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Local atomic structural changes on structural relaxation in a Pd82Si18 amorphous alloy
Materials Science and Engineering, A 179/A 180 (1994) 479-482
479
Local atomic structural changes on structural relaxation in a Pd828i18 amorphous alloy Kazunori Anazawa, Tadakatsu Ohkubo and Yoshihiko Hirotsu
Department of Mechanical Engineering, Nagaoka Universityof Technology, 1603-1, Kamitomioka, Nagaoka 940-21 (Japan)
Abstract The microstructural changes on low temperature annealing of amorphous Pd82Si18 ribbon specimens were observed using high-resolution electron microscopy and the results were correlated with the structural relaxation. At the initial stage of annealing up to 573 K, atomic medium-range order (MRO) develops and grows. At temperatures higher than 573 K but below the glass transition temperature, the MRO domains grow to f.c.c, nanocrystals which change the profile of the electron diffraction pattern, especially of the second halo ring. According to nano-beam elemental analysis on annealing using sputter-deposited Pd82Si18 thin films, the Si content in the MRO domains and nano-precipitates decreases as the domain or particle size increases. It is concluded that the atomic free volume introduced during amorphization is annihilated by atomic rearrangement in the formation and development of the MRO domains, and the chemical order must be developed in the dense random packing (DRP) matrix by the diffusion of Si from MRO into the matrix. The structural relaxation corresponds to these local atomic structural changes.
1. Introduction With respect to the structural relaxation of amorphous alloys on annealing, it is well known that a decrease in volume and increase in density accompany an exothermic reaction below the glass transition temperature Tg [1]. Other changes in physical and mechanical properties related to relaxation have been reported [2]. Structural changes on annealing below Tg have been investigated by X-ray diffraction [3, 4] and transmission electron microscopy (TEM) [5-7], especially in metal-non-metal systems. However, there have been no high-resolution electron microscopy (HREM) studies on the change in microstructure during structural relaxation and phase decomposition in amorphous alloys at an atomic level. Such studies are necessary for an understanding of the structure of amorphous alloys and changes in their physical and mechanical properties during annealing. In the present study, microstructural change on low temperature annealing of amorphous Pd82Si18 ribbon specimens was investigated using HREM. For the purpose of local nano-beam analysis, which includes nano-beam electron diffraction and elemental analysis, thin film Pd82Si18 amorphous specimens were prepared 0921-5093/94/$7.00 SSD10921-5093(93)05558-7
and their local structures and compositions on annealing were analysed.
2. Experimental details A m o r p h o u s Pd825i18 alloy ribbons were made by rapid quenching using a high-frequency induction furnace. The estimated cooling rate of the ribbons is about 5 × 105 K s -~. For the ribbon specimen, the exothermal reaction was measured by differential scanning calorimetry (DSC) using a heating rate of 20 K min- 1 in Ar gas. In order to investigate structural changes during almost the same annealing process as used for the DSC study, the ribbons were annealed by heating the specimens in Ar up to temperatures of 553, 573, 633 and 653 K at a heating rate of 20 K min-1, and then they were quenched. Ribbon specimens were also annealed in a high vacuum (approximately 3 x 10 -5 Pa) for 3.6 ks at temperatures of 553, 573, 633 and 653 K and were observed by TEM to check structural differences due to the difference in annealing conditions. Specimen thinning for HREM was done using a dual-gun ion mill equipped with a liquid N 2 specimen © 1994 - Elsevier Sequoia. All rights reserved
