Study on the reaction mechanism of (αααα-Al2O3+TiB2+TiC)/Al composites

fabricated by Al-TiO2-B4C system

Heguo Zhua, Jin Minb, Da Chuc and Huan Wangd

College of Materials Science and Engineering of Nanjing University of Science and Technology,

Nanjing, 210094, China

azhg1200@sina.com,bmihanxin1987@qq.com,cchuda_hi@126.com, dwanghuanxinzhi@163.com

Keywords: Exothermic Dispersion Synthesis, Thermal Analysis, Composites, Reaction Mechanism

Abstract The composites (α-Al2O3+TiB2+TiC)/Al has been fabricated by using exothermic

dispersion synthesis. Thermodynamic analysis indicated that the reaction between the Al and TiO2

can spontaneously occur due to the negative Gibbs free energy of the Al-TiO2 reaction system. With

the increase of B4C/TiO2 mole ratios, the exothermic peaks increase move to the higher temperature

and the corresponding ignite temperatures also increase. The reaction results indicate that when the

B4C/TiO2＝0, the reinforcements are composed of α-Al2O3, Al3Ti, with the increase of B4C/TiO2,

the amount of Al3Ti decreases and the TiC and TiB2 form simultaneously. When the B4C/TiO2

increases to 1/3, the Al3Ti almost disappear and the reinforcements of the composites are consisted

of α-Al2O3, TiC and TiB2.

Introduction

In-situ aluminum matrix composites have many excellent advantages such as clean particle-matrix

interface, fine and thermodynamically stable reinforcements, good compatibility and high bonding

strength between the reinforcements and the matrix and low fabrication costs [1-3]. So they have

been widely used in air navigation and automobile industries. The Al-TiO2 is a very interesting

system for fabricating the aluminum matrix composites [4]. But the reacted product Al3Ti is a bad

phase to increase the properties of the composites. So, the substances such as B, B2O3 and C are

added into the system [5-7]. As we known that the TiB2 and TiC all are good reinforcements of the

aluminum matrix composites [8-10]. In order to attain the good reinforcements TiB2 and TiC, the

B4C phase has been added into the Al-TiO2 system. In the present paper, we focus on investigating

the reaction mechanism of the Al-TiO2-B4C system by using DSC(difference scanning calorimetric),

SEM(scanning electronic microscope), and XRD(X-ray diffraction) methods. The reaction results

will also be helpful to understand the reaction mechanisms.

Experimental procedure

The B4C(99％ purity), TiO2 powders (99％ purity), and Al powders (99.0％ purity) with average

sizes of 20~30µm, 50~100µm and 20~30µm, respectively, were used as raw materials. According

to stoichiometric calculations, the four samples were prepared. The samples 1, 2 and 3 had the same

reinforcement volume fraction of 30% and different mole ratios of B4C/TiO2 of 0, 1/6 and 1/3,

respectively. The mixed powders were bal1-milled in a stainless steel vacuum jar for 2h, and then

cold compacted into green billets with a diameter of 20mm by a pressure of 120MPa. The compacts

Advanced Materials Research Vols. 150-151 (2011) pp 84-87Online available since 2010/Oct/27 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.150-151.84

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were heated in a vacuum furnace to a temperature to allow the combustion reactions to occur, then

held for about 10 min and subsequently cooled down to room temperature in the furnace. The

samples made from the reacted compacts were mechanically polished and then were investigated by

using methods of XRD and SEM. Some of the mixed powders were cold compacted into a slice

with the thickness of about 0.5mm by a pressure of 120MPa. A tiny block with the weight of

5~10mg cut from the slice was put into STA449C thermal analyzer to test the DSC curves at

heating rate of 20 /min.

