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

Abstract

The effect of diffusion annealing on the structure of superconducting layers of the Nb 3 Sn phase and the extent of transformation of niobium filaments of multifilamentary Nb/Cu-Sn superconductors obtained using the bronze process has been investigated by scanning and transmission electron microscopy. The sam ples differed in the titanium concentration and method of alloying. It has been shown that the thickness of layers of the Nb 3 Sn phase grows with an enhancement in the titanium concentration in Nb filaments and that it is composites with the alloyed bronze matrix that exhibit the most favorable structure and morphology from the viewpoint of reaching the maximum current carrying capacity of the conductor in all of the regimes of diffusion annealing under study. DERYAGINA et al.Notes: L is the width of the Nb 3 Sn layer; L eq is the width of the zone of Nb 3 Sn equiaxed grains; L col is the width of the zone of columnar grains; D col,max and D eq,max are the maximum longitudinal size of columnar grains and the maximum diameter of equiaxed grains; and Nb : Sn is the ratio between Nb and Sn (in at %) in Nb 3 Sn layers.

Specific Features of Statistical Analysis for Grain Nanostructures Formed by Solid-State Diffusion at Annealing

“…The results of the approximation of the experimental distributions by means of the models are summarized in Table 1. The various samples from [24][25][26][27] is represented as two sets. The first set consists of the samples after plastic deformation and the annealing at 575°С, 100 h (the numbers from 1 to 6).…”

“…Different Nb3Sn structures formed in Nb-Ti/Cu-Sn, Nb/Cu-Sn-Ti and Nb/Cu-Sn composite wires after various regimes of the diffusion annealing were selected for the analysis from [24][25][26][27], where the technologies of composite production and structures of superconducting layers in multifilamentary wires is described in detail. In accordance with the technology, the Nb3Sn grain structure is formed by solid-state reactive diffusion.…”

“…The experimental distributions from [24][25][26][27] were normalized in order to be in the uniform scale.…”

“…In contrast to the materials obtained by SPD, the structure of Nb3Sn layers in Nb/Cu-Sn composites, as material for the analysis which possesses a homogeneous nanostructure [24][25][26][27], is well suited. The formation of the Nb3Sn grain structure occurs during annealing of a pre-deformed composite in accordance with the technology of their production [24][25][26] and can be considered as a result of a single process.…”

Statistical Analysis of Histograms of Grain Size Distribution in Nanostructured Materials Processed by Severe Plastic Deformation

“…The smallest grain sizes that can be achieved by severe plastic deformation (SPD) were observed mainly in materials processed by the high-pressure torsion (HPT) [6][7][8][9][10][11]. Another example of materials with a homogeneous nanostructure is Nb 3 Sn composites in which a structure with similar characteristics is formed by annealing performed after large deformations in accordance with the technology of their production [12][13][14]. In all these works, the results are represented as histograms of size distribution of grains.…”