480
K. Anazawa et al.
/
Structural changes in Pds,Si/s
cold stage, and also by an ultra-microtome. The ion voltage and current for ion milling were 4 kV and 0.5 m A respectively. H R E M observations were done using a JEM-2010 electron microscope operating at 200 kV. To study local changes in composition on annealing by an analytical-type electron microscope as correctly as possible, amorphous Pd82Si~s films as thin as about 10 nm were prepared by Ar-beam sputtering (beam voltage 2 kV, beam current 40 mA) and were then annealed at temperatures of 373, 473, 523, 573, 623, 673 K for 3.6 ks in a high vacuum. To avoid contamination during film preparation, a cryo-pump system was used in the sputtering. The nano-beam analyis was done using an analytical-type high resolution electron microscope (HF-2000) operating at 200 kV equipped with an energy-dispersive X-ray (EDX) analysis system. 3. Results and discussion 3.1. Exothermic reaction on annealing A clear exothermic reaction on annealing the ribbon specimen was observed in DSC. The crystallization temperature T~ and the glass transition temperature T~ of the ribbon were 651 K and 638 K respectively. The heat of crystallization A H at structural relaxation was estimated roughly from the DSC curve by subtracting the exotherm for the specimen which was once annealed up to 7~ and quenched. The A H value was 0.1mcal mg 3.2. Structure of as-formed ribbon In rapidly quenched Pd-Cu-Si [8] and Pd-Si [9] alloys with eutectic compositions, small f.c.c.-like medium-range order (MRO) domains 1 nm in diameter were observed by H R E M . Figures l(a) and (b) are H R E M images taken from specimens prepared by ultra-microtome and ion milling respectively. These images were taken under a defocus condition of about 80 nm. Although diffuse halo rings are observed in the selected area electron diffraction (SAED) patterns (insets in (a) and (b)), M R O domains with lattice fringes extending about 1 nm are observed locally in these images. The M R O images, especially in the encircled areas, correspond well with those calculated for the Pds2Sijs structure with f.c.c. M R O embedded in the dense random packing (DRY') matrix [10]. In Fig. 2(a) and (b), calculated images under a suitable defocus condition (80 nm defocus) for the f.c.c. M R O models are shown as [101] oriented and [001] oriented f.c.c. MRO images respectively. In these models, the f.c.c. cluster is located in the middle of tetragonal cells (dimensions a = b = 3 nm and c = 9 nm). When thinning bulk specimens, the ultra-microtome thinning technique is believed to be the most reliable
Fig. 1. HREM images (80 nm underfocus) of as-quenched Pds2Sijs ribbon specimens prepared by ultra-microtome (a) and by ion milling with a specimen cooling stage (b). [I 01] oriented and [001] oriented f.c.c. MRO domains are observed locally in the encircled areas.
Fig. 2. Calculated images under a defocus condition (80 nm undeffocus) suitable for imaging of the f.c.c. MRO structure along the beam incidence of [101 ] f.c.c, cluster (a) and [001 [ f.c.c. cluster (b). The model structure includes 240 Pd and 49 Si atoms in the f.c.c, cluster and 4740 Pd and 1040 Si atoms in the enveloping DRP structure. The cell size is a = b = 3 nm and c = 9 nm. The images are along the tetragonal c-axis. The calculation was done using a multi-slice method (beam divergence 6 x 10 -4 rad, energy spread 7.5 nm, slice thickness 1.5 nm). for avoiding specimen degradation during thinning procedures. From these figures, it can be seen that there is no difference between the images taken from specimens prepared by the above two techniques. Because of this we thinned the specimens by ion
K. Anazawa et al. / Structural changes in Pdu2Si18 milling (with a specimen cold stage) for the H R E M and SAED observations after annealing. 3.3. Structural change on annealing After heating the specimen up to 553 K, M R O domains are found to grow to a size of 2-3 nm. In SAED patterns (selected area approximately 900 nm in diameter) from the annealed specimen, however, no appreciable change in intensity profile was observed over the entire area. We call this initial stage of annealing, stage I. On annealing up to 573 K, the intensity distribution in the second halo ring in the SAED starts to change. The second halo ring tends to change into two diffuse rings or to change into a fairly strong sharp ring with a diffuse tail along the higher scattering angle side, depending on the observation area. At this stage, diffuse halo diffraction patterns as observed in stage I were obtained occasionally. The size of M R O domains becomes as large as 5 nm at this stage. Figure 3 shows the H R E M image at this stage. An SAED pattern with the characteristic second ring is shown in the inset. This stage we call here stage II. In the specimen heated to 633 K (near Tg), the second halo ring in most of the SAED patterns splits completely into two rings and the width of the first halo ring starts to decrease. This stage is called stage III for the formation of nano-precipitates of a-Pd 8 nm in diameter. After heating up to 653 K (near Tx), the a-phase precipitates are observed quite frequently with a size larger than 8 nm. This is the growth stage of the aphase, recognizable as the crystallization stage by macroscopic structural characterization (such as X-ray diffraction). In Fig. 4, a H R E M image from the specimen heated to 633 K is shown, together with the SAED pattern.
