A review of long drop tubes as a supplement/alternative to space experiments

The 100 meter high drop tube at the Marshall Space Flight Center has proven to be a viable facility for studies of containerless solidification. Advantages are that experiments are inexpensive and large numbers of specimens can be processed rapidly. It would not be unusual to run ten specimens in a day. Another significant advantage is that the undercooling behavior can be followed with sufficient sensitivity to easily detect the onset of recalescence and subsequent events.Disadvantages are the restrictions on specimen sizes and types of alloys that can be run in a microgravity environment. Practical specimen sizes range between 50 mg and 500 mg depending on the type of furnace being used. Refractory alloys can be processed in a vacuum (about 10^?5 torr) and therefore at microgravity. Non-refractory alloys demand an atmosphere (about 200 torr) to obtain appreciable undercooling before impact at the bottom of the tube. Under these conditions significant g forces result.Because of the present limitations of the 100 meter drop tube, the most definitive work has been done on niobium based alloys. Large amounts of undercooling have been observed routinely and the effects of undercooling on microstructure have been characterized in detail. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy have been used to determine types of phases, amounts of phases, and compositions of phases. It is clear, as would be expected, that the results bear some resemblance to rapid solidification processing by quenching. However, there are dissimilarities due to the uniqueness of solidification by deep undercooling without quenching in long drop tubes and accompanying recalescence effects.