On the solidification of Al 62 Cu 20 Co 15 Si 3 and Al 61 Cu 19.5 Co 14.5 Si 5 alloys
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The solidification behavior of Al62Cu2oCoi5Si3 and Al 61 Cui 9 . 5 Co 145 Si5 alloys was studied by means of optical metallography, scanning and transmission electron microscopy, energy-dispersive x-ray analysis, powder x-ray diffraction, and differential thermal analysis. Slowly as well as rapidly cooled ingots of both alloys contained a decagonal quasicrystalline phase as the dominant phase with, additionally, several minor crystalline phases. The structure of the rapidly solidified Si-containing alloys was similar to that of the ternary Al65Cu2oCo15 alloy. In the slowly solidified alloys the substitution of 3 at. % Al by Si did not change the basic phase constitution. Si was only partially incorporated in the decagonal phase and a significant quantity of Si was found in elemental form. The increase of Si concentration to 5 at. % resulted in the appearance of new minor phases.
I. INTRODUCTION The discovery of a thermodynamically stable decagonal phase in A l - C u - C o alloys has stimulated extensive work on this alloy system. Following initial reports on this phase in alloys of Al65Cu2oCo15 and Al65Cui5Co2o composition,1-2 work concentrated mainly on these two alloys and an alloy of intermediate composition, i.e., AI65CU17.5C017.5.3 The decagonal phase was found in slowly cooled ingots as well as in heat-treated alloys1'2'4 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. Later it was observed that the decagonal phase was unstable below a certain minimum temperature at which it was transformed to a crystalline phase.3'4 In a recent publication it was reported that the decagonal phase was not detected at all in the pure ternary Al 65 Cui 5 Co 20 alloy and it was concluded that the addition of silicon was necessary for its formation.5 On the other hand, in an earlier study the decagonal phase was found in both Al 62 Cu 20 Coi 5 Si 3 and Al65Cu15Co2o.6 This observation implies that Si was not necessary for decagonal-phase formation. However, according to Ref. 6 an increase of the decagonal-phase grain size was induced by Si substitution. In this respect the observations of Ref. 7 are interesting, where an alloy of Al62Cui7 5Coi7 5Si3 composition was studied. It was reported that during cooling from the melt this alloy developed a decagonal structure at high temperature and transformed to a microcrystalline structure at room temperature. If the decagonal phase is indeed stable only within a certain high-temperature interval the observability of J. Mater. Res., Vol. 7, No. 10, Oct 1992
this phase at room temperature may critically depend on the cooling conditions.8 In order to obtain more insight into the evolution of the phase distribution in A l - C u - C o - S i during cooling, the solidification behavior of these alloys has to be studied in greater detail. In Ref. 9 the solidification of Si-free AI65CU20C015 and Al65Cu15Cso2o alloys was investigated. It was found that slow
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