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In-situ X-ray diffraction examination of nanocrystalline Ag37Cu63 powders synthesized by mechanical alloying
Journal of
ALLOY3 A~D COMPOLq~DS ELSEVIER
Journal of Alloys and Compounds 256 (1997) 230-233
In-situ X-ray diffraction examination of nanocrystalHne Ag37Cu63 powders
synthesized by mechanical alloying Y o n g Q i n * , Li C h e n , H u i S h e n Institute c~] Solid State Physics. Ac,demia Shtica P.O. Box 1129. Ht!fi'i 2.?(X131. P.R. China
Received 18 November 1996
Abstract Nanocrystalline Ag37Cu¢,3 powders have been synthesized by a mechanical alloying method although Ag and Cu are almost immiscible to each other. The microstructure changes of the as-synthesized powders with increasing temperature have been examined by XRD and TEM. Below 200 °C, the specimen is composed of an amorphous phase and a supersurated Cu-Ag solid solution with the starting composition. The Cu-8 at.%Ag solid solution is formed at 200 °C, so that an amorphous phase and two Cu-Ag solid solutions with different compositions are coexistent in the specimen. On further heating of the specimen to 3(X)°C and above, the amorphous phase and two Cu-Ag solid solutions vanish completely and transform into Cu- and Ag-phase. © 1997 Elsevier Science S.A. 1~¢3"words:
Microstructure; Mechanical alloying; Nanocrystalline AgCu
1, I n t r o d u c t i o n Considerable interest in synthesizing nanocrystalline materials has been increasing since the discovery of nano,:~ystalline materials which display interesting physical and mechanical properties. The nanocrystalline materials are initially synthesized by an inert gas condensation technique following in situ compaction. This method can synthesize bulk nanocrystalline materials with clean interface, and considerable effort has been expanded on nanocrystalline metals, ceramics and alloys [ ! - 4 ] . The inherent problem of this method, however, is that it is difficult to apply to high melting point materials and to large output, so that this method is limited in some applications. Mechanica~ alloying (MA) process [5], as a novel method for synthesizing materials by means of the solid state reaction from a mixture of elemental components, has been attracting attention and successfully used to synthesize a number of equilibrium and metastable materials, including amorphous alloys, supersaturated solid solutions, quasicrystals, immiscible alloys and high melting point intennetallics [6-10]. Especially, MA can be also suited for producing nanocrystalline materials [ I l]. It is well known that Ag and Cu are almost immiscible to each other at room temperature and the heat of mixing is positive. Nanocr~stalline AgCu, however, has been ob*Corresponding author. 0925-83881971517.00 © 1997 Elsevier Science S.A. All rights reserved P i l S0925-8388(96)03i i3-i
rained by MA. To the best of our knowledge, only a few works were done on the structural investigation of nanocrystalline AgCu prepared by MA. In this paper, the microstructure of mechanically alloyed nanocrystalline AgCu powder is examined by means of in-situ X-ray diffraction (XRD) as well as transmission electron microscopy (TEM).
2. E x p e r i m e n t a l Nanocrystalline Ag37Cu63 powders were synthesized by ball milling of elemental Ag (99.99% purity) and Cu (99.9% purity) powders with initial grain sizes of 50 and 45 p~m, respectively. MA was performed in a steel vial under Ar atmospberc using a planetary ball-mill at a ball-lo-powder weight ratio of 15:1. After milling for 50 h, the MA powders were removed for structural analysis. The MA powders were characterized by XRD analysis and TEM observation. The in situ XRD experiments during heating were carried out at successively increased temperature from room temperature to predetermined temperature (100, 200, 300, 400 and 500 °C) using CuKc¢ radiation with a Philips-PW 1706 X-ray diffractometer equipped with a HTK vacuum camera. The pressure of 3 × 10-" Pa was kept during measurement. The heating rate used between each XRD measurement was 1 0 ° C m i n -~ and the specimen was kept for 10 rain at every measure-
231
Y. Qin et al. / Jourmd of Alloys and Compound.~ 256 (19971 230-233
ment temperature for stability. The XRD line broadening method was also used in order to measure average grain sizes of MA powders. The TEM observation of MA powders was carried out with a JEOL-200CX electron microscope at an operating voltage of 200 kV. The TEM specimen of MA powders was prepared by putting a small drop of alcohol with dispersive MA powders on the microgrids with carbon films.
