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Processing of bulk nanostructured Ni-Fe materials
Scripta mater. 44 (2001) 1591–1594 www.elsevier.com/locate/scriptamat
PROCESSING OF BULK NANOSTRUCTURED Ni-Fe MATERIALS Jai-Sung Lee and Yun-Sung Kang Department of Metallurgy and Materials Science, Hanyang University, Ansan 425-791, Korea (Received August 21, 2000) (Accepted in revised form December 13, 2000) Keywords: Powder consolidation; Activation analysis; Soft magnetic; Bulk diffusion process
Introduction Nanocrystalline (nc) Ni-Fe alloys are promising soft magnetic materials that exhibit low coercivity and high permeability [1,2]. Especially, the retention of the metastable nanograined structure during the process is mandatory for preserving the superior properties of such nc Ni-Fe alloy. In recent years Lee and his colleagues [3–5] have attempted to find a new processing route for fabricating bulk nc ␥-Ni-Fe material by conventional (pressureless) sintering of mechano-chemical processed ␥-Ni-Fe nanoalloy powder from metal oxides. It was found that the inhomogeneity of the initial pore size distribution in nano-agglomerate ␥-Ni-Fe powder played a significant role for the full densification process accompanying nanograined structure [5]. Namely, as inhomogeneity in pore size distribution enlarges, full density is hardly achieved. In the present study, we attempted to find out how to optimize a full density processing of nc ␥-Ni-Fe alloy powder that effectively controls grain growth during the consolidation process. The densification mechanism has been explained on the basis of microstructural development and diffusion process.
Experimental Procedure Nc Ni-60wt%Fe powders consisting of ␥-Ni-Fe particles of 20 nm in size were prepared by the mechano-chemcial process described in detail elsewhere [3,4]. Fig. 1 shows SEM morphology of the nc Ni-Fe agglomerate powder which is composed of only ␥-Fe-Ni solid solution. Nano-agglomerate ␥-Ni-Fe powder was compacted with a pressure of 1250 MPa to have possibly a high green density with a minimum inter-agglomerate pore. The powder was compacted with a pressure of 1250 MPa (73%TD, theoretical density) and also 120 MPa (45%TD) for comparison. A sintering experiment was performed using laser-photo dilatometry in the course of heat-up to 1273 K at different heating rates of 5 ⬃ 20 K/min. The linear shrinkage during sintering was measured and apparent activation energy for densification at each sintering stage was calculated semi-empirically [5]. The density and grain size of ␥-Ni-Fe alloy powder were measured by using a gravimeter and X-ray line broadening method, respectively. Microstructure was examined by optical-, scanning electron- and transmission electron microscopy. 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00780-1
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Figure 1. SEM micrograph and X-ray diffraction pattern of ␥-Ni-Fe nano-agglomerate powder.
Results and Discussion Fig. 2 shows the microstructures of nc ␥-Ni-Fe powder compacts observed before and after heat-up sintering to 1273 K. A number of large inter-agglomerate pores are observed in the low pressure compact (a) while high pressure compact does not show any large inter-agglomerate pores (b). This indicates that the weakly bonding nano-agglomerate powder was broken and rearranged into a dense packing structure by high pressure compaction [6]. A difficult-to-sinter bimodal pore distribution, which is due to the coexistence of intra- and inter-agglomerate pores, leads to incomplete densification of the low pressure compact, as seen in Fig. 2(c). Thus it is easily expected that high pressure compaction which can minimize formation of inter-agglomerate pores effectively is the best way to reach full density through a homogeneous sintering process. This argument is confirmed by the full densified microstructure of Fig. 2(d). Fig. 3 reveals a surprising TEM micrograph of full densified nc Ni-Fe alloy shown in Fig. 2(d). It is observed that the microstructure is composed of sound grain boundary grains of less than 50 nm in size. Analysis of TEM SAD patterns confirmed that the alloy phase is a homogenous ␥-Ni-Fe solid solution which is consistent with the initial powder state. Taking into account the initial grain size of nano-agglomerate powder (⬃20 nm), the grains of this full density specimen only grew about 2.5 times
Figure 2. Microstructures of (a,b) green compact and (c,d) sintered compact of ␥-Ni-Fe powder, compacted with (a,c) 120 Mpa and (b,d) 1250 MPa and sintered during heat-up.
