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Nb5Si3 in situ composites by spark plasma sintering
Journal of Alloys and Compounds 471 (2009) 404–407
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effects of element proportions on microstructures of Nb/Nb5 Si3 in situ composites by spark plasma sintering BoWen Xiong a,∗ , WenYuan Long a , Zhe Chen b , Chun Xia a , Hong Wan a , YouWei Yan c a
College of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China Nanchang Institute of Technology, Nanchang 330013, PR China c State Key Laboratory of Die Technology, Huazhong University of Science & Technology, Wuhan 430074, PR China b
a r t i c l e
i n f o
Article history: Received 27 December 2007 Received in revised form 26 March 2008 Accepted 26 March 2008 Available online 6 May 2008 Keywords: High-temperature alloys Spark plasma sintering Microstructure Niobium silicide
a b s t r a c t Nb/Nb5 Si3 composites were successfully fabricated by a spark plasma sintering (SPS) technology. The effects of element (Nb and Si) proportions on the microstructures of the synthesized composites were mainly investigated. The results show that element proportions strongly affect the microstructures of Nb/Nb5 Si3 composites; Nb/Nb5 Si3 in situ composites up to 99.59% of theoretical density were fabricated by SPS technology, and the microstructures of the fabricated composites comprise the synthesized Nb5 Si3 and the uniformly distributed Nb particles. Moreover, with the increase in the Si content from 6% to 20% (atom fraction), the volume fraction of the synthesized Nb5 Si3 in the composites increases, whereas the content and size of Nb particles decrease obviously. The higher shrinkage and densification for Si content (16 at.% and 20 at.%) close to the eutectic composition take place. When sintering temperature over 1400 ◦ C, the shrinkage obviously retards and finally stops. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Niobium silicide based alloys have been extensively studied as high-temperature structural materials [1–3]. Nb5 Si3 has high melting point (2515 ◦ C), low density (7.16 g cm−3 ) and excellent specific mechanical properties at high temperature [4–7], but Nb5 Si3 has low fracture toughness at room temperature and poor deformability at high temperature; currently, these drawbacks hinder the application of Nb5 Si3 . Nb has high melting point (2472 ◦ C), excellent fracture toughness and good deformability. The two-phase Nb/Nb5 Si3 exhibits desirable mechanical and physical properties, excellent thermo-chemical stability and resistance to coarsening up to 1500 ◦ C [6,8]. In addition, the binary Nb–Si system equilibrium phase diagram shows that the eutectic reaction leads to the formations of Nbss and Nb3 Si at the melting point of 1915 ◦ C [4], the Nb3 Si transforms into Nbss and Nb5 Si3 through a eutectoid reaction at 1765 ◦ C [9]. The solubility of Si in Nb is 0.5–37.5 at.%, which implies that the volume fraction of Nb5 Si3 phase increases with the increase of Si content. Therefore, the microstructures of these compositions can be greatly changed by chemical element proportions and a synthesizing process. At present, researches have shown that Nb/Nb5 Si3 composites have been usually fabricated by vacuum arc-melting technology
∗ Corresponding author. Tel.: +86 791 3863021; fax: +86 791 3953325. E-mail address: [email protected] (B. Xiong). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.03.095
[10,11]. The Nb–Si system yields the end products consisting of primary Nbss and (Nb + Nb3 Si) eutectic compositions by vacuum arc-melting technology; however, the Nb3 Si is a metastable phase, which must be carried on a heat treatment 100 h at 1765 ◦ C, and transforms into Nb and Nb5 Si3 by the eutectoid reaction. Therefore, the vacuum arc-melting technology exhibits some restrictions leading to the consumption of a great deal of energy and prolonged production period. It is essential to develop a new technology to fabricate Nb/Nb5 Si3 composites. Recently, the spark plasma sintering (hereafter abbreviated as SPS) has been drawing attention as a new process for the fabrication of advanced materials [12]. The most important feature of SPS is that the pressed powders are heated by the spark plasma among the reactant particles; as a result, the samples can be sintered uniformly and rapidly, and the dense materials with fine grains can be obtained in a very short holding time [6,12]. In the present paper, Nb/Nb5 Si3 in situ composites were successfully fabricated by SPS. The objective of the present study was to experimentally investigate the effects of element proportions on densification and behavior microstructure of Nb/Nb5 Si3 composites. 2. Experimental procedure The Nb–6Si, Nb–10Si, Nb–16Si and Nb–20Si elemental powder mixtures (at. %) were used to produce the Nb/Nb5 Si3 composites by SPS, respectively. The raw materials used in the present study were Nb (−325 mesh) and Si (200 mesh) powders with purity of 99.9% and 99.6% (wt.%), respectively. These powders were dry mixed in a mechanical mixer for 24 h, and the mixed powders were packed into a graphite die with an inside diameter of 15 mm.
