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Phase identification of Al–B4C ceramic composites synthesized by reaction hot-press sintering
Int. Journal of Refractory Metals & Hard Materials 28 (2010) 297–300
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Phase identification of Al–B4C ceramic composites synthesized by reaction hot-press sintering P.C. Kang a,*, Z.W. Cao b, G.H. Wu a, J.H. Zhang b, D.J. Wei b, L.T. Lin b a b
Harbin Institute of Technology, Harbin 150001, People’s Republic of China Mudanjiang Jingangzuan Boron Carbide Co. Ltd., Mudanjiang 157009, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 16 June 2009 Accepted 7 November 2009
Keywords: Boron carbide Reaction hot sintering Al Chemical reaction Phase identification
a b s t r a c t A series of boron carbide (B4C) matrix composites with different contents of Al, were synthesized by reaction hot-press sintering with milled B4C and pure metallic Al powder at 1600 °C for 1 h. X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM) were used to identify the phase constituent of the milled powders and the composites. The results have shown that parts of B4C and Al particles were oxidized to boron oxide (B2O3) and alumina (Al2O3) during the milling. Thermit reaction occurred and B2O3 was reverted during hot-press sintering. A ternary phase of Al boron carbide (Al8B4C7) was found in the composites, and the B4C transformed to a rich boron phase (B6.5C) because of the superfluous boron in the system. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The unique combination of extremely high hardness, wear resistance, low specific gravity, and high chemical stability makes boron carbide (B4C) ceramics a candidate material for a variety of structural applications [1]. For instance, it is employed as lightweight armor plates for the ballistic properties, and employed as wear-resistant components for the tribological property. In addition, boron carbide can also be used as a neutron absorber in nuclear reactors associated with its high boron content [2–4]. However, the prevailing covalent character of the bonds in the crystal lattice of B4C determines both its valuable properties and low sinterability. Monolithic boron carbide, without applied pressure, cannot be sintered to obtain satisfactory densities even at the temperatures over 2280 °C [5]. Another major problem for B4C ceramic is extreme susceptibility to brittle fracture. Researchers have known that combining B4C with metal could solve the recognized difficulties with B4C [6–8]. They have focused on Al because of its lightweight, ready availability and reactivity with B4C under reasonable processing conditions [9,10]. Hence, B4C–Al composites have the potential to combine the high stiffness and hardness of B4C with the ductility of Al, and without defeating the goal of obtaining a strong and low density material. There are a variety of binary and ternary compounds in B4C–Al system. Arslan et al. [11] have reported that B4C–Al composites are composed of various combinations of Al3BC, AlB2, AlB12C2 and Al4C3 phases, when the compos* Corresponding author. Tel.: +86 451 86402372; fax: +86 451 86412164. E-mail address: [email protected] (P.C. Kang). 0263-4368/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2009.11.004
ites were fabricated by infiltration of Al into porous B4C between 985 and 1370 °C. Viala et al. [3] have also reported the chemical reactivity of Al with B4C by Al–B4C powder mixture at temperature ranging from 627 to 1000 °C, they found that besides Al3BC, the Al3B48C2 replaced AlB2 at temperature higher than 868 °C. For the pressure sintering, the temperature is often over 1600 °C, few reports were found about the chemical reactivity of Al with B4C at this temperature. Recently, we have synthesized Al–B4C composites at 1600 °C with the milled B4C and metallic Al particles, and found that the chemical reactivity of Al with B4C had been drastically changed at 1600 °C. The present study is concentrated on the reactant phase identification and the chemical reactivity of B4C with liquid Al at 1600 °C.
2. Experimental procedure Samples studied in this work were prepared from commercial powders of Al (purity 99.6 wt.%; grain size, d50 = 28.5 lm, Northeast Light Alloy Co., Ltd.), and boron carbide (stoichiometric B4C with C traces, purity 99.4 wt.%; grain size, d50 = 4.5 lm, Jingangzhuan, Co., Ltd.). These two kinds of powders were loaded in a stainless steel vial and milled for 5 h using a centrifugal planetary ball mill. A vacuum stainless steel vial (250 ml) and stainless steel balls (with the diameters of 10 and 60 mm) were used for milling. The rotational speed of the vial is 300 rpm. Four specimens with Al contents of about 5 wt.%, 10 wt.%, 20 wt.%, and 40 wt.% are pre-
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P.C. Kang et al. / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 297–300
pared, named BCA5, BCA10, BCA20, and BCA40, respectively. The milled powders were put into the graphite die and were sintered in the Ar gas at 1600 °C with 35 MPa of compressive stress for 1 h. Pure B4C sample was used as the reference. The phases of milled powders and the composites were identified by an XRD (Philips X’pert) diffractometer with Cu Ka radiation operated at 40 kV and 40 mA, the scanning speed was 5°/min with a scan step of 0.03°. The surface morphologies of milled powders and the fracture morphologies of the composites were observed
by a SEM (Hitachi S-4700). The microstructures of the composites were observed by a TEM (Philips, CM-12), operated at an acceleration voltage of 120 kV.