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Figure 3. TEM micrograph of bulk nc of ␥-Ni-Fe alloy corresponding to figure 2(d). (a) bright field image and (b) its SAD pattern.
larger than before sintering. This result is regarded as an outstanding achievement when considering the fact that nanosized particles, regardless of the material, hardly remain at a nano-grain size of less than 100 nm without any external pressure consolidation treatment [7]. Intuitively thinking, chemical stability and structural features of the nc ␥-Ni-Fe alloy powder used in this study may be responsible for this phenomena. The former factor is based on the fact that nc Ni-Fe alloy powder has already reached a chemically stable state during powder synthesis [3,8] and so we can neglect the DIGM (diffusion induced grain boundary migration) effect on grain growth during densification. Another factor affecting slow growth might be induced by structural features such as the large volume of pores pinning grain boundaries. These pores can act as an effective inhibitor against fast grain boundary migration. Now the key issue relating to processing of the nc Ni-Fe alloy powder should be dealt with based on the full density process of nc alloy powder in terms of atom diffusion. Fig. 4(a) shows the densification process of 73%TD compact during heat-up sintering at various heating rates. From this result, depending on the heating rate, the parameters required to calculate activation energy on the basis of Eq. [1] were obtained and plotted in Fig. 4(b) [5]. ln
冉冊 冉 冊
⌽i CR Q ⫺ n 2 ⫽ ln YQ RT i Ti
(1)
Figure 4. Densification process of the powder compact pressed with high pressure during heat-up sintering at different heating rates. (a) shrinkage behavior, (b,c) apparent activation energy for densification process with respect to sintering stage.
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where Q is activation energy, ⌽i is the ratio of the heating rate, Y ⬅ ‚L/L0 is the identical value of shrinkage, C is a pre-exponential constant, R is gas constant, and Ti is temperature, respectively. Fig. 4(c) represents apparent activation energy derived from the slopes of the plots in Fig. 4(b). By increasing the shrinkage value, in other words, as sintering proceeds, the activation energy gradually increases from ⬃50 kJ/mol to ⬃300 kJ/mol. It is quite reasonable that the steady state densification process appears to be controlled by high diffusion process such as grain boundary diffusion. Very recently this group of authors reported that Fe or Ni self diffusion in the same nc Ni-Fe alloy as this study, measured by the radiotracer technique, takes place along grain boundaries and triple junctions [9]. It seems likely that atom diffusion for densification in nc material predominantly occurs at such high diffusion paths, especially at triple junction points. Comparing the diffusion data with that of the densification process, activation energies for grain boundary (174 kJ/mol for Ni and 194 kJ/mol for Fe) and triple junction diffusion (138 kJ/mol for Ni and 145 kJ/mol for Fe) approximately correspond to that for the intermediate sintering stage of 85–90% (150 –200 kJ/mol). This implies that densification process of nc Ni-Fe powder is initiated by the diffusion process along high diffusion paths of grain boundaries and triple junctions. Conclusions Full density nanograined ␥-Ni-Fe bulk material was successfully fabricated by high pressure compaction and pressureless sintering of ␥-Ni-Fe nano-agglomerate powders. The elimination of interagglomerate pores by high pressure compaction was explained to be responsible for this full density process of which kinetic is mainly controlled by diffusion in grain boudaries and triple junctions. Remarkable slowdown of grain growth during sintering was discussed to result from chemical stability and pore pinning of grain boundary migration. Acknowledgment The authors greatly acknowledge the Alexander von Humboldt Foundation for financial support. References 1. 2. 3. 4. 5. 6. 7. 8. 9.
C. Suryanarayana, Int. Mater. Rev. 40, 41 (1995). X. Y. Qin, J. S. Lee, and J. G. Kim, J. Appl. Phys. 86, 2146 (1999). J. S. Lee, T. H. Kim, J. H. Yu, and S. W. Chung, Nanostruct. Mater. 9, 153 (1997). J. S. Lee, J. G. Nam, and P. Knorr, Metals Mater. 5, 115 (1999). P. Knorr, J. G. Nam, and J. S. Lee, Mater. Metall. Trans. 31A, 503 (2000). O. Dominguez and J. Bigot, Nanostruct. Mater. 6, 877 (1995). J. R. Groza and R. J. Dowding, Nanostruct. Mater. 7, 749 (1996). P. Knorr, B. S. Kim, and J. S. Lee, Mater. Res. Soc. Symp. Proc. 577, 409 (1999). F. Hisker, Y. S. Kang, J. S. Lee, and Chr. Herzig, Presented at DIMAT-2000, July 16, 2000, Paris.