B. Xiong et al. / Journal of Alloys and Compounds 471 (2009) 404–407
405
Fig. 1. Shrinkage curves of Nb–6Si, Nb–10Si, Nb–16Si and Nb–20Si samples sintered.
The sintering was carried out on a SPS machine manufactured by Sumitomo Coal Mining Co. Ltd., Japan. A uniaxial load of 6 kN acted on the powders to realize sintering and densification simultaneously. The heating speed of 150 ◦ C/min and the sintering temperature of 1500 ◦ C were adopted under fixed sintering holding time 10 min in the present study. The specimens were metallographically polished and etched using a reagent comprising 5 ml HNO3 , 10 ml HF, 15 ml H2 SO4 and 50 ml distilled water. The microstructures have been examined using a MeF-3 optical microscope and a JSMT200 scanning electron microscope. D/MAX-IIIB X-ray diffraction was performed on the bulk specimens for identification of phases. The distribution of phase and chemical species has been analyzed using JXA-800R electron microprobe analysis. The density of the fabricated composites was measured by Archimedes’ method.
3. Results and discussion 3.1. Effects of element proportions on densification behavior Fig. 1 shows the shrinkage curves of Nb–6Si, Nb–10Si, Nb–16Si and Nb–20Si samples sintered. It indicates that the shrinkage rate of different element proportions samples is obviously different from Fig. 1, but the trends of shrinkage curves have the same direction. At the sintering temperature between 900 ◦ C and 1400 ◦ C, the shrinkage rate is more rapid than under 900 ◦ C, and the shrinkage behavior obviously retards and finally stops when sintering temperature is raised to over 1400 ◦ C. With the increase in the Si content, the maximal shrinkage displacement of the specimen increases as indicated in Fig. 1; results after enhancing the density of Nb/Nb5 Si3 composites, Nb/Nb5 Si3 in situ composites with up to 99.59% of theoretical density were fabricated, as listed in Table 1. The local high temperature created by plasma and Joule heating, to the amount of molten silicon, increases and liquid phase sintering takes place under a uniaxial load of 6 kN; obviously, the more amount of silicon content results in the better degree of liquid phase sintering and the higher density of Nb/Nb5 Si3 in situ composites. So, it obtains the excellent microstructures of Nb/Nb5 Si3 in situ composites by validly controlling the element proportions.
Table 1 Effects of element proportions on the density of the composites Specimens
Experimental density (g cm−3 )
Theoretical density (g cm−3 )
Relative density (%)
Nb–6Si Nb–10Si Nb–16Si Nb–20Si
8.2640 8.1496 7.9617 7.8294
8.3708 8.2237 8.0122 7.8616
98.72 99.10 99.37 99.59
Fig. 2. XRD diffractograms of Nb–6Si, Nb–10Si, Nb–16Si and Nb–20Si composites.
