Nb3Sn Grains Research Articles

Unveiling the nucleation and growth of Zr oxide precipitates in internally oxidized Nb3Sn superconductors

We report on atomic-scale analyses of nucleation and growth of Zr oxide precipitates and the microstructural evolution of internally oxidized Nb3Sn wires for high-field superconducting magnet applications, utilizing atom probe tomography (APT), transmission electron microscopy (TEM), and first-principles calculations. APT analyses reveal that oxygen and zirconium are already segregated at grain boundaries (GBs) in the unreacted Nb-1Zr-4Ta (at%) alloy prior to forming Nb3Sn through reacting the Nb alloy with Sn and SnO2. After forming Nb3Sn, Zr oxide precipitates nucleate both at the Nb3Sn/Nb heterophase interfaces and in the Nb3Sn grains, driven by the small solubilities of Zr and O in Nb3Sn compared to their value in Nb. A high number density (Nv) of Zr oxide nanoprecipitates is observed in the Nb3Sn layers, ∼1023 m−3, with a mean diameter <10 nm for a heat treatment at 625 °C. Quantitative APT and TEM analyses of the Zr oxide precipitates in the reacted Nb3Sn layers elucidate details of the nucleation, growth, and coarsening processes of the Zr oxide precipitates in Nb3Sn. First-principles calculations and classical nucleation theory are employed to study the nucleation of Zr oxide precipitates in Nb3Sn and to estimate the maximum energy barrier and critical radius for nucleation. Our research unveils the kinetic pathways for nucleation and growth of Zr oxide precipitates and the microstructural evolution of Nb3Sn layers, which helps to understand and improve the superconducting properties of internally oxidized Nb3Sn wires for use in high-field superconducting magnets.

Comparative drawability and recrystallization evaluation of Nb4Ta and Nb4Ta1Hf alloys, and the beneficial influence of Hf on developing finer Nb3Sn grain size

Nb3Sn superconducting wires with high critical current density (Jc, 4.2 K) at fields greater than 15 T are needed for next-generation superconducting magnets for high-energy physics and nuclear magnetic resonance applications. It has been demonstrated that the addition of one atomic % Hf can significantly increase the high field Jc of Ta-doped Nb3Sn and offer the potential to meet the Future Circular Collider (FCC) specification of 1500 A/mm2 at 16 T, 4.2 K if the pure Nb filaments in conventional Nb3Sn composites can be replaced by Nb4Ta1Hf alloy. In this paper we demonstrate that a commercially produced Nb4Ta1Hf alloy is ductile and drawable in a Cu matrix to a large true strain of 15 without intermediate annealing. The hardness and work-hardening rates of Nb4Ta1Hf are shown to be higher than Nb and commercial Nb4Ta, but are comparable to ductile Nb47Ti alloy at high strains, which is significant because more than 1000 tons of Nb-Ti rods are co-drawn in Cu each year. Very importantly for the Nb3Sn properties is that just 1 at%Hf shifts the recrystallization curve of Nb4Ta to temperatures above the normal 600 °C −700 °C A15 range of reaction temperatures used for Nb3Sn wires. For the Hf-based 19 filament rod in tube (RIT) conductors of this study, we observe a refined (<100 nm) Nb3Sn grain size, and an improved upper critical field of 24.6 T (4.2 K), ∼1 T higher than that in a corresponding Nb4Ta composite. The demonstration of improved performance and good drawability with commercially produced Nb4Ta1Hf alloy indicates a pathway for Nb3Sn strand development for next-generation Nb3Sn magnets.

In-depth S/TEM observation of Ti–Hf and Ta–Hf-doped Nb3Sn layers

In superconducting Nb3Sn layers with coherence lengths of approximately 3 nm, grain boundaries act as effective pinning sites. Thus, grain refinement is an essential issue that directly affects the superconducting critical characteristics of the Nb3Sn layer. In recent years, Hf addition to Nb3Sn wires co-doped with Ta has attracted notable interest as a method that enables grain refinement down to several tens of nm. In-depth characterization of the Nb3Sn grain morphology in Hf-doping is crucially important to correlate the microstructure with the flux pinning characteristics. In this article, the grain morphologies of Ti–Hf and Ta–Hf-doped Nb3Sn layers were clarified by scanning transmission electron microscopy (STEM) and TEM-based automated crystal orientation mapping (ACOM-TEM). STEM/energy dispersive x-ray spectroscopy (EDS) revealed no significant oxide precipitates in our samples. The grain size distribution was attained by ACOM-TEM. Although Hf-doping attained a grain refinement effect in the Nb3Sn layer in both doping cases, the degree of this effect was relatively small for Ti–Hf. Kernel average misorientation analysis by scanning electron microscopy-electron backscattered diffraction unveiled no appreciable difference between the internal strain states of the Nb-alloy parent phases in Ti–Hf and Ta–Hf. One remarkable new finding through STEM/EDS was the presence of a Cu–Hf compound phase in the Nb3Sn layer. The Cu–Hf compound sounds analogous to the Cu–Ti compounds that form when Nb–47Ti with Cu matrix is heat treated. The STEM/EDS maps revealed a larger amount of Cu flow from the Cu–Sn side along the grain boundaries. The large Cu deposition on the grain boundaries might facilitate grain growth in Nb3Sn. Those findings make a novel contribution to the literature as they provide a deep insight into Nb3Sn phase formation via Hf doping.

