Icosahedral phase formation in rapidly quenched aluminum-ruthenium alloys
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KannanM. Krishnan National Centerfor Electron Microscopy, Lawrence Berkeley Laboratory, Berkeley, California 94720 (Received 6 April 1987; accepted 8 February 1988) Systematic rapid quenching experiments on the icosahedral phase-forming system A\l __ x Ru x were performed for x < 20 at. %. The solidified alloys have been studied by electron microscopy and x-ray diffraction to determine their composition, constituent phases, and phase morphology. It has been determined that the icosahedral phase must form directly from the liquid and, at these quench rates, is always found in the presence of second or third phases. The results have been summarized in a metastable phase diagram appropriate for rapid solidification of Alj _ x Ru x at piston and anvil quench rates. This metastable phase diagram describes the results for Ru concentrations less than Hat. %.
I. INTRODUCTION A novel material, containing precipitates displaying icosahedral point group symmetry in electron diffraction, along with quasiperiodic translational order, has recently been formed by rapid solidification.' There is a great deal of interest in studying this icosahedral phase to determine its atomic structure and physical properties.2 Naturally one is also interested in understanding and controlling the mechanisms responsible for the formation and growth of the icosahedral phase. With this in mind, we have performed rapid solidification experiments on one icosahedral phase-forming alloy system, with the intention of determining its endproduct phases as a function of composition. We have chosen the aluminum-ruthenium alloy system because of the relative simplicity of its equilibrium phase diagram in the icosahedral phase-forming region. The Al, x Ru x system has only three equilibrium phases for A ; < 2 5 at. % Ru, namely, Al13Ru4, Al6Ru, and the fee Al-Ru solid solution.3 In Ref. 3 it was also found that nucleation and growth of the Al 6 Ru phase was kinetically bypassed even at cooling rates of less than 20 K/s. Because of this, icosahedral phase formation proves to be relatively easy in the broad regime 2.4
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