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Nb5Si3 in situ composites synthesized by spark plasma sintering
Journal of Alloys and Compounds 413 (2006) 73–76
Influence of sintering temperature on microstructures of Nb/Nb5Si3 in situ composites synthesized by spark plasma sintering Zhe Chen ∗ , YouWei Yan State Key Laboratory of Die Technology, Huazhong University of Science & Technology, Wuhan 430074, PR China Received 25 April 2005; received in revised form 2 June 2005; accepted 2 June 2005 Available online 8 February 2006
Abstract Taking the use of Nb and Si elemental powders as raw materials, Nb/Nb5 Si3 composites were successfully fabricated by a spark plasma sintering (SPS) technology. The influence of sintering temperatures on microstructures of the synthesized composites was mainly investigated. The results show that there exists a critical sintering temperature Tc (1400 ◦ C) in the synthesis of Nb/Nb5 Si3 composites by SPS technology. When sintering temperature is lower than Tc , an intermediate Nb3 Si phase is produced in the fabricated samples. Moreover, the relative density of the samples is lower. Only if the sintering temperature is higher than Tc, the intermediate Nb3 Si phase is thoroughly eliminated, Nb particles become a finer size and desirable Nb/Nb5 Si3 composites up to 99.59% theoretic density are achieved. © 2006 Published by Elsevier B.V. Keywords: Spark plasma sintering; Nb/Nb5 Si3 composites; Sintering temperature; Microstructure
1. Introduction Nb5 Si3 has high potential as high-temperature structural materials due to its high melting, low density, and good creep resistance at elevated temperature [1–3]. However, poor fracture toughness at room temperature hinders its applications like many other intermetallic compounds. Recently, it is recognized that incorporation of a ductile phase into a brittle phase through microstructure control enhances the room temperature ductility or fracture toughness on many ceramics and intermetallics [4,5]. Nb possesses excellent toughness and high melting point (2472 ◦ C), moreover, according to Nb–Si binary phase diagram [6], Nb and Nb5 Si3 coexist in a wide range of temperature (room temperature ∼ 1770 ◦ C) and silicon content (0.5 ∼ 37.5 mol.%). These features of the phase diagram allow us to fabricate Nb/Nb5 Si3 in situ composites with a high degree of thermodynamical and microstructural stability at high temperature. Therefore, Nb/Nb5 Si3 has ∗
Corresponding author. Tel.: +86 27 87543876; fax: +86 27 87556262. E-mail address: [email protected] (Z. Chen).
0925-8388/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jallcom.2005.06.005
strong potential for developing high-temperature structure materials [7]. At present, vacuum arc melting is generally used for the fabrication of Nb/Nb5 Si3 composites, which promotes the development and application of Nb/Nb5 Si3 composites [8,9]. However, the arc-melting technology has some restrictions, such as remelting three or four times leading to consume a great deal of energy, heat treatment 100 h resulting in a prolonged production period, and needing secondary processing (for instance, hot-extrusion) after melting. Therefore, it is essential to develop a new technology to fabricate Nb/Nb5 Si3 composites. Recently, spark plasma sintering (hereafter abbreviated as SPS) has been drawing attention as a new process for the fabrication of advanced materials [10]. The most important feature of SPS is that the pressed powders are heated by the spark plasma between the reactant particles. As a result, the samples can be sintered uniformly and rapidly, and dense materials with fine grains can be obtained in a very short holding time. In the present paper, Nb/Nb5 Si3 in situ composites were successfully fabricated by SPS. The influence of sintering
Z. Chen, Y.W. Yan / Journal of Alloys and Compounds 413 (2006) 73–76
74
temperature on densification behavior and microstructure were evaluated.
sis. Density was measured via Archimedes’ method, and hardness was measured using HR150-A type hardness tester.
