On the stable Mg-Zn-Y quasicrystals
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I.
INTRODUCTION
U P to now, stable icosahedral and/or decagonal quasicrystals have been identified in AI-Cu-Li,[1.2,3] A1-Cu-Fe,t4] A1-Cu-Co, t5,61 A1-Cu-Mn, t71 A1-Mn-Pd, tSl A1-Pd-(Fe, Ru or Os),tg] and Ga-Mg-Znt1~ systems. Recently, we have reported on icosahedral quasicrystals in as-cast Mg-Zn-RE (RE = rare earth elements) alloystl21and identified the equilibrium Z phaset~3,~4] in the Mg-Zn-RE systems, which is similar to the T2 phase in the A1-Cu-Li system,tl.2,3j as a thermodynamically stable quasicrystal phase in the Mg-ZnRE ternary diagramsY 51 This article presents our further research work on the new stable Mg-Zn-Y quasicrystals.
II.
EXPERIMENTAL PROCEDURE
The experimental bulk materials were melted in an electric furnace at high temperature and slowly cooled in air. The final alloy compositions were obtained by chemical analysis in atomic percent. Powders of these alloys were prepared by milling from the bulk specimens and examined on a SIEMENS D-5000 X-ray diffractometer with a hightemperature attachment in l0 -3 Pa vacuum. The average heating rate was 10 K/min. During differential thermal analysis (DTA) with a scan rate of 20 K/min, a high-purity N 2 atmosphere was employed. Both samples before and after heating were prepared for transmission electron microscopy (TEM) analysis and were examined in H-800 and JEM 2010 transmission electron microscopes at an operating voltage of 200 kV. The computer simulation work was performed on an IBM PC* computer using Quick BA*IBM PC is a trademark of International Business Machines Corp., Armonk, NY.
SIC and FORTRAN languages.
Z.P. LUO, Research Engineer, is with the Beijing Institute of Aeronautical Materials, Beijing, and the Beijing Laboratory of Electron Microscopy, Chinese Academy of Sciences, Beijing. H.X. SUI, Postdoctoral Student, is with the Beijing Laboratory of Electron Microscopy, Chinese Academy of Sciences, Beijing 100080, China. S.Q. ZHANG, Professor, is with the Beijing Institute of Aeronautical Materials, Beijing 100095, China. Manuscript submitted December 5, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
III.
EXPERIMENTAL RESULTS AND DISCUSSION
A. Quasicrystals Figure 1 shows the powder x-ray diffraction (XRD) pattern of the Mg59.gZn356Y4.5 alloy prepared by conventional casting. It is found that most of the strong XRD peaks are from the icosahedral quasicrystal phase, while some weak peaks are from the crystal Mg7Zn3 phase, r161as indexed in Figure 1. The quasilattice constant aR is determined to be aR = 0.5203 nm according to the XRD peaks from the quasicrystal phase. This alloy has a larger amount of these stable quasicrystals than the alloy with low Zn and RE contents reported previously. El:] Figure 2 shows a high-resolution electron microscopy (HREM) micrograph of the Mg-Zn-Y quasicrystal projected along the fivefold axis. The bright dots show the clear fivefold symmetry. It can be constructed of pentagons, stars, boats, and rhombuses, and all of them form a two-dimensional Penrose tiling. It exhibits clear evidence of the prese
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