Solution calorimetry is the most accurate method available to determine heats of formation of metallic phases. During the calorimetry (1) a metallic phase is dissolved in a metallic melt, (2) the change in temperature of the melt is measured, and (3) the heat of solution is determined. Similarly, individual elements of the metallic phase under investigation are dissolved in the melt and the heat of solution for the elements is determined. The difference between these two heats of solution is the heat of formation of the phase.
If the metals (for example, chemical elements from the periodic table in (1) group IV, (2) group V, or (3) the Rare-Earth section) react with the crucible, then the heat of solution cannot be determined accurately because it is superimposed upon the heat of reaction. In the low-gravity environment, the calorimetry can take place in a crucible free system, thus eliminating these difficulties.
This TEXUS 2 experiment was the first in a series of investigations designed by Haessner et al. to study the effects of the low-gravity environment on the heats of formation of metallic phases. The specific objectives of the experiment were to determine (1) the time required for a solid metal sample to dissolve in a liquid melt, (2) the influence of the released heat of reaction on the solution, (3) the factors which disturb the measurement process, and (4) the diffusion-inhibitive internal surfaces which affect the process.
The experiment was performed in the TEXUS Experiment Module TEM 02 large muffle furnace. The furnace contained three steel plates (20 mm apart) attached to a central rod. Each plate held three Al samples contained in Al2O3 rings such that the Al samples protruded above the rings. Six different metals were chosen as alloying samples for the aluminum: copper, nickel, titanium, niobium, tantalum, and molybdenum. An alloying specimen was configured above each Al sample in a specimen holder. (The specimen holders were also attached to the central rod.) The entire apparatus was contained in a quartz tube. Video equipment recorded the experiment.
Prior to the rocket launch, the furnace was preheated to 600 ˇC. After launch, a battery was activated which further heated the furnace to between 796 and 816 ˇC and the Al samples melted forming liquid drops. Once the desired temperature and low-gravity conditions were achieved, the solid, pure metal alloying samples were pushed into the liquid Al drops and held for 190 seconds. Cooling was then initiated (while still under low-gravity conditions) by the introduction of a gas along the outside of the quartz furnace chamber. This external cooling procedure was employed to prevent damage of the specimens by the gas. The measured cooling rates were between 5 and 10 ˇC/sec.
Post-flight examination of the space samples indicated that "[the] space experiment [had] not fulfilled its expectations...." (1, p. 100). A leak in the quartz furnace chamber allowed cooling gas to be introduced into the furnace chamber resulting in two samples being blown from their holders. The remaining samples had been deformed by the force of the gas. The gas had also acted to separate the alloying specimens from the Al drops. Therefore, "[o]ne of the most important requirements of the solution experiment, i.e. solidification of alloys under gravity-free conditions was no longer fulfilled." (1, p. 99)
Metallographic and X-ray micro-analysis of the flight samples illustrated that "...no detectable localized alloying effect took place." (1, p. 98) This was probably due to the fact that "[t]he temperatures registered at the various sensing points did not indicate the actual specimen temperature...." (1, p. 99) It appears that the Al drops were not completely molten and merely deformed when the alloy specimen was inserted. Heat conduction by the sample holders would have prevented any further heating and subsequent melting of the Al drop. Examination by scanning electron microscopy did not contradict this deduction. It was further reported that the presence of a thin oxide layer on the Al samples could have also prevented alloying.
The specific reasons why the various metal combinations were chosen for the experiment were not detailed in the available publications.
(2) Haessner, F. and Hettwer, K. J.: Legierungsbildung unter Schwerelosigkeit. May 1979, 01 QV 168-ZA-SN-SLN 7785/2, DFVLR/BPT, Kšln, Porz, Germany. (in German)
(3) Haessner, F., Hemminger, W., and Lukas, H. L.: Solution Caloremetry in Zero Gravity for the Determination of Enthalpies of Formation of Reactive Metals. In AIAA/ASME 1974 Thermophysics and Heat Transfer Conference, Boston, Massachusetts, July 15-17, 1974, AIAA Paper #74-666. (preflight)
(4) Input received from Experiment Investigator, August 1988 and August 1993.