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Microstructure and phase decomposition of quasi-crystal Zn60Mg30Y10 alloy
Journal of Materials Processing Technology 123 (2002) 245–250
Microstructure and phase decomposition of quasi-crystal Zn60Mg30Y10 alloy Y.L. Cheung, K.C. Chan*, Y.H. Zhu Department of Manufacturing Engineering, The Hong Kong Polytechnic University, Yuk Choi Road, Hung Hom, Hong Kong, Hong Kong Received 31 October 2001
Abstract The microstructure and aging characteristics of a quasi-crystal Zn60Mg30Y10 alloy were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Typical structures of the icosahedral quasi-crystalline phase were characterized by three symmetries: 5-fold, 3-fold and 2-fold, based on which a structure model of this complex quasi-crystalline phase was proposed. Fine-dispersed precipitates were found inside the relatively stable quasi-crystalline phase during aging. Decomposition of the crystalline phase (MgZn2) was also investigated. # 2002 Published by Elsevier Science B.V. Keywords: Microstructure; Phase decomposition; Quasi-crystal Zn60Mg30Y10 alloy
1. Introduction Quasi-crystals were discovered in 1982 by Shechtman et al. [1] and first reported in 1984, changing the long-held belief that solid matter exists in only two states: amorphous and crystalline. In an amorphous substance, atoms exist in a random, disordered manner, whilst in crystalline substances, atoms form specific geometrical patterns that are repeated periodically in the material [2]. Quasi-crystals fall outside of these groups, containing atoms that are arranged in long range ordered manner and exhibit non-crystallographic symmetries (5, 8, 10 and 12-folds) but not a periodic structure. At first, quasi-crystals were found only in Al-transition metal alloys. Luo et al. [3] showed the existence of a stable icosahedral quasi-crystalline phase (i-phase) in the system Zn–Mg–RE alloys (RE: rare earth element or Y). A group of stable icosahedral quasi-crystals was identified by Nikura et al. [4] in Zn–Mg–RE with sharp diffraction peaks. This iphase was identified with an earlier mentioned Z-phase Zn6Mg3Y (Padezhnova et al. [5]), the structure of which could not be determined at that time. The ideal composition of i-phase was very close to Zn6Mg3Y [6]. Apparently, the present i-phase also obeyed the Hume–Rothery rule [7]. Structurally, a small amount (10 at.%) of RE atoms sitting in a perfect icosahedral lattice implies that the RE element should not be ignored [8]. However the structure model of * Corresponding author. Tel.: þ852-2766-4891; fax: þ852-2362-5267. E-mail address: [email protected] (K.C. Chan).
0924-0136/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 0 6 9 - 9
this complex quasi-crystalline phase has been not clear. The existence of the local magnetic moments of the rare earth ions in the i-phase should lead to interesting magnetic properties. It is expected to create new magnetic behavior because of the particular crystallographic environment surrounding the rare earths in the quasi-crystalline structure [9]. Therefore, further study on the microstructure and aging characteristics of the Zn–Mg–RE alloys is of significance, both theoretically and in respect of practical application. In this paper, the microstructures and phase decomposition of cast-annealed samples of a quasi-crystal Zn–Mg–Y alloy were studied during isothermal holding.
2. Experiment A Zn–Mg–Y alloy with a nominal composition of Zn60Mg30Y10 (at.%) was provided by ‘‘Deutsche Forschungsgemeinschaft’’ from the Germany Ministry of Science. The alloy was prepared from Zn (99.99%), Mg (99.99%) and Y (99.99%). The quasi-crystalline material was prepared by melting and pre-reacting the materials at 850 8C for 1 h in a Al2O3 crucible, which was sealed in a quartz tube under an argon atmosphere due to the high vapor pressure of Zn and Mg. A vertical-cylinder resistance furnace with air-atmosphere was used, the sample hangs on a wire. After pre-reacting, the sample was dropped into cold water to obtain a stable quasi-crystalline phase. Then the sample was annealed at 550 8C for 7 days in a normal resistance furnace.
