Undercooling studies on Nb-Pt and Nb-Si alloys using the 105 meter drop tube

Niobium-platinum samples of compositions ranging from 16 to 32 at. pct have been undercooled to as much as 540 K in the low gravity, containerless environment of a 105 meter drop tube. Undercooling was terminated in the Nb-Pt samples by the nucleation and growth of the Nb3Pt phase. In the 16-18 at. pct Pt samples, this resulted in samples which are completely Nb3Pt, in contrast to both the equilibrium phase diagram and the nonundercooled samples which formed with Nb dendrites and interdendritic Nb3Pt. Undercoolings for the Nb-Si samples were up to 670 K, which corresponds to 27 percent of the liquidus temperature or 80 percent of the estimated hypercooling limit. In the Nb-Si system, a coupled zone was identified as well as a metastable extension of the solubility limit of Si in Nb due to deep undercooling.