3. Results and discussion Fig. 1 shows SEM images of the milled powders of B4C mixed with different contents of Al. From Fig. 1 we can see the powders
Fig. 1. SEM images of the milled powders of (a) BCA5; (b) BCA10; (c) BCA20; (d) BCA40.
Fig. 2. XRD pattern of milled powder of BCA20.
Fig. 3. XRD patterns of composites of (a) BCA5; (b) BCA10; (c) BCA20; (d) BCA40.
P.C. Kang et al. / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 297–300
Fig. 4. Change of phase content with Al content in the mixed powder.
are homogenous and the angular B4C particles become smooth after milling. Al particles are ground to pieces and they stick on the surface of the B4C particle. With the Al content increasing, the B4C particles are enwrapped by more and more Al particles. The constituent of the milled powders were determined by XRD, and the XRD pattern of the milled powder BCA20 is shown in Fig. 2. The main diffraction peaks are from B4C and Al. Moreover, we can find a little of boron oxide (B2O3, according to PDF card 760781) and Al oxide (b-Al2O3, according to PDF card 10-0414) diffraction peaks in the XRD pattern. It indicates that parts of B4C and Al particles were oxidized during the milling. B2O3 will react with Al during the sintering and that will be beneficial to decrease the sintering temperature. The XRD patterns of the sintered composites are shown in Fig. 3. It is clear that the main phase of hot pressed composites is boron carbide, and the positions of the boron carbide diffraction peaks are unanimous. Meanwhile, Al diffraction peaks are also found in
299
Fig. 5. The comparison of XRD patterns of B4C with composites of BCA5.
all the composites, the intensities of Al diffraction peaks increase with Al content increasing. But the relative intensity of Al peaks declined evidently compared with that of B4C. During the hot-press sintering, the Al components existed in the form of melts and vapor, some liquid Al overflowed the graphite die and some Al vapor changed into solid particles when meet with the cool wall of the graphite die and stayed there, so that Al content in the composites decreased. Furthermore, the XRD patterns indicates that B2O3 diffraction peaks disappear, while, there are two kinds of Al oxide, a-Al2O3 (according to the PDF card 5-712) and h-Al2O3 (according to the PDF card 35-121) diffraction peaks were found, that means the following thermite reaction occurs during the sintering process.
B2 O3 þ 2Al ! Al2 O3 þ 2B
ð1Þ
A ternary phase of Al boron carbide (Al8B4C7), according to the PDF card (35-1216), is also found in these composites. The ternary phase, which increases with Al content, should be from the reac-
Fig. 6. The comparison of composites of BCA5 with PDF card (78-1547).
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P.C. Kang et al. / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 297–300
The SAD analysis further proves that boron carbide will react with Al and form principally Al8B4C7, rather than a multitude of phases at 1600 °C. Meanwhile, there is another reactant with evidence of melting at the triple grain junction of the B4C. The SAD pattern, as shown in Fig. 7c, indicates that the reactant is Al2O3. Although the sintering temperature is only 1600 °C, the thermite reaction of Eq. (1) is exothermic reaction, so as to cause Al2O3 melting during the hot-press sintering. We also performed SAD analysis for the boron carbide crystals, Fig 7d shows a typical set of the SAD pattern, the incident electron beam was along the (1 0 1) (0 0 3) and (1 0 4) crystal face directions. The interplanar distances of these crystal faces calculated in accordance with the SAD pattern are 0.45085, 0.40671 and 0.25848 nm, respectively. The calculated result is close to a boron-rich boron carbide phase (B6.5C), and that is also consistent with the XRD analysis. 4. Conclusion
Fig. 7. TEM micrographs and SAD patterns of the composites of BCA10. (a) TEM image of composite; (b) SAD pattern of interface phase; (c) SAD pattern of triple grain junction phase; (d) SAD pattern of matrix crystal.