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Phase identification and microstructure examination of the sample was carried out after various periods of aging at 150, 180 and 250 8C using X-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The XRD examination was performed on a diffractometer with nickel filtered Cu Ka radiation, scanning at a speed of 18/min, the characteristic peaks of the X-ray diffraction being collected within a diffraction angle (2y) range from 358 to 478. Microstructural analysis of the sample was performed with a Cambridge Stereoscan 360 scanning electron microscope equipped for composition analysis with energy dispersive X-ray spectroscopy (EDX) apparatus. Thin film samples for TEM examination were prepared by mechanical polishing and ion beam milling. The TEM investigation was carried out in a Jeol 2000 FX transmission electron microscope operating at 120 kV.
3. Results and discussion 3.1. Microstructure of the quasi-crystal Zn60Mg30Y10 alloy X-ray diffractograms of the as-annealed and aged Zn60Mg30Y10 quasi-crystal alloy samples are shown in Fig. 1. Three phases were observed in both as-annealed and aged samples. The main phase was identified as a facecentered icosahedral phase (f.c.i.-phase or simply i-phase), which was indexed in a reported metallurgical investigation indicated in Fig. 1 [5]. The intense diffraction peaks of the iphase were indexed according to a set of i-phase indices from Ref. [5]. The other crystalline phases MgZn2 and MgZn were identified as indicated in Fig. 1. Fig. 2 shows a back-scattered SEM micrograph of the asannealed quasi-crystal alloy Zn60Mg30Y10. The grain size of the quasi-crystalline phase has a large variation, ranging from about 5–100 mm. The as-annealed Zn60Mg30Y10 alloy consisted of five-phases: one quasi-crystalline phase (Zn6Mg3Y) and four binary phases (MgZn, MgZn2, Mg2Zn3 and Mg7Zn3), as shown in Fig. 3. Both the XRD and SEM results confirmed that the light imaged quasi-crystalline phase was the main phase in the alloy. The average chemical composition of the light imaged i-phase was Zn64Mg28Y8 (at.%). The icosahedral quasi-crystalline phase had a stoichiometric composition close to Zn6Mg3Y, as reported by Tsai et al. [6]. The volume fraction of this quasi-crystalline phase was about 90%. The Mg34Zn66 phase (at.%) corresponded to the MgZn2 phase (light gray region). The Mg49Zn51 phase (dark gray imaged region), the Mg40Zn60 (gray imaged region) and the Mg71Zn29 (black region) (at.%) corresponded to the crystalline MgZn phase, the Mg2Zn3 phase and Mg7Zn3 phase, as shown in Table 1. The Mg7Zn3 phase was shown to decompose upon eutectoid reaction, giving rise to a very fine lamellar structure with an average inter-lamellar spacing of 0.2 mm. Only three phases: the icosahedral quasi-crystalline phase Zn6Mg3Y, and the MgZn2 and MgZn phases, were identified
Fig. 1. X-ray diffraction pattern of the face-centered icosahedral phase of Zn60Mg30Y10 alloy: (a) the as-solidified state; (b) the aged state (held at 150 8C for 10.5 h); (c) the over-aged state (held at 180 8C for 35 h followed by 250 8C for 400 h); and (d) vertical bars represent the position and intensity of diffraction peaks in the Zn6Mg3Y-quasi-crystal.
in the Zn60Mg30Y10 alloy using XRD, as shown in Fig. 1(a). This was probably because of either the small volume fraction or the decomposition of the other two undetected phases. Distribution profiles of three elements Zn, Mg and Y in the various phases in the alloy were produced using SEM X-ray line scanning, as shown in Fig. 4. Very small amounts of Y
Fig. 2. Back-scattered SEM micrograph of the as-annealed quasi-crystal alloy Zn60Mg30Y10.
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Fig. 3. As-annealed Zn60Mg30Y10 alloy showing a five-phase structure.