2. Experimental Elemental powders of niobium and silicon were used as raw materials in this study. The niobium was 200 mesh and 99.9 mass% pure, the silicon was 325 mesh and 99.0 mass% pure. The powders were mixed in stoichiometric proportions according to the following equation: (5 − 4x)Nb + 3(1 − x)Si → (1 − x)Nb5 Si3 + xNb The powders were dry mixed in a mechanical mixer for 10 h. Then, the mixed powders were packed into a graphite die with an inside diameter of 15 mm. SPS was carried out on a SPS machine manufactured by Sumitomo Coal Mining Co. Ltd., Japan. Before sintering, the chamber was evacuated to about 6 Pa and a uniaxial load of 6 kN was applied to the powders to realize sintering and densification simultaneously. To study the effect of sintering temperature on microstructure, sintering temperatures of 1300, 1500, and 1600 ◦ C were adopted under fixed holding time of 10 min. The samples were etched with a solution of HF:HNO3 = 3:1 ratio. The microstructures were investigated using a MeF-3 optical microscope and a JSMT-200 scanning electron microscope. The samples were characterized by XRD using Cuk␣ radiation for bulk phase identification. The phase distribution and chemical species distribution were evaluated using JXA-8800R electron microprobe analy-
3. Results and analysis 3.1. Densification behavior during SPS process Fig. 1 shows the shrinkage curves of samples sintered at 1300, 1500, and 1600 ◦ C. Owing to uniaxial pressure and high temperature sintering, the samples shrunk rapidly to accelerate densification. At further heating and holding state, shrinkage behavior retards and stops finally. From Fig. 1, it is discernible that the rate of shrinkage at different sintering temperature is obviously different. The sample sintered at 1300 ◦ C continuously shrunk during heating and holding state, and its shrinkage rate was more sluggish than those sintered at 1500 and 1600 ◦ C. It can be also seen from Fig. 1(b and c) that, when temperature is raised to 1400 ◦ C (dashed line), the samples stop shrinking, which indicates that the samples are achieving nearly full density. The result of measured density supports the above explanation. The composites sintered at 1500 and 1600 ◦ C possess 99.17 and 99.59% of theoretical density, respectively, but the composites at 1300 ◦ C only 94.02% of theoretical density, as listed in Table 1. During SPS process, the action of local high temperature created by plasma as well as Joule heating by pulse current results in surface melting of silicon particles to promote shrinkage. When sintering temperature soared to 1400 ◦ C
Fig. 1. Effect of sintering temperatures on shrinkage characteristic of Nb/Nb5 Si3 in situ composites (a) 1300 ◦ C, (b) 1500 ◦ C, and (c) 1600 ◦ C.
Z. Chen, Y.W. Yan / Journal of Alloys and Compounds 413 (2006) 73–76 Table 1 Effect of sintering temperatures on relative density and hardness of SPS Nb/Nb5 Si3 in situ composites Sintering temperature (◦ C) Relative density (%) Hardness (HRC)
1300 94.02 35
1500 99.17 57
1600 99.59 61
Fig. 2. XRD patterns of Nb/Nb5 Si3 in situ composites at different sintering temperatures.
(near silicon melting point 1414 ◦ C), the amount of molten silicon increases and liquid phase sintering takes place to lead the sample finally to stop shrinkage and realize the sample densification. Thus, in the present study, it is proposed that 1400 ◦ C is regarded as a critical sintering temperature (Tc ) to fabricate Nb/Nb5 Si3 composites. Only if sintering temperature is higher than Tc , a nearly full density composite can be achieved. 3.2. Microstructures of Nb/Nb5 Si3 composites Fig. 2 shows XRD patterns of Nb/Nb5 Si3 composites sintered at various temperatures. Peaks of Nb and Nb5 Si3
75
