THE EFFECT OF SCANDIUM ON THE SOLIDIFICATION BEHAVIOUR OF ALUMINIUM ALLOYS

摘要

The solidification behaviour of binary Al-Sc alloys have been investigated for both hypo- and hyper-eutectic alloy compositions. Sc additions greater than 0.55 wt.% (the eutectic point) produce a substantial refinement in the grain size of aluminium castings, which is due to the formation of the primary Al_3Sc intermetallic phase during solidification. The refinement in grain size occurs because the L1_2 Al_3Sc phase is almost identical to the crystal structure and lattice parameter of Al, which nucleates epitaxially on its surface. The grain refinement is accompanied by a change in the growth morphology from dendritic in the large un-refined castings, to fine spherical grains in the refined castings. On cooling to room temperature, the Al-Sc supersaturated solid solution decomposes via a discontinuous precipitation reaction to form coherent rod-like precipitates of the L1_2 Al_3Sc phase.