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Ti2AlC composites prepared by spark plasma sintering
Materials Chemistry and Physics 75 (2002) 291–295
Investigation of TiAl/Ti2 AlC composites prepared by spark plasma sintering Bingchu Mei a,∗ , Yoshinari Miyamoto b a
The State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China b Joining and Welding Research Institute, Osaka University, Osaka, Japan
Abstract TiAl/Ti2 AlC composites were produced by spark plasma sintering technology from mixed powders of Ti, Al and TiC. X-ray diffraction (XRD) patterns showed that the sintered products mainly consisted of TiAl and Ti2 AlC phases. Electric probe microcosmic analysis (EPMA) photos indicated different microstructures depending on composition. In one case, when 7 vol.%TiC was mixed in the starting powders, the produced Ti2 AlC particles were uniformly dispersed in the TiAl matrix with the partial lamellar structure containing ␥ and lamellar phases (duplex structure). In another case, while 15 vol.%TiC was added, the composites consisted of interpenetrating networks of Ti2 AlC and TiAl phases. After the annealing, however, all of them changed to that of Ti2 AlC particles dispersed uniformly in the ␥(TiAl) matrix. The fracture toughness and bending strength of the composites were tested, respectively. © 2002 Published by Elsevier Science B.V. Keywords: TiAl matrix; Ti2 AlC; Spark plasma sintering
1. Introduction
2. Experimental procedure
Titanium aluminides are candidate materials for replacement of heavier superalloys [1,2]. These ordered intermetallic compounds can offer improved high-temperature properties, high specific strength and oxidation resistance. However, the ductility is still limited. Fabrication of titanium aluminide composites with ceramic particles is one of way to improve their properties [2,4]. For example, the incorporation of ceramic particles can stabilize a fine matrix grain size, which leads to higher toughness and strength. The Ti2 AlC was found to have both properties of metal and ceramic like ductility and hardness [5,6]. Therefore, introduction of a small amount of Ti2 AlC particles into the TiAl matrix may help to overcome these shortcomings without a large deterioration of its metallic properties. Spark plasma sintering (SPS) technique [7,8] was used to prepare the TiAl/Ti2 AlC composites in this work. The process can achieve the synthesis and densification of the materials simultaneously and rapidly. The main purpose of this work is to examine the possibility of fabrication of dense TiAl/Ti2 AlC composites by SPS and to investigate their microstructures and mechanical properties.
TiC, Ti and Al powders were used as the raw materials. Their particle sizes were about 1.5, 45 and 50 m, respectively. A chemical analysis (by weight) showed that the principal impurities of Ti powders were: O ∼2800 ppm, H ∼120 ppm, N ∼120 ppm, Cl <100 ppm, Fe ∼49 ppm, and Mg <30 ppm, those of Al powders were: Mg ∼21 ppm, Si ∼14 ppm, Fe ∼11 ppm, and Cu ∼11 ppm, and TiC has a purity of 99.9%. The powders were well mixed in 52 at.%Ti–48 at.%Al with 0, 7, 15 and 30 vol.%TiC expecting the reaction Ti + Al + TiC → TiAl + Ti2 AlC. The mixtures were then subjected to SPS at 873, 1173 and 1423 K in vacuum at 2 Pa for 6 min. The sintering pressure was 30 MPa. The SPS apparatus was schematically shown elsewhere [9]. Some products were heat treated at various temperatures in vacuum. X-ray diffraction (XRD) and electron probe micro-analysis (EPMA) was used to characterize the product phases and microstructures. The sintered samples were cut into strip specimens with 3 × 5 × 20 mm3 . The specimens were polished to a diamond surface finish of 3 m before testing. Some of the polished specimens were used for the measurement of the three-point bending strength with a span of 18 mm and a cross-head speed of 0.5 mm min−1 at room temperature and the measurement of the fracture toughness by a single-edge notched beam method [10]. The hardness of the samples
∗ Corresponding author. E-mail address: [email protected] (B. Mei).
0254-0584/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 0 7 8 - 0
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was tested by using a Vickers hardness testing machine with an indentation load of 196 N. The bend strength and toughness were determined with the following formulas, respectively, σ = 3PL/2bh2 , KIC = Y × 3PLα 1/2 /2bh2 , where P is the breaking load of the specimen, and L, b, h, Y, and α are, respectively, the span, width, height, form factor,
and notch depth of the specimen. The bending strength and fracture toughness were calculated by averaging three individual measurements, while the hardness was the average of 10 measurements performed on a single sample.
