Containerless Solidification
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| Schematic diagram of Cambridge 6.5 m drop-tube |
Molten droplets cooled out of contact with any container can attain large undercoolings prior to solidification. Heterogeneous nucleation on container walls would normally prevent this. One method for achieving containerless solidification is drop-tube processing. In the group we have a 6.5 m high drop tube, giving a free-fall time of around 1 second. The sample is melted by RF induction in the furnace chamber at the top, and then ejected through an orifice in the base of the crucible. The droplets, of diameter 10 microns to 1 mm solidify in free fall. The drop-tube is normally filled with helium to increase the cooling rate. The main column of the drop tube can be isolated from the furnace and collection chambers by gate valves; in this way there is no need to expose it to air, and cleanliness of the atmosphere is improved. Research using the drop-tube has focused on the novel microstructures arising when alloys undergo rapid solidification in highly undercooled droplets.
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| Micrograph of alloy droplet (cross-section) |
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