Formation of composites fabricated by exothermic dispersion reaction in Al-TiO₂-B₂O₃ system

ZHU He-guo(朱和国)¹, WANG Heng-zhi(王恒志)¹, GE Liang-qi(葛良琦)¹,

CHEN Shi(陈 湜)¹, WU Shen-qing(吴申庆)²

1. Department of Materials Science and Engineering, Nanjing University of Science and Technology,

Nanjing 210094, China;

2. Department of Materials Science and Engineering, Southeast University, Nanjing 210096, China

Received 29 May 2006; accepted 22 January 2007

Abstract: The formation of aluminum matrix composites fabricated by exothermic dispersion reaction in Al-TiO₂-B₂O₃ system was investigated. The thermal analysis results show that the reactions are spontaneous and exothermic. The Gibbs free energy of α-Al₂O₃ is the lowest among all the combustion products, followed by TiB₂ and Al₃Ti. It is noted that when the B₂O₃/TiO₂ mole ratio is below 1, the reaction products are composed of particle-like α-Al₂O₃, TiB₂ and rod-like Al₃Ti. The α-Al₂O₃ crystallites, resulting from the reaction between Al and TiO₂ or B₂O₃, are segregated at the grain boundaries due to a lower wettability with the matrix. SEM micrographs show that rod-like Al₃Ti phase distributes uniformly in the matrix. When the B₂O₃/TiO₂ mole ratio is around 1, the Al₃Ti phase almost disappears in the composites, and the distribution of α-Al₂O₃ particulates is improved evidently.

Key words: aluminum matrix composite; reaction mechanism; mole ratio; exothermic dispersion reaction

In situ synthesis techniques are derived from the self-propagating combustion, which is used for fabricating metal or ceramic matrix composites. As the reinforcements are generated directly from chemical reaction within the matrix, the composites are of many excellent advantages, such as clean reinforcement-matrix interface, fine and thermodynamically stable reinforcements, good compatibility and high bond strength between reinforcements and the matrix, and low fabrication costs. It is eventual and critical to select a suitable reaction system and corresponded reaction method. There are several methods reported previously, such as self-propagating high temperature synthesis (SHS), direct metal oxidation method (DIMOX), exothermic dispersion(XD), mechanical alloying(MA), and pressureless metal infiltration(PRRIMX)[1-3]. Recently, the XD method has been focused extensively for it can produce fine ceramic particles (＜1 mm) and the volume frication of the reinforcement can vary in a wide range. Among these reinforcements, titanium diboride (TiB₂) is compatible with aluminum matrix, and does not react with aluminum. In this case, it provides a method to avoid the formation of brittle products at the particles/matrix interface, which improves the interface bonding strength. Furthermore, TiB₂ exhibits very high stiffness and hardness. Considering such excellent characteristics, TiB₂ phase has been used increasingly as reinforcements in aluminum-based metal matrix composites(MMCs)[4-5].

Researchers have chosen the reaction systems like Al-Ti, Al-B, Al-Ti-C, Al-Ti-B and Al-Zr-O[6-10], for the fabrication of Al-based MMCs. To decrease the processing cost, the Ti and B were substituted with the compounds of TiO₂ and B₂O₃ respectively. Therefore, the Al-TiO₂-B₂O₃, Al-TiO₂-B and Al-TiO₂-C reaction systems were highlighted recently. This work aims to investigate the formation of Al₂O₃, Al₃Ti and TiB₂ in the Al-based MMCs fabricated by XD method in Al-TiO₂-B₂O₃ system.

The titanium dioxide TiO₂ powder (98% purity, manufactured by Guangdong Guanghua Chemistry Factory Co., Ltd., Guangdong, China), and pure aluminum Al powder (99.6% puring, supplied by Shanghai Refined Chemistry Industry Science & Technology Co., Ltd. Shanghai, China), and Boric oxide B₂O₃ powder (98.0% purity, made by Shanghai Tongya Refined Chemistry Industry Factory, Shanghai, China) with an average size of 3-5 mm, 30-50 mm and 20-30 mm, respectively, were used as raw materials. According to stoichiometric calculation, the mixed powders with 30% (volume fraction) reinforcements whose B₂O₃/TiO₂ mole ratios were 0, 0.5 and 1.0 respectively were mixed by a ball-milling in the stainless steel vacuum jar for 2 h, and then cold compacted into green billets with a diameter of 30 mm. When the compacts were heated in vacuum furnace one by one at about 1 073 K, the combustion reaction occurred and held for about 10 min, and then the combusted compacts were cooled down to room temperature in the furnace. The three samples A (r(B₂O₃/TiO₂)=0), B (r(B₂O₃/TiO₂)=0.5) and C (r(B₂O₃/TiO₂)=1) made from the reacted compacts were mechanically polished and then investigated by X-ray diffraction(XRD), scanning electron microscope(SEM) and energy dispersive spectrum(EDS).