tion of Al with B4C at the sintering temperature because AlB2 is the only other binary phase in B4C–Al system and unstable at the temperature over 868 °C [3]. Therefore, the chemical reaction equation could be suggested as follows:
7B4 C þ 8Al ! Al8 B4 C7 þ 24B
ð2Þ
A semi-quantitative estimation of the phase contents, which is shown in Fig. 4, was obtained with the peak area determination. One can see that phase content of Al8B4C7 increases with the Al content increasing and a-Al2O3 is more than h-Al2O3. Because the powders were milled in a sealed vial, the content of oxygen in the vial is stable, so the total sum of phase contents of h-Al2O3 and a-Al2O3 is basically the same, about 10% for all samples. For the BCA5, because the four samples with different Al content were sintered in the one graphite die, it is easy to absorb the overflowed Al, as above-mentioned, from the sample of higher content Al, thus cause the Al content higher unusually. Fig. 5 shows the comparison of XRD patterns of pure B4C with composites of BCA5. We can find that the diffraction peaks of boron carbide in the composites of BCA5 shift slightly to a smaller diffraction angle compared with those of pure B4C. Further analysis of the XRD pattern of the composites shows that the diffraction peaks fit another boron carbide (B6.5C) phase well, as shown in Fig. 6. According to the two reactions, there is superfluous boron in the system. So the superfluous boron will dissolve in B4C, and form a boron-rich boron carbide phase – B6.5C. In order to confirm the chemical reactions in the composites, we performed transmission electron microscopy (TEM) and the selected area electron diffraction (SAD) pattern. Fig. 7 shows the TEM image and SAD patterns of the specimen BCA10. From Fig. 7a we can see that there is interface reactant distributing between the two B4C grains, Fig. 7b shows a typical set of the SAD pattern, the indexing shows the interface reactant is an Al, B, C ternary phase, which corresponds to Al8B4C7.
The chemical reaction and the possible phases in the Al–B4C ceramic composites sintered at 1600 °C with milled B4C and Al powders have been analyzed. Based on the XRD, TEM and the SAD pattern analysis, we can conclude that parts of B4C and Al particles were oxidized to boron oxide (B2O3) and alumina (Al2O3) during the milling. Thermit reaction occurred during hot-press sintering, and B2O3 was reduced to B. B4C reacted with Al and formed a ternary phase Al8B4C7. The superfluous boron, which was from the interface reaction and thermite reaction, was dissolved in B4C and formed a boron-rich boron carbide phase B6.5C. Acknowledgement The corresponding author would like to appreciate the project supported by China Postdoctoral Science Foundation (No. 20060390787). References [1] Postel O, Heberlein J. Deposition of boron carbide thin film by supersonic plasma jet CVD with secondary discharge. Surf Coat Technol 1998;108– 109:247–52. [2] Mcclellan KJ, Chu F, Roper JM. Room temperature single crystal elastic constants of boron carbide. J Mater Sci 2001;36:3403–7. [3] Viala JC, Bouix J, Gonzalez G. Chemical reactivity of Al with boron carbide. J Mater Sci 1997;32:4559–73. [4] Sanchez-Coronado JE, Chung DDL, Martınez-Escandell M, Narciso J, RodrıguezReinoso F. Effect of boron carbide particle addition on the thermomechanical behavior of carbon matrix silicon carbide particle composites. Carbon 2003;41:1096–9. [5] Wu F, Lu J. Applications and the properties of boron carbide ceramic material. J Wuyi Univ (Nat Sci Ed) 2002;3:45–9. [6] Liu CH. Structure and properties of boron carbide with Al incorporation. Mater Sci Eng 2000;B72:23–6. [7] Ding S, Wen GW, Lei TQ, Zhou Y. Design and thermodynamics of in situ TiB2 reinforced B4C matrix composites. J Mater Sci Eng 2003;2:165–9. [8] Yin BY, Wang LS, Fang YC. Sintering mechanism of pure and carbon-doped boron carbide. J Chin Ceram Soc 2001;1:68–71. [9] Neshpor VS, Zaitsev GP, Zhuravlev SV, Kitsai AA. A composite material in Al–B– C system. Refract Ind Ceram 2003;44(2):89–91. [10] Gosset D, Provot B. Boron carbide as a potential inert matrix: an evaluation. Prog Nucl Energy 2001;38(3–4):263–6. [11] Arslan G, Kara F, Turan S. Quantitative X-ray diffraction analysis of reactive infiltrated boron carbide–aluminium composites. J Eur Ceram Soc 2003:23.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Phase identification of Al–B4C ceramic composites synthesized by reaction hot-press sintering P.C. Kang a,*, Z.W. Cao b, G.H. Wu a, J.H. Zhang b, D.J. Wei b, L.T. Lin b a b
Harbin Institute of Technology, Harbin 150001, People’s Republic of China Mudanjiang Jingangzuan Boron Carbide Co. Ltd., Mudanjiang 157009, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 16 June 2009 Accepted 7 November 2009
Keywords: Boron carbide Reaction hot sintering Al Chemical reaction Phase identification
