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Densification and microstructural development of nanocrystalline γ-Ni-Fe powders during sintering
NanoStructured
Materials,
Vol.
12. pp. 479482, 1999 Elsevier Science Ltd 0 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 0965.9773/99/$-see front matter
PI1 SO9659773(99)00163-4
DENSIFICATION AND MICROSTRUCTURAL DEVELOPMENT OF NANOCRYSTALLINE y-Ni-Fe POWDERS DURING SINTERING P. Knorr,
J.G. Nam and J.S. Lee
Department of Metallurgy and Materials Science Hanyang University, Ansan, 425-79 1 Korea
Abstract -- The sintering behavior of nanocrystalline (nc) y-Ni-Fe powders was investigated by laser-photo-dilatometry. The sinterability of nc powders was found to depend crucially on the state of agglomeration. The compacted FNi-Fe powders exhibited a bimodal pore distribution comprised of nanoscale intra-agglomerate pores and large, microscale interagglomerate pores. The results are discussed on the basis of the microstructural evolution during densifcation which was followed by optical microscopy and BET specific surface area measurements. 01999 Acta Metallurgica Inc. INTRODUCTION The potential application of nanoscale materials as novel structural or functional engineering materials largely depends on the consolidation of powders into bulk nanoscale solids. The retention of the metastable microstructure in the course of this consolidation process is mandatory for preserving the often superior mechanical, magnetic or catalytic properties of the material (1). Pressure-assisted sintering proves adequate for both reaching full density and preventing grain coarsening (2). Both objectives can hardly be achieved by pressureless sintering. However, pressureless sintering is inevitable for the consolidation of near-net-shaped powders as in the case of powder injection molding. The present paper reports on investigations of the pressureless sintering of nc y-Ni-Fe powders which were fabricated by mechano-chemical synthesis (3). Several communications have been dedicated to the synthesis of nc Ni-Fe solid solutions (4,5) mainly because of their improved magnetic properties compared to conventional permalloy. The densification process is discussed on the basis of the microstructural evolution which crucially depends on particle agglomeration.
EXPERIMENTAL NC y-Ni-Fe powders with a particle size of 30-50 nm were synthesized by a mechanochemical process described in detail elsewhere (3). For me sintering studies powder compacts of 45 % theoretical density were compressed in a cylindrical hardened-steel die. The sintering 479
480
FOURTH INTERNATIONAL
0.201
I.
CONFERENCE
ON NANOSTRUCTURED
MATERIALS
2K,,i,f’
Figure 1. Diametral shrinkage of nc y-Ni-Fe for different heating rates.
Figure 2. Diametral shrinkage rates as a function of temperature.
experiments were performed under hydrogen atmosphere in a tungsten lamp furnace allowing rapid heating rates. Diametral shrinkage was measured with a laser-photo dilatometer. For this purpose, the specimen was mounted on a mullite support with their front side facing the laser beam and the image of the specimen was magnified and recorded by a CCD camera. The displayed image was eventually evaluated with an image analyzer. To follow the evolution of the microstructure during sintering, a series of specimen was polished using standard metallographic techniques and photographed for examining the variation of inter-agglomerate porosity and inter-agglomerate pore size with temperature. Additionally, BET measurements were performed to follow the evolution of the intra-agglomerate pore size distribution.
