Correlation of superconducting and metallurgical properties of a Ti-20 at .% Nb alloy

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Abstract
The effects of heat treatment and cold work on the superconducting properties of Ti-20 at. % Nb have been studied by measurement of magnetization, M(H), critical current density, J c(H) and transition temperature, T c. The metallurgy has been determined by electron and optical microscopy and x-ray diffraction. Quenching the recrystallized alloy from the single phase β region above 500°C produces a martensitic α″ phase characterized by H c2 = 50 koe, T c = 6·5°K and a high J c arising from pinning of flux lines by the α″ structure. After very short ageing times at 330°C the α″ reverts to the β phase and H c2 and T c fall to 15 koe and 4·5 °K respectively and J c drops by several orders of magnitude. On further ageing ω-Ti precipitates and H c2 and T c increase because of Nb enrichment of the matrix. Pinning by the ω precipitate causes J c to increase. A pronounced peak effect in the J c(H) and M(H) curves has been studied in detail. There is strong evidence for a barrier to flux line entry into the pinning centres below the peak field. This barrier may be due to a Nb-rich shell surrounding the ω precipitate. If the quenched alloy is worked, the α″ phase is retained and T c and H c2 are unaltered. On ageing, the α″ reverts to strained β but T c, H c2 and J c do not drop as in the recrystallized alloy. Prolonged ageing produces a significant growth of α-Ti precipitate and T c, H c2 and J c all increase as in the recrystallized alloy.