A quasicrystalline transition state in an annealed Al 65 Cu 20 Fe 15 alloy
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A quasicrystalline transition state was found in an Al65Cu2oFei5 alloy after heat treatment at 700 CC and subsequent slow cooling of samples originally containing the icosahedral quasicrystalline phase. The transition state is characterized by the simultaneous presence of two types of quasicrystalline domains. Their structure can be obtained by the introduction of one-dimensional periodicity along one of the fivefold axes of the original icosahedral quasicrystal. The two types of domains differ in their period lengths which are obtained by multiplying a fundamental length of the icosahedral structure by the Fibonacci numbers 5 and 8.
I. INTRODUCTION Quasicrystalline phases in an alloy of approximate composition Al65Cu2oFei5 were first reported by Tsai et al.1 Ebalard and Spaepen2 and Devaud-Rzepski et al.3 found that the quasilattice in three dimensions can be described on the basis of an ordered face-centered superstructure of a hypercubic lattice in six-dimensional space. Detailed experimental work revealed that the morphology and atomic structure observed after cooling the alloy from the melt to room temperature depend sensitively on the temperature program and on subsequent heat treatments.4"7 In particular, Audier et al}'9 reported reversible transitions between a microcrystalline rhombohedral phase, a modulated icosahedral structure, and a perfect icosahedral quasicrystalline phase at 550-750 °C and 750-820 °C. It has been concluded from this work that the icosahedral quasicrystalline phase represents an equilibrium phase at temperatures between about 800 CC and about 860 CC. In the present paper we report on a study carried out by electron microscopy on a quasicrystalline state in heat-treated Al65Cu2oFei5, which can be considered as a transition state between the high-temperature icosahedral quasicrystalline phase and normal-crystalline phases stable at lower temperatures. It is characterized by the simultaneous presence of quasicrystalline domains exhibiting translational periodicity along one of the fivefold axes of the original icosahedral quasicrystal. II. EXPERIMENTAL
The alloy of composition Al65Cu2oFei5 was prepared by melting the high-purity elements in an arc fura)
On leave from Beijing Laboratory of Electron Microscopy, Academia Sinica, Beijing, People's Republic of China. b) On leave from Automobile Institute, Nanjing, People's Republic of China. 366
J. Mater. Res., Vol. 6, No. 2, Feb 1991
nace under an argon atmosphere. The as-cast ingot was placed in a quartz tube, sealed under argon, and heat treated at 700 °C for 24 h. Subsequently the temperature was lowered at a nearly constant rate to room temperature within about 9 h. The annealed alloy was cut into slices from which thin-foil specimens for transmission electron microscopy were prepared by mechanical thinning and subsequent ion milling in a Gatan ion mill equipped with a nitrogen-cooled stage. The ion energy (3 keV) and current (1 mA) were kept low in order to prevent beam-induced phase transformations in the specimen. The investigat
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