are detected but no peak of Si. The result demonstrates that the reaction has gone to completion. Fig. 2 also indicates that the sample sintered at 1300 ◦ C contains Nb3 Si but the sample at 1500 and 1600 ◦ C does not. Nevertheless, the Nb3 Si is a metastable phase, and its melting point (1975 ◦ C) is lower than that of Nb5 Si3 (2515 ◦ C). Therefore, the fabricated Nb/Nb5 Si3 composites for high-temperature structural application must avoid the existence of Nb3 Si. It is gratifying that, when sintering temperature is higher than 1500 ◦ C, intermediate phase Nb3 Si was thoroughly eliminated, as a result, the desired Nb/Nb5 Si3 composites were fabricated. Microstructures sintered at different temperatures are shown in Fig. 3. It is clear that the size of the particles sintered at 1300 ◦ C is larger than those sintered at 1500 and 1600 ◦ C, and a small amount of pores can be found in the microstructure at 1300 ◦ C, which is corresponding with the result of the measured density. With increasing sintering temperature, the size of particles reduces and pores disappear. As a result, the relative density is obviously increased, as listed in Table 1. Fig. 4 shows a scanning electron micrograph taken at high magnification and EDS images of the sample sintered at 1500 ◦ C. The result indicates that particle phase in Fig. 4(a) (i.e. point 1) only contains Nb (Fig. 4(b)), and the matrix (point 2) contains both Nb and Si (Fig. 4(c)). Thus, combined with the result of XRD (Fig. 2), it is concluded that the bright phase (point 1) is residual Nb and the dark matrix (point 2) is Nb5 Si3 synthesized during SPS. During the SPS process, the spark plasma caused the local temperature of the gaps between the powders instantly to increase so as to promote interfacial reaction taking place to synthesize Nb5 Si3 , as shown in Fig. 5. With increasing sintering temperature, the diffusion rate of Nb and Si atoms rapidly increased, leading to a strong interfacial reaction. As a result, the remaining Nb particles became finer and uniformly distributed in the synthesized Nb5 Si3 matrix (Fig. 3). The data of the measured hardness also indicate that the sample sintered at 1300 ◦ C exhibits a lower hardness, HRC35 and the samples sintered at 1500 and 1600 ◦ C exhibit a higher hardness, HRC57 and HRC61.
Fig. 3. Effect of sintering temperatures on microstructures of Nb/Nb5 Si3 in situ composites (a) 1300 ◦ C, (b) 1500 ◦ C, and (c) 1600 ◦ C.
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Fig. 4. SEM microstructure of Nb/Nb5 Si3 composite (a) and EDS images (b and c).
Fig. 5. SEM image between Nb and Si particles and area analysis of elements Nb and Si.
Thus, above the critical sintering temperature (Tc = 1400 ◦ C), the fabricated Nb/Nb5 Si3 composites possess not only an improved microstructure but also higher densification and hardness.
Acknowledgement
4. Summary
References
(1) Using Nb and Si elemental powders as raw materials, in situ Nb/Nb5 Si3 composites up to 99.59% of theoretical density were successfully fabricated, with Nb particles uniformly distributed in a Nb5 Si3 matrix. (2) A critical sintering temperature Tc (1400 ◦ C) exists for SPS to synthesize Nb/Nb5 Si3 composites. At sintering temperatures above Tc , dense Nb/Nb5 Si3 composites can be achieved. (3) The sintering temperature strongly affects the microstructure of the composites. With increasing sintering temperature, the size of the Nb particles decreases and the density of the composites increase.
This research was supported by a grant from the National Natural Science Foundation of China (No. 50276023).
[1] P.R. Subramanian, M.G. Mendiratta, D.M. Dimiduk, JOM 48 (1996) 33–38. [2] H.-Z. Fu, J. Aeronautical Mater. China 18 (1998) 52–60. [3] B.P. Bewlay, M.R. Jackson, H.A. Lipsitt, Metall. Trans. A V27A (1996) 3801–3808. [4] V.V. Kristic, P.S. Nicholson, J Am. Ceram. Soc. 64 (1981) 499–504. [5] D.R. Bloyer, K.T.V. Rao, R.O. Richie, Mater. Sci. Eng. A 26 (1996) 80–90. [6] P.B. Fernandes, G.C. Coelho, F. Ferreira, C.A. Nunes, B. Sundman, Intermetallics 10 (2002) 993–999. [7] S.-Y. Qu, R.-M. Wang, Y.-F. Han, Trans. Nonferrous Met. Soc. China 12 (2002) 691–694. [8] W.-Y. Kim, H. Tanaka, S. Hannda, Intermetallics 10 (2002) 625–634. [9] M.G. Mendiratta, J.J. Lewandowski, D.M. Dimiduk, Metall. Trans. A 22A (1991) 1573–1583. [10] O. Mamoru, Mater. Sci. Eng. A 287 (2000) 183–188.
