Dendritic and seaweed growth of proeutectic scandium tri-aluminide in hypereutectic Al-Sc undercooled melt

The morphologies of proeutectic Al3Sc growing in an undercooled hypereutectic Al-2 wt.% Sc alloy melt with varied cooling rates were carefully examined under electron microscopes, in order to reveal its different growth mechanisms. At a low cooling rate (similar to 1 K.s (- 1)), as expected, the formation of proeutectic Al3Sc was governed by the lateral growth, exposing six flat {100} facets. At an intermediate cooling rate (similar to 400 K.s (- 1)), proeutectic Al3Sc grew in a dendritic manner, with well-defined backbones extending in its eight < 111 > directions and paraboloidal dendrite tips, although the dendrite tips and side-branches turned into faceted steps at a late growth stage, when the lateral growth prevailed again. At a high cooling rate (similar to 1000 K.s (- 1)), proeutectic Al3Sc primarily solidified into an entirely seaweed particle, which was composed of interior compact seaweeds and exterior fractal seaweeds. The detailed formation mechanisms of the proeutectic Al3Sc in the latter two situations, to the author's knowledge, are first clarified with growth models in the present work. Various morphological stability criteria were used to verify the proposed dendritic and seaweed growth models, by examining characteristic length scales in the structure, such as the radius of the dendrite tip, the interspacing between seaweed branches, and the size of destabilized sphere at an early growth stage. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.