Results and Discussion

According to the thermodynamics analysis as done in the literature [4], the possible reaction in the

Al-TiO2 system can be described as follows:

TiAl3OAl2TiO313Al 3322 +−→+ α （1）

And the reaction process between Al and TiO2 consists of two step reactions as follows:

Ti][3OAlTiO32Al 322 +−→+ α TGT 5.275350380+−=∆ （2）

3[Ti] 3Al Al Ti+ → 0 144242 21T TG = − +∆ （3）

The Al firstly reacts with TiO2 to form the stable α-Al2O3 particles and active Ti atoms; and then

the active Ti atoms react with Al to form the Al3Ti. When B4C powders are added into Al-TiO2

system, the B4C can not react with the Al as proved in Fig.1. From Fig.1, we can see that only an

endothermic peak in the DSC curve during the heating process from room temperature to 1100 ,

obviously, it is corresponded to the melt process of the aluminum matrix. So, there is no reaction

occurred between the Al and B4C. Fig.2 shows the DSC curves of the B4C-TiO2 system and single

phase B4C and TiO2, respectively. it is evident that the DSC curve of the B4C-TiO2 system is the

simple superposition of the B4C and TiO2. It is further proved that the B4C can not react with TiO2.

In the Al-TiO2-B4C system, Al first reacts with TiO2 to form the active Ti atoms and α-Al2O3

ceramic particles, and then, the active Ti atoms leave the reacted area and diffuse into the matrix,

they can react with B4C or aluminum. All the possible reactions can be described as equation 3 and

following equation:

24 2TiBTiCCB3[Ti] +→+ TGT

78.377602340+−=∆ （4）

Fig.1 DSC curve of the Al-B4C

system at heating rate of 20 /min

Fig.2 DSC curves of the TiO2-B4C system and TiO2,

B4C, respectively, at heating rate of 20 /min

Advanced Materials Research Vols. 150-151 85

On the base of the standard Gibbs free energy variations changed with temperature of the equation

(3) and equation (4), it is concluded that the reaction (4) will happen previous to the reaction (3).

Certainly, when the content of the B4C is not enough to make all active Ti atoms consume over, the

active Ti atoms will react with the aluminum matrix to form the Al3Ti. Fig.3 shows the DSC curves

of the composites with different mole ratios of B4C/TiO2 at heating rate of 20 /min under argon

protective atmosphere, respectively. Form Fig.3, we can see that with the increase of the mole ratios

of B4C/TiO2, the slops of the exothermic peaks increase, so the reaction velocity and the initiation

temperature of the reactions increase obviously. The reason of this may be that the diffuse distance

of the Ti atoms decrease and the collection degree of the reaction promoted and released reaction

heat with the increase of the B4C content. So, the whole reactions of the Al-TiO2-B4C system will

change with the mole ratio of the B4C/TiO2 and they can be described as following equations:

rB4C/TiO2=1/6： 2 4 2 3 2 323 6 2 5 4Al TiO B C TiC TiB Al Ti Al Oα+ + → + + + − （5）

rB4C/TiO2=1/3： 2 4 2 2 34 3 2 2Al TiO B C TiC TiB Al Oα+ + → + + − （6）

(a),(d) rB4C/TiO2=0 (b), (e) rB4C/TiO2=1/6 (c), (f) rB4C/TiO2=1/3

Fig.4 SEM images and their respective XRD patterns of the composites varied with different molar

ratios of the B4C/ZrO2. (a), (d) rB4C/TiO2=0; (b), (e) rB4C/TiO2=1/6; (c), (f) rB4C/TiO2=1/3

Fig.3 DSC curves with different mole ratios of B4C/TiO2:1-rB4C/TiO2=0; 2-rB4C/TiO2=1/6; 3-rB4C/TiO2=1/3