i
Ag~Cu~,MA 50h o
"~~
•
°" o
o
°~~oo.c
3. Results and discussion The morphology and structure of MA Ag37Cu~,~ powders car, be obtained by means of TEM observation. Fig. I is the typical dark field image of the MA Ag3vCu~,3 powders and the corresponding electron diffraction pattern. The average grain size of the MA Ag3vCu6, powders measured from the dark field image is less than l0 nm. The electron diffraction pattern of MA Ag37Cu~3 powders is composed of amorphous haloes and sharp diffraction rings, which indicates the coexistence of amorphous and c~aystalline phase in the MA Ag37Cu6.~ powder specimen. The analysis of electron diffraction pattern confirms that this crystalline phase has a face-centered cubic (fcc) structure with the lattice parameter a=0.38 nm. It is known from the lattice parameter [10] that this crystalline phase is a supersaturated C u - A g solid solution with near the starting composition. Fig. 2 reveals the development of XRD patterns of the MA Ag,TCu63 powders during in situ heating, in which the different XRD patterns can be observed. This means that the microstructure clmnge of MA Cu37Cu6~ powders takes place with increasing temperature. It is clear from the XRD pattern at room temperature that a broad peak appears at 20-41 ° except for the amorphous phase. The broad peak is characteristic of the grain size refinement. The data of XRD reveals that this broad peak belongs to ( I l l ) diffraction of the C u - A g solid solution with fcc structure with the lattice parameter a=0.38 nm. This result coincides with that of the electron diffraction (Fig. Ib). A
Fig. I. (a) TEM dark field image and (b) corresponding electron diffraction pattern of the MA Ag~Cu6~ powders specimen. The bar is 100 nm long.
20O°C
~'0 ~
30
40
50 60 70 80 90 2 Theta (degrees)
tOO tin
Fig. 2. XRD patterns taken at different temperatures for the MA Ag,Cu,, powders specimen. *: Cu,O. +: Cu-Ag solid solution with near the starting composition. &: Cu-8 at.%Ag. O: Ag, and O: Cn.
weak Cu20 peak at 20-37 ° was also observed in the XRD patterns of MA Ag37Cu63 powder specimen and the Cu20 is considered to be from the raw material. From TEM and XRD result one can conclude that the nanocrystalline Ag37Cuo3 powders by MA are composed of an amorphous phase and a supersaturated C u - A g solid solution with near the starting composition. Compared with room temperature, the XRD pattern shows that no detectable new phase is found at about 100 °C, which implies that the microstructure at 100 °(2 is the same as that at room temperature for the MA Ag37Cu63 powder specimen. The XRD pattern begins to change at about 200 °C, in which another peak at 20-42.8 ° appears. This new phase is determined from the data of XRD to be a C u - A g solid solution with fcc structure with the lattice parameter a=0.366 nm. From the lattice parameter one can believe that this C u - A g solid solution contains about 8 at.%Ag [10]. Evidently, there are an amorphous and two C u - A g solid solutions with different composition in MA Ag37Cu63 powder specimen at about 200 °C. The amorphous phase and two C u - A g solid solutions completely vanish at about 300 °(2, instead Cu- and Ag-phase diffraction peaks are observed in the XRD pattern. Fu;ther heating the specimen to about 500 °C, no essential change is observed in the XRD pattern apart from the increase of diffraction peaks intensities of Cu- and Ag-phase, which means that the grains of Cu- and Ag-phase in the specimen grow up with increasing temperature. According to the above in situ XRD measurements, the
232
Y. Qin et al. I Jounud v f Alloys and Compounds 256 (1997) 230-233
