| Two different formation mechanisms of the icosahedral quasicrystalline phase in Al-Cu-Fe alloys were suggested according to the different cooling rate regimes. During conventional easting, where the cooling rate is as moderate as 1∼10²℃/s, the icosahedral phase is formed by a peritectic reaction between the λ-Al_(13)Fe⁴ primary phase and liquid. For melt spinning, where the cooling rate is as high as 10³∼10^6℃ /s, the icosahedral phase is formed by direct nucleation and grain growth from the undercooled melt. The relative amount of the icosahedral phase increased as the cooling rate decreased in conventional casting. During melt spinning, however, the relative amount of the icosahedral phase increased with an increase of the cooling rate. For an Al_(62.5)Cu_(25.5)Fe_(12) alloy, an almost single icosahedral quasicrystalline phase was obtained by conventional casting followed by a subsequent heat treatment at 750℃ for 3 hours. As the elemental Si was added up to 5 at.%, the relative volume fraction of the icosahedral phase gradually decreased in both as-cast and heat treated (Al, Si)-Cu-Fe alloys. For the as-cast Al_(50)Cu_(20)Fe_(15)Si_(15) alloy, however, the major phase was identified as an icosahedral phase related 1/1 cubic approximant. |
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