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structure and displacive phase transformations of small particles of In-Pb alloys
Scripta mater. 44 (2001) 2043–2046 www.elsevier.com/locate/scriptamat
STRUCTURE AND DISPLACIVE PHASE TRANSFORMATIONS OF SMALL PARTICLES OF In-Pb ALLOYS K. Asaka, E. Kitahata*, Y. Hirotsu**, K. Kifune***, Y. Kubota*** and T. Tadaki*** Graduate School, Division of Materials Science and Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan *Graduate School, Division of Natural Sciences, Osaka Women’s University, 2-1 Daisen-cho, Sakai, Osaka 590-0035, Japan **The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan ***Department of Environmental Sciences, Faculty of Science, Osaka Women’s University, 2-1 Daisen-cho, Sakai, Osaka 590-0035, Japanzcnyx (Received August 21, 2000) (Accepted November 22, 2000) Keywords: Structure; Small particle; Indium-lead alloy; Size effect; Phase transformation
Introduction The displacive first order phase transformations in alloys, i.e., martensitic transformations, are nucleated heterogeneously at lattice defects such as crystal surfaces, grain boundaries and dislocations (1,2). Distinctive lattice softening occurs as a precursory phenomenon of the transformations in some kinds of alloys (3). When the size of materials decreases, the proportion of surface atoms to the whole becomes larger, and in small particles lattice softening occurs which typically manifests itself as a remarkable decrease of melting temperature (4). It is thus expected that the nucleation of martensitic transformations becomes easier to take place in small particles due to the increase of their surface areas as a favorable nucleation site and the lattice softening. Recently, some of the present authors examined the phase transformation in nm-sized particles of Fe-Ni alloys which in bulk undergo the typical non-thermoelastic martensitic transformation but do not exhibit marked precursory lattice softening by means of transmission electron microscopy (TEM) and electron diffraction (ED) (5– 8). It was found, however, that the austenite phase was stabilized in the nm-sized particles, as previously reported for much larger Fe-Ni alloy particles (1,9 –13). In the present study we examined structure and the phase transformations of small particles of In-Pb alloys in the size ranging from about 100 nm to several nm. In-Pb alloys in bulk are reported to undergo face-centered tetragonal (fct)1 (c/a⬎1) (␣1) 䡠 face-centered orthorhombic (fco) 䡠 fct (c/a⬍1) (␣2) and fct (c/a⬍1) (␣2) 䡠 face-centered cubic (fcc) () thermoelastic martensitic transformations for 12⬃15 at.% Pb and 30⬃36 at.% Pb contents, respectively (14), and to exhibit the precursory lattice softening (15).
1
A face-centered tetragonal lattice should be expressed by a body-centered tetragonal lattice of the Bravais lattice, but crystal structures of ␣1 and ␣2 phases are taken as the former throughout the text, so that the simple lattice changes associated with the phase transformations among ␣1, ␣2 and  phases are intuitively understood. 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00860-0
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Experimental Procedure In-Pb alloys with several compositions ranging from 8 to 36 at.% Pb were made by melting pure In (99.99%) and Pb (99.99%) in sealed quartz tubes back-filled with argon gas. After melting, the alloy ingots were homogenized at 400 K for 24 h. Compositions of all the alloys made were determined by atomic absorption spectrochemical analysis. Small particles with various sizes ranging from about 100 nm to several nm on the average were prepared by vacuum-evaporating the master alloys on amorphous carbon films. The base pressure of the vacuum chamber was 8.0 ⫻ 10⫺5 Pa. The small particles were sandwiched by amorphous carbon films so as not to expose them to air before TEM observation. TEM and ED were performed by using JEM-100CX, 2010 and 3000F type transmission electron microscopes, operated at 100, 200 and 300 kV, respectively. Compositions of the particles were checked by the energy dispersive X-ray (EDX) spectroscopy, which was carried out by the JEM-3000F equipped with an EDX spectrometer.
