Containerless processing and rapid solidification of Nb-Si alloys of hypereutectic composition
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I.
INTRODUCTION
THE Nb-Si
system presents interesting aspects for rapid solidification. Significant research in the past was driven by the predictions that the metastable A15 Nb3Si stoichiometric compound would have a relatively high superconducting transition temperature. Ill The use of rapid solidification techniques has allowed researchers to form Nb3Si in the cubic A15 structure. However, the superconducting properties have been disappointing.12-51 From a different perspective, the refractory nature of the Nb-rich alloys makes them potential candidates for hightemperature applications. Containerless processing in drop tubes has shown that these alloys can undercool considerably in the bulk prior to nucleation. I6] This high undercooling ability is of special interest in the study of metastability. However, the recalescence of the undercooled drop after nucleation acts against the retention of the metastable structures. This effect can be mitigated by the employment of rapid quenching techniques. In this work, a combination of bulk undercooling in an electromagnetic levitation apparatus and splat quenching between two copper plates has been used to process Nb-Si alloys in an effort to maximize rapid solidification conditions and minimize the effects of recalescence. This technique was designed to promote the formation and retention of metastable microstructural configurations. The objective of this article is to report the results of this investigation with emphasis on the solidification characteristics of alloys in the 21 to 27 at. pct Si range of composition.
G.A. BERTERO, formerly Graduate Student, Vanderbilt University, is Graduate Student, Dep',mment of Materials Science and Engineering, Stanford University, Stanford, CA 94305. W.H. HOFMEISTER, Research Assistant Professor, and R.J. B A Y U Z I C K , Professor, are with the Materials Science and Engineering Department, Vanderbilt University, Nashville, TN 37235. M.B. ROBINSON, Scientist, is with Space Sciences, NASA George C. Marshall Space Flight Center, Huntsville, AL 35812. Manuscript submitted January 23, 1991. METALLURGICAL TRANSACTIONS A
II.
BACKGROUND
The NbsSi3 intermetallic has a high- and a lowtemperature modification. 171 The low-temperature phase is the tetragonal a-NbsSi3 isostructural with CrsB3, space group I 4 / m c m , and lattice constants a = 0.65698 nm and c = 1.18877 nm. A polymorphic transformation takes place between 1973 and 2213 K, and the stable structure at higher temperatures is the tetragonal /3-NbsSi3 (type Si3Ws) with lattice constants a = 1.00289 nm and c = 0.50698 nm. The equilibrium Nb3Si crystal structure is the tetragonal Ti3P type with space group P42/n and lattice constants a = 1.021 nm and c = 0.519 nm, very similar to the /3-NbsSi3 lattice parameters. I81 Deardorff et al. ISl showed that at 20 at. pet Si, the as-cast microstructure formed from the primary solidification of Nb3Si and the subsequent formation of the equilibrium a - N b + Nb3Si eutectic. At 25 at. pct Si, the as-cast microstructure showed no evide
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