3.1 Thermodynamic analysis

When the temperature of furnace is increased to about 1 073 K, the B₂O₃ and Al powders in the compact are melted firstly, and then Al-TiO₂ liquid-solid and Al-B₂O₃ liquid-liquid interfaces are formed. The reactions occur as follows:

The above reactions are exothermic and their theoretical combustion temperature can be calculated by the following formula:

where  T is the preheating temperature; T_ad  is the theoretic combustion temperature, n is the amount of substance, p is the products and R is the reactants.

The T_ad of the reactions (1) and (2) is 2 036 K and  2 213 K if not considering the aluminum matrix absorb- ing thermal. In fact, the T_ad of the reactions are 1 856 K and 2 045 K respectively which are lower than the previous figures. Therefore, the T_ad of the reactions (1) and (2) are all higher than the critical temperature 1 800 K that makes the combustion self- maintained. Due to the above reactions, an Al-Ti-B-α-Al₂O₃ reaction system is formed. Because the α-Al₂O₃ phase is very steady due to its low Gibbs free energy, the Al-Ti-B-α-Al₂O₃ quaternary system is equal to Al-Ti-B ternary system and the following reactions (3)-(8) are likely to occur in the compact:

Fig.1 shows the curves of the Gibbs free energy vs temperature of the above reactions. It is indicated that all the reactions can take place spontaneously due to their negative . It is also indicated that the stability of these products is in the following order: α-Al₂O₃＞  TiB₂＞AlB₁₂＞Al₃Ti＞TiB＞AlB₂＞TiAl. As shown in the Al-Ti phase diagram (see Fig.2), it can be inferred that Al and Ti can form many different kinds of products. But when the content of Ti is less than 36.5% (mass fraction) in the aluminum matrix, Ti will react with Al to form Al₃Ti. In this experiment, the content of Ti is much less than 36.5% (mass fraction), therefore, Al₃Ti is formed.

Fig.1 Gibbs free energies of possible combustion products vs temperature in Al-TiO₂- B₂O₃ system: 1 Al₂O₃; 2 TiB₂; 3 AlB₁₂; 4 TiB; 5 Ti₃Ti; 6 AlTi; 7 AlB₂

Fig.2 Al-Ti binary phase diagram

In the Al-TiO₂-B₂O₃ system, the reaction equations vary with the B₂O₃/TiO₂ mole ratios as follows:

When r(B₂O₃/TiO₂)=0,

13Al+3TiO₂→2α-Al₂O₃+3Al₃Ti;

When r(B₂O₃/TiO₂)=0.5,

23Al+6TiO₂+3B₂O₃→7α-Al₂O₃+3Al₃Ti+3TiB₂;

When r(B₂O₃/TiO₂)=1,

10Al+3TiO₂+3B₂O₃→5α-Al₂O₃+3TiB₂

From the above thermodynamic analysis, it can be concluded that when the B₂O₃/TiO₂ mole ratio is 0, the combustion results are composed of α-Al₂O₃ and Al₃Ti. With the increase of B₂O₃/TiO₂ mole ratio, the amount of Al₃Ti phase will decrease, and finally as the B₂O₃/TiO₂ mole ratio increases to 1, the Al₃Ti phase will disappear.

3.2 Results and analysis

Fig.3(a) displays that the reaction products in sample A are composed of fine particles and rod-like phase, and Fig.3(b) shows that the reaction products are α-Al₂O₃ and Al₃Ti. Figs.3(c) and (d) are their EDS patterns for the rod-like phase and the particle, respectively, which confirm that the rod-like phase is Al₃Ti and the particle is α-Al₂O₃. Each of Al₃Ti phase can act as an nuclei in the matrix during the solidification due to its good orientation relationships within aluminum matrix [11]: <110>//<110>{111}_Al; <210> //<110>{111}_Al. The preferential growing direction of the Al₃Ti is <110>[12], and then it grows into rod-like of several tens microns in length.