3.2. Microstructures of Nb/Nb5 Si3 in situ composites The XRD diffractograms of Nb–6Si, Nb–10Si, Nb–16Si and Nb–20Si composites are presented in Fig. 2; individual phases were identified by matching the characteristic XRD peaks against JCPDS data. It is obvious from Fig. 2 that the peaks of Nb and Nb5 Si3 are detected, but the peaks of Si and Nb3 Si are not detected, and with the increase in the Si content, the intensity of Nb peaks reduces and the intensity of Nb5 Si3 peaks increases. The reactant powders Si completely produce the desirable Nb5 Si3 composites. Scanning electron micrograph taken at high magnification and EPMA images of the fabricated Nb/Nb5 Si3 in situ composites are presented in Fig. 3. Microstructures of eutectic mixtures of Nbss and Nb5 Si3 are shown in Fig. 3(b), the microstructures contain the coarse and blocky primary Nbss . Furthermore, the eutectic mixtures observed in Fig. 3(b) may be categorized as coarse and fine, based on the size of lamellae of Nbss present. The result indicates that particle phase in Fig. 3(a) (i.e. point 1) only contains Nb (Fig. 3(c)), and the dark matrix (point 2) contains both Nb and Si (Fig. 3(d)). Table 2 shows the EDS results of Nb/Nb5 Si3 composites, thus, combining the EDS results with the analyzed results of XRD (Fig. 2), the atoms of Nb and Si in the dark matrix (point 2) take on the ration approach to 5:3. It is concluded that the bright phase (point 1) is residual Nb and the dark matrix (point 2) is Nb5 Si3 synthesized the binary phases Nb/Nb5 Si3 composites during SPS.
Table 2 EDS results of the fabricated composites
Point 1 in Fig. 3(a) Point 2 in Fig. 3(a)
Mass fraction (%)
Atom fraction (%)
Nb
Si
Nb
Si
100 85.66
0 14.34
100 64.37
0 35.63
406
B. Xiong et al. / Journal of Alloys and Compounds 471 (2009) 404–407
Fig. 3. SEM image between Nb and Si particles and area analysis of elements of Nb and Si.
Fig. 4. Effects of element proportions on the microstructure of Nb/Nb5 Si3 in situ composites (a) Nb–6Si, (b) Nb–10Si, (c) Nb–16Si and (d) Nb–20Si.
B. Xiong et al. / Journal of Alloys and Compounds 471 (2009) 404–407
407
Fig. 5. SEM image between Nb and Si particles and area analysis of the elements Nb and Si.
3.3. Effects of element proportions on microstructures of Nb/Nb5 Si3 in situ composites Fig. 4 shows the effects of different element proportions on the microstructures of Nb/Nb5 Si3 in situ composites by SPS. With the increase in the Si content, the amount and size of the bright phases of Nb decreases simultaneously. During SPS, the pulse current directly yielded by special power acts on the reactant powders, induces the rarefied gas ionization and discharges at the gaps of powders. The gaps in high temperature created by plasma as well as Joule heating by pulse current result in the surface melting of reactant powders, simultaneously, leading to gasification impurities, and the surface activation takes place at the reactant particle surfaces, otherwise, owing to uniaxial pressure and high temperature sintering; the interface reaction of Nb and Si take place to fabricate the Nb5 Si3 composites, as shown in Fig. 5. The Si powders surface is molten when the sintering temperature is up to 1400 ◦ C; the diffusion rate of Nb and Si atoms rapidly increases, which also leads to a strong interfacial reaction of Nb, and Si transforms into Nbss and Nb5 Si3 by eutectic or eutectoid reaction; with the increasing of Si content, the reduction of remaining Nb particles size takes place. As a result, the remaining Nb particles become finer and get uniformly distributed in the synthesized Nb/Nb5 Si3 composites. 4. Conclusions The following conclusions may be drawn, based on the results of the present study on the effects of element proportions of Nb–6Si, Nb–10Si, Nb–16Si and Nb–20Si on microstructures of Nb/Nb5 Si3 in situ composites by SPS. Using Nb and Si powders as raw materials, Nb/Nb5 Si3 in situ composites with up to 99.59% of theoretical
