Formation and structure of new quasicrystals of Ga16Mg32Zn52 and Al60Si20Cr20

Scripta METALLURGICA

Vol. 21, pp. 527-530, 1987 Printed in the U.S.A.

Pergamon Journals, Ltd All rights reserved

FORMATION AND STRUCTURE OF NEW QUASICRYSTALS OF Ga16Mg32Zn$2 AND AI60Si20Cr20 H. S. Chen and A. lnoue*

AT&T Bell Laboratories, Murray Hill, NJ 07974 *Research Institute for Iron and Steel, Tohoku Univ. Sendai 980 Japan

(Received January 8, 1987) (Revised February 2, 1987) Since the first report of an icosahedral phase in a melt-spun Als6Mn~4 (1), the icosahedral phase has been found • in melt-spun AI-(Mn, Cr, Fe, V, W ...), A1-Si-Mn, Mg3Zn3A12, Mg4CuA16, (Ti, V)2Ni and A16Li3Cu alloys. Most of the icosahedral phases form in composition ranges where equilibrium phases are of the Frank-Kasper type in which icosahedral coordination shells dominate in the structure (2). Thus the formation of an icosahedral quasicrystal is rel;~ted to the structural similarity to the equilibrium state (3-4). We report here the formation of new quasicrystals of Ga16Mg32Zns: and A16oSi20Cr2o, and their structure as examined with TEM and x-ray diffraction. We found that the x-ray diffraction patterns are distinctively different between these two icosahedral (i-) quasicrystais. The i-GaMgZn shows the x-ray line intensities being very similar to those of i-Mg3Zn3Al 2, while the i-AlSiCr to i-A174Si6Mn20. Ga16Mg3:Zn52, A16oSi20Cr2o, Al25Mg37.sZn3~.s and Al~4Si6Mn:0 alloys were prepared by induction melting high parity elemeats in an argon atmosphere. Thin ribbons of about 1 ram width and 30 p.m thickness were produced by melt-spinning with a copper wheel - 2 0 cm diameter rotating at 2000 rpm in an enclosed argon atmosphere. The melt-spun ribbons had several electron transparent regions and consequently could be examined without further preparation. The specimens were examined with a Philips EM 420 transmission electron microscope. X-ray diffraction on powdered samples were done using a Philips APD 6300 automated diffractometer with Cu Kc~ radiation. The instrumental resolution was approximately 0.04 run-1 half width at half maximum (HWHM). The GaMgZn specimens show a typical rosette microstructure (Fig. la), which has been seen in i-Als6Mn14 (1,6). The average grain size was about 0.5p.m. The AISiCr specimens show a much finer grain size of - 2 0 0 nm (Fig. lb). The icosahedral domains have the appearance of containing fine speckles with irregular domain boundaries. The corresponding diffraction patterns (shown in Figs. lc and ld) exhibit spotty rings which can be indexed to an icosahedral phase. However, the intensity modulations of diffraction peaks are significantly different between these two alloys. In particular, the strong 222 100 peak (shown by an arrow) in the GaMgZn is nearly extinct in the AlSiCr (see also Fig. 2). Convergent beam electron diffraction (CBED) pattern taken along a 5-fold axes in the GaMgZn (Fig. le) exhibits tea-spot rings, characteristics of icosahedral crystals. Fig. 2 shows the x-ray diffraction patterns of the melt-spun GaMgZn and AlSiCr. Also shown in Fig. 2, for comparison, are those of A12~Mg~7.sZn37.s and A174Si6Mn:0. The relative peak positions Q,(= IQ:~ml/lQI), integrated intensity I and half width at half maximum (HWHM) ~Q are listed in Table 1. The calculated rhombohedral edge length a R (= 13.308/[Q21 ~ 11tl) and the average interspacing d211 lu are also shown. Here we use the indexing scheme of Elser (7). The peak positions observed are in a good agreement with the calculated values within 0.2%, except for lower Q region (Q~ > 1.5), where larger scatters ~ 0.5% are seen. The line inter~ities of the GaMgZn and A1MgZn are very similar and are distinctively different from those of the A1SiCr and AlSiMn. Besides the about 10% increase in lattice parameters as, the 222 100 and 311 111 peaks, which are barely observable in the i-Al-transition metals type alloys, are very strong in the i-GaMgZn and i-AlMgZn, while the 322 101 peak which shows up in the i-Al-transition metal alloys is now absent. Similar features are found in i-Al6Li~Cu and other ternary alloys (8). We note that the strongest peak, for all four alloys including the AlSiCr and AlSiMn, is at 221 001 instead of 211 111 as reported previously for i-AlMn binary alloys (9). Recently Knowles and Stobbs (10) demonstrated that the computed intensities of diffraction patterns from model structures of three-dimensional Penrose tilling (3DPT) are very sensitive to the atomic decoration of the underlying rhombohedral unit cells. The present results suggest that the icosahedral GaMgZn and AlSiCr have the same structure but different degrees of atomic decoration (4). It is believed that in the GaMgZn and A1MgZn system the heavy atoms Ga and Zn decorate the edges of the 3DPT, while in the AlSiCr and AlSiMn the heavy atoms Cr and

527 0036-9748/87 $3.00 + .00 Copyright (c) 1987 Pergamon Journals Ltd.