Influence of sintering temperature on microstructures of Nb/Nb5Si3 in situ composites synthesized by spark plasma sintering Zhe Chen ∗ , YouWei Yan State Key Laboratory of Die Technology, Huazhong University of Science & Technology, Wuhan 430074, PR China Received 25 April 2005; received in revised form 2 June 2005; accepted 2 June 2005 Available online 8 February 2006
Abstract Taking the use of Nb and Si elemental powders as raw materials, Nb/Nb5 Si3 composites were successfully fabricated by a spark plasma sintering (SPS) technology. The influence of sintering temperatures on microstructures of the synthesized composites was mainly investigated. The results show that there exists a critical sintering temperature Tc (1400 ◦ C) in the synthesis of Nb/Nb5 Si3 composites by SPS technology. When sintering temperature is lower than Tc , an intermediate Nb3 Si phase is produced in the fabricated samples. Moreover, the relative density of the samples is lower. Only if the sintering temperature is higher than Tc, the intermediate Nb3 Si phase is thoroughly eliminated, Nb particles become a finer size and desirable Nb/Nb5 Si3 composites up to 99.59% theoretic density are achieved. © 2006 Published by Elsevier B.V. Keywords: Spark plasma sintering; Nb/Nb5 Si3 composites; Sintering temperature; Microstructure
1. Introduction Nb5 Si3 has high potential as high-temperature structural materials due to its high melting, low density, and good creep resistance at elevated temperature [1–3]. However, poor fracture toughness at room temperature hinders its applications like many other intermetallic compounds. Recently, it is recognized that incorporation of a ductile phase into a brittle phase through microstructure control enhances the room temperature ductility or fracture toughness on many ceramics and intermetallics [4,5]. Nb possesses excellent toughness and high melting point (2472 ◦ C), moreover, according to Nb–Si binary phase diagram [6], Nb and Nb5 Si3 coexist in a wide range of temperature (room temperature ∼ 1770 ◦ C) and silicon content (0.5 ∼ 37.5 mol.%). These features of the phase diagram allow us to fabricate Nb/Nb5 Si3 in situ composites with a high degree of thermodynamical and microstructural stability at high temperature. Therefore, Nb/Nb5 Si3 has ∗
Corresponding author. Tel.: +86 27 87543876; fax: +86 27 87556262. E-mail address: [email protected] (Z. Chen).
0925-8388/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jallcom.2005.06.005
strong potential for developing high-temperature structure materials [7]. At present, vacuum arc melting is generally used for the fabrication of Nb/Nb5 Si3 composites, which promotes the development and application of Nb/Nb5 Si3 composites [8,9]. However, the arc-melting technology has some restrictions, such as remelting three or four times leading to consume a great deal of energy, heat treatment 100 h resulting in a prolonged production period, and needing secondary processing (for instance, hot-extrusion) after melting. Therefore, it is essential to develop a new technology to fabricate Nb/Nb5 Si3 composites. Recently, spark plasma sintering (hereafter abbreviated as SPS) has been drawing attention as a new process for the fabrication of advanced materials [10]. The most important feature of SPS is that the pressed powders are heated by the spark plasma between the reactant particles. As a result, the samples can be sintered uniformly and rapidly, and dense materials with fine grains can be obtained in a very short holding time. In the present paper, Nb/Nb5 Si3 in situ composites were successfully fabricated by SPS. The influence of sintering
Z. Chen, Y.W. Yan / Journal of Alloys and Compounds 413 (2006) 73–76
74
temperature on densification behavior and microstructure were evaluated.
sis. Density was measured via Archimedes’ method, and hardness was measured using HR150-A type hardness tester.
