Low-Temperature Structural Transformations in Powdered Samples of Nb3Sn

Abstract

Recent observations of the effects of stress on the properties of superconductors1 have focussed attention on the low-temperature martensitic phase transformations that occur in β-W compounds such as V3Si2 and Nb3Sn.3 In both of these alloy systems, the transformation is known to be sensitive to materials characteristics such as composition and the presence of strain. Thus, Mailfert, Batterman, and Hanak,3 Vieland,4 and King, Cocks, and Pollock5 all failed to observe the transformation in samples of Nb3Sn, prepared by vapour deposition or from reaction with molten Sn, when the room-temperature lattice parameter of the cubic structure was in the range 5.2950 – 5.2960 A. After these samples were annealed in air or vacuum for long periods at 900 – 1000°C, however, significant amounts of Sn boiled off, reducing the room-temperature lattice parameter to the region of 5.290 A. On subsequent cooling, the low-temperature phase transformation was observed.3–5 Vieland, Wickland, and White4,6 have also shown that the transformation in Nb3Sn can be suppressed by the addition of H2 or Al, and it is significant to note that both of these alloying elements increase the lattice parameter of Nb3Sn. The addition of Sb, which lowers the lattice parameter of Nb3Sn, is also of interest, because this causes the sign of the tetragonality of the martensite phase to be reversed.7