RESULTS
AND DISCUSSION
The dilatometer results for different heating rates are depicted in Figure 1. The shrinkage curves show that active sintering occurs in the temperature range between 950 K and 1250 K. For all heating rates, the diametral shrinkage AL/L, reached a maximum linear dimensional change of 0.18. Densification proved to be fairly isotropic, as was revealed by a comparison of the linear shrinkage AL/Lo with the volume shrinkage dV/V,. The final densities, achieved by the sintering experiments, are limited to about 80 % theoretical density. Previous experiments on pressureless sintering of nc (40 run) iron powders led to a reduction of the onset of sintering from about 900 K for conventional powder to 500 K (6). Sintering of nc iron powder under a load of 10 MPa showed active sintering to begin at temperatures as low as 400 K to 500 K and yielded final densities well above 90 % (7). In comparison with the quoted references (6,7), the consolidation characteristics of nc y-Ni-Fe powders appear to be
FOURTH INTERNATIONAL CONFERENCE
ON NAN~STRUCTURED
MATERIALS
481
Figure 3. Polished cross section of nc y-Ni-Fe (a) before sintering, (b) sintered to 1100 K, (c) sinteredto 1300KatarateoflOKmin’. rather poor. The cause of this incomplete densification is accounted for by an examination of the microstructure which is shown in Figures 3a to 3c. It is seen from the micrographs that the compacts consist of large agglomerates of polygonal appearance, the size of which varies from 20 urn to as large as 80 pm. The arrangement of the agglomerates is such that small agglomerates reside in the interstices of the packing of the large ones. The high level of agglomeration clearly leads to a difficult-to-sinter bimodal pore size distribution: nanometerscale, intru-agglomerate pores in the interstices of the bonded nanoparticles, and micrometersized, inter-agglomerate pores between the clusters of small particles. The first two micrographs (Figures 3a and b) reveal a fine network of intra-agglomerate pores which has disappeared after densitication (Figure 3c) indicating that most of the i&a-agglomerate porosity has been eliminated in the course of sintering. This finding is reflected by the BET results on the i&a-agglomerate pore size distribution (Figure 4). It is seen that the intraagglomerate porosity successively decreases until virtually no porosity remains. In contrast to the almost complete elimination of intra-agglomerate porosity, the micrographs show that inter-agglomerate porosity has remained to a large extent. As revealed by quantitative microscopy, out of a 33 % decrease of total porosity in the course of sintering, the contribution of inter-agglomerate densitication is merely 8 %, whereas the decrease in intra-agglomerate porosity amounts to 25 %. Figure 5 shows the evolution of the inter-agglomerate pore size distribution during sintering. Obviously, significant reduction of the pore size takes place only at elevated temperatures. Figure 2 shows a plot of the shrinkage rate versus temperature. Depending on heating rate, the shrinkage rates vary from 3 x lo-’ to 3 x lOA S’ which is in line with shrinkage rates observed previously for nc iron powders (6,7). A common feature of the sintering experiments on nc iron conducted by Bourell et al. (7) and Dominguez et al. (6) and the present study on the sintering behavior of y-Ni-Fe is the occurrence of two distinct maxima in the temperature dependence of the shrinkage rate. The occurrence of maxima may cautiously be attributed to shrinkage and interheating-rate dependent, relative variations of intra-agglomerate agglomerate shrinkage which both superpose to the experimentally determined overall
FOURTH INTERNATIONAL
CONFERENCE
1.0
ON NANOSTRUCTURED
MATERIAIS
-7
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Figure 4. Irma-agglomerate pore distribution measuredby BET for specimenssinteredto different temperatures.
Figure 5. Inter-agglomerate pore distribution measuredby quantitative microscopy for specimenssinteredto different temperatures.
shrinkage rate. On the basis of theseresults the objective of further researchwill be to reduce particle agglomeration induced by the mechano-chemicalproduction method. A systematic study for finding improved synthesisconditions is under way.
ACKNOWLEDGEMENTS Financial support of the Korean Ministry of Education Research Fund for Advanced Materials in 1997 is gratefully acknowledged. One of the authors (P.K.) wishes to thank the Alexander-von-Humboldt Stiftung (Bonn) for supporting this work.
REFERENCES 1. 2. 3. 4. 5. 6. 7.
Suryanarayana, ht. Mat. Reviews, 1995,@, 41. Groza, J.R. and Dowding, R.J., Nanostr. Mat., 1996,1,749. Lee, J.S., Kim, T.H., Yu, J.H. andchung, S.W., Nanostr. Mat., 1997,% 153. Cheung, C., Djuanda, F., Erb, IJ. and Palumbo, G., Nanostr. Mat., 1995,3,513. Eroglu, S., Zhang, S.C. and Messing, G.L., J. Mat. Res., 1996,l-L 2131. Dominguez, 0. and Bigot, J., Nanostr. Mat., 1995,& 877. Bourell, D.L. and Kaysser, W.A., Metall. and Mat. Trans. A, 1994,z, 677.
Materials,
Vol.