Fig. 3. HREM image and SAED pattern taken from the Pd828ils ribbon after heating to 573 K. Largely grown MROs are seen.
481
After examination of H R E M images and SAED patterns from specimens annealed at 553, 573, 633 and 653 K for 3.6 ks in a high vacuum, it was concluded that there is no appreciable difference between H R E M images and SAED patterns taken from the above specimens annealed in vacuum and from specimens heated to the corresponding temperature ranges in Ar. 3.4. Local compositional change on annealing In order to investigate the Si composition change in the M R O domains and nano-precipitates, nano-beam electron diffraction (NBED) and nano-beam E D X analysis were done for the annealed Pd82Si18 thin film specimens. From lattice parameter measurements using NBED with a beam size of 4 nm, it was found that the M R O grown in stage II possibly includes a large amount of Si at interstitial sites in the f.c.c, domains, and that the Si content decreases when the f.c.c. domains grow further to nano-precipitates in stage III [11]. To study the Si composition in M R O domains, nano-precipitates and matrix, nano-beam E D X analysis was used, with a beam size of 1 nm. There was no clear compositional change in the specimens until they were annealed at temperatures higher than 573 K. Figure 5 shows the change in Si composition in the f.c.c. M R O domains and nano-precipitates on annealing as a function of the domain or particle size. The Si composition tends to decrease as the domain or the particle size increases. The figure also shows that the Si composition in the M R O structure in the asformed specimens must be large, as in the average composition Pds2Si18. The Si content in the matrix regions increases with increasing annealing temperature. 3.5. Microstructural change and structural relaxation During annealing, the following two structural changes are supposed to occur as structural relaxation
Fig. 4. HREM image and SAED pattern from the Pd82Si18 ribbon specimen after heating to 633 K. Nano-precipitates with a size of approximately 8 nm are seen.
482
K. Anazawa et al.
20
~
Structural changes" in l'd~.Si>
TABLE 1. Structure parameters of DRP and MRO structure models and changes in cell size after relaxation using LennardJones type atomic potentials
o 473K <: 573K • 623K
<
o~
/
()
10
E o 1 _ ~
I
5
I ~[
l
I ~
1
10
I
15
I
I
I
20
DRP model
MRO model
I)RP model
Number of total atoms Number of MRO atoms Unit cell size before relaxation (nm) Unit cell size after relaxation (nml
200(t 240 3.161
2000 0 3.161
3.139
3.151
Particle size [nrn]
Fig. 5. Change in Si content in the f.c.c. MRO domains and f.c.c. nano-precipitates measured by nano-beam EDX analysis.
[12]: (1) annihilation of free volume in the amorphous structure and (2) the development of atomic shortrange ordering. T h e local structural changes in the present amorphous Pds2Si 1~ alloys on annealing, which follow the exothermic annealing process up to crystallization, are thought to be closely related to the structural relaxation process. Atomic rearrangement for the formation and growth of M R O domains contributes to the development of local topological order, and may contribute to annihilation of the atomic free volume by increasing the atomic packing density. Also, the local compositional change, namely the diffusion of Si from the M R O domains to the surrounding matrix, develops the chemical ordering in the matrix. In this study, we tried to estimate the volume change before and after the formation of M R O by computer simulation. T h e following two extreme structures were assumed: one is a DRP structure with composition PdszSi~s and the other is a structure with a spherical f.c.c. Pd cluster (diameter 2 nm) without Si, e m b e d d e d in a DRP structure with Si composition greater than 18 at.%. T h e former corresponds to the as-quenched specimen and the latter to the annealed specimen. In these models, the total numbers of Pd and Si atoms and their ratios were kept equal. After making these models, the structures were relaxed using LennardJones type atomic potentials [13]. In order to eliminate the surface effect, the periodic boundary condition was applied. T h e M R O volume fraction in the model is about 15%, which is close to the fraction of the asformed specimens estimated from image simulation of H R E M images. In Table 1, the structural parameters and the calculated structural data are listed. It is seen that the difference in volume between the two models after relaxation is about 0.4%. In reality, a considerable amount of Si is still in the MRO, especially in the smaller M R O domains, and in most amorphous alloys, the M R O structure is already
introduced in the as-formed state. Therefore, the experimental value of the volume change up to "I~ must be smaller than the calculated value. An experimental volume change determined from amorphous P d - C u - S i alloy on annealing up to T~ is about 0.1% [ 1 ].
