The variation in the microstructure by aging was investigated by means of electron microscopy in heavily cold-worked Nb-Ti-Zr alloys on the Nb-Ti side. The variations in the critical current density,
Jc, by aging reported in the former paper were correlated with the microstructure changes observed in the present study, and the pinning mechanism was examined. The results are summarized as follows:
(1) On aging at 500°C, an equilibrium phase α precipitates directly from a β phase, whereas on aging at 350°C, a transition phase ω is formed prior to the precipitation of the α phase.
(2) The α phase takes a hexagonal closed-packed structure with lattice parameters
a=0.296 and
c=0.471 nm and has the orientation relationships (0001)
α\varparallel(1\bar10)
β and [11\bar20]
α\varparallel[111]
β. The ω phase takes a hexagonal structure with lattice parameters
a=0.463 and
c=0.284 nm and has the orientation relationships (\bar2110)
ω\varparallel(1\bar10)
β and [0001]
ω\varparallel[111]
β.
(3) When the heavily cold-drawn wire with fibered structure is aged, precipitation occurs preferentially at the fiber boundaries and these precipitates grow coarser than those within the fibers. So,the structure of aged wire grows coarser with increasing cold-reduction prior to aging.
(4) In the precipitation of the α phase from the β supersaturated solid solution, the precipitates become finer and numerous at a lower aging temperature or with a higher solute Ti or O
2 content in the β phase.
(5) The effective fluxoid pinners are the α and ω, precipitates in specimens aged after cold-working. The highest
Jc is obtained by aging which produces a fine dispersion of precipitates. The
Jc reaches a maximum value as the size of precipitate is on the order of 10 nm.
(6) The peak of the curve in pinning force density versus applied magnetic field increases in height and shifts to lower field with isothermal aging at relatively low temperatures (\lesssim400°C).
(7) It is found that Kramer’s model seems to account for flux pinning in specimens with a fine distribution of precipitates.
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