Solidification Studies of Nb-Ge Alloys at Large Degrees of Supercooling
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SOLIDIFICATION STUDIES OF Nb-Ge ALLOYS AT LARGE DEGREES OF SUPERCOOLING L. L. LACY*, M.B. ROBINSON+, T.J. RATHZ+, N. D. EVANSt, AND R. J. BAYUZICKt + NASA, Marshall Space Flight Center, * Exxon Corporation, Houston, Texas, Alabama; tVanderbilt University, Nashville, Tennessee ABSTRACT A 32 meter evacuated drop tube has been used to investigate the solidification of Nb-Ge alloys after deep undercooling. Samples have been supercooled as much as 500 0K below the liquidus by using free-fall conditions to eliminate crucible induced nucleation. Final microstructures are dependent on the quenching rates at the bottom of the drop tube with a striking extension of the a phase solubility limit at the higher quenching rates. INTRODUCTION Lacey, Rathz, and Robinson have recently developed a 32 meter evacuated drop tube for the study of the effects of low gravity containerless processing on the degree of supercooling and resultant properties [1,2]. The first extensive application of the facility, reported here, has been in the Nb-Ge alloy system where alloys containing 13, 18, 22, and 25a/o Ge have been investigated. Figure 1 shows the relevant phase diagram [3] in which both peritectic and eutectic reactions are exhibited in the Nb rich region. Hence, Nb rich compositions are particularly interesting since effects of drop tube solidification can be compared in alloys with two of the most important invariant temperature transformations. The 8 phase deserves special consideration in that it can have an extremely high superconducting transition temperature (230 K) when its solubility limit is extended to the neighborhood of 25a/o Ge. This can be accomplished through procedures such as sputtering or splat-cooling to obtain non-equilibrium Nb-Ge
thin films [4,5]. Up till now, the metastable extension has not been produced in bulk form. However, supercooling in a drop tube as a method for extending the solubility limit in bulk has been recognized as an intriguing possibility. PROCEDURE Alloys were prepared from pure materials (99.999 Nb and 99.9999 Ge) by arcmelting. Discs were cut from the cast rods for remelting and solidification in the drop tube as previously reported [2]. Most of the specimens were caught and quenched at the bottom of the drop tube in Convalex-lO vacuum oil (quenching rate = 700°K/sec) as indicated in Table I. However, one of the Nb-22a/o Ge specimens was caught in DC-704 vacuum oil (quenching rate = 1700OK/sec) at the bottom of the drop tube and, for comparison, another Nb-22a/o Ge specimen was caught at the top of the drop tube in DC-704 to provide quenching In addition, a Nb-25a/o Ge specimen was caught at without supercooling the bottom of the drop tube on a copper block. The latter three specimens are also listed in Table I. All specimens were examined by optical metallography. Qualitative analysis by x-ray energy dispersive spectrometry (EDS) of the metallographic specimens in a scanning electron microscope was used to aid in phase identification. Further confirmation of phase identification
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