were detected from the binary crystalline phases, which indicated that large amount of Y atoms compensated for creating the f.c.i.-quasi-crystalline phase. In order to clarify the microstructural features of the Zn60Mg30Y10 quasi-crystal alloy, TEM observations were made after annealing and various periods of aging. Fig. 5 shows a bright field image of the f.c.i.-quasi-crystalline phase in the as-annealed alloy sample. Fine dispersed precipitates of about 10 nm diameter were detected in the f.c.i.quasi-crystalline phase in the as-annealed Zn60Mg30Y10 alloy. This kind of continuous precipitation implied that the relatively stable f.c.i.-phase had decomposed after annealing. Kikuchi lines were produced from the face-centered icosahedral type Zn–Mg–Y quasi-crystalline phase using convergent beam electron diffraction (CBED), shown in Fig. 6(a). The Kikuchi bands were symmetrical and the camera length is 30 cm. The selected area diffraction (SAD) patterns from the f.c.i.-quasi-crystalline phase were taken along the tracks of the Kikuchi bands, with the incident beams parallel to the 5-fold, 3-fold and the 2-fold axes, respectively, the typical SAD patterns with three symmetries of 5-fold, 3-fold and 2-fold being shown in Fig. 6(b). It was noticed that the diffraction spots were sharp, and the distortion in the spots was clear with small innermost spots observed for all major zone axes. This suggested that the distortion of the spots originated from phason strain in this quasi-crystal sample, with the phason strains being created during the growth process of a quasi-crystal grain [10,11]. Based on the SAD patterns of the three symmetric folds
Fig. 4. (a) A relative SEM photograph; and (b) the distribution profile of elements Zn, Mg and Y in linescans analysis for Zn60Mg30Y10 sample.
(5-fold, 3-fold and 2-fold) along with the Kikuchi diffraction patterns, a structure model of the f.c.i.-quasi-crystalline phase was proposed, as shown in Fig. 6(c). The same symmetrical diffraction patterns were observed in the over-aged samples.
Table 1 Analyzed compositions of the five phases in the quasi-crystal alloy Phases
Analyzed compositions (at.%)
Zn6Mg3Y MgZn MgZn2 Mg2Zn3 Mg7Zn3
Zn64Mg28Y8 Mg49Zn51 Mg34Zn66 Mg40Zn60 Mg71Zn29
Fig. 5. The quasi-crystalline phase with fine dispersed particle (precipitation) morphology in the as-annealed stage.
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Fig. 6. (a) CBED patterns; (b) SAD patterns; and (c) an icosahedral unit cell in the cubic reference frame of a ZnMgY as-annealed quasi-crystalline alloy, taken along the 3-fold (3-f), 5-fold (5-f), 2-fold (2-f) and pseudo-2-fold (p2-f) axes. The solid lines and the dashed lines in (a) and (b) are in correspondence with that in (c).
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3.2. Aging characteristics of quasi-crystal Zn60Mg30Y10 alloy 3.2.1. Precipitation inside the f.c.i.-quasi-crystalline phase After aging at 180 8C for 30 h, the continuous precipitation became visible in SEM micrographs. Nearly spherical fine precipitates of about 0.3 mm diameter were dispersed in the f.c.i.-phase, as shown in Fig. 7. This indicated that the precipitates had grown from the 10 nm of the as-annealed state to about 0.3 mm after 30 h aging at 180 8C. In previous studies, it was reported that the f.c.i.-phase was stable [3,4]. Therefore, the decomposition of the f.c.i.-phase during aging would be an interesting topic for further investigation. 3.2.2. Decomposition of crystalline MgZn2 phase The crystalline MgZn2 phase is one of the co-existing phases with the f.c.i.-phase in the as-annealed Zn60Mg30Y10 alloy, as shown in Fig. 3. The detailed microstructure of the MgZn2 phase in the as-annealed Zn60Mg30Y10 alloy is shown in Fig. 8(a). As can be seen in this SEM micrograph, needle-like shaped transitional precipitates grow in two apparent directions to form rhomboid networks with a cross-angle of 1358. This was a typical structure of the
Fig. 7. Dispersion of precipitates in the quasi-crystalline phase of the aged stage at 180 8C for 30 h.
transitional phase, which appeared during diffusional decomposition of the supersaturated phases in the Al-base and Zn-Al base alloys [12]. The precipitates at the asannealed state were very fine. They were approximately 0.5–1.5 mm long, of 20–50 nm diameter, with the interspacing between precipitates being about 0.3–0.8 mm. The
Fig. 8. SEM micrographs of transitional precipitates in the MgZn2 phase: (a) as-solidified stage; (b) aged at 180 8C for 20 h; (c) aged at 180 8C for 35 h followed by 250 8C for 15 min; (d) aged at 180 8C for 35 h followed by 250 8C for 8 h; (e) aged at 180 8C for 35 h followed by 250 8C for 216 h; and (f) aged at 180 8C for 35 h followed by 250 8C for 400 h.
