Containerless processing and rapid solidification of Nb-Si alloys in the niobium-rich eutectic range
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INTRODUCTION
C O N T A I N E R L E S S processing has proven to be an effective method for studying phase relations, metastability, undercooling, and solidification in refractory alloy systems, m2,31 Specifically, containerless processing in a clean, controlled environment provides ideal conditions for promoting large bulk undercoolings prior to nucleation of the solid phase. 131 Electromagnetic levitation has been used in this study to process niobium-silicon alloys of compositions from 16 to 27 at. pct Si. Liquid metal drops were undercooled or superheated to various temperatures in a levitation field and subsequently splat quenched between two copper plates. The objective of this article is to present results of the investigations in the eutectic composition range (16 to 20 at. pct Si) with emphasis on the microstructural evolution as a function of processing conditions. In a subsequent article, the results of the work performed on alloys from 22 to 26 at. pct Si will be presented. 141 II.
BACKGROUND
Spaepen I51 studied the niobium-silicon system using pulsed laser quenching of thin alloy films ( - 1 0 0 0 ,~) to achieve estimated cooling rates from 109 to >1012 K / s . At the highest cooling rates ( > 10 j2 K / s ) , the amorphous phase was formed from 8 to 27 at. pct Si. At 1012 K / s , the amorphous phase plus a disordered face-centered cubic (fcc) solid solution were observed in the same range of compositions. In the 10 j~ to 10 lz K / s range, a supersaturated body-centered cubic (bcc) a-Nb solid solution extending to 18 at. pct Si was reported, whereas beyond 18 at. pct Si, amorphous phase was found, and at 25 at. pct Si, an ordered fcc L12-type phase was reported. In the 109 to 10 ~~ K / s range, a hexagonal and an unidentified phase were present within compositions close to 25 at. pct Si. G.A. BERTERO, formerly Graduate Student, Vanderbilt University, is Graduate Student, Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305. W.H. HOFMEISTER, Research Assistant Professor, and R.J. BAYUZICK, 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 April 9, 1990. METALLURGICAL TRANSACTIONS A
An extensive microstructural study of melt-spun ribbons of Nb-Si from 12 to 25 at. pct Si was performed by Bendersky et al. E6] The results showed no evidence of the Nb3Si structure (tetragonal or A15) at any composition. Glass formation was reported; however, the amorphous phase was limited to a thin region in contact with the wheel. At 12 at. pct Si, the microstructure consisted of c~-Nb dendrites and NbsSi3 as the interdendritic phase. At 20 at. pct Si, a rod eutectic of a - N b + NbsSi3 phases was identified with a minimum inter-rod spacing of 30 nm. Closer to the chill surface, a highly metastable two-phase equiaxed grain structure of a - N b + NbsSi3 was present. The NbsSi3 phase was
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