Rod Eutectic Growth of Al-Al 3Sc in Al-2 Wt. % Sc Undercooled Melt

Abstract

The lack of a thorough understanding of Al-Al3Sc eutectic growth motivates the present work. We observed a rod-typed Al 3 Sc eutectic phase existing prevalently in an as-cast Al-2 wt. % Sc alloy that solidified via both slow cooling in air (~1 oC·s−1) and rapid cooling in a wedge-shaped copper mold (up to ~3000 oC·s−1), different from reported results. Al-Al3Sc eutectic dendrites were identified within a narrow region near the edge of the wedge. The eutectic dendrites have an equiaxed dendritic contour and a rod eutectic structure inside. The condition for the growth of the eutectic dendrites was assessed by using appropriate analytical models. It was revealed that an interface undercooling of 48.2 oC is required to form the eutectic dendrites, or to enter the coupled zone of the Al-Al3Sc phase diagram. A phenomenon of scientific interest is that when crystallizing under a near-equilibrium condition, the eutectic Al3Sc phase formed a non-faceted morphology, in contradiction to its faceted nature. Based on the competitive growth criterion, we deduced that the non-faceting of the eutectic Al3Sc phase essentially reduces the interface undercooling for the resultant regular eutectic, in comparison to an otherwise irregular eutectic that contains a faceted eutectic Al3Sc phase.