Acknowledgments T h e authors would like to express their thanks to Mr. Y. Ueki at Hitachi Instrument Eng. Co. Ltd. for operating the HF-2000, and to Mr. S. Aida at J E O L Ltd. for preparing T E M specimens by ultra-microtome. T h e authors also thank Mr. M. Seda for his help in the experiment.
References 1 H.S. Chen, d. Appl. Phys., 40(1978)3289. 2 E E. Luborsky (ed.), Amorphous Metallic Alloys, Butterworths Monographs in Materials, Butterworths, London, 1983, Chapters 12-18. 3 Y. Waseda, The Structure of Non-(O'stallitw Materials, McGraw-Hill, New York, 1980, Chapter 4. 4 T. Egami, Y. Mater. Sci., 13 (1978) 2587. 5 Y. Hirotsu, R. Akada and A. Onishi, J. Non-C~st. Solids', 74 (1985) 97. 6 T. Masumoto and R. Maddin, Acta Metall., 19( 1971 ) 725. 7 T. Masumoto, H. Kimura, A. lnoue and Y. Waseda, Mater. Sci. Eng., 2,?(1976) 141. 8 Y. Hirotsu, M. Uehara and M. Ueno, J. AppL l'hys., 59 (1986) 3081. 9 Y. Hirotsu, N. Imai and T. Hirahara, Proc. Int. Syrup. on Non-Equilibrium Solid Phase of Metals and Alloys, in Suppl. Jpn. Inst. Met. (1988) 131. 10 K. Anazawa, Y. Hirotsu and Y. lnoue, Acta Metall., in press. 1 l K. Anazawa, Y. Hirotsu and Y. Ichinose, J. Non-Co'st. Solids, 150-158 (1993) 196. 12 T. Egami, Mater. Res. Bull., 13(1978) 557. 13 J. M. Dubois, R H. Gaskell and G. Le Ca6r, Proc. R. Soc. London, Ser. A, 402(1985)323.
479
Local atomic structural changes on structural relaxation in a Pd828i18 amorphous alloy Kazunori Anazawa, Tadakatsu Ohkubo and Yoshihiko Hirotsu
Department of Mechanical Engineering, Nagaoka Universityof Technology, 1603-1, Kamitomioka, Nagaoka 940-21 (Japan)
Abstract The microstructural changes on low temperature annealing of amorphous Pd82Si18 ribbon specimens were observed using high-resolution electron microscopy and the results were correlated with the structural relaxation. At the initial stage of annealing up to 573 K, atomic medium-range order (MRO) develops and grows. At temperatures higher than 573 K but below the glass transition temperature, the MRO domains grow to f.c.c, nanocrystals which change the profile of the electron diffraction pattern, especially of the second halo ring. According to nano-beam elemental analysis on annealing using sputter-deposited Pd82Si18 thin films, the Si content in the MRO domains and nano-precipitates decreases as the domain or particle size increases. It is concluded that the atomic free volume introduced during amorphization is annihilated by atomic rearrangement in the formation and development of the MRO domains, and the chemical order must be developed in the dense random packing (DRP) matrix by the diffusion of Si from MRO into the matrix. The structural relaxation corresponds to these local atomic structural changes.
1. Introduction With respect to the structural relaxation of amorphous alloys on annealing, it is well known that a decrease in volume and increase in density accompany an exothermic reaction below the glass transition temperature Tg [1]. Other changes in physical and mechanical properties related to relaxation have been reported [2]. Structural changes on annealing below Tg have been investigated by X-ray diffraction [3, 4] and transmission electron microscopy (TEM) [5-7], especially in metal-non-metal systems. However, there have been no high-resolution electron microscopy (HREM) studies on the change in microstructure during structural relaxation and phase decomposition in amorphous alloys at an atomic level. Such studies are necessary for an understanding of the structure of amorphous alloys and changes in their physical and mechanical properties during annealing. In the present study, microstructural change on low temperature annealing of amorphous Pd82Si18 ribbon specimens was investigated using HREM. For the purpose of local nano-beam analysis, which includes nano-beam electron diffraction and elemental analysis, thin film Pd82Si18 amorphous specimens were prepared 0921-5093/94/$7.00 SSD10921-5093(93)05558-7
and their local structures and compositions on annealing were analysed.