12. pp. 479482, 1999 Elsevier Science Ltd 0 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 0965.9773/99/$-see front matter
PI1 SO9659773(99)00163-4
DENSIFICATION AND MICROSTRUCTURAL DEVELOPMENT OF NANOCRYSTALLINE y-Ni-Fe POWDERS DURING SINTERING P. Knorr,
J.G. Nam and J.S. Lee
Department of Metallurgy and Materials Science Hanyang University, Ansan, 425-79 1 Korea
Abstract -- The sintering behavior of nanocrystalline (nc) y-Ni-Fe powders was investigated by laser-photo-dilatometry. The sinterability of nc powders was found to depend crucially on the state of agglomeration. The compacted FNi-Fe powders exhibited a bimodal pore distribution comprised of nanoscale intra-agglomerate pores and large, microscale interagglomerate pores. The results are discussed on the basis of the microstructural evolution during densifcation which was followed by optical microscopy and BET specific surface area measurements. 01999 Acta Metallurgica Inc. INTRODUCTION The potential application of nanoscale materials as novel structural or functional engineering materials largely depends on the consolidation of powders into bulk nanoscale solids. The retention of the metastable microstructure in the course of this consolidation process is mandatory for preserving the often superior mechanical, magnetic or catalytic properties of the material (1). Pressure-assisted sintering proves adequate for both reaching full density and preventing grain coarsening (2). Both objectives can hardly be achieved by pressureless sintering. However, pressureless sintering is inevitable for the consolidation of near-net-shaped powders as in the case of powder injection molding. The present paper reports on investigations of the pressureless sintering of nc y-Ni-Fe powders which were fabricated by mechano-chemical synthesis (3). Several communications have been dedicated to the synthesis of nc Ni-Fe solid solutions (4,5) mainly because of their improved magnetic properties compared to conventional permalloy. The densification process is discussed on the basis of the microstructural evolution which crucially depends on particle agglomeration.
EXPERIMENTAL NC y-Ni-Fe powders with a particle size of 30-50 nm were synthesized by a mechanochemical process described in detail elsewhere (3). For me sintering studies powder compacts of 45 % theoretical density were compressed in a cylindrical hardened-steel die. The sintering 479
480
FOURTH INTERNATIONAL
0.201
I.
CONFERENCE
ON NANOSTRUCTURED
MATERIALS
2K,,i,f’
Figure 1. Diametral shrinkage of nc y-Ni-Fe for different heating rates.
Figure 2. Diametral shrinkage rates as a function of temperature.
experiments were performed under hydrogen atmosphere in a tungsten lamp furnace allowing rapid heating rates. Diametral shrinkage was measured with a laser-photo dilatometer. For this purpose, the specimen was mounted on a mullite support with their front side facing the laser beam and the image of the specimen was magnified and recorded by a CCD camera. The displayed image was eventually evaluated with an image analyzer. To follow the evolution of the microstructure during sintering, a series of specimen was polished using standard metallographic techniques and photographed for examining the variation of inter-agglomerate porosity and inter-agglomerate pore size with temperature. Additionally, BET measurements were performed to follow the evolution of the intra-agglomerate pore size distribution.
