The quantity, composition, and distribution of pinning cen­ter as well as the composition of the matrix are limited, in the conventional process, by the thermodynamics of the Nb-Ti phase diagram. Additional precipitate can be pro­duced by increasing Ti content of the alloy (as shown in Figure 10), but that is more than offset by the decrease in Hc2 (Fig. 2). The result is a critical current limit in con­ventionally processed Nb-Ti superconductors of approxi­mately 3800 A/mm2, at 4.2 K and 5 T. An alternative ap­

proach is to fabricate the microstructure by mechanically assembling the desired components of the microstructure at large size and reducing the microstructure to the ap­propriate size by extrusion and cold drawing (58, 59). The engineered microstructural rods can be restacked into a composite just as for a conventional Nb-Ti superconduc­tor, but no precipitation heat treatments are required. An intermediate approach developed by Supercon, Inc. (60) uses a low-temperature diffusion heat treatment to mod­ify a densely packed microstructure fabricated from layer of pure Nb and Ti. The diffusion-modified APC has been successfully used in solenoid, model dipole (61), and MRI magnets (62). Round-wire APC superconductors and multi­layers have developed zero-field Jc up to 10% of the theoret­ical upper limit provided by the depairing current density Jd. (Jd ~ Hc/X) (e. g., Refs. 63 and 64), where X is the pen­etration depth. APC superconductors fabricated with Nb pins perform particularly well at low fields (up to about 5 T to 7 T), and Jc values approaching 7500 A/mm[1] at 3 T (65, 66) have been achieved (25% of Nb pinning center in an Nb-47 wt. % Ti matrix). Nb has been a preferred pin­ning material because of its mechanical compatibility with the Nb-Ti matrix. Even using Nb, however, poor workabil­ity and increased costs associated with assembly and yield have so far limited the commercial application ofAPC com­posites. The components of an engineered microstructure must initially be large enough to be stacked by hand (or possibly machine); consequently the engineered pins must undergo a far greater deformation to reach optimum size than for a-Ti precipitates which start at 100 nm to 200 nm in diameter. The larger deformation and multiple extru­sions and the restacks required by the APC process result in a microstructure that can be much less uniform than for the conventional process (67). For this reason, processes that can use smaller cross-sectional starting dimension, such as stacked or wrapped sheet, can result in superior properties such as the Jc of 4250 A/mm2 at 5 T and 4.2 K, achieved by Matsumoto et al. (68) with stacked sheets of Nb-50 wt. % Ti and 28 vol. % of Nb sheets. Because of the large amount of cold work in the engineered microstruc­ture, it is extremely sensitive to heating during extrusion; the highest round-wire Jc (5 T, 4.2 K) of 4600 A/mm2 was achieved by Heussner et al. (69). For the Nb pins, similar volumes of pinning material are required as for conven­tionally processed materials; but by using ferromagnetic pins (Fe or Ni) the required pin volume to achieve high critical current density has been reduced to only 2 vol. % (70). Such developments suggest that there are still ex­citing advances that can be made in the development of ductile Nb-Ti-based superconductors.

that the critical current density ofproduction LHC strands, from five different sources, when measured at 1.9 K and 9 T (2275-2376 A/mm2) is approximately that of the same strands at 4.2 K and only 6 T.

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