Solidification of Al 65 Cu 20 Co 15 and Al 65 Cu 15 Co 20 alloys
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means of scanning electron microscopy, energy-dispersive x-ray analysis, powder x-ray diffraction, and differential thermal analysis. Slowly cooled as well as rapidly cooled ingots of both alloys contained a decagonal quasicrystalline phase as dominant phase with additions of several minor normal-crystalline phases. The decagonal phase was found to solidify in a temperature range limited by two peritectic reactions involving phases of CsCl-type structure. Formation of AlCu and Al2Cu phases was also observed.
I. INTRODUCTION Recently it was reported that a thermodynamically stable decagonal quasicrystalline phase exists in alloys of Al65Cu2oCo15 and Al65Cu15Co2o composition.1"3 This phase was found in slowly cooled ingots where it had solidified in the form of decaprismatic grains of some millimeters in length with the long axis parallel to the periodic direction of the decagonal quasicrystalline structure. Since the discovery of quasicrystalline phases the growth of large single-quasicrystals has been considered an important goal of metallurgy. If quasicrystalline states do indeed occur as equilibrium phases, this could in principle be achieved by slow solidification of a liquid alloy of proper composition. Indeed, a coarsegrained structure with grain diameters of up to 4 mm was obtained by a Bridgman-type technique.2 The decagonal phase was reported for Al 65 Cu 20 Co 15 and Al65Cu15Co2o alloys. However, these compositions do not necessarily correspond to single-phase alloys. Indeed, it was reported that the decagonal phase melts incongruently at about 1000 °C. 4 Resolidification of an incongruently melted phase can result in significant deviations from the initial composition and, as a consequence, the formation of additional phases. Also the decagonal phase could be stable in a compositional range which, in principle, may lie outside the nominal alloy composition. Indeed, further work is required that allows a better understanding of alloy solidification behavior and phase evolution. For instance, it was concluded5 that, under equilibrium conditions, the decagonal phase should be unstable below a certain minimum temperature. Moreover, it was reported that the decagonal phase could not be found in the pure ternary Al65Cui5Co2o alloy and that Si addition was necessary for its formation.6 Our knowledge of the ternary A l - C u - C o phase diagram is incomplete, especially in the relevant composition range. A single isothermal section for 600 °C J. Mater. Res., Vol. 6, No. 12, Dec 1991
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and some suggestions concerning the 800 °C section were published in Ref. 7. There the compositions of the decagonal phase reported in the recent literature fall in the range of existence of a phase with unknown structure denoted "X-phase". At 600 °C this phase covers a region of about 3 to 4 at. % in radius around the composition of Al 6 3Cui 9 Co 18 . Although this phase diagram provides us with some insights into possible phase evolution, an equilibrium phase diagram at 600
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