528

QUASICRYSTALS

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No.

Mn occupy the vertices of these basic units that make up the icosahedral structure. It should be emphasized that the discovery of the OaMgZn quasicrystal is of interest because it does not contain the element Al which is the major constituent of nearly all i-phase forming alloys reported, except Pd6oSi2oU2o (11) and (Ti, V)2Ni (12). However, in the PdSiU the i-phase was obtained by the thermal transformation of melt-spun glassy phase, and in the (Ti, V)2Ni as precipitates embedded in a glassy matrix. One of the authors (HSC) thanks S. Nakahara, and H. M. O'Bryan, Jr. for valuable advice. Table 1. The peak position Q,(=IQ21Lml/[O[), the integrated intensity I, and half width at hal:[ m a x i m u m AQ(10- Into-i) of icosahedral quasicrystals. The latticeparameter and the average spacing (aa, d::: m in urn) are shown in parenthesis.

Index

Q,s,I.

111 000 111 100 211 100 211 III 221 001 222 100 311 111 322 101 332 002 333 101

1.777 1.539 1.163 1.000 0.951 0.838 0.809 0.689 0.588 0.558

Ga:6Mg2zZns2 (0.5094, 0.2404)

Al2sMg37.sZn37 s (0.5188, 0.2445)

Al6oSi2oCr2o (0.4601, 0.2171)

Alv4Si6Mn2o (0.4607, 0.2174)

Q,

Or

Q,

Q,

I

AQ

1.784 8.6 4.5 1.550 2.8 5.9 . . . . . 1.000 58 2.9 0.950 100 3.3 0.838 55 5.9 0.808 19 5.0 . . . . 0.588 23 4.2 0.558 7.6 4.9

I

AQ

1.782 3.4 5.6 . . . . . . 1.001 56 3.7 0.950 100 3.3 0.839 45 4.3 0.807 --. . 0.587 31 3.3 0.556 7.0 4.9

1.778 . 1.159 1.001 0.951 . . 0.808 0.688 0.587 . .

I

AQ

12

4.2

I

1.784 13 1.540 3.4 2.1 2.5 1.169 2.1 76 2.7 1.000 75 100 3.6 0.950 100 . . . . 4.4 3.5 0.809 4.6 11 4.6 0.689 12 34 4.0 0.587 30 . . . .

AQ 2.7 4.0 3.0 1.9 1.6 4.2 3.3 2.8

REFERENCES

1. D. Shechtman, I. Bleck, D. Gratias and 3. W. Calm, Phys. Rev. Lett. 53 1951 (1984). 2. W . B . Pearson, "The Crystal Chemistry and Physics of Metals and Alloys" (New York: Wiley, 1972) p. 515. 3. V. Eiser and C. L. Henley, Phys. Rev. Lett. 55, 2883 (1985); P. Guyot and M. Audier, Phil. Mag. 52B, L15 (1985). 4. C . L . Henley and V. Eiser, Phil. Mag. 53B, L59 (1986). 5. P. Ramachandrarao and G. V. S. Sastry, Pramana 25, L225 (1985). 6. H . S . Chan, C. H. Chen, A. Inoue and J'. T. Krause, Phys. Rev. B32 1940 (1985). 7. V. Elser, Phys. Rev. B 32, 4892 (1985). 8. H . S . Chen, J. C. Phillips, P. Pillar and A. R. Kortan, unpublished. 9. P. A. Bancel, P. A. Heiney, P. W. Stephans, A. I. Feldman and P. M. Horn, Phys. Rev. Lctt. 54, 2422 (1985). 10. K . M . Knowles and W. M. Stobbs, Nature, 323, 313 (1986). 11. S. J'. Pooh, A. I. Drehman and K. R. Lawless, Phys. Rev. Latt. 55, 2324 (1985). 12. Z. Zhang, H. Q. Ye and K. H. Kuo, Phil. Mag. Lett. 52, L49 (1985).

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F;g. I. TEM micrographs of melt-spun Ga16Mg3:Zns2 (a) and Al60Si2oCr2o, (b) and corresponding diffraction patterns (c and d), and CBED pattern (e) of the GaMgZn.

QUASICRYSTALS

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Vol. 21, No,

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Fig. 2. X-ray powder diffraction patterns of the melt.spun alloys.

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