2. Experimental details A m o r p h o u s Pd825i18 alloy ribbons were made by rapid quenching using a high-frequency induction furnace. The estimated cooling rate of the ribbons is about 5 × 105 K s -~. For the ribbon specimen, the exothermal reaction was measured by differential scanning calorimetry (DSC) using a heating rate of 20 K min- 1 in Ar gas. In order to investigate structural changes during almost the same annealing process as used for the DSC study, the ribbons were annealed by heating the specimens in Ar up to temperatures of 553, 573, 633 and 653 K at a heating rate of 20 K min-1, and then they were quenched. Ribbon specimens were also annealed in a high vacuum (approximately 3 x 10 -5 Pa) for 3.6 ks at temperatures of 553, 573, 633 and 653 K and were observed by TEM to check structural differences due to the difference in annealing conditions. Specimen thinning for HREM was done using a dual-gun ion mill equipped with a liquid N 2 specimen © 1994 - Elsevier Sequoia. All rights reserved
480
K. Anazawa et al.
/
Structural changes in Pds,Si/s
cold stage, and also by an ultra-microtome. The ion voltage and current for ion milling were 4 kV and 0.5 m A respectively. H R E M observations were done using a JEM-2010 electron microscope operating at 200 kV. To study local changes in composition on annealing by an analytical-type electron microscope as correctly as possible, amorphous Pd82Si~s films as thin as about 10 nm were prepared by Ar-beam sputtering (beam voltage 2 kV, beam current 40 mA) and were then annealed at temperatures of 373, 473, 523, 573, 623, 673 K for 3.6 ks in a high vacuum. To avoid contamination during film preparation, a cryo-pump system was used in the sputtering. The nano-beam analyis was done using an analytical-type high resolution electron microscope (HF-2000) operating at 200 kV equipped with an energy-dispersive X-ray (EDX) analysis system. 3. Results and discussion 3.1. Exothermic reaction on annealing A clear exothermic reaction on annealing the ribbon specimen was observed in DSC. The crystallization temperature T~ and the glass transition temperature T~ of the ribbon were 651 K and 638 K respectively. The heat of crystallization A H at structural relaxation was estimated roughly from the DSC curve by subtracting the exotherm for the specimen which was once annealed up to 7~ and quenched. The A H value was 0.1mcal mg 3.2. Structure of as-formed ribbon In rapidly quenched Pd-Cu-Si [8] and Pd-Si [9] alloys with eutectic compositions, small f.c.c.-like medium-range order (MRO) domains 1 nm in diameter were observed by H R E M . Figures l(a) and (b) are H R E M images taken from specimens prepared by ultra-microtome and ion milling respectively. These images were taken under a defocus condition of about 80 nm. Although diffuse halo rings are observed in the selected area electron diffraction (SAED) patterns (insets in (a) and (b)), M R O domains with lattice fringes extending about 1 nm are observed locally in these images. The M R O images, especially in the encircled areas, correspond well with those calculated for the Pds2Sijs structure with f.c.c. M R O embedded in the dense random packing (DRY') matrix [10]. In Fig. 2(a) and (b), calculated images under a suitable defocus condition (80 nm defocus) for the f.c.c. M R O models are shown as [101] oriented and [001] oriented f.c.c. MRO images respectively. In these models, the f.c.c. cluster is located in the middle of tetragonal cells (dimensions a = b = 3 nm and c = 9 nm). When thinning bulk specimens, the ultra-microtome thinning technique is believed to be the most reliable
Fig. 1. HREM images (80 nm underfocus) of as-quenched Pds2Sijs ribbon specimens prepared by ultra-microtome (a) and by ion milling with a specimen cooling stage (b). [I 01] oriented and [001] oriented f.c.c. MRO domains are observed locally in the encircled areas.
