In situ transformation behavior of icosahedral and decagonal quasicrystalline phases
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I. INTRODUCTION There has been an intense research activity concerning the formation and characterization of quasicrystalline phases during the last two years.l>2 This followed the revolutionary discovery by Shechtman et al? of an icosahedral phase in a rapidly solidified Al-14 at. % Mn alloy and a decagonal phase by Chattopadhyay et al.4 Although these icosahedral (fivefold) and decagonal (tenfold) quasicrystalline phases have so far been detected in over 40 alloy systems,5 only a limited number of investigations have been carried out to understand their decomposition behavior.6'7 Even among these, transmission electron microscopic investigations have been very few and in situ transformation studies have been still limited.8 In this article we report results of the hot-stage electron microscopic investigations on the transformation behavior of icosahedral Mg 32 (Al,Zn) 49 and decagonal Al-Co phases.
wt. % Co alloys. The alloys were rapidly solidified using the technique of melt spinning. The resultant ribbons were thin enough for direct observation in the transmission electron microscope without further thinning. A
An icosahedral phase was observed in rapidly solidified Mg 32 (Al,Zn) 49 . 910 It was reported that the asquenched structure showed nodular morphology and that each nodule contained sectors that showed different diffraction conditions. The formation of a decagonal phase was reported in a rapidly solidified Al-26 wt. % Co alloy" through transmission electron microscopy techniques. Dunlap and Dini12 identified this phase as an icosahedral phase through x-ray diffraction techniques, but electron microscopic investigations confirm this as decagonal. A rosette morphology was reported for the decagonal phase, and its cobalt content was determined through microanalysis as 35 wt. % (22.5 at. % ) . "
II. EXPERIMENTAL PROCEDURES The alloys were prepared from high-purity constituent elements by melting them in an argon arc melting furnace to produce Mg 32 (Al,Zn) 49 and Al-35 34
J. Mater. Res. 3(1), Jan/Feb 1988
http://journals.cambridge.org
FIG. 1. (a) Bright-field electron micrograph of the icosahedral phase in rapidly solidified Mg 32 (Al,Zn) 49 . (b) Fivefold diffraction pattern from the above. The diffuse intensity in between the most intense spots may be noticed.
0003-6951 /88/010034-06S01.75
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© 1988 Materials Research Society
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Suryanarayana, Chandra, and Menon: Quasicrystalline phases
JEOL JEM-200 CX electron microscope operating at 200 kV and fitted with a goniometer stage, and a hot stage was used to characterize the melt-spun foils. A W 5%Re/W-26%Re thermocouple was used to measure the temperature during in situ heating, and a heating rate of about 5 K s~' was employed.
III. RESULTS The nodular morphology of the icosahedral Mg 32 (Al,Zn) 49 quasicrystal has been confirmed in the present investigation [Fig. l ( a ) ] . However, it was noticed that the whole nodule lighted up uniformly under dark-field conditions suggesting that the entire nodul
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