Intermediate Phases in Superconducting Niobium-Tin Alloys

In attempting to produce superconducting wire of the niobium sheath Nb3Sn core type, it became apparent that results were generally unpredictable. Metallographic examination showed that such materials are heterogeneous and contain a number of intermediate phases. Detailed metallographic studies were made on diffusion zones in which tin had been allowed to react with porous niobium blocks, with fused niobium rod, and with niobium wires, and on a number of reacted powder mixtures. The phases produced were identified by anodizing to characteristic colors and by microspot analysis, supplemented by some hot-stage microscope and thermal analysis tests. On the basis of these observations, a tentative revised diagram is offered to illustrate the types of reactions which occur in the system. The presumably desired phase, Nb3Sn, is found to lie between the more easily formed phases Nb4Sn and Nb2Sn3, which are stable to temperatures well above the peritectoid decomposition of the Nb3Sn. At lower temperatures the compound Nb2Sn is formed. It is indicated that the high-temperature treatment to react niobium and tin should be followed either by very slow cooling or by an anneal in the 600 to 700 °C range to form Nb3Sn.