Influence of a rare-earth element on the solidification behaviour and mechanical properties of undercooled Al-Si alloys

The influence of undercooling and a rare earth (RE) metal modifier on the solidification behaviour of hypereutectic Al-Si alloys was investigated by the electromagnetic levitation technique combining analysis of high-speed video and scanning electron microscopy. The results show that, for Al-Si alloys without RE addition, the morphology of the primary silicon transforms from dendrites to fragmented bulks and granular grains and then to equiaxed grains with increasing degree of undercooling; the nucleation frequency increases exponentially, while the average grain size decreases exponentially. The effect of undercooling on microhardness was also analysed. On the addition of an RE metal to the Al-Si alloys, the critical undercooling of the morphological transition and the grain size decrease, while the nucleation number and nucleation frequency increase. A comparison of the microhardness before and after the RE addition reveals that the microhardness is also improved at similar degrees of undercooling.