3. Results and discussion Fig. 1 shows XRD profiles of the products obtained from the starting composition of (Ti–48 at.%Al)–7 vol.%TiC which was sintered at different temperatures. When the sintering temperature was 873 K, lower than the melting point of aluminium, the product contained TiAl3 , TiAl, Ti3 Al, Ti and TiC phases (Fig. 1a). At 1173 K, the diffraction peaks of Ti2 AlC appeared, but there still existed TiC phase in the products (Fig. 1b). When the sintering temperature was 1423 K, the products contained the phase TiAl, Ti3 Al and Ti2 AlC, which suggests that TiC was transformed into Ti2 AlC. When the product sintered at 1173 K was subjected
Fig. 1. XRD patterns of the samples with the starting composition (Ti–48 at.%Al)–7 vol.%TiC: (a) sintered at 873 K; (b) sintered at 1173 K; (c) annealed at 1423 K for 2 h after sintering at 1173 K; (d) sintered at 1423 K; (e) annealed at 1473 K for 2 h after sintering at 1423 K.
Fig. 2. EPMA micrographs of the samples with the starting composition (Ti–48 at.%Al)–7 vol.%TiC: (a) and (b) sintered at 1423 K; (c) and (d) annealed at 1473 K for 2 h after sintering at 1423 K.
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Fig. 2. (Continued ).
to annealing at 1423 K (Fig. 1c), it consisted of the same phases as the product sintered at 1423 K (Fig. 1d). This shows that TiC can be completely transformed to Ti2 AlC at 1423 K or a lower temperature. The Ti3 Al peaks observed in samples sintered at 1423 K disappeared when these samples were subjected to an additional annealing at 1473 K (Fig. 1e), which indicated that the annealing might result in the matrix phase approaching to the initial composition. EPMA micrographs of the products are shown in Figs. 2 and 3, which respectively, correspond to the samples with the original compositions of (Ti–48 at.%Al)–7 vol.%TiC and (Ti–48 at.%Al)–15 vol.%TiC, and sintered at 1423 K. These photos show completely different microstructures. Before annealing (Fig. 2a and b), the Ti2 AlC particles with a fine size (2–5 m) of spherical and/or columnar forms are uniformly dispersed in the TiAl matrix. The matrix exhibits a duplex structure consisting of the lamellar phases containing ␣2 (Ti3 Al) + ␥(TiAl) and colonies equiaxed ␥(TiAl) grain. Upon annealing (Fig. 2c and d), the lamellar structure of the matrix disappears, while the morphology of
Fig. 3. EPMA micrographs of the samples with the starting composition (Ti–48 at.%Al)–15 vol.%TiC: (a) sintered at 1423 K; (b) and (c) annealed at 1473 K for 2 h after sintering at 1423 K.
Ti2 AlC is still very similar to those shown in Fig. 2a and b. In the latter sample with 15 vol.%TiC, it was observed that the Ti2 AlC phase and ␥ phase link together so that an interpenetrating network structure was formed before annealing as seen in Fig. 3a and b. After the sample was
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Fig. 4. EPMA micrographs of Ti–48 at.%Al: (a) sintered at 1423 K; (b) annealed at 1473 K for 2 h after sintering at 1423 K.
subjected to annealing, the Ti2 AlC phase changed to the separated particles dispersed in the matrix consisting of the single ␥ phase, as shown in Fig. 3c. The microstructure of the sintered sample with the starting composition (Ti–48 at.%Al)–30 vol.%TiC, which is not shown here, was similar to that of (Ti–48 at.%Al)–15 vol.%TiC. These microstructure evolution depends on the starting compositions and processing conditions needs to be studied in more detail. To make a comparison between the microstructure of TiAl/Ti2 AlC composites and pure TiAl, Fig. 4a and b indicates, respectively, the sample Ti–48 at.%Al sintered at 1423 K and then annealed at 1473 K. The former offers the dual equiaxed structure (black area is ␥ phase and light area is ␣2 phase), but the latter has the full lamellar structure.
