Cold-Worked Microstructures

In order to achieve high critical current densities a fine and homogeneous dispersion of flux pinning material must be introduced that is of sufficient volume for significant pin­ning but does not deleteriously affect the other Hc2 or Tc. The process by which the first high critical current density microstructures were achieved was arrived at empirically before the resulting microstructures were characterized (12). The processing involved a high cold-work strain fol­lowed by three or more heat treatments in the a + в phase range, each separated by additional cold work with the fi­nal heat treatment being followed by another large cold – work strain. An understanding of the microstructural de­velopment was key, however, to the further optimization of Nb-Ti and the reproducible production of high critical cur­rent strand. Initial observation of the microstructure was hindered by the difficulty in preparing transverse cross sec­tions of micron-sized filaments suitable for examination by transmission electron microscopy (TEM). Once techniques had been developed to prepare the TEM specimens, it be­came clear that folded sheets of a-Ti precipitates were the dominant microstructural features of the final strand (see Ref. 13). Systematic analysis of the production process (as in Ref. 14) revealed that the high prestrain heat treat­ments produced a-Ti precipitates only at the intersections of grain boundaries. The location of precipitation at the grain boundary triple points meant that the precipitation was homogeneously distributed if alloy composition and grain size were uniform. The grain boundary triple-point a-Ti was also sufficiently ductile that it could be drawn down to the nanometer scale with breaking up or caus­ing the strand itself to become difficult to draw. This con­trasted with the other commonly observed a-Ti precipitate morphology, Widmanstatten a-Ti, which formed in densely packed rafts in the interior of grains and resulted in a great increase in the filament hardness. The next section reviews each step of the process in more detail.

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