microstructm!e changes of MA Ag37Cu63 powders with different temperatures can be described as follows. Below 200°C the MA Ag37Cu~3 powders are composed of an amorphous plhase and a supersaturated Cu-Ag solid solution with neatr the starting composition. Another supersaturated Cu-Ag solid solution containing about 8 at.%Ag is formed at about 200 °(2 so that an amorphous phase and two Cu-Ag solid solutions with different compositions coexist. The amorphous phase and Cu-Ag solid solutions completely decompose into Cu- and Ag-phase with increasing temperature to 300 °C. The supersaturated Cu-Ag solid solution with the starting composition has been found during MA over a wide composition range. The formation of Cu-8 ,~t.%Ag solid solution has been also observed in the compacted MA Ag37Cu63specimens, but not been reported in the MA Ag37Cut,3 powder specimen. Li et al. [12] examined the microstructure of the MA Ag37Cut,3 specimens compacted at 1 and 3 GPa by XRD. But it is found that the microstructural change of the compacted specimen is different from that of the powder specimen. For the former, apart from the MA powder phases (an amorphous phase and a~ Cu-Ag solid solution with the starting composition) the Cu-8 at.RAg solid solution is also observed at room temperature, while for the latter the Cu-8 at.%Ag solid solution is not observed. At about 200 °C, the powder phases discompose into Ag and Cu-8 at.%Ag phase for the compacted specimen; while the powder phases and Cu-8 at.%Ag phase coexist for the powder specimen. At about 300 °(2, the Cu-8 at.RAg-phase partly transforms in,to Cu- and Ag-phases so that the compacted specimen contains the Cu-8 at.%Ag, Cu- and Ag-phases; while in the powder specimen, the powder phases and Cu-8 at.RAg solid solution completely transforms into Cuand Ag-phases. Not only the formation of Cu-8 at.RAg solid solution in the compacted specimen is easier than that in the powder specimen, but also the Cu-8 at.RAg solid solution is more stable in the compacted specimen than in the powder sl~ecimen. Since the compacted specimens are obtained by compacting same MA Ag37Cu63 powders as those in present experiment, it is clear that the compaction has a strong influence on the microstructure of the resulting materials. From XRD pattern of MA Ag37Cu63 powders, additionally, it is found that the Cu-8 at.%Ag solid solution has a small content and unstable structure, so that its discovery is difficult in the MA powder specimen. More than 300°C, the XRD confirms that the MA Ag37Cu~3 powder specimen is composed of Cu- and Agphases. This is supported by TEM observation. Fig. 3 shows the bright field image and the corresponding electron diffraction pattern, which was taken at room temperature ~fter heating MA Ag37Cu63 powder specimen at 500 °C. It can be seen by comparing Fig. 3 with Fig. ! that their electron diffraction pattern has an obvious difference apart from the growth of grains with increasing temperature. The electron diffraction pattern is composed
Fig. 3. (a) TEM bright field image and (b) corresponding electron diffraction pattern of the MA Ag,,Cu~, powders specimen annealed at 500°C. The bar is 100 nm long.
of an amorphous phase and a Cu-Ag solid solution for the original MA specimen, while Cu- and Ag-phases for the heated MA specimen. On the other hand, it is known from the shape of XRD peaks that the average grain size of Cu- and Ag-phase increases during heating. The average grain sizes of Cuand Ag-phase at different temperatures have been measured using in situ XRD line broadening method on Cu(200) and Ag(220) peaks, and the measured result is shown in Fig. 4 where the variation of grain size with temperature can be seen. The grains of Ag-phase grow
Y. Qin et al. I Journal of Alloys and Compounds 2.56 (1997) 230-233
Ag,~7Cu63 powder specimen. The MA powders are composed of an amorphous phase and a supersaturated C u - A g solid solution with near the starting composition, whereas the MA powder phases partly decompose into Cu-8 at.%Ag solid solution at about 200°C. The amorphous phase and two C u - A g solid solutions with different composition completely transform into Uu- and Ag-phase with heating the MA powder specimen to 300°(=. The average grain size of MA powders increases during heating, whereas the Ag-phase has smaller grain size than the C,l-phase.