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directional precipitate networks was one of the reasons for the high hardness of the quasi-crystal alloy. The transitional precipitates grew during aging. Fig. 8(b)– (f) are the back-scattered SEM micrographs of the transitional phase after various periods of aging at 180 and 250 8C. After aging at 180 8C for 20 h, the precipitates had apparently grown and coarsened, as shown in Fig. 8(b). After aging at 180 8C for 35 h followed by 250 8C for 15 min, precipitates with one orientation started to disappear, as shown in Fig. 8(c). After aging at 180 8C for 35 h followed by 250 8C for 8 h, the precipitates diminished considerably, accordingly the inter-spacing further increasing, as shown in Fig. 8(d). With prolonged aging at 250 8C for up to 216 and 400 h, the networks of the precipitates completely disappeared, as shown in Fig. 8(e) and (f). The transitional precipitates had transformed to relatively stable particles in the prolonged over-aged stage. Accompanying to the microstructural change of the crystalline MgZn2 phase observed by SEM, the X-ray diffraction of the phase decreased after aging at 150 8C for 10.5 h, as shown in Fig. 1(b). After aging at 180 8C for 35 h followed by 250 8C for 400 h, the X-ray diffraction of the MgZn2 phase diminished considerably, as shown in Fig. 1(c).
4. Conclusions Face-centered icosahedral (f.c.i.)-quasi-crystalline phase was the main phase in the annealed Zn60Mg30Y10 alloy. Typical structures of the f.c.i.-quasi-crystalline phase were characterized as being of three symmetries: 5-fold, 3-fold and 2-fold; based on which a structure model of this complex phase was proposed. Fine dispersed continuous precipitates formed in the relatively quasi-crystalline phase after annealing and aging. Small amount of the four binary crystalline phases MgZn, MgZn2, Mg2Zn3 and Mg7Zn3 phases were
also observed in the annealed Zn60Mg30Y10 alloy. The crystalline phase (MgZn2) decomposed during aging.
Acknowledgements The authors gratefully acknowledge Mr. Roland Sterzel for providing the samples used in this study. Furthermore, we thank Mr. T.K. Cheung, Mr. W.S. Lee and Ms. B. Leticia for their help in the experimental work and Professor Dong Chuang for helpful discussions. Finally, we thank The Research Committee of The Hong Kong Polytechnic University for financial support (Project No. GV905). References [1] D. Shechtman, I. Blech, D. Gratias, J.W. Cahn, Phys. Rev. Lett. 53 (1984) 1951. [2] Z. Shen, P.J. Pinhero, T.A. Lograsso, D.W. Delaney, C.J. Jenks, A.P. Thei, Surf. Sci. 385 (1997) L923. [3] Z. Lou, S. Zhang, Y. Tang, D. Zhao, Scripta Metall. 28 (1993) 1513. [4] A. Nikura, A.P. Tsai, A. Inoue, T. Masumoto, Phil. Mag. Lett. 69 (1994) 351. [5] E.M. Padezhnova, E.V. Melnik, R.A. Miliyevskiy, T.V. Dobatikina, V.V. Kinzhibalo, Russ. Metall. (English Translation) 3 (1982) 185. [6] A.P. Tsai, A. Nikkura, A. Inoue, T. Masumoto, in: J. Christian, M. Remy (Eds.), Proceedings of the Fifth International Conference on Quasicrystals, World Scientific, Singapore, 1995, p. 628. [7] F.S. William, Principles of Materials Science and Engineering, McGraw-Hill, New York, p. 439. [8] A.P. Tsai, in: A.I. Goldman, D.J. Sordelet, A.P. Tsai, J.M. Dubois (Eds.), New Horizons in Quasicrystals Research and Applications, Singapore, 1996, p. 1. [9] B. Charrier, D. Schmitt, J. Magn. Magn. Mater. 171 (1997) 106. [10] H.S. Chen, A.R. Kortan, J.M. Parsey, Phys. Rev. B 38 (1988) 1654. [11] A. Nikkura, A.P. Tsai, A. Inoue, T. Masumoto, Jpn. J. Appl. Phys. 33 (1994) 1538. [12] Y.H. Zhu, S.J. Murphy, Mater. Sci. Technol. 3 (1987) 261.
