The solidification behaviour of dilute aluminium–scandium alloys

Abstract

The solidification behaviour of dilute Sc containing Al alloys has been investigated. In binary Al–Sc alloys, Sc additions greater than the eutectic composition (0.55wt%) were found to produce a remarkable refinement in the grain size of aluminium castings, of two orders of magnitude, due to the formation of the primary Al3Sc intermetallic phase during solidification. The refinement in grain size only occurred in hypereutectic compositions and was shown to be far greater than can be achieved by conventional Al grain refiners. Grain refinement by the addition of Sc is accompanied by a change in growth morphology from dendritic, in the large unrefined grains, to fine spherical grains with a divorced eutectic appearing on the grain boundaries in the refined castings. Similar levels of refinement were observed in Al–Sc–Zr and Al–Cu–Sc alloys. In the latter, a change in the segregation behaviour of Cu was observed, from a strongly interdendritic segregation pattern to a more homogeneous distribution. The supersaturated Al–Sc solid solution can decompose via a discontinuous precipitation reaction to form coherent rod-like precipitates of the L12 Al3Sc phase.