100 oCu *Ag
/
/
/~
-N -~o o,,,i
0
233
Illllllll|lllllllll[lllllllll
250
350
Temperature
450
550
(°12)
References
Fig. 4. Vafiation ofave~geg~insize withtem~tu~. slowly in the range of the given experimental temperature. Different from the variation of grain size of Ag-phase, the grains of Cu-phase grow slowly below 400 °(2, while rapid growth of the grains occurs at 400 °C. The average grain size of Ag-phase is smaller than that of Cu-phase in the present experiment for the MA Ag37Cu63 powder specimen. It is supposed here that the Ag crystals have a higher activation energy of growth than the Cu crystals.
4. Conclusion The microstructure of nanocrystalline Ag37Cu6. ~ powders synthesized by MA was characterized by means of XRD and TEM. The experimental results indicate that the microstructure change takes place with heating the MA
[I] H. Gleiter, Prog. Mater. Sci.. 33 (1989) 223. [2] R. Birringer, Mater. 5el. Eng. A, 117(1989) 33. 13] Z. Li, S. Ramasamy, H. Hahn and R.W. Siegel, Mater. Left., 6 (1988) 195. [4] R. Bohn. T. Haubold. R. Birringer and H. Gleiter, $cripm Metall.. 25 (1991) 811. [5] I.S. Benjamin, Metal/. Trans., I (1970) 2943. 16] C.C. Koch, O.B. Cavin, C.G. Mckamey and J.O. Scarbrough, Appl. Phy,~. Lett., 43 (1983) 1017. [7~ H.J. Fecht, G. Han, Z. Fu and W.L. Johnson, J. Appl. Phys., 67 (1990) 1744. [8] J. Eckert, L. Schultz and K. Urban, Acta Metall. Mater.. 39 (1991) 1497. [9] L. Liu, F. Padella,W. Guo and M. Magini, Acta Metal/. Mater., 43 (1995) 3755. [IO] K. Uenishi, K.F. Kobay&shi,K.N. Ishihara and P.H. Shingu, Mater. Sci. Eng. A, /34 (1991) 1342. [11] T. Christman and M, Jain. Scripta Metall., 25 (1991) 767. [12] Z.Q. Li. H. Shen, L. Chen. Y. Li and Giinther, Phil. Mag. A. 72 (1995) 1485.
ALLOY3 A~D COMPOLq~DS ELSEVIER
Journal of Alloys and Compounds 256 (1997) 230-233
In-situ X-ray diffraction examination of nanocrystalHne Ag37Cu63 powders
synthesized by mechanical alloying Y o n g Q i n * , Li C h e n , H u i S h e n Institute c~] Solid State Physics. Ac,demia Shtica P.O. Box 1129. Ht!fi'i 2.?(X131. P.R. China
Received 18 November 1996
Abstract Nanocrystalline Ag37Cu¢,3 powders have been synthesized by a mechanical alloying method although Ag and Cu are almost immiscible to each other. The microstructure changes of the as-synthesized powders with increasing temperature have been examined by XRD and TEM. Below 200 °C, the specimen is composed of an amorphous phase and a supersurated Cu-Ag solid solution with the starting composition. The Cu-8 at.%Ag solid solution is formed at 200 °C, so that an amorphous phase and two Cu-Ag solid solutions with different compositions are coexistent in the specimen. On further heating of the specimen to 3(X)°C and above, the amorphous phase and two Cu-Ag solid solutions vanish completely and transform into Cu- and Ag-phase. © 1997 Elsevier Science S.A. 1~¢3"words:
Microstructure; Mechanical alloying; Nanocrystalline AgCu
1, I n t r o d u c t i o n Considerable interest in synthesizing nanocrystalline materials has been increasing since the discovery of nano,:~ystalline materials which display interesting physical and mechanical properties. The nanocrystalline materials are initially synthesized by an inert gas condensation technique following in situ compaction. This method can synthesize bulk nanocrystalline materials with clean interface, and considerable effort has been expanded on nanocrystalline metals, ceramics and alloys [ ! - 4 ] . The inherent problem of this method, however, is that it is difficult to apply to high melting point materials and to large output, so that this method is limited in some applications. Mechanica~ alloying (MA) process [5], as a novel method for synthesizing materials by means of the solid state reaction from a mixture of elemental components, has been attracting attention and successfully used to synthesize a number of equilibrium and metastable materials, including amorphous alloys, supersaturated solid solutions, quasicrystals, immiscible alloys and high melting point intennetallics [6-10]. Especially, MA can be also suited for producing nanocrystalline materials [ I l]. It is well known that Ag and Cu are almost immiscible to each other at room temperature and the heat of mixing is positive. Nanocr~stalline AgCu, however, has been ob*Corresponding author. 0925-83881971517.00 © 1997 Elsevier Science S.A. All rights reserved P i l S0925-8388(96)03i i3-i
rained by MA. To the best of our knowledge, only a few works were done on the structural investigation of nanocrystalline AgCu prepared by MA. In this paper, the microstructure of mechanically alloyed nanocrystalline AgCu powder is examined by means of in-situ X-ray diffraction (XRD) as well as transmission electron microscopy (TEM).