density were fabricated, with Nb particles uniformly distributed in Nb5 Si3 matrix. The element proportions strongly affect the microstructures of Nb/Nb5 Si3 composites; with the increasing of Si content, the volume fraction of the synthesized Nb5 Si3 in the composites increases and the content and size of Nb phase decreases simultaneously. With the increasing of Si content, the maximal shrinkage displacement of the specimen increases and results in enhanced the density of Nb/Nb5 Si3 composites. Acknowledgements This research was financially supported by the National Natural Science Foundation of China (Grant Number: 50276023), Nanchang Hangkong University Foundation of China (Grant Number: EC200701021), and Jiangxi Provincial department of Science and Technology Foundation of China (Grant Number: DB200701049). References [1] W.-Y. Kim, H. Tanaka, S. Hannda, Intermetallics 10 (2002) 625–634. [2] H.-Z. Fu, J. Aeronautical Mater. Chin. 18 (1998) 52–60. [3] T. Murakami, C.N. Xu, A. Kitahara, M. Kawahara, Y. Takahashi, H. Inui, M. Yamaguchi, Intermetallics 7 (9) (1999) 1043–1048. [4] C.L. Yeh, W.H. Chen, J. Alloy Compd. 425 (2006) 216–222. [5] C.L. Yeh, W.H. Chen, J. Alloy Compd. 402 (2005) 118–123. [6] Z. Chen, Y.-W. Yan, J. Alloy Compd. 413 (2006) 73–76. [7] J. Geng, P. Tsakiropoulos, G. Shao, Intermetallics 15 (2007) 69–76. [8] K. Chattopadhyay, G. Balachandran, R. Mitra, K.K. Ray, Intermetallics 14 (2006) 1452–1460. [9] M.R. Jackson, B.P. Bewlay, R.G. Rowe, JOM 48 (1996) 39–46. [10] B.P. Bewlay, M.R. Jackson, H.A. Lipsitt, Mater. Sci. Eng. A V27A (1996) 89– 90. [11] M.G. Mendiratta, J.J. Lewandowski, D.M. Dimiduk, Metall. Trans. A 22A (1991) 1573–1583. [12] O. Mamoru, Mater. Sci. Eng. A 287 (2000) 183–188.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effects of element proportions on microstructures of Nb/Nb5 Si3 in situ composites by spark plasma sintering BoWen Xiong a,∗ , WenYuan Long a , Zhe Chen b , Chun Xia a , Hong Wan a , YouWei Yan c a
College of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China Nanchang Institute of Technology, Nanchang 330013, PR China c State Key Laboratory of Die Technology, Huazhong University of Science & Technology, Wuhan 430074, PR China b
a r t i c l e
i n f o
Article history: Received 27 December 2007 Received in revised form 26 March 2008 Accepted 26 March 2008 Available online 6 May 2008 Keywords: High-temperature alloys Spark plasma sintering Microstructure Niobium silicide
a b s t r a c t Nb/Nb5 Si3 composites were successfully fabricated by a spark plasma sintering (SPS) technology. The effects of element (Nb and Si) proportions on the microstructures of the synthesized composites were mainly investigated. The results show that element proportions strongly affect the microstructures of Nb/Nb5 Si3 composites; Nb/Nb5 Si3 in situ composites up to 99.59% of theoretical density were fabricated by SPS technology, and the microstructures of the fabricated composites comprise the synthesized Nb5 Si3 and the uniformly distributed Nb particles. Moreover, with the increase in the Si content from 6% to 20% (atom fraction), the volume fraction of the synthesized Nb5 Si3 in the composites increases, whereas the content and size of Nb particles decrease obviously. The higher shrinkage and densification for Si content (16 at.% and 20 at.%) close to the eutectic composition take place. When sintering temperature over 1400 ◦ C, the shrinkage obviously retards and finally stops. