Solidification sequence of the icosahedral quasicrystal forming Al-Cu-Fe alloys

The solidification sequence of the icosahedral quasicrystal forming Al-Cu-Fe alloys was investigated as functions of cooling rate during solidification, compositional variation, and additional Si amount. Annealing treatment was also carried out to obtain the single icosahedral quasicrystalline phase. Two different formation mechanisms of the icosahedral quasicrystalline phase can be suggested as a function of cooling rate. For a moderate cooling rate regime, the icosahedral phase is formed by a peritectic reaction, which is consistent with the previous reports. For a high cooling rate regime, where large undercooling of the liquid is obtainable, the i-phase (icosahedral phase) is formed directly from the undercooled melt, without the formation of the primary λ or τ phases. The τ phase was formed at the inter-dendritic region of the i-phase grains from the remaining liquid at the final stage of solidification. As the Si element was added up to 5 at.%, the relative volume fraction of the i-phase gradually decreased in both as-cast and heat treated Al-Cu-Fe alloys. For an as-cast Al₅₀Cu₂₀Fe₁₅Si₁₅ alloy, however, the major phase was identified as an i-phase related 1/1 simple cubic approximant.