2. E x p e r i m e n t a l Nanocrystalline Ag37Cu63 powders were synthesized by ball milling of elemental Ag (99.99% purity) and Cu (99.9% purity) powders with initial grain sizes of 50 and 45 p~m, respectively. MA was performed in a steel vial under Ar atmospberc using a planetary ball-mill at a ball-lo-powder weight ratio of 15:1. After milling for 50 h, the MA powders were removed for structural analysis. The MA powders were characterized by XRD analysis and TEM observation. The in situ XRD experiments during heating were carried out at successively increased temperature from room temperature to predetermined temperature (100, 200, 300, 400 and 500 °C) using CuKc¢ radiation with a Philips-PW 1706 X-ray diffractometer equipped with a HTK vacuum camera. The pressure of 3 × 10-" Pa was kept during measurement. The heating rate used between each XRD measurement was 1 0 ° C m i n -~ and the specimen was kept for 10 rain at every measure-
231
Y. Qin et al. / Jourmd of Alloys and Compound.~ 256 (19971 230-233
ment temperature for stability. The XRD line broadening method was also used in order to measure average grain sizes of MA powders. The TEM observation of MA powders was carried out with a JEOL-200CX electron microscope at an operating voltage of 200 kV. The TEM specimen of MA powders was prepared by putting a small drop of alcohol with dispersive MA powders on the microgrids with carbon films.
i
Ag~Cu~,MA 50h o
"~~
•
°" o
o
°~~oo.c
3. Results and discussion The morphology and structure of MA Ag37Cu~,~ powders car, be obtained by means of TEM observation. Fig. I is the typical dark field image of the MA Ag3vCu~,3 powders and the corresponding electron diffraction pattern. The average grain size of the MA Ag3vCu6, powders measured from the dark field image is less than l0 nm. The electron diffraction pattern of MA Ag37Cu~3 powders is composed of amorphous haloes and sharp diffraction rings, which indicates the coexistence of amorphous and c~aystalline phase in the MA Ag37Cu6.~ powder specimen. The analysis of electron diffraction pattern confirms that this crystalline phase has a face-centered cubic (fcc) structure with the lattice parameter a=0.38 nm. It is known from the lattice parameter [10] that this crystalline phase is a supersaturated C u - A g solid solution with near the starting composition. Fig. 2 reveals the development of XRD patterns of the MA Ag,TCu63 powders during in situ heating, in which the different XRD patterns can be observed. This means that the microstructure clmnge of MA Cu37Cu6~ powders takes place with increasing temperature. It is clear from the XRD pattern at room temperature that a broad peak appears at 20-41 ° except for the amorphous phase. The broad peak is characteristic of the grain size refinement. The data of XRD reveals that this broad peak belongs to ( I l l ) diffraction of the C u - A g solid solution with fcc structure with the lattice parameter a=0.38 nm. This result coincides with that of the electron diffraction (Fig. Ib). A
Fig. I. (a) TEM dark field image and (b) corresponding electron diffraction pattern of the MA Ag~Cu6~ powders specimen. The bar is 100 nm long.
