Containerless processing and rapid solidification of Nb-Si alloys of hypereutectic composition
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Niobium-silicon alloys from 21 to 27 at. pct Si were rapidly solidified employing a combination of electromagnetic levitation and splat-quenching techniques. Levitated liquid drops were over-heated or undercooled to different temperatures in the electromagnetic lévitation field and sub-sequently released into a splat-quenching apparatus. Some undercooled drops were allowed to solidify in the coil. Analytical scanning (SEM) and transmission electron microscopy (TEM) as well as X-ray diffraction were used to characterize the microstructures of the processed samples. In the range of compositions studied, the splat-quenched drops always formed the tetragonal Nb3Si phase directly from the liquid. On the other hand, drops solidified in the coil were characterized by the presence of the primary intermetallic Nb5Si3 and the absence of both peritectic Nb3Si and the equilibrium eutectic. In these cases, a metastable α-Nb + Β-Nb5Si3 eutectic formed. The cubic A15 Nb3Si structure was not observed in any of these experiments. The results are discussed in terms of possible metastable configurations of the Nb-Si phase diagram as well as concepts of nucleation and growth kinetics applied to the Nb3Si and Nb5Si3 intermetallics.
KeywordsMetallurgical Transaction Convergent Beam Electron Diffraction Electromagnetic Levitation Chill Surface Equilibrium Eutectic
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