Fig.3 SEM micrograph (a) and XRD pattern (b) of produced sample A as well as EDS patterns (c, d) of rod-like phase and particle, respectively

Owing to the small fraction of Al₃Ti phase, it is hard to refine the matrix considerably. The α-Al₂O₃ particles are formed directly from reactions (1) and (2). They segregated on the grain boundaries because of the following three reasons: 1) the size of α-Al₂O₃ particles is 2-3 mm and it is difficult for the α-Al₂O₃ particles to enter the matrix during the solidification; 2) the viscosity of matrix increases quickly due to the reaction temperature decreasing rapidly after combustion; 3) the low wettability between the α-Al₂O₃ and aluminum matrix. The needed outer work for the α-Al₂O₃ particles entering into matrix is reported previously as[13]

where  W is the outer work; f_P is the particle volume fraction; R is the particle radius; σ_lg is the liquid-solid interface energy; θ is the wetting angle. Here θ is 118? [14], therefore W of α-Al₂O₃ particles is very high, which makes α-Al₂O₃ particles hard to enter into Al matrix.

As adding B₂O₃ powders, B₂O₃ powders will be melt at the temperature of 633 K and then enter into the aluminum powders by the capillary force. Reaction (2) will occur at the temperature of about 1 000 K and produce active B atoms. Then the active B will react with active Ti atoms produced by reaction (1) to form TiB₂ particles. When the B₂O₃/TiO₂ mole ratio is 0.5(sample B), the SEM micrograph of the combustion results indicates that the amount of Al₃Ti sticks decreases, as shown in Fig.4(a), which is consistent with the XRD result in Fig.4(c). When the B₂O₃/TiO₂ mole ratio is 1 (sample C), the Al₃Ti phase disappears, as shown in Fig.4(b), and the corresponding XRD spectrum in Fig.4(d) also shows that there is no diffraction peaks of Al₃Ti, which is in agreement with the conclusion of above thermodynamic analysis.

Fig.4 SEM micrographs and XRD patterns of reaction products: (a), (c) Sample A; (b), (d) Sample B

In the Al-TiO₂-B reaction system, YANG et al[15] suggested that B powder can be combined directly with liquid Al to form AlB₁₂, and then AlB₁₂ is decomposed as following equation: AlB₁₂→12[B]+Al, to form active B and Al. At the same time, TiO₂ powder reacts with Al to produce active Ti. The active B atoms react with Ti atoms to form TiB₂. But in the Al-TiO₂-B₂O₃ reaction system, Al reacts with TiO₂ and B₂O₃ respectively to form active B and active Ti, then the active Ti will be combined with the active B to form thermodynamically steady phase TiB₂, and there are no intermediate phases AlB₁₂. When the B₂O₃/TiO₂ mole ratio is below 1, the active Ti atoms are excess in the reaction, and then the remained Ti atoms will react with Al to form Al₃Ti. With the increase of B₂O₃/TiO₂ mole ratio, the amount of remained Ti atoms decreases. And when the B₂O₃/TiO₂ mole ratio is 1, there is no remained Ti atom to combine with Al to form Al₃Ti. Therefore the reaction route in the Al-TiO₂-B₂O₃ system can be shown as follows:

It can be noted that the SEM micrographs of Al₃Ti and the reaction process of TiB₂ in the Al-TiO₂-B₂O₃ are different from those in the Al-TiO₂-B system. In the Al-TiO₂-B system, when the B/TiO₂ mole ratio is less than 2, the products consist of α-Al₂O₃, Al₃Ti and TiB₂, and firstly B will react with Al to form Al₁₂B and then Al₁₂B will decompose to produce active B. The active B will diffuse to the surfaces of Al₃Ti and react with Al₃Ti to form TiB₂, whereas the active B is not enough to reduce all the Al₃Ti. Therefore, the surfaces of the remained Al₃Ti are not smooth[13]. When the B/TiO₂ mole ratio is increased to 2, the Al₃Ti sticks are reduced entirely and no Al₃Ti is observed in the results. According to the previous analysis, the formation time of TiB₂ is later than that of Al₃Ti, and the surfaces of the remained Al₃Ti are not smooth in the Al-TiO₂-B system. However, in the Al-TiO₂-B₂O₃ system, Al can react with TiO₂ and B₂O₃ simultaneously, and then produce active Ti and active B atoms in the matrix. Therefore they can form TiB₂ directly by diffusion. When the B₂O₃/TiO₂ mole ratio is below 1, the remained Ti atoms will react with Al to form Al₃Ti phase, which is smooth surface as shown in Fig.4(a). Therefore, the formation time of TiB₂ is prior to that of Al₃Ti, and the surfaces of the remained Al₃Ti are smooth in the Al-TiO₂-B₂O₃ system.

