Microstructure selections in the undercooled hypereutectic Al-Si alloys

Experimental work is described in undercoolings and microstructures of hypereutectic Al-Si alloy droplets emulsified in salt bath. By controlling experimental variables, measured undercoolings ranged up to approximately 125 K (from the liquidus line). For near-eutectic Al-13 wt%Si alloys, microstructure transitions, which include a phase change from eutectic to primary α-Al dendrite plus eutectic and a morphological change from α-Al dendrite to equiaxed Al grains, were observed with increasing undercooling levels. For hypereutectic Al-Si alloys, microstructure transitions, which include a morphological change of primary and eutectic Si crystals, were also obtained quantitatively by increasing undercooling. The physical mechanism of phase change is analyzed on the basis of the LKT theory for dendrite growth and the JH theory for lamellar eutectic growth. Growth kinetics of Si crystals is also considered for the analysis of Si crystal morphology changes. Microstructural changes upon undercooling and composition are used to build microstructure selection maps for the hypereutectic Al-Si alloy.