Results and Discussion Figures 1(a) to (d) show TEM images taken from the small particles with different average sizes, produced from an In-31.0 at.% Pb alloy, the average sizes being 125, 67, 32 and 27 nm, respectively. Since the compositions of these aggregations of small particles checked by EDX were only a few at.% Pb different from that of the master alloy, the compositions of the aggregations were regarded as substantially the same as that of the master alloy. Figures 1(a⬘) to (d⬘) are ED patterns taken from Figs. 1(a) to (d), respectively. The Debye rings appearing in Fig. 1(a⬘) were consistently indexed by the fct structure with the axial ratio c/a ⫽ 0.94 of the ␣2 phase at room temperature in bulk (14). It was observed that the ␣2 phase particles were transformed into the fcc  phase between 383 and 393 K on heating. This transformation temperature range was roughly the same as in bulk (14). As seen in Figs. 1(a⬘) to (d⬘), the separations of 200 and 002 reflections and of 220 and 022 reflections of the fct ␣2 phase decreased with decreasing average particle size. When the average size was reduced down to about 27 nm, the separation became undiscernible, that is, the axial ratio c/a became unity, as seen in Fig. 1(d⬘). It was thus concluded that the crystal structure of the small particles at room temperature changes from fct to fcc as the particle size varies from about 100 nm to a few tens of nm. This result indicated that despite the increasing surface area and the lattice softening possibly enhanced the transformation temperature for the  3 ␣2 phase transformation is lowered below room temperature when the particle size is reduced to a few tens of nm or less. The lowering of the transformation temperature would possibly be due to the decrease in the thermo-dynamical equilibrium temperature, T0, between the  and ␣2 phases, of which the free energies in the small particle state of the alloy would be definitely different from those in the bulk state owing to surface effects. Figure 2(a) shows a TEM image of a particle about 45 nm in size produced from an In-12.1 at.% Pb alloy. In Fig. 2(a⬘) is shown a Fourier transform (FT) pattern of the encircled area in Fig. 2(a), where lattice fringes with spacings of 0.23 and 0.27 nm are visible. The EDX indicated that the composition of the particle was In-11.9 at.% Pb. The angular relations among intensity maxima appearing in the FT pattern were well consistent with those in an ED pattern of an ⬍011⬎ orientation of the fct structure with the axial ratio c/a ⫽ 1.08 of the ␣1 phase in bulk, as indexed. Figure 2(b) shows a TEM image of a particle about 10 nm in size produced from the same alloy as in Fig. 2(a), and in Fig. 2(b⬘) is shown the FT pattern of the whole area of the particle. The composition of this particle examined by EDX was In-11.5 at.% Pb. In Fig. 2(b) are visible lattice fringes with spacings of 0.24 and 0.28 nm. Angular relations among the intensity maxima in the FT pattern were well explained by an fcc structure in an ⬍110⬎ orientation, as indexed. It was thus seen from Figs. 2(b) and (b⬘) that the particle possesses an
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Figure 1. (a) to (d): TEM images of small particles with various average sizes produced from an In-31.0 at.% Pb alloy. (a⬘) to (d⬘): ED patterns taken from the TEM images (a) to (d), respectively.
fcc structure with the lattice parameter a ⫽ 0.48 nm. Although an intermediate fco phase was found in In-Pb alloys with Pb contents around 12 at.% in bulk (14), which was not observed in the small particles, no fcc phase has been reported so far to exist in the composition range. It was thus conceivable that the fcc phase found in this composition range is intrinsic of small particles with size about 10 nm or less. It was reported that pure In metal changed its crystal structure from fct to fcc when the size was reduced to a few nm (16,17). Then, it seemed that the critical size for the structure change in pure In metal becomes roughly several times larger by being alloyed with Pb.
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Figure 2. (a) and (b): TEM images of small particles with different sizes, but with almost the same compositions produced from an In-12.1 at.% Pb alloy. (a⬘) and (b⬘): FT patterns of the encircled area in (a) and of the whole area in (b), respectively.