4 Conclusions

1) In the Al-TiO₂-B₂O₃ reaction system, when the B₂O₃/TiO₂ mole ratio is below 1, the combustion results are composed of α-Al₂O₃, TiB₂ and Al₃Ti. The α-Al₂O₃ phase is the most stable due to its lowest Gibbs free energy in the products.

2) The α-Al₂O₃ particles can not enter Al-based grains and segregate in the matrix grain boundaries. The Al₃Ti phase distributes uniformly in the matrix. The TiB₂ particles are compatible with the matrix and can become the nuclei of the matrix during the solidification, and as a result, the matrix grains can be refined with uniform distribution of the α-Al₂O₃ particles in the matrix.

3) As the B₂O₃/TiO₂ mole ratio increases, the fraction of Al₃Ti will decrease, and if the ratio reaches 1, the Al₃Ti phase disappears in the composites.

4) In the Al-TiO₂-B₂O₃ reaction system, the active Ti and B produced by reactions (1) and (2) respectively can form TiB₂ directly by diffusion. When the B₂O₃/TiO₂ mole ratio is less than 1, there are still some active Ti atoms remained, and then these active Ti atoms will react with Al to form Al₃Ti phases whose surfaces are smooth.

References

[1] MERZHANOV A G. Review paper: History and recent development in SHS [J]. Ceramic International, 1995, 21: 371-379.

[2] TIONG S C, MA Z Y. Microstructural and mechanical characteristic of in situ metal matrix composites [J]. Materials Science and Engineering, 2000, 29: 49-113.

[3] WEN G, LI S B , ZHANG B S, GUO Z X. Reaction synthesis of TiB₂-TiC composites with enhanced toughness [J]. Acta Mater, 2001, 49: 1463-1470.

[4] LIU Y H, YIN S, GUO Z M, LAI H Y. Aluminum boride in the combustion synthesis of alumina/boride composite [J]. Journal of Materials Research. 1998, 13(7): 1749-1752.

[5] MEYERS M A, OLEVSKY E A, MA J, JAMET M. Combustion synthesis/densification of an Al₂O₃-TiB₂ composite [J]. Materials Science and Engineering A, 2001, 311: 83-99.

[6] ISIL K. Production of TiC reinforced aluminum composite with the addition of elemental carbon [J]. Materials Letters, 2005, 59: 3795-3800.

[7] LI H, SRIHARAN T, LAM Y M, LENG N Y. Effects of processing parameters on the performance of the Al grain refinement master alloys Al-Ti and Al-B in small ingot [J]. Journal of Materials Processing Technology, 1997, 66: 253-257.

[8] NASSIK M, CHRIFI-ALAOUI F Z, MAHDOUK K, GACHCN J C. Calorimetric study of the aluminum-titanium system [J]. Journal of Alloys and Compounds, 2003, 350: 151-154.

[9] EMAMY M, MAHTA M, RASIZADEH J. Formation of TiB₂ particles during dissolution of Al₃Ti in Al-TiB₂ metal matrix composites using an in situ technique [J]. Composites Science and Technology, 2006, 26: 1063-1066.

[10] ZHAO Y T, DAI Q X, CHEN X N. Microstructure characterization of reinforcements in in situ synthesized compound of Al-Zr-O system [J]. Trans Nonferrous Met Soc China, 2005, 15(1): 109-112.

[11] MARKE E, DAVID S. Grain refinement of aluminum alloys (Part 1): The nucleant and solute paradigms [J]. Metallurgical and Materials Transactions A, 1999, 30(6): 1613-1623.

[12] MOORE J. Review self-propagating high-temperature combustion synthesis of powder compacted materials [J]. Journal of Materials Science, 1990, 25: 1159-1168.

[13] STEFANESCU D M, DHINDAW B K, KACAR S A, MOITRA A. Behavior of ceramic particles at the solid-liquid metal interface in metal matrix composites [J]. Metallurgical Transaction A, 1992, 23: 2847-2855.

[14] KSIAZEK M, SOBCZAK N, MIKULOWSKI B, RADZIWILL W, SUROWIAK I. Wetting and bonding strength in Al/Al₂O₃ system [J]. Materials Science and Engineering A, 2002, 324: 162-167.

[15] YANG B, WANG Y Q, ZHOU B L. The mechanism of formation of TiB₂ particles prepared by in situ reaction in molten aluminum [J]. Metallurgical and Materials Transactions B, 1998, 29: 635-640.

Foundation item: Project(BK2006207) supported by the Natural Science Foundation of Jiangsu Province, China

Corresponding author: ZHU He-guo; Tel: +86-25-84315979; E-mail: zhg1200@sina.com

(Edited by YUAN Sai-qian)