a b s t r a c t A series of boron carbide (B4C) matrix composites with different contents of Al, were synthesized by reaction hot-press sintering with milled B4C and pure metallic Al powder at 1600 °C for 1 h. X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM) were used to identify the phase constituent of the milled powders and the composites. The results have shown that parts of B4C and Al particles were oxidized to boron oxide (B2O3) and alumina (Al2O3) during the milling. Thermit reaction occurred and B2O3 was reverted during hot-press sintering. A ternary phase of Al boron carbide (Al8B4C7) was found in the composites, and the B4C transformed to a rich boron phase (B6.5C) because of the superfluous boron in the system. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The unique combination of extremely high hardness, wear resistance, low specific gravity, and high chemical stability makes boron carbide (B4C) ceramics a candidate material for a variety of structural applications [1]. For instance, it is employed as lightweight armor plates for the ballistic properties, and employed as wear-resistant components for the tribological property. In addition, boron carbide can also be used as a neutron absorber in nuclear reactors associated with its high boron content [2–4]. However, the prevailing covalent character of the bonds in the crystal lattice of B4C determines both its valuable properties and low sinterability. Monolithic boron carbide, without applied pressure, cannot be sintered to obtain satisfactory densities even at the temperatures over 2280 °C [5]. Another major problem for B4C ceramic is extreme susceptibility to brittle fracture. Researchers have known that combining B4C with metal could solve the recognized difficulties with B4C [6–8]. They have focused on Al because of its lightweight, ready availability and reactivity with B4C under reasonable processing conditions [9,10]. Hence, B4C–Al composites have the potential to combine the high stiffness and hardness of B4C with the ductility of Al, and without defeating the goal of obtaining a strong and low density material. There are a variety of binary and ternary compounds in B4C–Al system. Arslan et al. [11] have reported that B4C–Al composites are composed of various combinations of Al3BC, AlB2, AlB12C2 and Al4C3 phases, when the compos* Corresponding author. Tel.: +86 451 86402372; fax: +86 451 86412164. E-mail address: [email protected] (P.C. Kang). 0263-4368/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2009.11.004
ites were fabricated by infiltration of Al into porous B4C between 985 and 1370 °C. Viala et al. [3] have also reported the chemical reactivity of Al with B4C by Al–B4C powder mixture at temperature ranging from 627 to 1000 °C, they found that besides Al3BC, the Al3B48C2 replaced AlB2 at temperature higher than 868 °C. For the pressure sintering, the temperature is often over 1600 °C, few reports were found about the chemical reactivity of Al with B4C at this temperature. Recently, we have synthesized Al–B4C composites at 1600 °C with the milled B4C and metallic Al particles, and found that the chemical reactivity of Al with B4C had been drastically changed at 1600 °C. The present study is concentrated on the reactant phase identification and the chemical reactivity of B4C with liquid Al at 1600 °C.
2. Experimental procedure Samples studied in this work were prepared from commercial powders of Al (purity 99.6 wt.%; grain size, d50 = 28.5 lm, Northeast Light Alloy Co., Ltd.), and boron carbide (stoichiometric B4C with C traces, purity 99.4 wt.%; grain size, d50 = 4.5 lm, Jingangzhuan, Co., Ltd.). These two kinds of powders were loaded in a stainless steel vial and milled for 5 h using a centrifugal planetary ball mill. A vacuum stainless steel vial (250 ml) and stainless steel balls (with the diameters of 10 and 60 mm) were used for milling. The rotational speed of the vial is 300 rpm. Four specimens with Al contents of about 5 wt.%, 10 wt.%, 20 wt.%, and 40 wt.% are pre-
298
P.C. Kang et al. / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 297–300
pared, named BCA5, BCA10, BCA20, and BCA40, respectively. The milled powders were put into the graphite die and were sintered in the Ar gas at 1600 °C with 35 MPa of compressive stress for 1 h. Pure B4C sample was used as the reference. The phases of milled powders and the composites were identified by an XRD (Philips X’pert) diffractometer with Cu Ka radiation operated at 40 kV and 40 mA, the scanning speed was 5°/min with a scan step of 0.03°. The surface morphologies of milled powders and the fracture morphologies of the composites were observed
by a SEM (Hitachi S-4700). The microstructures of the composites were observed by a TEM (Philips, CM-12), operated at an acceleration voltage of 120 kV.