Microstructure and phase decomposition of quasi-crystal Zn60Mg30Y10 alloy Y.L. Cheung, K.C. Chan*, Y.H. Zhu Department of Manufacturing Engineering, The Hong Kong Polytechnic University, Yuk Choi Road, Hung Hom, Hong Kong, Hong Kong Received 31 October 2001
Abstract The microstructure and aging characteristics of a quasi-crystal Zn60Mg30Y10 alloy were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Typical structures of the icosahedral quasi-crystalline phase were characterized by three symmetries: 5-fold, 3-fold and 2-fold, based on which a structure model of this complex quasi-crystalline phase was proposed. Fine-dispersed precipitates were found inside the relatively stable quasi-crystalline phase during aging. Decomposition of the crystalline phase (MgZn2) was also investigated. # 2002 Published by Elsevier Science B.V. Keywords: Microstructure; Phase decomposition; Quasi-crystal Zn60Mg30Y10 alloy
1. Introduction Quasi-crystals were discovered in 1982 by Shechtman et al. [1] and first reported in 1984, changing the long-held belief that solid matter exists in only two states: amorphous and crystalline. In an amorphous substance, atoms exist in a random, disordered manner, whilst in crystalline substances, atoms form specific geometrical patterns that are repeated periodically in the material [2]. Quasi-crystals fall outside of these groups, containing atoms that are arranged in long range ordered manner and exhibit non-crystallographic symmetries (5, 8, 10 and 12-folds) but not a periodic structure. At first, quasi-crystals were found only in Al-transition metal alloys. Luo et al. [3] showed the existence of a stable icosahedral quasi-crystalline phase (i-phase) in the system Zn–Mg–RE alloys (RE: rare earth element or Y). A group of stable icosahedral quasi-crystals was identified by Nikura et al. [4] in Zn–Mg–RE with sharp diffraction peaks. This iphase was identified with an earlier mentioned Z-phase Zn6Mg3Y (Padezhnova et al. [5]), the structure of which could not be determined at that time. The ideal composition of i-phase was very close to Zn6Mg3Y [6]. Apparently, the present i-phase also obeyed the Hume–Rothery rule [7]. Structurally, a small amount (10 at.%) of RE atoms sitting in a perfect icosahedral lattice implies that the RE element should not be ignored [8]. However the structure model of * Corresponding author. Tel.: þ852-2766-4891; fax: þ852-2362-5267. E-mail address: [email protected] (K.C. Chan).
0924-0136/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 0 6 9 - 9
this complex quasi-crystalline phase has been not clear. The existence of the local magnetic moments of the rare earth ions in the i-phase should lead to interesting magnetic properties. It is expected to create new magnetic behavior because of the particular crystallographic environment surrounding the rare earths in the quasi-crystalline structure [9]. Therefore, further study on the microstructure and aging characteristics of the Zn–Mg–RE alloys is of significance, both theoretically and in respect of practical application. In this paper, the microstructures and phase decomposition of cast-annealed samples of a quasi-crystal Zn–Mg–Y alloy were studied during isothermal holding.
2. Experiment A Zn–Mg–Y alloy with a nominal composition of Zn60Mg30Y10 (at.%) was provided by ‘‘Deutsche Forschungsgemeinschaft’’ from the Germany Ministry of Science. The alloy was prepared from Zn (99.99%), Mg (99.99%) and Y (99.99%). The quasi-crystalline material was prepared by melting and pre-reacting the materials at 850 8C for 1 h in a Al2O3 crucible, which was sealed in a quartz tube under an argon atmosphere due to the high vapor pressure of Zn and Mg. A vertical-cylinder resistance furnace with air-atmosphere was used, the sample hangs on a wire. After pre-reacting, the sample was dropped into cold water to obtain a stable quasi-crystalline phase. Then the sample was annealed at 550 8C for 7 days in a normal resistance furnace.