PROCESSING OF BULK NANOSTRUCTURED Ni-Fe MATERIALS Jai-Sung Lee and Yun-Sung Kang Department of Metallurgy and Materials Science, Hanyang University, Ansan 425-791, Korea (Received August 21, 2000) (Accepted in revised form December 13, 2000) Keywords: Powder consolidation; Activation analysis; Soft magnetic; Bulk diffusion process
Introduction Nanocrystalline (nc) Ni-Fe alloys are promising soft magnetic materials that exhibit low coercivity and high permeability [1,2]. Especially, the retention of the metastable nanograined structure during the process is mandatory for preserving the superior properties of such nc Ni-Fe alloy. In recent years Lee and his colleagues [3–5] have attempted to find a new processing route for fabricating bulk nc ␥-Ni-Fe material by conventional (pressureless) sintering of mechano-chemical processed ␥-Ni-Fe nanoalloy powder from metal oxides. It was found that the inhomogeneity of the initial pore size distribution in nano-agglomerate ␥-Ni-Fe powder played a significant role for the full densification process accompanying nanograined structure [5]. Namely, as inhomogeneity in pore size distribution enlarges, full density is hardly achieved. In the present study, we attempted to find out how to optimize a full density processing of nc ␥-Ni-Fe alloy powder that effectively controls grain growth during the consolidation process. The densification mechanism has been explained on the basis of microstructural development and diffusion process.
Experimental Procedure Nc Ni-60wt%Fe powders consisting of ␥-Ni-Fe particles of 20 nm in size were prepared by the mechano-chemcial process described in detail elsewhere [3,4]. Fig. 1 shows SEM morphology of the nc Ni-Fe agglomerate powder which is composed of only ␥-Fe-Ni solid solution. Nano-agglomerate ␥-Ni-Fe powder was compacted with a pressure of 1250 MPa to have possibly a high green density with a minimum inter-agglomerate pore. The powder was compacted with a pressure of 1250 MPa (73%TD, theoretical density) and also 120 MPa (45%TD) for comparison. A sintering experiment was performed using laser-photo dilatometry in the course of heat-up to 1273 K at different heating rates of 5 ⬃ 20 K/min. The linear shrinkage during sintering was measured and apparent activation energy for densification at each sintering stage was calculated semi-empirically [5]. The density and grain size of ␥-Ni-Fe alloy powder were measured by using a gravimeter and X-ray line broadening method, respectively. Microstructure was examined by optical-, scanning electron- and transmission electron microscopy. 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00780-1
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Figure 1. SEM micrograph and X-ray diffraction pattern of ␥-Ni-Fe nano-agglomerate powder.
Results and Discussion Fig. 2 shows the microstructures of nc ␥-Ni-Fe powder compacts observed before and after heat-up sintering to 1273 K. A number of large inter-agglomerate pores are observed in the low pressure compact (a) while high pressure compact does not show any large inter-agglomerate pores (b). This indicates that the weakly bonding nano-agglomerate powder was broken and rearranged into a dense packing structure by high pressure compaction [6]. A difficult-to-sinter bimodal pore distribution, which is due to the coexistence of intra- and inter-agglomerate pores, leads to incomplete densification of the low pressure compact, as seen in Fig. 2(c). Thus it is easily expected that high pressure compaction which can minimize formation of inter-agglomerate pores effectively is the best way to reach full density through a homogeneous sintering process. This argument is confirmed by the full densified microstructure of Fig. 2(d). Fig. 3 reveals a surprising TEM micrograph of full densified nc Ni-Fe alloy shown in Fig. 2(d). It is observed that the microstructure is composed of sound grain boundary grains of less than 50 nm in size. Analysis of TEM SAD patterns confirmed that the alloy phase is a homogenous ␥-Ni-Fe solid solution which is consistent with the initial powder state. Taking into account the initial grain size of nano-agglomerate powder (⬃20 nm), the grains of this full density specimen only grew about 2.5 times
Figure 2. Microstructures of (a,b) green compact and (c,d) sintered compact of ␥-Ni-Fe powder, compacted with (a,c) 120 Mpa and (b,d) 1250 MPa and sintered during heat-up.