The fracture toughness (KIC ) of the composites produced is shown in Fig. 5. For the as-sintered product containing 7 vol.%TiC, the toughness was 43 MPa m1/2 which was 15% higher than that of the as-sintered sample of TiAl matrix. After annealing, the toughness decreased to 29 MPa m1/2 , a decrease of 20% relative to the value of TiAl which was annealed. However, it can be still compared with that of the as-sintered TiAl (30 MPa m1/2 ). For the sample containing 15 vol.%TiC, the fracture toughness was evidently lower than that of TiAl, although it increased to 23 MPa m1/2 by annealing. The sample with 30 vol.%TiC was very brittle such that its toughness is not shown here. These results suggest that the introduction of small quantities of Ti2 AlC can improve the fracture toughness, probably due to the dispersion of hard particles [3], however, further increase of
Fig. 5. Fracture toughness of the samples produced in this work: (A) starting composition of Ti–48 at.%Al, sintered at 1423 K; (A1 ) sample (A), treated at 1473 K for 2 h; (B) starting composition of (Ti–48 at.%Al)–7 vol.%TiC, sintered at 1423 K; (B1 ) sample (B), treated at 1473 K for 2 h; (C) starting composition (Ti–48 at.%Al)–15 vol.%TiC, sintered at 1423 K; (C1 ) sample (C), annealed at 1473 K for 2 h.
B. Mei, Y. Miyamoto / Materials Chemistry and Physics 75 (2002) 291–295
Fig. 6. The bending strength of TiAl/Ti2 AlC composites as a function of TiC addition in the starting mixtures.
Ti2 AlC content tends to form a network structure of the brittle Ti2 AlC phase resulting in an increase in toughness. Both the lamellar structure of TiAl and the appropriate content of Ti2 AlC dispersion are required for an improved toughness. The bending strength of samples is shown in Fig. 6. The sample containing 7 vol.%TiC exhibited a higher strength than that of TiAl. When the TiC content is more than 7 vol.%, the strength decreased as the fracture toughness decreased. The heat treatment had no significant effect on the strength.
4. Conclusions (1) TiAl/Ti2 AlC composites could be obtained from the starting powders of TiC, elemental Ti and Al by SPS. The reaction Ti + Al + TiC → TiAl + Ti2 AlC took place above the melting point of Al, while the tempe-
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rature at which TiC was completely changed to Ti2 AlC was between 1173 and 1423 K. (2) The microstructure depends on the heat treatment and TiC contents in the starting mixture. Two different microstructures were observed. In one case, the sample with 7 vol.%TiC, Ti2 AlC particles were uniformly dispersed in the TiAl matrix with a partial lamellar structure. In another case for the sample containing 15 vol.%TiC, the composites consisted of interpenetrating networks of Ti2 AlC and TiAl phases. After the heat treatment, all of them were transformed into the same microstructure consisting of dispersed Ti2 AlC particles in TiAl. (3) Introduction of small quantities of Ti2 AlC phase could result in an increase of both fracture toughness and strength, owing to the dispersion effect of hard particles. The lamellar structure leads to a higher fracture toughness. References [1] C.T. Liu, J.O. Stiegler, Mater. Eng. 100 (5) (1984) 29. [2] M.L. Adams, S.L. Kampe, L. Christodoulou, Int. J. Powder Metall. 26 (2) (1990) 105. [3] S.L. Kampe, J.D. Bryant, L. Christodoulou, Metall. Trans. A 22 (2) (1991) 447. [4] J.D. Whittenberger, R.K. Viswanadham, S.K. Mannan, B. Sprissler, J. Mater. Sci. 25 (1990) 35. [5] M.W. Barsoum, T. El-Raghy, J. Am. Ceram. Soc. 79 (1996) 1731. [6] R. Ramaseshan, S.K. Senshadri, N.G. Nair, et al., J. Jpn. Soc. Powder Powder Metall. 45 (4) (1998) 330. [7] M. Tokita, J. Soc. Powder Technol. Jpn. 30 (11) (1993) 790. [8] T. Nishimura, M. Mitomo, H. Hirotsuru, M. Kawahara, J. Mater. Sci. 14 (1995) 1046. [9] B.C. Mei, Y. Miyamoto, Metall. Mater. Trans. A 32 (3) (2001) 843. [10] B.C. Mei, R.Z. Yuan, X.L. Duan, J. Mater. Res. 8 (11) (1993) 2830.
