Restricted , Novel Heat Treatments for Obtaining High

The effect of greatly restricting heat treatment time and temperature, as well as the strain space between heat treatments was investigated for a high homogeneity Nb46.5wt% composite. The number of heat treatments was 3 or 6. It was found that the crucial heat treatment was the final one and that the initial ones could be considerably restricted. A strong linear relationship was found between Jc (5T, 8T) and the % α-Ti for Jc (5T) values ranging from ~ 950-3050 A/mm and α-Ti contents ranging from 2.5 to 21 vol%. INTRODUCTION The fabrication process for a high Jc Nb-Ti superconductor is illustrated in Figure 1 in terms of cold work strain. Strain will be defined here in terms of true strain, e — In (A0/A), where A0 and A are the original last recrystallization and final cross-sectional areas of the Nb-Ti alloy. A cold work prestrain, ep, of approximately 5 is required before the first heat treatment of Nb-46.5wt%Ti alloy to ensure precipitation of α-Ti at /β-Nb-Ti triple points. The exact prestrain depends on heat treatment temperature, alloy composition and alloy homogeneity. Additional heat treatments are applied at strain intervals of 1.15; these heat treatments increase the amount of α-Ti and improve the uniformity of the microstructure. Increasing the number of heat treatments and their duration increases the Jc and the quantity of precipitate.' After final heat treatment a large final drawing strain is applied. This strain distorts the α-Ti precipitates into a densely folded array of ribbons typically 1-2 nm thick, 4-8 nm apart. The final strain ranges from 4-5 with the higher strains being required to optimize composites having longer and/or higher temperature heat treatments (and higher peak Jc). In practice the total strain available for these operations is limited and is reduced by any warm or hot working process that reduces the stored work. As normal commercial practice involves at least one warm extrusion using temperatures above 500°C, the loss in available strain space may be considerable. Methods of more fully utilizing the strain space include using hydrostatic extrusion in place of extrusion or using NbTi rod annealed at a larger size. An alternative approach is to reduce the strain space between heat treatments. This experiment compares schedules using 3 heat treatments (HT) with one using 6 HT within a total heat treatment strain space of 3.4. The duration of heat treatment is normally kept the same throughout the process and is typically 40 hrs or 80 hrs. Previous monofilament studies have shown that 80 hrs is superior to 40 hrs and that 160 hrs may also offer improved ultimate Jc. Long heat treatments such as these, however, require excessive furnace time and may result in deleterious intermetallic formation at the filament surface if no diffusion barrier or one of insufficient thickness is used. Examining the amounts of α-Ti produced in previous heat treatment studies 2 there seems little advantage in using long initial heat treatments. For example, 10 hrs at 405°C produced 9 volume % α-Ti , whereas 80 hrs at 420°C produced only 11 volume % α-Ti at the first H.T. Accordingly the principal thrust-of the present work was to understand how the initial heat treatment(s) could be minimized, without unacceptably degrading the final Jc. The lowest heat treatment time and temperature used was 3 hrs at 300°C; this heat treatment, often used as a Cu anneal, has been Advances in Cryogenic Engineering (Materials), Vol. 36 Edited by R. P. Reed and F. R. Fickett Plenum Press, New York, 1990 found to only produce a thin grain boundary film of α-Ti in Nb-46.5wt%Ti. The grain boundary film, typically less than 4 nm thick, is also produced at the higher temperature heat treatments. By using a heat treatment of 3 hrs at 300° C in some schedules, the role of the grain boundary film in terms of triple-point α-Ti precipitate nucleation and microstructural refinement could be examined. Previous work suggested that considerable precipitate coarsening occurs in later heat treatments. If heat treatment times are minimized, a reduction in precipitate size would be expected. Monofilament studies have shown that long heat treatment times result in longer final strains being required for peak Jc. If this is a consequence of the larger precipitate size at final HT size, then it might be speculated that εf can be reduced by minimizing heat treatment time.