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The quasicrystalline transformation in the AlCr system D. A. Lilienfeld, M. Nastasi, H. H. Johnson, D. G. Ast, and J. W. Mayer Department ofMaterials Science, Cornell University, Ithaca, New York 14853 (Received 1 October 1985; accepted 17 February 1986) Amorphous Al go Cr 20 films were made by coevaporation and by room temperature ion irradiation of the coevaporated films. The amorphous phase was transformed into the quasicrystalline state through two routes: thermal and ion beam assisted anneal. The intensity of the quasicrystalline electron diffraction pattern increases continuously within the annealing temperature range from 547° to 607 °C. The starting state of the films (as-deposited or ionirradiated codeposited) had no effect on the thermal transformation to the quasicrystalline state. Ion irradiation of the amorphous phase at 200 °C produces a more complete set of icosahedral diffraction lines. Icosahedral AlCr has the same reciprocal lattice spacings as icosahedral AlMn.

I. INTRODUCTION

II. EXPERIMENTAL METHOD

The quasicrystalline state of matter has been studied intensively in the last year. This state is characterized by long-range icosahedral order and was first reported by Shechtman etal.1 and has since been the topic of many experimental studies.2"9 In most of these studies the quasicrystalline state was prepared by the rapid quenching from the liquid state using splat cooling1"7 or by a scanned electron beam.7 Recently ion beam techniques have been used by several research groups to create the quasicrystalline state in the AlMn system.8"10 At least six binary systems are known to transform into states that appear to have icosahedral symmetry.3'4 The AlMn system has been concentrated on since the first reports of the formation of quasicrystalline material. Production of several different quasicrystalline systems by the same technique would help to define their similarities or differences in composition and structure. Thus it is of interest to extend the ion irradiation studies to other systems. The AlCr system is a good candidate for the next system for an ion irradiation study because Cr is a first row neighbor in the periodic table, possesses complex phases in the icosahedral composition range, has a similar atomic volume to Mn, and is known to transform to the quasicrystalline state. We report the observation of the formation of the quasicrystalline state in the AlCr system by a solid-state polymorphic transformation from the amorphous phase by either thermal annealing or ion beam irradiation. Although both processing techniques are capable of producing this transformation, only the ion irradiation-induced transformation generates a nearly complete set of icosahedral powder diffraction lines. The diffraction lines for the AlCr system occur at the same reciprocal lattice values as in the AlMn system suggesting that both phases possess the same structure.

Codeposited amorphous Al 80 Cr 20 films were deposited on NaCl substrates in a cryopumped vacuum system using dual e-beam evaporation. The pres