Investigation of TiAl/Ti2 AlC composites prepared by spark plasma sintering Bingchu Mei a,∗ , Yoshinari Miyamoto b a
The State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China b Joining and Welding Research Institute, Osaka University, Osaka, Japan
Abstract TiAl/Ti2 AlC composites were produced by spark plasma sintering technology from mixed powders of Ti, Al and TiC. X-ray diffraction (XRD) patterns showed that the sintered products mainly consisted of TiAl and Ti2 AlC phases. Electric probe microcosmic analysis (EPMA) photos indicated different microstructures depending on composition. In one case, when 7 vol.%TiC was mixed in the starting powders, the produced Ti2 AlC particles were uniformly dispersed in the TiAl matrix with the partial lamellar structure containing ␥ and lamellar phases (duplex structure). In another case, while 15 vol.%TiC was added, the composites consisted of interpenetrating networks of Ti2 AlC and TiAl phases. After the annealing, however, all of them changed to that of Ti2 AlC particles dispersed uniformly in the ␥(TiAl) matrix. The fracture toughness and bending strength of the composites were tested, respectively. © 2002 Published by Elsevier Science B.V. Keywords: TiAl matrix; Ti2 AlC; Spark plasma sintering
1. Introduction
2. Experimental procedure
Titanium aluminides are candidate materials for replacement of heavier superalloys [1,2]. These ordered intermetallic compounds can offer improved high-temperature properties, high specific strength and oxidation resistance. However, the ductility is still limited. Fabrication of titanium aluminide composites with ceramic particles is one of way to improve their properties [2,4]. For example, the incorporation of ceramic particles can stabilize a fine matrix grain size, which leads to higher toughness and strength. The Ti2 AlC was found to have both properties of metal and ceramic like ductility and hardness [5,6]. Therefore, introduction of a small amount of Ti2 AlC particles into the TiAl matrix may help to overcome these shortcomings without a large deterioration of its metallic properties. Spark plasma sintering (SPS) technique [7,8] was used to prepare the TiAl/Ti2 AlC composites in this work. The process can achieve the synthesis and densification of the materials simultaneously and rapidly. The main purpose of this work is to examine the possibility of fabrication of dense TiAl/Ti2 AlC composites by SPS and to investigate their microstructures and mechanical properties.
TiC, Ti and Al powders were used as the raw materials. Their particle sizes were about 1.5, 45 and 50 m, respectively. A chemical analysis (by weight) showed that the principal impurities of Ti powders were: O ∼2800 ppm, H ∼120 ppm, N ∼120 ppm, Cl <100 ppm, Fe ∼49 ppm, and Mg <30 ppm, those of Al powders were: Mg ∼21 ppm, Si ∼14 ppm, Fe ∼11 ppm, and Cu ∼11 ppm, and TiC has a purity of 99.9%. The powders were well mixed in 52 at.%Ti–48 at.%Al with 0, 7, 15 and 30 vol.%TiC expecting the reaction Ti + Al + TiC → TiAl + Ti2 AlC. The mixtures were then subjected to SPS at 873, 1173 and 1423 K in vacuum at 2 Pa for 6 min. The sintering pressure was 30 MPa. The SPS apparatus was schematically shown elsewhere [9]. Some products were heat treated at various temperatures in vacuum. X-ray diffraction (XRD) and electron probe micro-analysis (EPMA) was used to characterize the product phases and microstructures. The sintered samples were cut into strip specimens with 3 × 5 × 20 mm3 . The specimens were polished to a diamond surface finish of 3 m before testing. Some of the polished specimens were used for the measurement of the three-point bending strength with a span of 18 mm and a cross-head speed of 0.5 mm min−1 at room temperature and the measurement of the fracture toughness by a single-edge notched beam method [10]. The hardness of the samples
∗ Corresponding author. E-mail address: [email protected] (B. Mei).
0254-0584/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 0 7 8 - 0
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was tested by using a Vickers hardness testing machine with an indentation load of 196 N. The bend strength and toughness were determined with the following formulas, respectively, σ = 3PL/2bh2 , KIC = Y × 3PLα 1/2 /2bh2 , where P is the breaking load of the specimen, and L, b, h, Y, and α are, respectively, the span, width, height, form factor,
and notch depth of the specimen. The bending strength and fracture toughness were calculated by averaging three individual measurements, while the hardness was the average of 10 measurements performed on a single sample.
