THE ROLE OF OXYGEN AND ZIRCONIUM IN THE FORMATION AND GROWTH OF NB3SNGRAINS

One method of producing Nb3Sn is to react a molten tin alloy with a so lid niobium alloy. Using this process, the addition of zirconium and o xygen to the niobium foil has been found to dramatically reduce the Nb 3Sn grain size and affect the Nb3Sn superconducting critical current p roperties. Nb3Sn grains grow semicoherently on the niobium alloy foil. The initial grain size is about 50 nm. These initial Nb3Sn grains coa rsen rapidly to become equiaxed grains about 0.2 mum in diameter. The equiaxed Nb3Sn grains away from the Nb/Nb3Sn interface are completely surrounded by a tin alloy phase that would have been liquid at the rea ction temperature. Based on transmission electron microscopy observati on and electrical properly characterization, it is concluded that ZrO2 clusters, less than 10 angstrom in size, form in the niobium alloy fo il during processing. These clusters combine at the Nb/Nb3Sn interface to form ZrO2 precipitates. The ZrO2 precipitates are found in all of the Nb3Sn grains that have formed from a reaction between the liquid t in and the solid niobium at the Nb/Nb3Sn interface. The precipitates a re coherent with their host Nb3Sn grains. During Nb3Sn grain growth, t he ZrO2 precipitates dissolve in shrinking grains and reprecipitate in growing grains, as the migrating grain boundary intersects the precip itate. This dissolution/reprecipitation process slows the growth of Nb 3Sn grains.