UNDERCOOLING AND SOLIDIFICATION BEHAVIOR OF MELTS OF THE QUASI-CRYSTAL-FORMING ALLOYS AL-CU-FE AND AL-CU-CO

Al-Cu-Fe, Al-Fe and Al-Cu-Co melts of different compositions were unde rcooled by containerless processing in an electromagnetic levitation f acility. The phase selection during solidification from the undercoole d melt was determined by direct measurements of the temperature change s during recalescence. Complementarily, the phase selection and micros tructure development was studied by scanning-and transmission electron microscopy (SEM, TEM) and X-ray diffraction (XRD) on the as-solidifie d samples with the undercooling and the alloy composition as experimen tal parameters. For comparison, rapidly quenched samples of the same a lloys were produced by splat-cooling and investigated by TEM and XRD. The undercooling results were analyzed within the framework of classic al 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 phase s with polytetrahedral symmetry elements (Al13Fe4 and Al5Fe2), but lar ge for the cubic phase of Al-50(CuCo)(50) with non-polytetrahedral cry stalline symmetry. These results are explained assuming of an icosahed ral short-range order that prevails in the undercooled melt and gives rise to an interfacial energy decreasing with increasing degree of pol ytetrahedral order in the solid nucleus. (C) 1998 Acta Metallurgica In c.