Containerless processing and rapid solidification of Nb-Si alloys of hypereutectic composition
A combination of bulk undercooling in an electromagnetic levitation apparatus and splat quenching between two copper plates is used to process Nb-Si alloys in order to maximize rapid solidification conditions and minimize the effects of recalescence, with emphasis on the solidification of characteristics of alloys in the 21 to 27 at. pct Si range of composition. SEM and TEM as well as X-ray diffraction are 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. 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 alpha-Nb + beta-Nb5Si3 eutectic formed. 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.