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Phase identification and microstructure examination of the sample was carried out after various periods of aging at 150, 180 and 250 8C using X-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The XRD examination was performed on a diffractometer with nickel filtered Cu Ka radiation, scanning at a speed of 18/min, the characteristic peaks of the X-ray diffraction being collected within a diffraction angle (2y) range from 358 to 478. Microstructural analysis of the sample was performed with a Cambridge Stereoscan 360 scanning electron microscope equipped for composition analysis with energy dispersive X-ray spectroscopy (EDX) apparatus. Thin film samples for TEM examination were prepared by mechanical polishing and ion beam milling. The TEM investigation was carried out in a Jeol 2000 FX transmission electron microscope operating at 120 kV.
3. Results and discussion 3.1. Microstructure of the quasi-crystal Zn60Mg30Y10 alloy X-ray diffractograms of the as-annealed and aged Zn60Mg30Y10 quasi-crystal alloy samples are shown in Fig. 1. Three phases were observed in both as-annealed and aged samples. The main phase was identified as a facecentered icosahedral phase (f.c.i.-phase or simply i-phase), which was indexed in a reported metallurgical investigation indicated in Fig. 1 [5]. The intense diffraction peaks of the iphase were indexed according to a set of i-phase indices from Ref. [5]. The other crystalline phases MgZn2 and MgZn were identified as indicated in Fig. 1. Fig. 2 shows a back-scattered SEM micrograph of the asannealed quasi-crystal alloy Zn60Mg30Y10. The grain size of the quasi-crystalline phase has a large variation, ranging from about 5–100 mm. The as-annealed Zn60Mg30Y10 alloy consisted of five-phases: one quasi-crystalline phase (Zn6Mg3Y) and four binary phases (MgZn, MgZn2, Mg2Zn3 and Mg7Zn3), as shown in Fig. 3. Both the XRD and SEM results confirmed that the light imaged quasi-crystalline phase was the main phase in the alloy. The average chemical composition of the light imaged i-phase was Zn64Mg28Y8 (at.%). The icosahedral quasi-crystalline phase had a stoichiometric composition close to Zn6Mg3Y, as reported by Tsai et al. [6]. The volume fraction of this quasi-crystalline phase was about 90%. The Mg34Zn66 phase (at.%) corresponded to the MgZn2 phase (light gray region). The Mg49Zn51 phase (dark gray imaged region), the Mg40Zn60 (gray imaged region) and the Mg71Zn29 (black region) (at.%) corresponded to the crystalline MgZn phase, the Mg2Zn3 phase and Mg7Zn3 phase, as shown in Table 1. The Mg7Zn3 phase was shown to decompose upon eutectoid reaction, giving rise to a very fine lamellar structure with an average inter-lamellar spacing of 0.2 mm. Only three phases: the icosahedral quasi-crystalline phase Zn6Mg3Y, and the MgZn2 and MgZn phases, were identified
Fig. 1. X-ray diffraction pattern of the face-centered icosahedral phase of Zn60Mg30Y10 alloy: (a) the as-solidified state; (b) the aged state (held at 150 8C for 10.5 h); (c) the over-aged state (held at 180 8C for 35 h followed by 250 8C for 400 h); and (d) vertical bars represent the position and intensity of diffraction peaks in the Zn6Mg3Y-quasi-crystal.
in the Zn60Mg30Y10 alloy using XRD, as shown in Fig. 1(a). This was probably because of either the small volume fraction or the decomposition of the other two undetected phases. Distribution profiles of three elements Zn, Mg and Y in the various phases in the alloy were produced using SEM X-ray line scanning, as shown in Fig. 4. Very small amounts of Y
Fig. 2. Back-scattered SEM micrograph of the as-annealed quasi-crystal alloy Zn60Mg30Y10.
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Fig. 3. As-annealed Zn60Mg30Y10 alloy showing a five-phase structure.