RESULTS
AND DISCUSSION
The dilatometer results for different heating rates are depicted in Figure 1. The shrinkage curves show that active sintering occurs in the temperature range between 950 K and 1250 K. For all heating rates, the diametral shrinkage AL/L, reached a maximum linear dimensional change of 0.18. Densification proved to be fairly isotropic, as was revealed by a comparison of the linear shrinkage AL/Lo with the volume shrinkage dV/V,. The final densities, achieved by the sintering experiments, are limited to about 80 % theoretical density. Previous experiments on pressureless sintering of nc (40 run) iron powders led to a reduction of the onset of sintering from about 900 K for conventional powder to 500 K (6). Sintering of nc iron powder under a load of 10 MPa showed active sintering to begin at temperatures as low as 400 K to 500 K and yielded final densities well above 90 % (7). In comparison with the quoted references (6,7), the consolidation characteristics of nc y-Ni-Fe powders appear to be
FOURTH INTERNATIONAL CONFERENCE
ON NAN~STRUCTURED
MATERIALS
481
Figure 3. Polished cross section of nc y-Ni-Fe (a) before sintering, (b) sintered to 1100 K, (c) sinteredto 1300KatarateoflOKmin’. rather poor. The cause of this incomplete densification is accounted for by an examination of the microstructure which is shown in Figures 3a to 3c. It is seen from the micrographs that the compacts consist of large agglomerates of polygonal appearance, the size of which varies from 20 urn to as large as 80 pm. The arrangement of the agglomerates is such that small agglomerates reside in the interstices of the packing of the large ones. The high level of agglomeration clearly leads to a difficult-to-sinter bimodal pore size distribution: nanometerscale, intru-agglomerate pores in the interstices of the bonded nanoparticles, and micrometersized, inter-agglomerate pores between the clusters of small particles. The first two micrographs (Figures 3a and b) reveal a fine network of intra-agglomerate pores which has disappeared after densitication (Figure 3c) indicating that most of the i&a-agglomerate porosity has been eliminated in the course of sintering. This finding is reflected by the BET results on the i&a-agglomerate pore size distribution (Figure 4). It is seen that the intraagglomerate porosity successively decreases until virtually no porosity remains. In contrast to the almost complete elimination of intra-agglomerate porosity, the micrographs show that inter-agglomerate porosity has remained to a large extent. As revealed by quantitative microscopy, out of a 33 % decrease of total porosity in the course of sintering, the contribution of inter-agglomerate densitication is merely 8 %, whereas the decrease in intra-agglomerate porosity amounts to 25 %. Figure 5 shows the evolution of the inter-agglomerate pore size distribution during sintering. Obviously, significant reduction of the pore size takes place only at elevated temperatures. Figure 2 shows a plot of the shrinkage rate versus temperature. Depending on heating rate, the shrinkage rates vary from 3 x lo-’ to 3 x lOA S’ which is in line with shrinkage rates observed previously for nc iron powders (6,7). A common feature of the sintering experiments on nc iron conducted by Bourell et al. (7) and Dominguez et al. (6) and the present study on the sintering behavior of y-Ni-Fe is the occurrence of two distinct maxima in the temperature dependence of the shrinkage rate. The occurrence of maxima may cautiously be attributed to shrinkage and interheating-rate dependent, relative variations of intra-agglomerate agglomerate shrinkage which both superpose to the experimentally determined overall
FOURTH INTERNATIONAL
CONFERENCE
1.0
ON NANOSTRUCTURED
MATERIAIS
-7
;; sJ
-n-
gwnconpact
-o-
IlCXIK(lOKnin-‘)
-A-
13COK(lOKnin-‘)
-o-
1120K(3OKnin~‘)
1 j
-v-
lXX)K(3OKnin”)
p
1
Moo- A
--o-greencoIlpact
1200-
-I)---A-
llOOK(lOKnin-‘) 13OOK(lOKnin-‘)
-* lemg $
Jzj 0.5
SW
si
0.0 1 poresg[mri]
Figure 4. Irma-agglomerate pore distribution measuredby BET for specimenssinteredto different temperatures.
Figure 5. Inter-agglomerate pore distribution measuredby quantitative microscopy for specimenssinteredto different temperatures.
shrinkage rate. On the basis of theseresults the objective of further researchwill be to reduce particle agglomeration induced by the mechano-chemicalproduction method. A systematic study for finding improved synthesisconditions is under way.
ACKNOWLEDGEMENTS Financial support of the Korean Ministry of Education Research Fund for Advanced Materials in 1997 is gratefully acknowledged. One of the authors (P.K.) wishes to thank the Alexander-von-Humboldt Stiftung (Bonn) for supporting this work.
REFERENCES 1. 2. 3. 4. 5. 6. 7.
Suryanarayana, ht. Mat. Reviews, 1995,@, 41. Groza, J.R. and Dowding, R.J., Nanostr. Mat., 1996,1,749. Lee, J.S., Kim, T.H., Yu, J.H. andchung, S.W., Nanostr. Mat., 1997,% 153. Cheung, C., Djuanda, F., Erb, IJ. and Palumbo, G., Nanostr. Mat., 1995,3,513. Eroglu, S., Zhang, S.C. and Messing, G.L., J. Mat. Res., 1996,l-L 2131. Dominguez, 0. and Bigot, J., Nanostr. Mat., 1995,& 877. Bourell, D.L. and Kaysser, W.A., Metall. and Mat. Trans. A, 1994,z, 677.