Abstract
Aluminum-iron alloys with Fe contents of 1.8, 2.4, 3.9, 5.1, 6.2, 7.7 mol% were rapidly solidified to foils with about 50 μm thickness and 6 mm width by the planar flow casting method.
Microstructural changes across the thickness of foils were investigated as a function of Fe content in aluminum. All the foils examined exhibit two zones: zone A on the substrate side of the foil with a slight etching response and zone B on the air side of the foil with a strong one. The foils with Fe contents of 5∼6 mol% have a maximum area fraction of zone A, the value of which is about 80% of the cross-sectional area of the foil. As for the foils with Fe contents of up to 6.2 mol%, the zone A region reveals the microstructure of a fine α-Al cell whose spacing is about 100 nm, while the zone B region reveals that of a coarse α-Al dendritic cell whose spacing is about 350∼1000 nm depending on Fe content. The zone A region of the foil with a Fe content of 7.7 mol% reveals the microstructure of a fine lamellar eutectic whose spacing is about 100 nm and the zone B region consists of primary spherical compounds surrounded by a coarser lamellar eutectic. It is found in all the foils regardless of Fe content that the spacings at the transition regions from zone A to zone B are approximately constant around a value of 220 nm which corresponds to the limit of resolution of a microscope used.
On the basis of the calculation of a solidification problem considering the kinetics of constrained cellular growth of α-Al, the origin of the microstructural change from a fine cell to a coarse dendritic cell is interpreted as a sudden drop in the heat transfer coefficient at the foil-substrate interface. Values of the heat transfer coefficient and of the initial undercooling at the onset of solidification are estimated through most suitably fitting the calculated results to the experimental ones.