were detected from the binary crystalline phases, which indicated that large amount of Y atoms compensated for creating the f.c.i.-quasi-crystalline phase. In order to clarify the microstructural features of the Zn60Mg30Y10 quasi-crystal alloy, TEM observations were made after annealing and various periods of aging. Fig. 5 shows a bright field image of the f.c.i.-quasi-crystalline phase in the as-annealed alloy sample. Fine dispersed precipitates of about 10 nm diameter were detected in the f.c.i.quasi-crystalline phase in the as-annealed Zn60Mg30Y10 alloy. This kind of continuous precipitation implied that the relatively stable f.c.i.-phase had decomposed after annealing. Kikuchi lines were produced from the face-centered icosahedral type Zn–Mg–Y quasi-crystalline phase using convergent beam electron diffraction (CBED), shown in Fig. 6(a). The Kikuchi bands were symmetrical and the camera length is 30 cm. The selected area diffraction (SAD) patterns from the f.c.i.-quasi-crystalline phase were taken along the tracks of the Kikuchi bands, with the incident beams parallel to the 5-fold, 3-fold and the 2-fold axes, respectively, the typical SAD patterns with three symmetries of 5-fold, 3-fold and 2-fold being shown in Fig. 6(b). It was noticed that the diffraction spots were sharp, and the distortion in the spots was clear with small innermost spots observed for all major zone axes. This suggested that the distortion of the spots originated from phason strain in this quasi-crystal sample, with the phason strains being created during the growth process of a quasi-crystal grain [10,11]. Based on the SAD patterns of the three symmetric folds
Fig. 4. (a) A relative SEM photograph; and (b) the distribution profile of elements Zn, Mg and Y in linescans analysis for Zn60Mg30Y10 sample.
(5-fold, 3-fold and 2-fold) along with the Kikuchi diffraction patterns, a structure model of the f.c.i.-quasi-crystalline phase was proposed, as shown in Fig. 6(c). The same symmetrical diffraction patterns were observed in the over-aged samples.
Table 1 Analyzed compositions of the five phases in the quasi-crystal alloy Phases
Analyzed compositions (at.%)
Zn6Mg3Y MgZn MgZn2 Mg2Zn3 Mg7Zn3
Zn64Mg28Y8 Mg49Zn51 Mg34Zn66 Mg40Zn60 Mg71Zn29
Fig. 5. The quasi-crystalline phase with fine dispersed particle (precipitation) morphology in the as-annealed stage.
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Fig. 6. (a) CBED patterns; (b) SAD patterns; and (c) an icosahedral unit cell in the cubic reference frame of a ZnMgY as-annealed quasi-crystalline alloy, taken along the 3-fold (3-f), 5-fold (5-f), 2-fold (2-f) and pseudo-2-fold (p2-f) axes. The solid lines and the dashed lines in (a) and (b) are in correspondence with that in (c).
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3.2. Aging characteristics of quasi-crystal Zn60Mg30Y10 alloy 3.2.1. Precipitation inside the f.c.i.-quasi-crystalline phase After aging at 180 8C for 30 h, the continuous precipitation became visible in SEM micrographs. Nearly spherical fine precipitates of about 0.3 mm diameter were dispersed in the f.c.i.-phase, as shown in Fig. 7. This indicated that the precipitates had grown from the 10 nm of the as-annealed state to about 0.3 mm after 30 h aging at 180 8C. In previous studies, it was reported that the f.c.i.-phase was stable [3,4]. Therefore, the decomposition of the f.c.i.-phase during aging would be an interesting topic for further investigation. 3.2.2. Decomposition of crystalline MgZn2 phase The crystalline MgZn2 phase is one of the co-existing phases with the f.c.i.-phase in the as-annealed Zn60Mg30Y10 alloy, as shown in Fig. 3. The detailed microstructure of the MgZn2 phase in the as-annealed Zn60Mg30Y10 alloy is shown in Fig. 8(a). As can be seen in this SEM micrograph, needle-like shaped transitional precipitates grow in two apparent directions to form rhomboid networks with a cross-angle of 1358. This was a typical structure of the
Fig. 7. Dispersion of precipitates in the quasi-crystalline phase of the aged stage at 180 8C for 30 h.
transitional phase, which appeared during diffusional decomposition of the supersaturated phases in the Al-base and Zn-Al base alloys [12]. The precipitates at the asannealed state were very fine. They were approximately 0.5–1.5 mm long, of 20–50 nm diameter, with the interspacing between precipitates being about 0.3–0.8 mm. The
Fig. 8. SEM micrographs of transitional precipitates in the MgZn2 phase: (a) as-solidified stage; (b) aged at 180 8C for 20 h; (c) aged at 180 8C for 35 h followed by 250 8C for 15 min; (d) aged at 180 8C for 35 h followed by 250 8C for 8 h; (e) aged at 180 8C for 35 h followed by 250 8C for 216 h; and (f) aged at 180 8C for 35 h followed by 250 8C for 400 h.
