Phase Diagram of the Nb—Sn System
The phase diagram of the Nb-Sn system is studied by metallographic analysis, x-ray diffraction, x-ray spectral microanalysis, and also thermal and chemical analysis. The alloys are studied in the cast and quenched states and also after prolonged annealing (up to 2000 h). There intermetallic compounds occur inthesystem: Nb3Sn(ß phase, about 75at.% Nb), Nb3Sn2 (γ phase, about 58 at.% Nb), and NbSn2 (δ phase, about 33.5 at.% Nb). The compounds Nb3Sn2 and NbSn2 are formed by a peritectic reaction at 910 t 10° and 840 ± 10°C respectively. The compounds Nb3Sn and NbSn2 are stable down to room temperatures; the range of stability of Nb3Sn2 lies between 820 and 910°C. The crystal structure of the δ phase is studied for the case of single crystals. The compound has an orthorhombic structure with parameters a = 5.65 Å, b = 9.85 Å, and c = 19.2 Å. The structure of the γ phase has not been established. There is a range of homogeneity in the compound Nb3Sn; at 950°C the solubility of tin in Nb3Sn equals 0.6 at.% and that of niobium 1.3 at.%; at 1500°C these values rise to 1.02 at.% Sn and 1.0 at.% Nb. The solubility of tin in solid niobium at 1000 to 1500°C is no greater than 2.5 at.%.Sn.
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