This SPAR 5 experiment was the third in a series of experiments designed by Papazian et al. to study the low-gravity solidification behavior of a polycrystalline material (see Papazian, SPAR 1, Experiment 74-37, and SPAR 4). The specific objective of this experiment was to study columnar-to-equiaxed transition (CET) during polycrystalline solidification.
Prior to the mission, two solutions of NH4Cl-H2O were prepared: (1) 325 grams/liter NH4Cl (liquidus temperature of 35 ¡C) and (2) 362.5 grams/liter H4Cl (liquidus temperature of 55 ¡C). (Two samples of each solution were prepared for the experiment.)
The experimental apparatus consisted of (1) a sample chamber of four independent, semi-cylindrical pockets contained between Plexiglas faces, (2) an electronics box, (3) a motor-driven camera (230 exposures, 1 frame/second), (4) a freon reservoir designed to deliver freon for sample quench, (5) a support structure, and (6) heaters (details of the experimental apparatus may be located in Reference (1)).
Prior to launch, the sample chamber was heated and maintained at a temperature of 80 ¡C. At launch, the heaters were switched off. One hundred seconds after launch (low-gravity conditions), a freon sample quench was initiated and solidification occurred throughout the next 200 seconds. During this time the solidification process was photographed. Corresponding ground-based studies were performed for comparison.
Post-flight examination of the flight samples' thermal data indicated "...that because of reduced convective transport of heat, the liquid portion of the sample was significantly warmer. This led to a steeper thermal gradient in the liquid, but more significantly, this did not allow for the growth of any equiaxed grains ahead of the columnar interface. No gravity driven convective mechanisms occurred to transport nuclei into the liquid, but this is thought to be of secondary significance. Even if nuclei had been present the thermal conditions would not have allowed them to grow. In order for an equiaxed zone to form, growth of the equiaxed grains must occur faster than growth of the columnar grains. This was not possible in the reduced gravity environment." (1, p. V-17)
Examination of the photographic records and solidified samples led to the following conclusions:
(1) Significant convection driven by thermal inversion was absent in the flight experiment.
(2) Grain multiplication by showering (growth of dendritic fragments at the open surface of an ingot which shower through the melt and settle on the columnar interface) was not observed.
(3) Grain multiplication, driven by either thermal inversion or compositional inversion, did not occur.
(4) Completely columnar structures were obtained in all flight samples, while the ground-processed samples exhibited equiaxed structures making up 25% to 100% of the cast material.
(5) In the flight samples, (a) the thermal gradient was steeper and (b) the rate of cooling of the liquid was slower than those observed in the 1-g samples.
(6) The crystalline growth rate was not changed by low-gravity processing.
(7) Bubbles were not pushed by the advancing solidification front.
(8) An equiaxed structure did not result from an induced "big bang" nucleation process (see Reference (1)).
See Papazian, SPAR 4.
(2) Papazian, J. M. and Kattamis, T. Z.: Effect of Reduced Gravity on the Solidification Microstructures of NH4CL: H2O Alloys. Metallurgical Transactions A, Vol. 11A, March 1980 pp. 483-93.
(3) Papazian, J. M. and Kattamis, T. Z: SPAR V Technical Report for Experiment 74-37 Contained Polycrystalline Solidification in Low-G. Grumman Corporate Research Center Report RE-569, February 1979, 29 pp. and addendum, May 1979, 4 pp.
(4) Naumann, R. J. (editor): Contained Polycrystalline Solidification in Low Gravity. In Descriptions of Space Processing Applications Rocket (SPAR) Experiments, NASA TM-78217, January 1979, pp. 19-20. (post-flight)
(5) Input received from Principal Investigator J. M. Papazian, December 1987, September 1988, and August 1993.