Solidification of Nb-Ge alloys in long drop tubes
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I.
WEIGHT % Ge
INTRODUCTION
THE 30 meter and
100 meter long drop tubes at the Marshall Space Flight Center are being used to study the solidification of alloys in free fall. 1.2 Use of these long drop tubes provides an inexpensive low-gravity containerless environment which permits deep undercooling of the melt. The objective of the research is to investigate low-gravity solidification from a deeply undercooled state as a process to produce bulk materials with unique microstructures, metastable phases, and special properties as compared to normal "earthbound" processes. Collaterally, there is a strong desire to gain insights into space processing as a method of producing new and unique alloys with novel properties. Thus far, the work has concentrated on Nb-Ge alloysTM having nominal compositions of Nb-18 at. pct Ge, Nb22 at. pet Ge, Nb-25 at. pct Ge, and Nb-27 at. pet Ge. The equilibrium phase diagram, 5 shown in Figure 1, reveals that the compositions selected provide a series with a variegation in solidification behavior. The 18 at. pct alloy is at a peritectic point; the 22 at. pet alloy intersects the right edge of the peritectic horizontal; the 25 at. pet alloy is hypoeutectic; the 27 at. pet alloy has the eutectic composition. This binary system is important technically due to the characteristics of the/3 phase. At equilibrium, the Ge-rich solubility limit of this phase at 1000 ~ and below is about 20 at. pet Ge as shown in Figure 1. Being Ge deficient, the phase has a defect A-15 crystal structure and no noteworthy properties. However, thin films formed from sputtering techniques have extended the solubility limit of Ge in the/3 phase to 25 at. pct Ge or even slightly higher. 6"7 The resuiting metastable, but stoichiometric Nb3Ge A-15 phase has the highest superconducting transition temperature yet observed, about 23 K. 8 There is considerable interest in the possibility of achieving similar behavior in bulk materials.
II.
EXPERIMENTAL PROCEDURE
Most bulk specimens discussed here were processed in the 30 m drop tube and are identified with the prefix "DT". Those obtained using the 100 m drop tube have identiN. D. EVANS and W. H. tlOFMEISTER, Graduate Students, and R. J. BAYUZICK, Professor, are with the Department of Mechanical and Materials Engineering, Vanderbilt University, Nashville, TN 37235. M . B . ROBINSON is Research Scientist, Marshall Space Flight Center, Mail Code ES72, Huntsville, AL 35812. Manuscript submitted February 25, 1985. METALLURGICAL TRANSACTIONS A
NbLb
10
2600i'
20
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2180
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