Fig. 2. Calculated images under a defocus condition (80 nm undeffocus) suitable for imaging of the f.c.c. MRO structure along the beam incidence of [101 ] f.c.c, cluster (a) and [001 [ f.c.c. cluster (b). The model structure includes 240 Pd and 49 Si atoms in the f.c.c, cluster and 4740 Pd and 1040 Si atoms in the enveloping DRP structure. The cell size is a = b = 3 nm and c = 9 nm. The images are along the tetragonal c-axis. The calculation was done using a multi-slice method (beam divergence 6 x 10 -4 rad, energy spread 7.5 nm, slice thickness 1.5 nm). for avoiding specimen degradation during thinning procedures. From these figures, it can be seen that there is no difference between the images taken from specimens prepared by the above two techniques. Because of this we thinned the specimens by ion
K. Anazawa et al. / Structural changes in Pdu2Si18 milling (with a specimen cold stage) for the H R E M and SAED observations after annealing. 3.3. Structural change on annealing After heating the specimen up to 553 K, M R O domains are found to grow to a size of 2-3 nm. In SAED patterns (selected area approximately 900 nm in diameter) from the annealed specimen, however, no appreciable change in intensity profile was observed over the entire area. We call this initial stage of annealing, stage I. On annealing up to 573 K, the intensity distribution in the second halo ring in the SAED starts to change. The second halo ring tends to change into two diffuse rings or to change into a fairly strong sharp ring with a diffuse tail along the higher scattering angle side, depending on the observation area. At this stage, diffuse halo diffraction patterns as observed in stage I were obtained occasionally. The size of M R O domains becomes as large as 5 nm at this stage. Figure 3 shows the H R E M image at this stage. An SAED pattern with the characteristic second ring is shown in the inset. This stage we call here stage II. In the specimen heated to 633 K (near Tg), the second halo ring in most of the SAED patterns splits completely into two rings and the width of the first halo ring starts to decrease. This stage is called stage III for the formation of nano-precipitates of a-Pd 8 nm in diameter. After heating up to 653 K (near Tx), the a-phase precipitates are observed quite frequently with a size larger than 8 nm. This is the growth stage of the aphase, recognizable as the crystallization stage by macroscopic structural characterization (such as X-ray diffraction). In Fig. 4, a H R E M image from the specimen heated to 633 K is shown, together with the SAED pattern.
Fig. 3. HREM image and SAED pattern taken from the Pd828ils ribbon after heating to 573 K. Largely grown MROs are seen.
481
After examination of H R E M images and SAED patterns from specimens annealed at 553, 573, 633 and 653 K for 3.6 ks in a high vacuum, it was concluded that there is no appreciable difference between H R E M images and SAED patterns taken from the above specimens annealed in vacuum and from specimens heated to the corresponding temperature ranges in Ar. 3.4. Local compositional change on annealing In order to investigate the Si composition change in the M R O domains and nano-precipitates, nano-beam electron diffraction (NBED) and nano-beam E D X analysis were done for the annealed Pd82Si18 thin film specimens. From lattice parameter measurements using NBED with a beam size of 4 nm, it was found that the M R O grown in stage II possibly includes a large amount of Si at interstitial sites in the f.c.c, domains, and that the Si content decreases when the f.c.c. domains grow further to nano-precipitates in stage III [11]. To study the Si composition in M R O domains, nano-precipitates and matrix, nano-beam E D X analysis was used, with a beam size of 1 nm. There was no clear compositional change in the specimens until they were annealed at temperatures higher than 573 K. Figure 5 shows the change in Si composition in the f.c.c. M R O domains and nano-precipitates on annealing as a function of the domain or particle size. The Si composition tends to decrease as the domain or the particle size increases. The figure also shows that the Si composition in the M R O structure in the asformed specimens must be large, as in the average composition Pds2Si18. The Si content in the matrix regions increases with increasing annealing temperature. 3.5. Microstructural change and structural relaxation During annealing, the following two structural changes are supposed to occur as structural relaxation
Fig. 4. HREM image and SAED pattern from the Pd82Si18 ribbon specimen after heating to 633 K. Nano-precipitates with a size of approximately 8 nm are seen.
