Ti Redistribution in Multifilamentary Nb/Cu-Sn Composites

Abstract

Multifilamentary bronze-processed Nb3Sn-based composites have been studied by the methods of TEM and SEM. Ti as a doping element required for an enhancement of superconducting characteristics, especially in high magnetic fields, was inserted either in a bronze matrix, or in Nb filaments of a composite. It has been found that Ti diffuses into the growing Nb3Sn layer in both cases, and affects positively its structure and superconducting characteristics of a composite as a whole, especially in case of the doped matrix. When Ti is added to Nb filaments, it forms fine particles of intermetallic compounds with Sn in the nanocrystalline diffusion layer. When these particles are formed, grain boundaries of the diffusion Nb3Sn layer purify from segregations, and grains in the vicinity of these particles coarsen, which negatively affects the current-carrying capacity of a composite. That’s why an optimal amount of Ti in Nb should be chosen, when Ti mainly dissolves in the Nb3Sn phase increasing its superconducting properties and not deteriorating its grain structure.

Effect of Multifilamentary Nb/Cu-Sn Wire Diameter on the Nb₃Sn Diffusion Layers Structure

“…4c, in which these superstructural reflections are indicated by arrows). We previously observed more than once such super- structural reflections in the diffusion layers of A 3 B phases forming in bronze-processed superconducting composites of various types (see, for example, [6,8,9]), and in [10] we suggested that their appearance is due to deviation of the diffusion layers composition from stoichiometry, namely, the lack of component B (Sn or Ga), which results in the additional ordering of vacancies in the element B sub-lattice.…”

Evolution of the nanocrystalline structure of Nb3Sn superconducting layers upon two-stage annealing of Nb/Cu-Sn composites alloyed with titanium

“…Upon the employment of artificial alloying of Nb filaments with titanium, the width of the zone of columnar grains and their sizes decrease as the concen tration of this alloying element increases to 1.55 wt %. However, it is necessary to remember that, with growth of the Ti concentration in Nb filaments, sizes of equiaxed grains of the superconducting phase may increase, as is shown in this work, even at the first stage of diffusion annealing and, especially, as we found ear lier [22,23], upon long term two stage annealing. The necessity of limitation of the upper value of the Ti con centration is additionally caused by a narrow range of the positive action of this alloying addition on the upper critical field, which is maximum when the tita nium concentration is 1.55-1.75% Ti [6].…”

“…Upon alloying of Nb filaments, the increase in grain size and in the grain size scatter can be observed already at the first stage of annealing at an enhanced (1.51, 1.75%) con tent of this alloying addition. Earlier, we revealed this effect upon two stage diffusion annealing in [22,23]. After two stage diffusion annealing, the amount of residual Nb decreases considerably, especially in alloyed conductors; the dimensions of grains in Nb 3 Sn continuous diffusion layers become larger by a factor of about 1.5; and the spread of grain size increases ( Table 4).…”

“…In both cases, Ti penetrates into the Nb 3 Sn diffusion layer, and in the latter case its content in the supercon ducting layer is higher than in the former case [20]. In [21], the existence of intense diffusion fluxes of this element from one component of the composite into another is explained, and in [22,23] it is shown that the amount of Ti, especially upon its introduction into Nb filaments, should not exceed some optimum value, since at enhanced Ti concentrations the nanocrystal line structure of the superconducting layers may become worse.…”

“…In [22,23,29], we established a correlation between the sizes of grains of the superconducting phase and the critical current density in multifilamentary Nb 3 Sn based conductors (the smaller the average grain size, the higher the critical current). On the basis of data obtained in this work, it can be expected that the com posites subjected to a shorter annealing, as compared to the earlier adopted regime for ITER, will possess suffi ciently high superconducting characteristics.…”