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Niobium silicide based alloys have been extensively studied as high-temperature structural materials [1–3]. Nb5 Si3 has high melting point (2515 ◦ C), low density (7.16 g cm−3 ) and excellent specific mechanical properties at high temperature [4–7], but Nb5 Si3 has low fracture toughness at room temperature and poor deformability at high temperature; currently, these drawbacks hinder the application of Nb5 Si3 . Nb has high melting point (2472 ◦ C), excellent fracture toughness and good deformability. The two-phase Nb/Nb5 Si3 exhibits desirable mechanical and physical properties, excellent thermo-chemical stability and resistance to coarsening up to 1500 ◦ C [6,8]. In addition, the binary Nb–Si system equilibrium phase diagram shows that the eutectic reaction leads to the formations of Nbss and Nb3 Si at the melting point of 1915 ◦ C [4], the Nb3 Si transforms into Nbss and Nb5 Si3 through a eutectoid reaction at 1765 ◦ C [9]. The solubility of Si in Nb is 0.5–37.5 at.%, which implies that the volume fraction of Nb5 Si3 phase increases with the increase of Si content. Therefore, the microstructures of these compositions can be greatly changed by chemical element proportions and a synthesizing process. At present, researches have shown that Nb/Nb5 Si3 composites have been usually fabricated by vacuum arc-melting technology
∗ Corresponding author. Tel.: +86 791 3863021; fax: +86 791 3953325. E-mail address: [email protected] (B. Xiong). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.03.095
[10,11]. The Nb–Si system yields the end products consisting of primary Nbss and (Nb + Nb3 Si) eutectic compositions by vacuum arc-melting technology; however, the Nb3 Si is a metastable phase, which must be carried on a heat treatment 100 h at 1765 ◦ C, and transforms into Nb and Nb5 Si3 by the eutectoid reaction. Therefore, the vacuum arc-melting technology exhibits some restrictions leading to the consumption of a great deal of energy and prolonged production period. It is essential to develop a new technology to fabricate Nb/Nb5 Si3 composites. Recently, the spark plasma sintering (hereafter abbreviated as SPS) has been drawing attention as a new process for the fabrication of advanced materials [12]. The most important feature of SPS is that the pressed powders are heated by the spark plasma among the reactant particles; as a result, the samples can be sintered uniformly and rapidly, and the dense materials with fine grains can be obtained in a very short holding time [6,12]. In the present paper, Nb/Nb5 Si3 in situ composites were successfully fabricated by SPS. The objective of the present study was to experimentally investigate the effects of element proportions on densification and behavior microstructure of Nb/Nb5 Si3 composites. 2. Experimental procedure The Nb–6Si, Nb–10Si, Nb–16Si and Nb–20Si elemental powder mixtures (at. %) were used to produce the Nb/Nb5 Si3 composites by SPS, respectively. The raw materials used in the present study were Nb (−325 mesh) and Si (200 mesh) powders with purity of 99.9% and 99.6% (wt.%), respectively. These powders were dry mixed in a mechanical mixer for 24 h, and the mixed powders were packed into a graphite die with an inside diameter of 15 mm.
B. Xiong et al. / Journal of Alloys and Compounds 471 (2009) 404–407
405
Fig. 1. Shrinkage curves of Nb–6Si, Nb–10Si, Nb–16Si and Nb–20Si samples sintered.
The sintering was carried out on a SPS machine manufactured by Sumitomo Coal Mining Co. Ltd., Japan. A uniaxial load of 6 kN acted on the powders to realize sintering and densification simultaneously. The heating speed of 150 ◦ C/min and the sintering temperature of 1500 ◦ C were adopted under fixed sintering holding time 10 min in the present study. The specimens were metallographically polished and etched using a reagent comprising 5 ml HNO3 , 10 ml HF, 15 ml H2 SO4 and 50 ml distilled water. The microstructures have been examined using a MeF-3 optical microscope and a JSMT200 scanning electron microscope. D/MAX-IIIB X-ray diffraction was performed on the bulk specimens for identification of phases. The distribution of phase and chemical species has been analyzed using JXA-800R electron microprobe analysis. The density of the fabricated composites was measured by Archimedes’ method.