3. Results and discussion Fig. 1 shows SEM images of the milled powders of B4C mixed with different contents of Al. From Fig. 1 we can see the powders
Fig. 1. SEM images of the milled powders of (a) BCA5; (b) BCA10; (c) BCA20; (d) BCA40.
Fig. 2. XRD pattern of milled powder of BCA20.
Fig. 3. XRD patterns of composites of (a) BCA5; (b) BCA10; (c) BCA20; (d) BCA40.
P.C. Kang et al. / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 297–300
Fig. 4. Change of phase content with Al content in the mixed powder.
are homogenous and the angular B4C particles become smooth after milling. Al particles are ground to pieces and they stick on the surface of the B4C particle. With the Al content increasing, the B4C particles are enwrapped by more and more Al particles. The constituent of the milled powders were determined by XRD, and the XRD pattern of the milled powder BCA20 is shown in Fig. 2. The main diffraction peaks are from B4C and Al. Moreover, we can find a little of boron oxide (B2O3, according to PDF card 760781) and Al oxide (b-Al2O3, according to PDF card 10-0414) diffraction peaks in the XRD pattern. It indicates that parts of B4C and Al particles were oxidized during the milling. B2O3 will react with Al during the sintering and that will be beneficial to decrease the sintering temperature. The XRD patterns of the sintered composites are shown in Fig. 3. It is clear that the main phase of hot pressed composites is boron carbide, and the positions of the boron carbide diffraction peaks are unanimous. Meanwhile, Al diffraction peaks are also found in
299
Fig. 5. The comparison of XRD patterns of B4C with composites of BCA5.
all the composites, the intensities of Al diffraction peaks increase with Al content increasing. But the relative intensity of Al peaks declined evidently compared with that of B4C. During the hot-press sintering, the Al components existed in the form of melts and vapor, some liquid Al overflowed the graphite die and some Al vapor changed into solid particles when meet with the cool wall of the graphite die and stayed there, so that Al content in the composites decreased. Furthermore, the XRD patterns indicates that B2O3 diffraction peaks disappear, while, there are two kinds of Al oxide, a-Al2O3 (according to the PDF card 5-712) and h-Al2O3 (according to the PDF card 35-121) diffraction peaks were found, that means the following thermite reaction occurs during the sintering process.
B2 O3 þ 2Al ! Al2 O3 þ 2B
ð1Þ
A ternary phase of Al boron carbide (Al8B4C7), according to the PDF card (35-1216), is also found in these composites. The ternary phase, which increases with Al content, should be from the reac-
Fig. 6. The comparison of composites of BCA5 with PDF card (78-1547).
300
P.C. Kang et al. / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 297–300
The SAD analysis further proves that boron carbide will react with Al and form principally Al8B4C7, rather than a multitude of phases at 1600 °C. Meanwhile, there is another reactant with evidence of melting at the triple grain junction of the B4C. The SAD pattern, as shown in Fig. 7c, indicates that the reactant is Al2O3. Although the sintering temperature is only 1600 °C, the thermite reaction of Eq. (1) is exothermic reaction, so as to cause Al2O3 melting during the hot-press sintering. We also performed SAD analysis for the boron carbide crystals, Fig 7d shows a typical set of the SAD pattern, the incident electron beam was along the (1 0 1) (0 0 3) and (1 0 4) crystal face directions. The interplanar distances of these crystal faces calculated in accordance with the SAD pattern are 0.45085, 0.40671 and 0.25848 nm, respectively. The calculated result is close to a boron-rich boron carbide phase (B6.5C), and that is also consistent with the XRD analysis. 4. Conclusion
Fig. 7. TEM micrographs and SAD patterns of the composites of BCA10. (a) TEM image of composite; (b) SAD pattern of interface phase; (c) SAD pattern of triple grain junction phase; (d) SAD pattern of matrix crystal.
