Undercooling and solidification behaviour of melts of the quasicrystal-forming alloys Al-Cu-Fe and Al-Cu-Co

Al-Cu-Fe, Al-Fe and Al-Cu-Co melts of different compositions were undercooled by containerless processing in an electromagnetic levitation facility. The phase selection during solidification from the undercooled melt was determined by direct measurements of the temperature changes during recalescence. Complementarily, the phase selection and microstructure development was studied by scanning-and transmission electron microscopy (SEM, TEM) and X-ray diffraction (XRD) on the as-solidified samples with the undercooling and the alloy composition as experimental parameters. For comparison, rapidly quenched samples of the same alloys were produced by splat-cooling and investigated by TEM and XRD. The undercooling results were analyzed within the framework of classical nucleation theory. The activation threshold for the nucleation was found to be small for the icosahedral quasicrystalline phase in Al-Cu-Fe, medium for the decagonal D-phase in Al-Cu-Co and crystalline phases with polytetrahedral symmetry elements (Al13Fe4 and Al5Fe2), but large for the cubic phase of Al-50(CuCo)(50) with non-polytetrahedral crystalline symmetry. These results are explained assuming of an icosahedral short-range order that prevails in the undercooled melt and gives rise to an interfacial energy decreasing with increasing degree of polytetrahedral order in the solid nucleus. (C) 1998 Acta Metallurgica Inc.