Acknowledgment The present study was supported by the Grant-in-Aid for Scientific Research on Priority Areas (1997–99) of the Ministry of Education, Science, Sports and Culture, Japan. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
R. E. Cech and D. Turnbull, Trans. AIME. 206, 124 (1956). T. Saburi and S. Nenno, in Proceedings of the International Conference on Martensitic Transformations, Nara, Japan, Japan Institute of Metals, 671 (1986). L. Delaey, P. F. Gobin, G. Guenin, and H. Warlimont, in Proceedings of the International Conference on Martensitic Transformations, Boston, MIT, 400 (1979). M. Takagi, J. Phys. Soc. Jpn. 9, 359 (1954). K. Asaka, Y. Hirotsu, and T. Tadaki, J. Electron Microsc. 48, 387 (1999). K. Asaka, Y. Hirotsu, and T. Tadaki, in Proceedings of the International Conference on Solid-Solid phase Transformations’99 (JIMIC-3), ed. M. Koiwa, K. Otsuka, and T. Miyazaki, p. 1068, Japan Institute of Metals, Kyoto (1999). K. Asaka, Y. Hirotsu, and T. Tadaki, Mater. Sci. Eng. A273–275, 262 (1999). K. Asaka, Y. Hirotsu, and T. Tadaki, Mater. Sci. Forum. 327/328, 409 (2000). S. Kachi, Y. Bando, and S. Higuchi, Jpn. J. Appl. Phys. 6, 307 (1962). Y. H. Zhou, M. Harmelin, and J. Bigot, Mater. Sci. Eng. A124, 241 (1990). S. Kajiwara, S. Ohno, and K. Honma, Phil. Mag. A63, 625 (1991). Y. H. Zhou, M. Harmelin, and J. Bigot, Mater. Sci. Eng. A133, 775 (1991). Y. Chen, G. Deng, H. Lu, J. Wang, and G. Li, Jpn. J. Appl. Phys. 34, 113 (1995). Y. Koyama, T. Ukena, and O. Nittono, Trans. JIM. 23, 518 (1982). Y. Koyama and O. Nittono, J. Inst. Met. 45, 869 (1981). A. Yokozeki and G. D. Stein, J. Appl. Phys. 49, 2224 (1978). M. Tanaka, M. Takeguchi, and K. Furuya, Surf. Sci. 433– 434, 491 (1999).
STRUCTURE AND DISPLACIVE PHASE TRANSFORMATIONS OF SMALL PARTICLES OF In-Pb ALLOYS K. Asaka, E. Kitahata*, Y. Hirotsu**, K. Kifune***, Y. Kubota*** and T. Tadaki*** Graduate School, Division of Materials Science and Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan *Graduate School, Division of Natural Sciences, Osaka Women’s University, 2-1 Daisen-cho, Sakai, Osaka 590-0035, Japan **The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan ***Department of Environmental Sciences, Faculty of Science, Osaka Women’s University, 2-1 Daisen-cho, Sakai, Osaka 590-0035, Japanzcnyx (Received August 21, 2000) (Accepted November 22, 2000) Keywords: Structure; Small particle; Indium-lead alloy; Size effect; Phase transformation
Introduction The displacive first order phase transformations in alloys, i.e., martensitic transformations, are nucleated heterogeneously at lattice defects such as crystal surfaces, grain boundaries and dislocations (1,2). Distinctive lattice softening occurs as a precursory phenomenon of the transformations in some kinds of alloys (3). When the size of materials decreases, the proportion of surface atoms to the whole becomes larger, and in small particles lattice softening occurs which typically manifests itself as a remarkable decrease of melting temperature (4). It is thus expected that the nucleation of martensitic transformations becomes easier to take place in small particles due to the increase of their surface areas as a favorable nucleation site and the lattice softening. Recently, some of the present authors examined the phase transformation in nm-sized particles of Fe-Ni alloys which in bulk undergo the typical non-thermoelastic martensitic transformation but do not exhibit marked precursory lattice softening by means of transmission electron microscopy (TEM) and electron diffraction (ED) (5– 8). It was found, however, that the austenite phase was stabilized in the nm-sized particles, as previously reported for much larger Fe-Ni alloy particles (1,9 –13). In the present study we examined structure and the phase transformations of small particles of In-Pb alloys in the size ranging from about 100 nm to several nm. In-Pb alloys in bulk are reported to undergo face-centered tetragonal (fct)1 (c/a⬎1) (␣1) 䡠 face-centered orthorhombic (fco) 䡠 fct (c/a⬍1) (␣2) and fct (c/a⬍1) (␣2) 䡠 face-centered cubic (fcc) () thermoelastic martensitic transformations for 12⬃15 at.% Pb and 30⬃36 at.% Pb contents, respectively (14), and to exhibit the precursory lattice softening (15).