3. Results and discussion Fig. 1 shows XRD profiles of the products obtained from the starting composition of (Ti–48 at.%Al)–7 vol.%TiC which was sintered at different temperatures. When the sintering temperature was 873 K, lower than the melting point of aluminium, the product contained TiAl3 , TiAl, Ti3 Al, Ti and TiC phases (Fig. 1a). At 1173 K, the diffraction peaks of Ti2 AlC appeared, but there still existed TiC phase in the products (Fig. 1b). When the sintering temperature was 1423 K, the products contained the phase TiAl, Ti3 Al and Ti2 AlC, which suggests that TiC was transformed into Ti2 AlC. When the product sintered at 1173 K was subjected
Fig. 1. XRD patterns of the samples with the starting composition (Ti–48 at.%Al)–7 vol.%TiC: (a) sintered at 873 K; (b) sintered at 1173 K; (c) annealed at 1423 K for 2 h after sintering at 1173 K; (d) sintered at 1423 K; (e) annealed at 1473 K for 2 h after sintering at 1423 K.
Fig. 2. EPMA micrographs of the samples with the starting composition (Ti–48 at.%Al)–7 vol.%TiC: (a) and (b) sintered at 1423 K; (c) and (d) annealed at 1473 K for 2 h after sintering at 1423 K.
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Fig. 2. (Continued ).
to annealing at 1423 K (Fig. 1c), it consisted of the same phases as the product sintered at 1423 K (Fig. 1d). This shows that TiC can be completely transformed to Ti2 AlC at 1423 K or a lower temperature. The Ti3 Al peaks observed in samples sintered at 1423 K disappeared when these samples were subjected to an additional annealing at 1473 K (Fig. 1e), which indicated that the annealing might result in the matrix phase approaching to the initial composition. EPMA micrographs of the products are shown in Figs. 2 and 3, which respectively, correspond to the samples with the original compositions of (Ti–48 at.%Al)–7 vol.%TiC and (Ti–48 at.%Al)–15 vol.%TiC, and sintered at 1423 K. These photos show completely different microstructures. Before annealing (Fig. 2a and b), the Ti2 AlC particles with a fine size (2–5 m) of spherical and/or columnar forms are uniformly dispersed in the TiAl matrix. The matrix exhibits a duplex structure consisting of the lamellar phases containing ␣2 (Ti3 Al) + ␥(TiAl) and colonies equiaxed ␥(TiAl) grain. Upon annealing (Fig. 2c and d), the lamellar structure of the matrix disappears, while the morphology of
Fig. 3. EPMA micrographs of the samples with the starting composition (Ti–48 at.%Al)–15 vol.%TiC: (a) sintered at 1423 K; (b) and (c) annealed at 1473 K for 2 h after sintering at 1423 K.
Ti2 AlC is still very similar to those shown in Fig. 2a and b. In the latter sample with 15 vol.%TiC, it was observed that the Ti2 AlC phase and ␥ phase link together so that an interpenetrating network structure was formed before annealing as seen in Fig. 3a and b. After the sample was
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Fig. 4. EPMA micrographs of Ti–48 at.%Al: (a) sintered at 1423 K; (b) annealed at 1473 K for 2 h after sintering at 1423 K.
subjected to annealing, the Ti2 AlC phase changed to the separated particles dispersed in the matrix consisting of the single ␥ phase, as shown in Fig. 3c. The microstructure of the sintered sample with the starting composition (Ti–48 at.%Al)–30 vol.%TiC, which is not shown here, was similar to that of (Ti–48 at.%Al)–15 vol.%TiC. These microstructure evolution depends on the starting compositions and processing conditions needs to be studied in more detail. To make a comparison between the microstructure of TiAl/Ti2 AlC composites and pure TiAl, Fig. 4a and b indicates, respectively, the sample Ti–48 at.%Al sintered at 1423 K and then annealed at 1473 K. The former offers the dual equiaxed structure (black area is ␥ phase and light area is ␣2 phase), but the latter has the full lamellar structure.
