During the solidification of a casting, two types of grain structure may form: (1) equiaxed (roughly spherical grains of nearly the same size) and (2) columnar (cylindrical grains). A zone of equiaxed grain structure will form if (1) nuclei for equiaxed grains are present throughout the melt and (2) growth of the equiaxed grains is greater than that of the columnar grains. Both of these conditions (presence of nuclei and growth of eqiaxed grains) are influenced by gravity. For example, gravity-induced convection (driven by composition gradients that arise from rejection of solute at the solidification interface) can contribute to the formation of an equiaxed grain structure.
Metals melting experiments performed in the low-gravity environment such as (1) Skylab experiment M551 (see Poorman, Skylab SL-2) and (2) drop tower experiments illustrated an unexpected occurrence of fine-grained equiaxed microstructures in the cast samples. These structures were formed despite the reduction of gravity-induced convection. Therefore, further investigation of the gravitational effects on columnar-to-equiaxed transition was initiated.
This SPAR 1 experiment was the first in a series of investigations designed by Papazian et al. to study the low-gravity solidification behavior of a polycrystalline material. (The experiment also represented one of two investigations performed by Papazian during the SPAR 1 sounding rocket mission (see Papazian, SPAR 1, Experiment 74-36).) The objectives of Experiment 74-37 were to (1) directly measure the effect of reduced gravity on the width of the solute enriched zone ahead of a solidification interface and (2) investigate the columnar-to-equiaxed transition during polycrystalline solidification.
In preparation for the experiment, a sample of transparent cyclohexanol mixed with fluorescein dye was placed in a silica cuvette. (Use of this material allowed the observation of the buildup of the colored solute (fluorescein, green in color) ahead of the solidification interface.) During the mission, solidification of the material was accomplished via a thermoelectric cooling device which was placed at the bottom of the cuvette. Reflected illumination from a tungsten filament microscope illuminator provided lighting for 35 mm photographic recording of the process.
It was intended that a sequence of 220 photographs would record the solidification process and buildup of solute. "Unfortunately, during flight, the government furnished (GFE) camera for this experiment malfunctioned. We obtained 4 test exposures before lift-off, followed by 13 exposures which seem to have been taken at the beginning of the low-gravity interval and 56 exposures which were probably taken toward the end of the cooling sequence. The first 14 low-gravity frames show that no crystallization had yet occurred; this is consistent with our ground-based results in which the first solid is observed at approximately 20 s. The 56 subsequent frames show approximately 6 mm of solid present...." (1, p. VIII-9)
Further post-flight examination of the photographs revealed that the solute-enriched zone ahead of the interface was wider and more irregular than that observed for ground-based tests. The solidified flight sample consisted of an equiaxed grain structure, while the 1-g processed samples consisted of a columnar grain morphology. The grain size of the flight sample was 1.2 mm. The grain sizes of the ground-processed samples (width of columnar grain) ranged between 2 and 3.5 mm. The average growth rate of the flight sample was reported as 60 microns/sec which was significantly higher than the growth rate of the ground samples (20 to 30 microns/sec).
The smaller grain size, equiaxed grain morphology, and a larger average macroscopic growth rate of the flight sample were attributed to parasitic nucleation ahead of the interface (that was not observed in the ground-based samples). Several possibilities for this different behavior were reported: (1) the different thermal histories of the flight and ground-processed samples, (2) contamination of the flight sample, (3) perturbation of the heat and fluid flow caused by the presence of a small bubble in the flight sample, (4) launch-induced fluid motion, and (5) inadequate superheating of the flight sample prior to launch. Subsequent ground-based investigations led to the conclusion that the parasitic nucleation "...may be attributed to ordered islands within the liquid, which survived remelting because of the low degree of superheating (approximately 1.5 ¡C), and did not settle because of reduced gravity and acted as nuclei during cooling." (1, p. VIII-19)
(2) Toth, S. and Frayman, M.: Measurement of Acceleration Forces Experienced by Space Processing Applications. Goddard Space Flight Center, Contract No. NAS5-23438, Mod. 23, ORI, Inc., Technical Report 1308, March 1978. (acceleration measurements, SPAR 1-4)
(3) Input received from Principal Investigator J. M. Papazian, December 1987, September 1988, and August 1993.