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directional precipitate networks was one of the reasons for the high hardness of the quasi-crystal alloy. The transitional precipitates grew during aging. Fig. 8(b)– (f) are the back-scattered SEM micrographs of the transitional phase after various periods of aging at 180 and 250 8C. After aging at 180 8C for 20 h, the precipitates had apparently grown and coarsened, as shown in Fig. 8(b). After aging at 180 8C for 35 h followed by 250 8C for 15 min, precipitates with one orientation started to disappear, as shown in Fig. 8(c). After aging at 180 8C for 35 h followed by 250 8C for 8 h, the precipitates diminished considerably, accordingly the inter-spacing further increasing, as shown in Fig. 8(d). With prolonged aging at 250 8C for up to 216 and 400 h, the networks of the precipitates completely disappeared, as shown in Fig. 8(e) and (f). The transitional precipitates had transformed to relatively stable particles in the prolonged over-aged stage. Accompanying to the microstructural change of the crystalline MgZn2 phase observed by SEM, the X-ray diffraction of the phase decreased after aging at 150 8C for 10.5 h, as shown in Fig. 1(b). After aging at 180 8C for 35 h followed by 250 8C for 400 h, the X-ray diffraction of the MgZn2 phase diminished considerably, as shown in Fig. 1(c).
4. Conclusions Face-centered icosahedral (f.c.i.)-quasi-crystalline phase was the main phase in the annealed Zn60Mg30Y10 alloy. Typical structures of the f.c.i.-quasi-crystalline phase were characterized as being of three symmetries: 5-fold, 3-fold and 2-fold; based on which a structure model of this complex phase was proposed. Fine dispersed continuous precipitates formed in the relatively quasi-crystalline phase after annealing and aging. Small amount of the four binary crystalline phases MgZn, MgZn2, Mg2Zn3 and Mg7Zn3 phases were
also observed in the annealed Zn60Mg30Y10 alloy. The crystalline phase (MgZn2) decomposed during aging.
Acknowledgements The authors gratefully acknowledge Mr. Roland Sterzel for providing the samples used in this study. Furthermore, we thank Mr. T.K. Cheung, Mr. W.S. Lee and Ms. B. Leticia for their help in the experimental work and Professor Dong Chuang for helpful discussions. Finally, we thank The Research Committee of The Hong Kong Polytechnic University for financial support (Project No. GV905). References [1] D. Shechtman, I. Blech, D. Gratias, J.W. Cahn, Phys. Rev. Lett. 53 (1984) 1951. [2] Z. Shen, P.J. Pinhero, T.A. Lograsso, D.W. Delaney, C.J. Jenks, A.P. Thei, Surf. Sci. 385 (1997) L923. [3] Z. Lou, S. Zhang, Y. Tang, D. Zhao, Scripta Metall. 28 (1993) 1513. [4] A. Nikura, A.P. Tsai, A. Inoue, T. Masumoto, Phil. Mag. Lett. 69 (1994) 351. [5] E.M. Padezhnova, E.V. Melnik, R.A. Miliyevskiy, T.V. Dobatikina, V.V. Kinzhibalo, Russ. Metall. (English Translation) 3 (1982) 185. [6] A.P. Tsai, A. Nikkura, A. Inoue, T. Masumoto, in: J. Christian, M. Remy (Eds.), Proceedings of the Fifth International Conference on Quasicrystals, World Scientific, Singapore, 1995, p. 628. [7] F.S. William, Principles of Materials Science and Engineering, McGraw-Hill, New York, p. 439. [8] A.P. Tsai, in: A.I. Goldman, D.J. Sordelet, A.P. Tsai, J.M. Dubois (Eds.), New Horizons in Quasicrystals Research and Applications, Singapore, 1996, p. 1. [9] B. Charrier, D. Schmitt, J. Magn. Magn. Mater. 171 (1997) 106. [10] H.S. Chen, A.R. Kortan, J.M. Parsey, Phys. Rev. B 38 (1988) 1654. [11] A. Nikkura, A.P. Tsai, A. Inoue, T. Masumoto, Jpn. J. Appl. Phys. 33 (1994) 1538. [12] Y.H. Zhu, S.J. Murphy, Mater. Sci. Technol. 3 (1987) 261.