The role of oxygen and zirconium in the formation and growth of Nb3sn grains

One method of producing Nb₃Sn is to react a molten tin alloy with a solid niobium alloy. Using this process, the addition of zirconium and oxygen to the niobium foil has been found to dramatically reduce the Nb₃Sn grain size and affect the Nb₃Sn superconducting critical current properties. Nb₃Sn grains grow semicoherently on the niobium alloy foil. The initial grain size is about 50 nm. These initial Nb₃Sn grains coarsen rapidly to become equiaxed grains about 0.2 μm 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 reaction temperature. Based on transmission electron microscopy observation and electrical property characterization, it is concluded that ZrO₂ clusters, less than 10 Å in size, form in the niobium alloy foil 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 tin and the solid niobium at the Nb/Nb₃Sn interface. The precipitates are coherent with their host Nb₃Sn grains. During Nb₃Sn grain growth, the ZrO₂ precipitates dissolve in shrinking grains and reprecipitate in growing grains, as the migrating grain boundary intersects the precipitate. This dissolution/reprecipitation process slows the growth of Nb₃Sn grains. Formerly with GE Corporate Research & Development, Schenectady, NY,