0924-0136(03)00815-X [17] Kennedy, A.R., Brampton, B., The Reactive Wetting and Incorporation of B4C Particles into Molten

Aluminium, Scripta Mater., 44 (2001)1077-1082

doi:10.1016/S1359-6462(01)00658-3 [18] Halverson, D.C., Pyzik, A.J., Aksay, I.A., Snowden, W.E., Processing of Boron Carbide-Aluminum

Composites, J. Am. Ceram. Soc., 72 [5] 775-80 (1989)

doi:10.1111/j.1151-2916.1989.tb06216.x [19] Kennedy, A.R., Karantzalis, A.E., The incorporation of ceramic particles in molten aluminium and the

relationship to contact angle data, Mater. Sci. & Eng. A264 (1999) 122–129

doi:10.1016/S0921-5093(98)01102-2 [22] Shen, P., Zou, B., Jin, S., Jiang, Q., Reaction mechanism in self-propagating high temperature synthesis

of TiC-TiB2/Al composites from an Al-Ti-B4C system, Mater. Sci. & Eng. A 454–455 (2007) 300–309

doi:10.1016/j.msea.2006.11.055 [2] Lopez, V.H., Scoles, A., Kennedy, A.R., The thermal stability of TiC particles in an l7wt.%Si alloy,

Mater. Sci. & Eng. A356 (2003) 316-325

doi:10.1016/S0921-5093(03)00143-6 [3] Khan, K.B., Kutty, T.R.G., Surappa, M.K., Hot hardness and indentation creep study on Al– % Mg alloy

matrix–B4C particle reinforced composites, Mater. Sci. & Eng. A 427 (2006) 6–82

doi:10.1016/j.msea.2006.04.015 [4] Lee, K.B., Ahn, J.P., Kwon, H., Characteristics of AA6061/BN composite fabricated by ressureless

infiltration technique, Metall. Mater. Trans. A 32A (2001) (4), 1007–1018

doi:10.1007/s11661-001-0358-5 [5] Kerti, I., Toptan, F., Microstructural variations in cast B4C-reinforced aluminium matrix omposites

(AMCs), Mater. Lett. 62 (2008) 1215–1218

doi:10.1016/j.matlet.2007.08.015 [6] Ipek, R., Adhesive wear behaviour of B4C and SiC reinforced 4147 Al matrix composites

Al/B4C–Al/SiC), J. Mater. Procng. Technol. 162–163 (2005) 71–75

doi:10.1016/j.jmatprotec.2005.02.207 [7] Bedir, F., Characteristic properties of Al–Cu–SiCp and Al–Cu–B4Cp composites produced by ot pressing

method under nitrogen atmosphere, Mater. & Design 28 (2007) 1238–1244

doi:10.1016/j.matdes.2006.01.003 [8] Kalkanli, A., Yılmaz, S., Synthesis and characterization of aluminum alloy 7075 reinforced ith silicon

carbide particulates, Mater. & Design 29 (2008) 775–780

doi:10.1016/j.matdes.2007.01.007 [9] Kerti, I. , Production of TiC reinforced-aluminum composites with the addition of elemental arbon, Mater.

Lett. 59 (2005) 3795–3800

doi:10.1016/j.matlet.2005.06.032 [10] Zhang, H., Ramesh, K. T., Chin, E.S.C., High Strain Rate Response of Aluminium 6092/B4C omposites,

Mater. Sci. and Eng., A 384 (2004) 26-34

doi:10.1016/j.msea.2004.05.027 [12] Jung, J., Kang, S., Advances in Manufacturing Boron Carbide–Aluminum Composites, J. Am. eram.

Soc., 87 [1] 47–54 (2004)

doi:10.1111/j.1551-2916.2004.00047.x [13] Zhu, X., Dong, H., Lu, K., Coating different thickness nickel–boron nanolayers onto boron arbide

particles, Surf. & Coat. Technol. 202 (2008) 2927–2934

doi:10.1016/j.surfcoat.2007.10.021 [15] Zhang, Z., Chen, X.-G., Charette, A., Particle distribution and interfacial reactions of Al– %Si–10%B4C

die casting composite, J. Mater. Sci. (2007) 42:7354–7362

doi:10.1007/s10853-007-1554-5 [16] Shorowordi, K.M., Laoui, T., Haseeb, A. S. M. A., Celis, J. P., Froyen, L., Microstructure nd interface

characteristics of B4C, SiC and Al2O3 reinforced Al matrix composites: a omparative study, J. Mater.

Procng. Technol. 142 (2003) 738–743

doi:10.1016/S0924-0136(03)00815-X [18] Halverson, D.C., Pyzik, A.J., Aksay, I.A., Snowden, W.E., Processing of Boron Carbide- luminum

Composites, J. Am. Ceram. Soc., 72 [5] 775-80 (1989)

doi:10.1111/j.1151-2916.1989.tb06216.x [19] Kennedy, A.R., Karantzalis, A.E., The incorporation of ceramic particles in molten aluminium nd the

relationship to contact angle data, Mater. Sci. & Eng. A264 (1999) 122–129

doi:10.1016/S0921-5093(98)01102-2 [22] Shen, P., Zou, B., Jin, S., Jiang, Q., Reaction mechanism in self-propagating high temperature ynthesis

of TiC-TiB2/Al composites from an Al-Ti-B4C system, Mater. Sci. & Eng. A 454– 55 (2007) 300–309

doi:10.1016/j.msea.2006.11.055