1
A face-centered tetragonal lattice should be expressed by a body-centered tetragonal lattice of the Bravais lattice, but crystal structures of ␣1 and ␣2 phases are taken as the former throughout the text, so that the simple lattice changes associated with the phase transformations among ␣1, ␣2 and  phases are intuitively understood. 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00860-0
2044
In-Pb SMALL PARTICLES
Vol. 44, Nos. 8/9
Experimental Procedure In-Pb alloys with several compositions ranging from 8 to 36 at.% Pb were made by melting pure In (99.99%) and Pb (99.99%) in sealed quartz tubes back-filled with argon gas. After melting, the alloy ingots were homogenized at 400 K for 24 h. Compositions of all the alloys made were determined by atomic absorption spectrochemical analysis. Small particles with various sizes ranging from about 100 nm to several nm on the average were prepared by vacuum-evaporating the master alloys on amorphous carbon films. The base pressure of the vacuum chamber was 8.0 ⫻ 10⫺5 Pa. The small particles were sandwiched by amorphous carbon films so as not to expose them to air before TEM observation. TEM and ED were performed by using JEM-100CX, 2010 and 3000F type transmission electron microscopes, operated at 100, 200 and 300 kV, respectively. Compositions of the particles were checked by the energy dispersive X-ray (EDX) spectroscopy, which was carried out by the JEM-3000F equipped with an EDX spectrometer.
Results and Discussion Figures 1(a) to (d) show TEM images taken from the small particles with different average sizes, produced from an In-31.0 at.% Pb alloy, the average sizes being 125, 67, 32 and 27 nm, respectively. Since the compositions of these aggregations of small particles checked by EDX were only a few at.% Pb different from that of the master alloy, the compositions of the aggregations were regarded as substantially the same as that of the master alloy. Figures 1(a⬘) to (d⬘) are ED patterns taken from Figs. 1(a) to (d), respectively. The Debye rings appearing in Fig. 1(a⬘) were consistently indexed by the fct structure with the axial ratio c/a ⫽ 0.94 of the ␣2 phase at room temperature in bulk (14). It was observed that the ␣2 phase particles were transformed into the fcc  phase between 383 and 393 K on heating. This transformation temperature range was roughly the same as in bulk (14). As seen in Figs. 1(a⬘) to (d⬘), the separations of 200 and 002 reflections and of 220 and 022 reflections of the fct ␣2 phase decreased with decreasing average particle size. When the average size was reduced down to about 27 nm, the separation became undiscernible, that is, the axial ratio c/a became unity, as seen in Fig. 1(d⬘). It was thus concluded that the crystal structure of the small particles at room temperature changes from fct to fcc as the particle size varies from about 100 nm to a few tens of nm. This result indicated that despite the increasing surface area and the lattice softening possibly enhanced the transformation temperature for the  3 ␣2 phase transformation is lowered below room temperature when the particle size is reduced to a few tens of nm or less. The lowering of the transformation temperature would possibly be due to the decrease in the thermo-dynamical equilibrium temperature, T0, between the  and ␣2 phases, of which the free energies in the small particle state of the alloy would be definitely different from those in the bulk state owing to surface effects. Figure 2(a) shows a TEM image of a particle about 45 nm in size produced from an In-12.1 at.% Pb alloy. In Fig. 2(a⬘) is shown a Fourier transform (FT) pattern of the encircled area in Fig. 2(a), where lattice fringes with spacings of 0.23 and 0.27 nm are visible. The EDX indicated that the composition of the particle was In-11.9 at.% Pb. The angular relations among intensity maxima appearing in the FT pattern were well consistent with those in an ED pattern of an ⬍011⬎ orientation of the fct structure with the axial ratio c/a ⫽ 1.08 of the ␣1 phase in bulk, as indexed. Figure 2(b) shows a TEM image of a particle about 10 nm in size produced from the same alloy as in Fig. 2(a), and in Fig. 2(b⬘) is shown the FT pattern of the whole area of the particle. The composition of this particle examined by EDX was In-11.5 at.% Pb. In Fig. 2(b) are visible lattice fringes with spacings of 0.24 and 0.28 nm. Angular relations among the intensity maxima in the FT pattern were well explained by an fcc structure in an ⬍110⬎ orientation, as indexed. It was thus seen from Figs. 2(b) and (b⬘) that the particle possesses an
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In-Pb SMALL PARTICLES
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Figure 1. (a) to (d): TEM images of small particles with various average sizes produced from an In-31.0 at.% Pb alloy. (a⬘) to (d⬘): ED patterns taken from the TEM images (a) to (d), respectively.