tion of Al with B4C at the sintering temperature because AlB2 is the only other binary phase in B4C–Al system and unstable at the temperature over 868 °C [3]. Therefore, the chemical reaction equation could be suggested as follows:
7B4 C þ 8Al ! Al8 B4 C7 þ 24B
ð2Þ
A semi-quantitative estimation of the phase contents, which is shown in Fig. 4, was obtained with the peak area determination. One can see that phase content of Al8B4C7 increases with the Al content increasing and a-Al2O3 is more than h-Al2O3. Because the powders were milled in a sealed vial, the content of oxygen in the vial is stable, so the total sum of phase contents of h-Al2O3 and a-Al2O3 is basically the same, about 10% for all samples. For the BCA5, because the four samples with different Al content were sintered in the one graphite die, it is easy to absorb the overflowed Al, as above-mentioned, from the sample of higher content Al, thus cause the Al content higher unusually. Fig. 5 shows the comparison of XRD patterns of pure B4C with composites of BCA5. We can find that the diffraction peaks of boron carbide in the composites of BCA5 shift slightly to a smaller diffraction angle compared with those of pure B4C. Further analysis of the XRD pattern of the composites shows that the diffraction peaks fit another boron carbide (B6.5C) phase well, as shown in Fig. 6. According to the two reactions, there is superfluous boron in the system. So the superfluous boron will dissolve in B4C, and form a boron-rich boron carbide phase – B6.5C. In order to confirm the chemical reactions in the composites, we performed transmission electron microscopy (TEM) and the selected area electron diffraction (SAD) pattern. Fig. 7 shows the TEM image and SAD patterns of the specimen BCA10. From Fig. 7a we can see that there is interface reactant distributing between the two B4C grains, Fig. 7b shows a typical set of the SAD pattern, the indexing shows the interface reactant is an Al, B, C ternary phase, which corresponds to Al8B4C7.
The chemical reaction and the possible phases in the Al–B4C ceramic composites sintered at 1600 °C with milled B4C and Al powders have been analyzed. Based on the XRD, TEM and the SAD pattern analysis, we can conclude that parts of B4C and Al particles were oxidized to boron oxide (B2O3) and alumina (Al2O3) during the milling. Thermit reaction occurred during hot-press sintering, and B2O3 was reduced to B. B4C reacted with Al and formed a ternary phase Al8B4C7. The superfluous boron, which was from the interface reaction and thermite reaction, was dissolved in B4C and formed a boron-rich boron carbide phase B6.5C. Acknowledgement The corresponding author would like to appreciate the project supported by China Postdoctoral Science Foundation (No. 20060390787). References [1] Postel O, Heberlein J. Deposition of boron carbide thin film by supersonic plasma jet CVD with secondary discharge. Surf Coat Technol 1998;108– 109:247–52. [2] Mcclellan KJ, Chu F, Roper JM. Room temperature single crystal elastic constants of boron carbide. J Mater Sci 2001;36:3403–7. [3] Viala JC, Bouix J, Gonzalez G. Chemical reactivity of Al with boron carbide. J Mater Sci 1997;32:4559–73. [4] Sanchez-Coronado JE, Chung DDL, Martınez-Escandell M, Narciso J, RodrıguezReinoso F. Effect of boron carbide particle addition on the thermomechanical behavior of carbon matrix silicon carbide particle composites. Carbon 2003;41:1096–9. [5] Wu F, Lu J. Applications and the properties of boron carbide ceramic material. J Wuyi Univ (Nat Sci Ed) 2002;3:45–9. [6] Liu CH. Structure and properties of boron carbide with Al incorporation. Mater Sci Eng 2000;B72:23–6. [7] Ding S, Wen GW, Lei TQ, Zhou Y. Design and thermodynamics of in situ TiB2 reinforced B4C matrix composites. J Mater Sci Eng 2003;2:165–9. [8] Yin BY, Wang LS, Fang YC. Sintering mechanism of pure and carbon-doped boron carbide. J Chin Ceram Soc 2001;1:68–71. [9] Neshpor VS, Zaitsev GP, Zhuravlev SV, Kitsai AA. A composite material in Al–B– C system. Refract Ind Ceram 2003;44(2):89–91. [10] Gosset D, Provot B. Boron carbide as a potential inert matrix: an evaluation. Prog Nucl Energy 2001;38(3–4):263–6. [11] Arslan G, Kara F, Turan S. Quantitative X-ray diffraction analysis of reactive infiltrated boron carbide–aluminium composites. J Eur Ceram Soc 2003:23.