86 Advances in Composites

Fig.4 shows the SEM micrographs and XRD patterns of the composites with different molar

ratios of B4C/TiO2 and with the reinforcement volume fraction of 30%. When the B4C/TiO2 mole

ratio is 0, the reaction products are composed of rod-like phase, fine particles and the matrix as

shown in Fig.4 (a). Its corresponded XRD patterns as shown in Fig.4 (d) proves that compounded

phases of the composites are Al, Al3Ti and α-Al2O3. The EDS patterns confirm that the block-like

phase is Al3Ti [4], so, the particle phase is α-Al2O3. As the B4C/TiO2 mole ratio increases to 1/6, the

amount of the rod-like phase Al3Ti decreases as shown in Fig. 4 (b). Its corresponded XRD patterns

as shown in Fig. 4 (e) indicates that there are new phases TiC and TiB2 occurred in the final

products except for the former phases of the Al, Al3Ti and α-Al2O3, which is accordance with the

thermodynamic analysis in the section 3.1. When the B4C/TiO2 mole ratio is 1/3, the rod-like phase

Al3Ti almost disappears as shown in Fig. 4 (c). Its corresponded XRD pattern shows that the

reaction products are Al, α-Al2O3, TiC and TiB2 as shown in Fig. 4 (f). From the Fig. 4 (c), we can

see that the distribution of the reinforcements improves greatly. Therefore, the results of the

combustion reactions are compatible with the conclusions of the above thermodynamic analysis.

Conclusions

1. In the Al-TiO2-B4C reaction system, when the B4C/TiO2 mole ratio is 0, the Al first reacts with

TiO2 to form the α-Al2O3 and Al3Ti. With increasing the B4C/TiO2 mole ratios, the amount of rod

Al3Ti decreases gradually. When the B4C/TiO2 mole ratio reaches 1/3, the Al3Ti rods almost

disappear and the reinforcements of the composites are consisted of α-Al2O3, TiC and TiB2.

2. DSC curves indicates that the B4C can not react with TiO2 and that with the increase of the mole

ratios of B4C/TiO2, the reaction velocity and the ignite temperature of the reactions increase

obviously.

Acknowledgments

This work was supported by grant No.BK2006207 from the Natural Science Foundation of Jiangsu

Province.

References

[1] C.F. Feng and L. Froyen: Composites Part A Vol. 31(2000), P. 385

[2] G. Naveen Kumar, R. Narayanasamy , S. Natarajan, S.P. Kumaresh Babu, K. Sivaprasad, S.

Sivasankaran, Materials Design Vol. 31 (2010), P. 1526

[3] M.S. Song, M.X. Zhang, S.G. Zhang B Huang, J.G. Li: Materials Science & Engineering A

Vol. 473(2008), P. 166

[4] H.G. Zhu, J. Min, Y.L. Ai and Q. Wu: Advanced Material Research Vol. 97-101(2010), P. 1624

[5] H.G. Zhu, H.Z. Wang and L.Q. Ge: Wear Vol.264(2008), P. 967

[6] H.G. Zhu and H.Z. Wang: Materials Science and Engineering A Vol. 478(2008), P. 87

[7] S.K. Mishra and S.K. Das: Composites Science and Technology Vol. 67(2007), P. 2447

[8] S.C. Tjong, Z.Y. Ma, R.K.Y. Li: Materials Letter Vol.18(1999), P. 39

[9] H. Ross, Plovnick, A. Elizabeth: Materials Research Bulletin Vol. 36(2001), P. 1487

[10] J.H. Lee, S.K. Ko, C.W. Won: Materials Research Bulletin Vol. 36(2001), P. 1157

Advanced Materials Research Vols. 150-151 87

Advances in Composites 10.4028/www.scientific.net/AMR.150-151 Study on the Reaction Mechanism of (α-Al2O3+TiB2+TiC)/Al Composites Fabricated by Al-TiO2-B4C

System 10.4028/www.scientific.net/AMR.150-151.84

DOI References

[2] G. Naveen Kumar, R. Narayanasamy , S. Natarajan, S.P. Kumaresh Babu, K. Sivaprasad, S. ivasankaran,

Materials Design Vol. 31 (2010), P. 1526

doi:10.1016/j.matdes.2009.09.017 [2] G. Naveen Kumar, R. Narayanasamy , S. Natarajan, S.P. Kumaresh Babu, K. Sivaprasad, S. Sivasankaran,

Materials Design Vol. 31 (2010), P. 1526

doi:10.1016/j.matdes.2009.09.017