Microstructures of niobium-germanium alloys processed in inert gas in the 100 meter drop tube
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I.
INTRODUCTION
INTEREST in reduced gravity containerless processing in space has led to the investigation of earth-based experiments utilizing the 100 meter drop tube at NASA's Marshall Space Flight Center in Huntsville, Alabama. The mechanics of the drop tube have been reported earlier as well as experiments in vacuum on niobium and niobium-germanium alloys. 1-5 The experiments reported herein are niobium-germanium alloy drops processed in a 200 torr helium environment. This condition sacrifices the zero gravity aspect of drop tube experiments for higher cooling rates (300 to 400 K/sec) due to convective heat transfer. 6 Under these conditions alloys are completely solidified during free fall in the drop tube as evidenced by their spherical shape. Due to the containerless aspect of the drop tube, large degrees of undercooling are obtained prior to solidification. The possibility of metastable phase formation is increased by this undercooling 7 but it is interesting to apply the model of Levi and Mehrabian, for undercooled metal droplets, to drop tube samples. Their model indicates that the rate of recalescence in drop tube samples at large undercooling will cause the solid-liquid interface to approach the melting temperature at the moderate cooling rates experienced in the drop tube) Some metastable states may therefore be eliminated by the release of the latent heat of fusion during solidification. Even with the heat flow restrictions, microstructural evidence of rapid solidification is apparent in drop tube samples.
was recorded by a series of silicon infrared detectors in the drop tube. The detectors "see" the flash of recalescence accompanying solidification. The undercooling was calculated, using droplet cooling models reported elsewhere, 5'6 with an accuracy of -+70 K. Processed samples were analyzed by optical microscopy, energy dispersive X-ray analysis, X-ray diffraction, and transmission electron microscopy. Scanning electron microscopy and energy dispersive analysis were carried out on a Hitachi X-650 Scanning Electron Microanalyzer with a PGT System 4 X-Ray Analyzer. The PGT uses a FRAME C9 analysis program that performs background subtraction and ZAF matrix corrections. Sample spectra and standard spectra of pure niobium and pure germanium were collected for 100 seconds at 25 KV accelerating voltage, 38 deg takeoff angle, and 0.25 nanoamp beam current. The count rate ranged from 1100 to 2100 counts per second with a dead time of 18 to 20 pct. The collection windows were 2.1125 to 2.287 KeV for Nb L~.t~and 9.780 to 9.980 KeV for GeK~. The spectrometer calibration was maintained at less than 0.1 pct error. Samples were very lightly etched before analysis. Compositional analysis is accurate to ---1 at. pct. The transmission electron microscopy was performed with a Philips EM400T located at the Oak Ridge National Laboratory. The instrument is equipped with a high brightness field emission gun and an EDAX 9100 energy dispersive analysis system (EDS).
III. II.
EXPERIMENTAL PROCEDURE
Near spherical sa
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