fcc structure with the lattice parameter a ⫽ 0.48 nm. Although an intermediate fco phase was found in In-Pb alloys with Pb contents around 12 at.% in bulk (14), which was not observed in the small particles, no fcc phase has been reported so far to exist in the composition range. It was thus conceivable that the fcc phase found in this composition range is intrinsic of small particles with size about 10 nm or less. It was reported that pure In metal changed its crystal structure from fct to fcc when the size was reduced to a few nm (16,17). Then, it seemed that the critical size for the structure change in pure In metal becomes roughly several times larger by being alloyed with Pb.
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Figure 2. (a) and (b): TEM images of small particles with different sizes, but with almost the same compositions produced from an In-12.1 at.% Pb alloy. (a⬘) and (b⬘): FT patterns of the encircled area in (a) and of the whole area in (b), respectively.
Acknowledgment The present study was supported by the Grant-in-Aid for Scientific Research on Priority Areas (1997–99) of the Ministry of Education, Science, Sports and Culture, Japan. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
R. E. Cech and D. Turnbull, Trans. AIME. 206, 124 (1956). T. Saburi and S. Nenno, in Proceedings of the International Conference on Martensitic Transformations, Nara, Japan, Japan Institute of Metals, 671 (1986). L. Delaey, P. F. Gobin, G. Guenin, and H. Warlimont, in Proceedings of the International Conference on Martensitic Transformations, Boston, MIT, 400 (1979). M. Takagi, J. Phys. Soc. Jpn. 9, 359 (1954). K. Asaka, Y. Hirotsu, and T. Tadaki, J. Electron Microsc. 48, 387 (1999). K. Asaka, Y. Hirotsu, and T. Tadaki, in Proceedings of the International Conference on Solid-Solid phase Transformations’99 (JIMIC-3), ed. M. Koiwa, K. Otsuka, and T. Miyazaki, p. 1068, Japan Institute of Metals, Kyoto (1999). K. Asaka, Y. Hirotsu, and T. Tadaki, Mater. Sci. Eng. A273–275, 262 (1999). K. Asaka, Y. Hirotsu, and T. Tadaki, Mater. Sci. Forum. 327/328, 409 (2000). S. Kachi, Y. Bando, and S. Higuchi, Jpn. J. Appl. Phys. 6, 307 (1962). Y. H. Zhou, M. Harmelin, and J. Bigot, Mater. Sci. Eng. A124, 241 (1990). S. Kajiwara, S. Ohno, and K. Honma, Phil. Mag. A63, 625 (1991). Y. H. Zhou, M. Harmelin, and J. Bigot, Mater. Sci. Eng. A133, 775 (1991). Y. Chen, G. Deng, H. Lu, J. Wang, and G. Li, Jpn. J. Appl. Phys. 34, 113 (1995). Y. Koyama, T. Ukena, and O. Nittono, Trans. JIM. 23, 518 (1982). Y. Koyama and O. Nittono, J. Inst. Met. 45, 869 (1981). A. Yokozeki and G. D. Stein, J. Appl. Phys. 49, 2224 (1978). M. Tanaka, M. Takeguchi, and K. Furuya, Surf. Sci. 433– 434, 491 (1999).