3. Results and discussion 3.1. Effects of element proportions on densification behavior Fig. 1 shows the shrinkage curves of Nb–6Si, Nb–10Si, Nb–16Si and Nb–20Si samples sintered. It indicates that the shrinkage rate of different element proportions samples is obviously different from Fig. 1, but the trends of shrinkage curves have the same direction. At the sintering temperature between 900 ◦ C and 1400 ◦ C, the shrinkage rate is more rapid than under 900 ◦ C, and the shrinkage behavior obviously retards and finally stops when sintering temperature is raised to over 1400 ◦ C. With the increase in the Si content, the maximal shrinkage displacement of the specimen increases as indicated in Fig. 1; results after enhancing the density of Nb/Nb5 Si3 composites, Nb/Nb5 Si3 in situ composites with up to 99.59% of theoretical density were fabricated, as listed in Table 1. The local high temperature created by plasma and Joule heating, to the amount of molten silicon, increases and liquid phase sintering takes place under a uniaxial load of 6 kN; obviously, the more amount of silicon content results in the better degree of liquid phase sintering and the higher density of Nb/Nb5 Si3 in situ composites. So, it obtains the excellent microstructures of Nb/Nb5 Si3 in situ composites by validly controlling the element proportions.
Table 1 Effects of element proportions on the density of the composites Specimens
Experimental density (g cm−3 )
Theoretical density (g cm−3 )
Relative density (%)
Nb–6Si Nb–10Si Nb–16Si Nb–20Si
8.2640 8.1496 7.9617 7.8294
8.3708 8.2237 8.0122 7.8616
98.72 99.10 99.37 99.59
Fig. 2. XRD diffractograms of Nb–6Si, Nb–10Si, Nb–16Si and Nb–20Si composites.
3.2. Microstructures of Nb/Nb5 Si3 in situ composites The XRD diffractograms of Nb–6Si, Nb–10Si, Nb–16Si and Nb–20Si composites are presented in Fig. 2; individual phases were identified by matching the characteristic XRD peaks against JCPDS data. It is obvious from Fig. 2 that the peaks of Nb and Nb5 Si3 are detected, but the peaks of Si and Nb3 Si are not detected, and with the increase in the Si content, the intensity of Nb peaks reduces and the intensity of Nb5 Si3 peaks increases. The reactant powders Si completely produce the desirable Nb5 Si3 composites. Scanning electron micrograph taken at high magnification and EPMA images of the fabricated Nb/Nb5 Si3 in situ composites are presented in Fig. 3. Microstructures of eutectic mixtures of Nbss and Nb5 Si3 are shown in Fig. 3(b), the microstructures contain the coarse and blocky primary Nbss . Furthermore, the eutectic mixtures observed in Fig. 3(b) may be categorized as coarse and fine, based on the size of lamellae of Nbss present. The result indicates that particle phase in Fig. 3(a) (i.e. point 1) only contains Nb (Fig. 3(c)), and the dark matrix (point 2) contains both Nb and Si (Fig. 3(d)). Table 2 shows the EDS results of Nb/Nb5 Si3 composites, thus, combining the EDS results with the analyzed results of XRD (Fig. 2), the atoms of Nb and Si in the dark matrix (point 2) take on the ration approach to 5:3. It is concluded that the bright phase (point 1) is residual Nb and the dark matrix (point 2) is Nb5 Si3 synthesized the binary phases Nb/Nb5 Si3 composites during SPS.
Table 2 EDS results of the fabricated composites
Point 1 in Fig. 3(a) Point 2 in Fig. 3(a)
Mass fraction (%)
Atom fraction (%)
Nb
Si
Nb
Si
100 85.66
0 14.34
100 64.37
0 35.63
406
B. Xiong et al. / Journal of Alloys and Compounds 471 (2009) 404–407
Fig. 3. SEM image between Nb and Si particles and area analysis of elements of Nb and Si.
Fig. 4. Effects of element proportions on the microstructure of Nb/Nb5 Si3 in situ composites (a) Nb–6Si, (b) Nb–10Si, (c) Nb–16Si and (d) Nb–20Si.