2. Experimental Elemental powders of niobium and silicon were used as raw materials in this study. The niobium was 200 mesh and 99.9 mass% pure, the silicon was 325 mesh and 99.0 mass% pure. The powders were mixed in stoichiometric proportions according to the following equation: (5 − 4x)Nb + 3(1 − x)Si → (1 − x)Nb5 Si3 + xNb The powders were dry mixed in a mechanical mixer for 10 h. Then, the mixed powders were packed into a graphite die with an inside diameter of 15 mm. SPS was carried out on a SPS machine manufactured by Sumitomo Coal Mining Co. Ltd., Japan. Before sintering, the chamber was evacuated to about 6 Pa and a uniaxial load of 6 kN was applied to the powders to realize sintering and densification simultaneously. To study the effect of sintering temperature on microstructure, sintering temperatures of 1300, 1500, and 1600 ◦ C were adopted under fixed holding time of 10 min. The samples were etched with a solution of HF:HNO3 = 3:1 ratio. The microstructures were investigated using a MeF-3 optical microscope and a JSMT-200 scanning electron microscope. The samples were characterized by XRD using Cuk␣ radiation for bulk phase identification. The phase distribution and chemical species distribution were evaluated using JXA-8800R electron microprobe analy-
3. Results and analysis 3.1. Densification behavior during SPS process Fig. 1 shows the shrinkage curves of samples sintered at 1300, 1500, and 1600 ◦ C. Owing to uniaxial pressure and high temperature sintering, the samples shrunk rapidly to accelerate densification. At further heating and holding state, shrinkage behavior retards and stops finally. From Fig. 1, it is discernible that the rate of shrinkage at different sintering temperature is obviously different. The sample sintered at 1300 ◦ C continuously shrunk during heating and holding state, and its shrinkage rate was more sluggish than those sintered at 1500 and 1600 ◦ C. It can be also seen from Fig. 1(b and c) that, when temperature is raised to 1400 ◦ C (dashed line), the samples stop shrinking, which indicates that the samples are achieving nearly full density. The result of measured density supports the above explanation. The composites sintered at 1500 and 1600 ◦ C possess 99.17 and 99.59% of theoretical density, respectively, but the composites at 1300 ◦ C only 94.02% of theoretical density, as listed in Table 1. During SPS process, the action of local high temperature created by plasma as well as Joule heating by pulse current results in surface melting of silicon particles to promote shrinkage. When sintering temperature soared to 1400 ◦ C
Fig. 1. Effect of sintering temperatures on shrinkage characteristic of Nb/Nb5 Si3 in situ composites (a) 1300 ◦ C, (b) 1500 ◦ C, and (c) 1600 ◦ C.
Z. Chen, Y.W. Yan / Journal of Alloys and Compounds 413 (2006) 73–76 Table 1 Effect of sintering temperatures on relative density and hardness of SPS Nb/Nb5 Si3 in situ composites Sintering temperature (◦ C) Relative density (%) Hardness (HRC)
1300 94.02 35
1500 99.17 57
1600 99.59 61
Fig. 2. XRD patterns of Nb/Nb5 Si3 in situ composites at different sintering temperatures.
(near silicon melting point 1414 ◦ C), the amount of molten silicon increases and liquid phase sintering takes place to lead the sample finally to stop shrinkage and realize the sample densification. Thus, in the present study, it is proposed that 1400 ◦ C is regarded as a critical sintering temperature (Tc ) to fabricate Nb/Nb5 Si3 composites. Only if sintering temperature is higher than Tc , a nearly full density composite can be achieved. 3.2. Microstructures of Nb/Nb5 Si3 composites Fig. 2 shows XRD patterns of Nb/Nb5 Si3 composites sintered at various temperatures. Peaks of Nb and Nb5 Si3
75