20O°C
~'0 ~
30
40
50 60 70 80 90 2 Theta (degrees)
tOO tin
Fig. 2. XRD patterns taken at different temperatures for the MA Ag,Cu,, powders specimen. *: Cu,O. +: Cu-Ag solid solution with near the starting composition. &: Cu-8 at.%Ag. O: Ag, and O: Cn.
weak Cu20 peak at 20-37 ° was also observed in the XRD patterns of MA Ag37Cu63 powder specimen and the Cu20 is considered to be from the raw material. From TEM and XRD result one can conclude that the nanocrystalline Ag37Cuo3 powders by MA are composed of an amorphous phase and a supersaturated C u - A g solid solution with near the starting composition. Compared with room temperature, the XRD pattern shows that no detectable new phase is found at about 100 °C, which implies that the microstructure at 100 °(2 is the same as that at room temperature for the MA Ag37Cu63 powder specimen. The XRD pattern begins to change at about 200 °C, in which another peak at 20-42.8 ° appears. This new phase is determined from the data of XRD to be a C u - A g solid solution with fcc structure with the lattice parameter a=0.366 nm. From the lattice parameter one can believe that this C u - A g solid solution contains about 8 at.%Ag [10]. Evidently, there are an amorphous and two C u - A g solid solutions with different composition in MA Ag37Cu63 powder specimen at about 200 °C. The amorphous phase and two C u - A g solid solutions completely vanish at about 300 °(2, instead Cu- and Ag-phase diffraction peaks are observed in the XRD pattern. Fu;ther heating the specimen to about 500 °C, no essential change is observed in the XRD pattern apart from the increase of diffraction peaks intensities of Cu- and Ag-phase, which means that the grains of Cu- and Ag-phase in the specimen grow up with increasing temperature. According to the above in situ XRD measurements, the
232
Y. Qin et al. I Jounud v f Alloys and Compounds 256 (1997) 230-233
microstructm!e changes of MA Ag37Cu63 powders with different temperatures can be described as follows. Below 200°C the MA Ag37Cu~3 powders are composed of an amorphous plhase and a supersaturated Cu-Ag solid solution with neatr the starting composition. Another supersaturated Cu-Ag solid solution containing about 8 at.%Ag is formed at about 200 °(2 so that an amorphous phase and two Cu-Ag solid solutions with different compositions coexist. The amorphous phase and Cu-Ag solid solutions completely decompose into Cu- and Ag-phase with increasing temperature to 300 °C. The supersaturated Cu-Ag solid solution with the starting composition has been found during MA over a wide composition range. The formation of Cu-8 ,~t.%Ag solid solution has been also observed in the compacted MA Ag37Cu63specimens, but not been reported in the MA Ag37Cut,3 powder specimen. Li et al. [12] examined the microstructure of the MA Ag37Cut,3 specimens compacted at 1 and 3 GPa by XRD. But it is found that the microstructural change of the compacted specimen is different from that of the powder specimen. For the former, apart from the MA powder phases (an amorphous phase and a~ Cu-Ag solid solution with the starting composition) the Cu-8 at.RAg solid solution is also observed at room temperature, while for the latter the Cu-8 at.%Ag solid solution is not observed. At about 200 °C, the powder phases discompose into Ag and Cu-8 at.%Ag phase for the compacted specimen; while the powder phases and Cu-8 at.%Ag phase coexist for the powder specimen. At about 300 °(2, the Cu-8 at.RAg-phase partly transforms in,to Cu- and Ag-phases so that the compacted specimen contains the Cu-8 at.%Ag, Cu- and Ag-phases; while in the powder specimen, the powder phases and Cu-8 at.RAg solid solution completely transforms into Cuand Ag-phases. Not only the formation of Cu-8 at.RAg solid solution in the compacted specimen is easier than that in the powder specimen, but also the Cu-8 at.RAg solid solution is more stable in the compacted specimen than in the powder sl~ecimen. Since the compacted specimens are obtained by compacting same MA Ag37Cu63 powders as those in present experiment, it is clear that the compaction has a strong influence on the microstructure of the resulting materials. From XRD pattern of MA Ag37Cu63 powders, additionally, it is found that the Cu-8 at.%Ag solid solution has a small content and unstable structure, so that its discovery is difficult in the MA powder specimen. More than 300°C, the XRD confirms that the MA Ag37Cu~3 powder specimen is composed of Cu- and Agphases. This is supported by TEM observation. Fig. 3 shows the bright field image and the corresponding electron diffraction pattern, which was taken at room temperature ~fter heating MA Ag37Cu63 powder specimen at 500 °C. It can be seen by comparing Fig. 3 with Fig. ! that their electron diffraction pattern has an obvious difference apart from the growth of grains with increasing temperature. The electron diffraction pattern is composed
Fig. 3. (a) TEM bright field image and (b) corresponding electron diffraction pattern of the MA Ag,,Cu~, powders specimen annealed at 500°C. The bar is 100 nm long.