482
K. Anazawa et al.
20
~
Structural changes" in l'd~.Si>
TABLE 1. Structure parameters of DRP and MRO structure models and changes in cell size after relaxation using LennardJones type atomic potentials
o 473K <: 573K • 623K
<
o~
/
()
10
E o 1 _ ~
I
5
I ~[
l
I ~
1
10
I
15
I
I
I
20
DRP model
MRO model
I)RP model
Number of total atoms Number of MRO atoms Unit cell size before relaxation (nm) Unit cell size after relaxation (nml
200(t 240 3.161
2000 0 3.161
3.139
3.151
Particle size [nrn]
Fig. 5. Change in Si content in the f.c.c. MRO domains and f.c.c. nano-precipitates measured by nano-beam EDX analysis.
[12]: (1) annihilation of free volume in the amorphous structure and (2) the development of atomic shortrange ordering. T h e local structural changes in the present amorphous Pds2Si 1~ alloys on annealing, which follow the exothermic annealing process up to crystallization, are thought to be closely related to the structural relaxation process. Atomic rearrangement for the formation and growth of M R O domains contributes to the development of local topological order, and may contribute to annihilation of the atomic free volume by increasing the atomic packing density. Also, the local compositional change, namely the diffusion of Si from the M R O domains to the surrounding matrix, develops the chemical ordering in the matrix. In this study, we tried to estimate the volume change before and after the formation of M R O by computer simulation. T h e following two extreme structures were assumed: one is a DRP structure with composition PdszSi~s and the other is a structure with a spherical f.c.c. Pd cluster (diameter 2 nm) without Si, e m b e d d e d in a DRP structure with Si composition greater than 18 at.%. T h e former corresponds to the as-quenched specimen and the latter to the annealed specimen. In these models, the total numbers of Pd and Si atoms and their ratios were kept equal. After making these models, the structures were relaxed using LennardJones type atomic potentials [13]. In order to eliminate the surface effect, the periodic boundary condition was applied. T h e M R O volume fraction in the model is about 15%, which is close to the fraction of the asformed specimens estimated from image simulation of H R E M images. In Table 1, the structural parameters and the calculated structural data are listed. It is seen that the difference in volume between the two models after relaxation is about 0.4%. In reality, a considerable amount of Si is still in the MRO, especially in the smaller M R O domains, and in most amorphous alloys, the M R O structure is already
introduced in the as-formed state. Therefore, the experimental value of the volume change up to "I~ must be smaller than the calculated value. An experimental volume change determined from amorphous P d - C u - S i alloy on annealing up to T~ is about 0.1% [ 1 ].
Acknowledgments T h e authors would like to express their thanks to Mr. Y. Ueki at Hitachi Instrument Eng. Co. Ltd. for operating the HF-2000, and to Mr. S. Aida at J E O L Ltd. for preparing T E M specimens by ultra-microtome. T h e authors also thank Mr. M. Seda for his help in the experiment.
References 1 H.S. Chen, d. Appl. Phys., 40(1978)3289. 2 E E. Luborsky (ed.), Amorphous Metallic Alloys, Butterworths Monographs in Materials, Butterworths, London, 1983, Chapters 12-18. 3 Y. Waseda, The Structure of Non-(O'stallitw Materials, McGraw-Hill, New York, 1980, Chapter 4. 4 T. Egami, Y. Mater. Sci., 13 (1978) 2587. 5 Y. Hirotsu, R. Akada and A. Onishi, J. Non-C~st. Solids', 74 (1985) 97. 6 T. Masumoto and R. Maddin, Acta Metall., 19( 1971 ) 725. 7 T. Masumoto, H. Kimura, A. lnoue and Y. Waseda, Mater. Sci. Eng., 2,?(1976) 141. 8 Y. Hirotsu, M. Uehara and M. Ueno, J. AppL l'hys., 59 (1986) 3081. 9 Y. Hirotsu, N. Imai and T. Hirahara, Proc. Int. Syrup. on Non-Equilibrium Solid Phase of Metals and Alloys, in Suppl. Jpn. Inst. Met. (1988) 131. 10 K. Anazawa, Y. Hirotsu and Y. lnoue, Acta Metall., in press. 1 l K. Anazawa, Y. Hirotsu and Y. Ichinose, J. Non-Co'st. Solids, 150-158 (1993) 196. 12 T. Egami, Mater. Res. Bull., 13(1978) 557. 13 J. M. Dubois, R H. Gaskell and G. Le Ca6r, Proc. R. Soc. London, Ser. A, 402(1985)323.