are detected but no peak of Si. The result demonstrates that the reaction has gone to completion. Fig. 2 also indicates that the sample sintered at 1300 ◦ C contains Nb3 Si but the sample at 1500 and 1600 ◦ C does not. Nevertheless, the Nb3 Si is a metastable phase, and its melting point (1975 ◦ C) is lower than that of Nb5 Si3 (2515 ◦ C). Therefore, the fabricated Nb/Nb5 Si3 composites for high-temperature structural application must avoid the existence of Nb3 Si. It is gratifying that, when sintering temperature is higher than 1500 ◦ C, intermediate phase Nb3 Si was thoroughly eliminated, as a result, the desired Nb/Nb5 Si3 composites were fabricated. Microstructures sintered at different temperatures are shown in Fig. 3. It is clear that the size of the particles sintered at 1300 ◦ C is larger than those sintered at 1500 and 1600 ◦ C, and a small amount of pores can be found in the microstructure at 1300 ◦ C, which is corresponding with the result of the measured density. With increasing sintering temperature, the size of particles reduces and pores disappear. As a result, the relative density is obviously increased, as listed in Table 1. Fig. 4 shows a scanning electron micrograph taken at high magnification and EDS images of the sample sintered at 1500 ◦ C. The result indicates that particle phase in Fig. 4(a) (i.e. point 1) only contains Nb (Fig. 4(b)), and the matrix (point 2) contains both Nb and Si (Fig. 4(c)). Thus, combined with the result of XRD (Fig. 2), it is concluded that the bright phase (point 1) is residual Nb and the dark matrix (point 2) is Nb5 Si3 synthesized during SPS. During the SPS process, the spark plasma caused the local temperature of the gaps between the powders instantly to increase so as to promote interfacial reaction taking place to synthesize Nb5 Si3 , as shown in Fig. 5. With increasing sintering temperature, the diffusion rate of Nb and Si atoms rapidly increased, leading to a strong interfacial reaction. As a result, the remaining Nb particles became finer and uniformly distributed in the synthesized Nb5 Si3 matrix (Fig. 3). The data of the measured hardness also indicate that the sample sintered at 1300 ◦ C exhibits a lower hardness, HRC35 and the samples sintered at 1500 and 1600 ◦ C exhibit a higher hardness, HRC57 and HRC61.
Fig. 3. Effect of sintering temperatures on microstructures of Nb/Nb5 Si3 in situ composites (a) 1300 ◦ C, (b) 1500 ◦ C, and (c) 1600 ◦ C.
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Fig. 4. SEM microstructure of Nb/Nb5 Si3 composite (a) and EDS images (b and c).
Fig. 5. SEM image between Nb and Si particles and area analysis of elements Nb and Si.
Thus, above the critical sintering temperature (Tc = 1400 ◦ C), the fabricated Nb/Nb5 Si3 composites possess not only an improved microstructure but also higher densification and hardness.
Acknowledgement
4. Summary
References
(1) Using Nb and Si elemental powders as raw materials, in situ Nb/Nb5 Si3 composites up to 99.59% of theoretical density were successfully fabricated, with Nb particles uniformly distributed in a Nb5 Si3 matrix. (2) A critical sintering temperature Tc (1400 ◦ C) exists for SPS to synthesize Nb/Nb5 Si3 composites. At sintering temperatures above Tc , dense Nb/Nb5 Si3 composites can be achieved. (3) The sintering temperature strongly affects the microstructure of the composites. With increasing sintering temperature, the size of the Nb particles decreases and the density of the composites increase.
This research was supported by a grant from the National Natural Science Foundation of China (No. 50276023).
[1] P.R. Subramanian, M.G. Mendiratta, D.M. Dimiduk, JOM 48 (1996) 33–38. [2] H.-Z. Fu, J. Aeronautical Mater. China 18 (1998) 52–60. [3] B.P. Bewlay, M.R. Jackson, H.A. Lipsitt, Metall. Trans. A V27A (1996) 3801–3808. [4] V.V. Kristic, P.S. Nicholson, J Am. Ceram. Soc. 64 (1981) 499–504. [5] D.R. Bloyer, K.T.V. Rao, R.O. Richie, Mater. Sci. Eng. A 26 (1996) 80–90. [6] P.B. Fernandes, G.C. Coelho, F. Ferreira, C.A. Nunes, B. Sundman, Intermetallics 10 (2002) 993–999. [7] S.-Y. Qu, R.-M. Wang, Y.-F. Han, Trans. Nonferrous Met. Soc. China 12 (2002) 691–694. [8] W.-Y. Kim, H. Tanaka, S. Hannda, Intermetallics 10 (2002) 625–634. [9] M.G. Mendiratta, J.J. Lewandowski, D.M. Dimiduk, Metall. Trans. A 22A (1991) 1573–1583. [10] O. Mamoru, Mater. Sci. Eng. A 287 (2000) 183–188.