of an amorphous phase and a Cu-Ag solid solution for the original MA specimen, while Cu- and Ag-phases for the heated MA specimen. On the other hand, it is known from the shape of XRD peaks that the average grain size of Cu- and Ag-phase increases during heating. The average grain sizes of Cuand Ag-phase at different temperatures have been measured using in situ XRD line broadening method on Cu(200) and Ag(220) peaks, and the measured result is shown in Fig. 4 where the variation of grain size with temperature can be seen. The grains of Ag-phase grow
Y. Qin et al. I Journal of Alloys and Compounds 2.56 (1997) 230-233
Ag,~7Cu63 powder specimen. The MA powders are composed of an amorphous phase and a supersaturated C u - A g solid solution with near the starting composition, whereas the MA powder phases partly decompose into Cu-8 at.%Ag solid solution at about 200°C. The amorphous phase and two C u - A g solid solutions with different composition completely transform into Uu- and Ag-phase with heating the MA powder specimen to 300°(=. The average grain size of MA powders increases during heating, whereas the Ag-phase has smaller grain size than the C,l-phase.
100 oCu *Ag
/
/
/~
-N -~o o,,,i
0
233
Illllllll|lllllllll[lllllllll
250
350
Temperature
450
550
(°12)
References
Fig. 4. Vafiation ofave~geg~insize withtem~tu~. slowly in the range of the given experimental temperature. Different from the variation of grain size of Ag-phase, the grains of Cu-phase grow slowly below 400 °(2, while rapid growth of the grains occurs at 400 °C. The average grain size of Ag-phase is smaller than that of Cu-phase in the present experiment for the MA Ag37Cu63 powder specimen. It is supposed here that the Ag crystals have a higher activation energy of growth than the Cu crystals.
4. Conclusion The microstructure of nanocrystalline Ag37Cu6. ~ powders synthesized by MA was characterized by means of XRD and TEM. The experimental results indicate that the microstructure change takes place with heating the MA
[I] H. Gleiter, Prog. Mater. Sci.. 33 (1989) 223. [2] R. Birringer, Mater. 5el. Eng. A, 117(1989) 33. 13] Z. Li, S. Ramasamy, H. Hahn and R.W. Siegel, Mater. Left., 6 (1988) 195. [4] R. Bohn. T. Haubold. R. Birringer and H. Gleiter, $cripm Metall.. 25 (1991) 811. [5] I.S. Benjamin, Metal/. Trans., I (1970) 2943. 16] C.C. Koch, O.B. Cavin, C.G. Mckamey and J.O. Scarbrough, Appl. Phy,~. Lett., 43 (1983) 1017. [7~ H.J. Fecht, G. Han, Z. Fu and W.L. Johnson, J. Appl. Phys., 67 (1990) 1744. [8] J. Eckert, L. Schultz and K. Urban, Acta Metall. Mater.. 39 (1991) 1497. [9] L. Liu, F. Padella,W. Guo and M. Magini, Acta Metal/. Mater., 43 (1995) 3755. [IO] K. Uenishi, K.F. Kobay&shi,K.N. Ishihara and P.H. Shingu, Mater. Sci. Eng. A, /34 (1991) 1342. [11] T. Christman and M, Jain. Scripta Metall., 25 (1991) 767. [12] Z.Q. Li. H. Shen, L. Chen. Y. Li and Giinther, Phil. Mag. A. 72 (1995) 1485.