The fracture toughness (KIC ) of the composites produced is shown in Fig. 5. For the as-sintered product containing 7 vol.%TiC, the toughness was 43 MPa m1/2 which was 15% higher than that of the as-sintered sample of TiAl matrix. After annealing, the toughness decreased to 29 MPa m1/2 , a decrease of 20% relative to the value of TiAl which was annealed. However, it can be still compared with that of the as-sintered TiAl (30 MPa m1/2 ). For the sample containing 15 vol.%TiC, the fracture toughness was evidently lower than that of TiAl, although it increased to 23 MPa m1/2 by annealing. The sample with 30 vol.%TiC was very brittle such that its toughness is not shown here. These results suggest that the introduction of small quantities of Ti2 AlC can improve the fracture toughness, probably due to the dispersion of hard particles [3], however, further increase of
Fig. 5. Fracture toughness of the samples produced in this work: (A) starting composition of Ti–48 at.%Al, sintered at 1423 K; (A1 ) sample (A), treated at 1473 K for 2 h; (B) starting composition of (Ti–48 at.%Al)–7 vol.%TiC, sintered at 1423 K; (B1 ) sample (B), treated at 1473 K for 2 h; (C) starting composition (Ti–48 at.%Al)–15 vol.%TiC, sintered at 1423 K; (C1 ) sample (C), annealed at 1473 K for 2 h.
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Fig. 6. The bending strength of TiAl/Ti2 AlC composites as a function of TiC addition in the starting mixtures.
Ti2 AlC content tends to form a network structure of the brittle Ti2 AlC phase resulting in an increase in toughness. Both the lamellar structure of TiAl and the appropriate content of Ti2 AlC dispersion are required for an improved toughness. The bending strength of samples is shown in Fig. 6. The sample containing 7 vol.%TiC exhibited a higher strength than that of TiAl. When the TiC content is more than 7 vol.%, the strength decreased as the fracture toughness decreased. The heat treatment had no significant effect on the strength.
4. Conclusions (1) TiAl/Ti2 AlC composites could be obtained from the starting powders of TiC, elemental Ti and Al by SPS. The reaction Ti + Al + TiC → TiAl + Ti2 AlC took place above the melting point of Al, while the tempe-
295
rature at which TiC was completely changed to Ti2 AlC was between 1173 and 1423 K. (2) The microstructure depends on the heat treatment and TiC contents in the starting mixture. Two different microstructures were observed. In one case, the sample with 7 vol.%TiC, Ti2 AlC particles were uniformly dispersed in the TiAl matrix with a partial lamellar structure. In another case for the sample containing 15 vol.%TiC, the composites consisted of interpenetrating networks of Ti2 AlC and TiAl phases. After the heat treatment, all of them were transformed into the same microstructure consisting of dispersed Ti2 AlC particles in TiAl. (3) Introduction of small quantities of Ti2 AlC phase could result in an increase of both fracture toughness and strength, owing to the dispersion effect of hard particles. The lamellar structure leads to a higher fracture toughness. References [1] C.T. Liu, J.O. Stiegler, Mater. Eng. 100 (5) (1984) 29. [2] M.L. Adams, S.L. Kampe, L. Christodoulou, Int. J. Powder Metall. 26 (2) (1990) 105. [3] S.L. Kampe, J.D. Bryant, L. Christodoulou, Metall. Trans. A 22 (2) (1991) 447. [4] J.D. Whittenberger, R.K. Viswanadham, S.K. Mannan, B. Sprissler, J. Mater. Sci. 25 (1990) 35. [5] M.W. Barsoum, T. El-Raghy, J. Am. Ceram. Soc. 79 (1996) 1731. [6] R. Ramaseshan, S.K. Senshadri, N.G. Nair, et al., J. Jpn. Soc. Powder Powder Metall. 45 (4) (1998) 330. [7] M. Tokita, J. Soc. Powder Technol. Jpn. 30 (11) (1993) 790. [8] T. Nishimura, M. Mitomo, H. Hirotsuru, M. Kawahara, J. Mater. Sci. 14 (1995) 1046. [9] B.C. Mei, Y. Miyamoto, Metall. Mater. Trans. A 32 (3) (2001) 843. [10] B.C. Mei, R.Z. Yuan, X.L. Duan, J. Mater. Res. 8 (11) (1993) 2830.