B. Xiong et al. / Journal of Alloys and Compounds 471 (2009) 404–407
407
Fig. 5. SEM image between Nb and Si particles and area analysis of the elements Nb and Si.
3.3. Effects of element proportions on microstructures of Nb/Nb5 Si3 in situ composites Fig. 4 shows the effects of different element proportions on the microstructures of Nb/Nb5 Si3 in situ composites by SPS. With the increase in the Si content, the amount and size of the bright phases of Nb decreases simultaneously. During SPS, the pulse current directly yielded by special power acts on the reactant powders, induces the rarefied gas ionization and discharges at the gaps of powders. The gaps in high temperature created by plasma as well as Joule heating by pulse current result in the surface melting of reactant powders, simultaneously, leading to gasification impurities, and the surface activation takes place at the reactant particle surfaces, otherwise, owing to uniaxial pressure and high temperature sintering; the interface reaction of Nb and Si take place to fabricate the Nb5 Si3 composites, as shown in Fig. 5. The Si powders surface is molten when the sintering temperature is up to 1400 ◦ C; the diffusion rate of Nb and Si atoms rapidly increases, which also leads to a strong interfacial reaction of Nb, and Si transforms into Nbss and Nb5 Si3 by eutectic or eutectoid reaction; with the increasing of Si content, the reduction of remaining Nb particles size takes place. As a result, the remaining Nb particles become finer and get uniformly distributed in the synthesized Nb/Nb5 Si3 composites. 4. Conclusions The following conclusions may be drawn, based on the results of the present study on the effects of element proportions of Nb–6Si, Nb–10Si, Nb–16Si and Nb–20Si on microstructures of Nb/Nb5 Si3 in situ composites by SPS. Using Nb and Si powders as raw materials, Nb/Nb5 Si3 in situ composites with up to 99.59% of theoretical
density were fabricated, with Nb particles uniformly distributed in Nb5 Si3 matrix. The element proportions strongly affect the microstructures of Nb/Nb5 Si3 composites; with the increasing of Si content, the volume fraction of the synthesized Nb5 Si3 in the composites increases and the content and size of Nb phase decreases simultaneously. With the increasing of Si content, the maximal shrinkage displacement of the specimen increases and results in enhanced the density of Nb/Nb5 Si3 composites. Acknowledgements This research was financially supported by the National Natural Science Foundation of China (Grant Number: 50276023), Nanchang Hangkong University Foundation of China (Grant Number: EC200701021), and Jiangxi Provincial department of Science and Technology Foundation of China (Grant Number: DB200701049). References [1] W.-Y. Kim, H. Tanaka, S. Hannda, Intermetallics 10 (2002) 625–634. [2] H.-Z. Fu, J. Aeronautical Mater. Chin. 18 (1998) 52–60. [3] T. Murakami, C.N. Xu, A. Kitahara, M. Kawahara, Y. Takahashi, H. Inui, M. Yamaguchi, Intermetallics 7 (9) (1999) 1043–1048. [4] C.L. Yeh, W.H. Chen, J. Alloy Compd. 425 (2006) 216–222. [5] C.L. Yeh, W.H. Chen, J. Alloy Compd. 402 (2005) 118–123. [6] Z. Chen, Y.-W. Yan, J. Alloy Compd. 413 (2006) 73–76. [7] J. Geng, P. Tsakiropoulos, G. Shao, Intermetallics 15 (2007) 69–76. [8] K. Chattopadhyay, G. Balachandran, R. Mitra, K.K. Ray, Intermetallics 14 (2006) 1452–1460. [9] M.R. Jackson, B.P. Bewlay, R.G. Rowe, JOM 48 (1996) 39–46. [10] B.P. Bewlay, M.R. Jackson, H.A. Lipsitt, Mater. Sci. Eng. A V27A (1996) 89– 90. [11] M.G. Mendiratta, J.J. Lewandowski, D.M. Dimiduk, Metall. Trans. A 22A (1991) 1573–1583. [12] O. Mamoru, Mater. Sci. Eng. A 287 (2000) 183–188.