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Figure 3. TEM micrograph of bulk nc of ␥-Ni-Fe alloy corresponding to figure 2(d). (a) bright field image and (b) its SAD pattern.
larger than before sintering. This result is regarded as an outstanding achievement when considering the fact that nanosized particles, regardless of the material, hardly remain at a nano-grain size of less than 100 nm without any external pressure consolidation treatment [7]. Intuitively thinking, chemical stability and structural features of the nc ␥-Ni-Fe alloy powder used in this study may be responsible for this phenomena. The former factor is based on the fact that nc Ni-Fe alloy powder has already reached a chemically stable state during powder synthesis [3,8] and so we can neglect the DIGM (diffusion induced grain boundary migration) effect on grain growth during densification. Another factor affecting slow growth might be induced by structural features such as the large volume of pores pinning grain boundaries. These pores can act as an effective inhibitor against fast grain boundary migration. Now the key issue relating to processing of the nc Ni-Fe alloy powder should be dealt with based on the full density process of nc alloy powder in terms of atom diffusion. Fig. 4(a) shows the densification process of 73%TD compact during heat-up sintering at various heating rates. From this result, depending on the heating rate, the parameters required to calculate activation energy on the basis of Eq. [1] were obtained and plotted in Fig. 4(b) [5]. ln
冉冊 冉 冊
⌽i CR Q ⫺ n 2 ⫽ ln YQ RT i Ti
(1)
Figure 4. Densification process of the powder compact pressed with high pressure during heat-up sintering at different heating rates. (a) shrinkage behavior, (b,c) apparent activation energy for densification process with respect to sintering stage.
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where Q is activation energy, ⌽i is the ratio of the heating rate, Y ⬅ ‚L/L0 is the identical value of shrinkage, C is a pre-exponential constant, R is gas constant, and Ti is temperature, respectively. Fig. 4(c) represents apparent activation energy derived from the slopes of the plots in Fig. 4(b). By increasing the shrinkage value, in other words, as sintering proceeds, the activation energy gradually increases from ⬃50 kJ/mol to ⬃300 kJ/mol. It is quite reasonable that the steady state densification process appears to be controlled by high diffusion process such as grain boundary diffusion. Very recently this group of authors reported that Fe or Ni self diffusion in the same nc Ni-Fe alloy as this study, measured by the radiotracer technique, takes place along grain boundaries and triple junctions [9]. It seems likely that atom diffusion for densification in nc material predominantly occurs at such high diffusion paths, especially at triple junction points. Comparing the diffusion data with that of the densification process, activation energies for grain boundary (174 kJ/mol for Ni and 194 kJ/mol for Fe) and triple junction diffusion (138 kJ/mol for Ni and 145 kJ/mol for Fe) approximately correspond to that for the intermediate sintering stage of 85–90% (150 –200 kJ/mol). This implies that densification process of nc Ni-Fe powder is initiated by the diffusion process along high diffusion paths of grain boundaries and triple junctions. Conclusions Full density nanograined ␥-Ni-Fe bulk material was successfully fabricated by high pressure compaction and pressureless sintering of ␥-Ni-Fe nano-agglomerate powders. The elimination of interagglomerate pores by high pressure compaction was explained to be responsible for this full density process of which kinetic is mainly controlled by diffusion in grain boudaries and triple junctions. Remarkable slowdown of grain growth during sintering was discussed to result from chemical stability and pore pinning of grain boundary migration. Acknowledgment The authors greatly acknowledge the Alexander von Humboldt Foundation for financial support. References 1. 2. 3. 4. 5. 6. 7. 8. 9.
C. Suryanarayana, Int. Mater. Rev. 40, 41 (1995). X. Y. Qin, J. S. Lee, and J. G. Kim, J. Appl. Phys. 86, 2146 (1999). J. S. Lee, T. H. Kim, J. H. Yu, and S. W. Chung, Nanostruct. Mater. 9, 153 (1997). J. S. Lee, J. G. Nam, and P. Knorr, Metals Mater. 5, 115 (1999). P. Knorr, J. G. Nam, and J. S. Lee, Mater. Metall. Trans. 31A, 503 (2000). O. Dominguez and J. Bigot, Nanostruct. Mater. 6, 877 (1995). J. R. Groza and R. J. Dowding, Nanostruct. Mater. 7, 749 (1996). P. Knorr, B. S. Kim, and J. S. Lee, Mater. Res. Soc. Symp. Proc. 577, 409 (1999). F. Hisker, Y. S. Kang, J. S. Lee, and Chr. Herzig, Presented at DIMAT-2000, July 16, 2000, Paris.