Formation of columnar grains during diffusional growth of Nb3Sn layer and its suppression

The manufacturing process of multifilamentary Nb3Sn superconducting wire includes annealing for diffusional growth of the Nb3Sn intermetallic compound layer. During the process, the formation of columnar Nb3Sn grains should be suppressed to maximize the flux pinning effect. However, the detailed mechanism underlying the formation of columnar Nb3Sn grains has not been clarified yet, and is an obstacle to finding optimal process conditions. In the present work, we investigated the fundamental reason for the columnar grain formation, using a novel Monte Carlo Potts model which can simultaneously predict grain growth and interfacial reactions. We found that a decrease in thermodynamic driving force due to insufficient Sn at the interface causes the formation of columnar Nb3Sn grains. Accordingly, columnar grain formation is expected to be suppressed when abundant Sn atoms are supplied rapidly to the reaction front. Based on the fundamental understanding of columnar grain formation, possible ways to suppress columnar grain formation are suggested. We especially focused on the difference in the columnar grain fraction depending on the wire strand type (bronze processed wire vs. internal-tin processed wire). Based on simulations and experimental analyses, we suggest a brief guideline for the design of wire strand geometry to avoid columnar grain formation.

Effects of the oxygen source configuration on the superconducting properties of internally-oxidized internal-Sn Nb3Sn wires

We successfully manufactured 12-filament rod-in-tube Nb3Sn wires with oxide nanoparticles formed by the internal oxidation method. We employed Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt% Hf alloys along with oxygen sources (OSs) in two different configurations—in the core of Nb filaments (coreOS) and at the boundary between the filaments and the Cu tube (annularOS)—to assess the influence of the OS layout on the superconducting properties and grain size. The simultaneous presence of the OS and of Hf or Zr reduced the average Nb3Sn grain size to around 50 nm, leading to an enhancement of the layer critical current density (Jc ) up to 3000 A mm−2 at 4.2 K and 16 T for the Hf-annularOS wire. Samples manufactured with an OS show a shift toward higher reduced magnetic fields of the position of the maximum in pinning-force density, this shift being more pronounced when SnO2 is added in the annularOS configuration, and for the Hf-containing samples. This enhanced pinning at higher magnetic field is beneficial for high-field magnet applications. Moreover, we measured a very high upper critical field, reaching 29.3 T at 4.2 K in the Hf-annularOS samples.

Measuring Three-Dimensional Strain in Nb3Sn Grains by Combining ZOLZ and HOLZ diffraction.

Measuring Three-Dimensional Strain in Nb3Sn Grains by Combining ZOLZ and HOLZ diffraction.

Fabrication and performance of Nb3Sn superconducting joints

Nb3Sn superconducting joints were developed, and the joint characteristics were investigated. The joints were prepared by powder metallurgy to sinter the precursor powders that were mechanically alloyed. The optimized parameters were confirmed through x-ray diffraction studies to confirm the stoichiometry of Nb, Sn, and Cu. First, the precursor powders after mechanical alloying were heat treated without the Nb3Sn wires. Electron backscatter diffraction was used to quantify the crystal morphology, phase distribution, grain boundaries, physical grain orientation, and so on. Small equiaxed Nb3Sn grains were formed after heat treatment. The high-angle grain boundaries in Nb3Sn indicated the complete solid-state diffusion reaction. After the Nb3Sn was successfully prepared, and the superconducting characteristics were measured, we introduce the Nb3Sn wires to prepare Nb3Sn joints and measure their electrical properties. To use the space as economically as possible, the joint was tested under different background fields to estimate the joint resistance under various locations in the magnet. The experimental method and the test results are reported in this work. The joint resistance was tested by using the current decay measure method. The electrical resistance of the Nb3Sn–Nb3Sn joints was on the 10−13 Ω level under the 5.5 T magnetic field.

Atomic behavior of Ti in A15 Nb3Sn and its effects on diffusional growth of Nb3Sn layer

Atomic details of the dramatic increase in the diffusional growth rate of A15 Nb3Sn superconducting compound with Ti addition have been investigated using atomistic simulations and experimental observations. Ti atoms preferentially locate at Nb sites in the Nb3Sn bulk while they do not affect the volume diffusion of Sn significantly. In addition, Ti is predicted to segregate on Nb3Sn grain boundaries, inducing the segregation of Sn, but cannot enhance the grain boundary diffusion of Sn enough to explain the dramatic increase in the growth rate of the Nb3Sn layer. On the other hand, grain refinement is observed when Ti is added, which is expected to substantially promote the effective diffusion of Sn. Consequently, grain refinement due to the grain boundary segregation of Ti is revealed as the most important mechanism underlying the alloying effect of Ti. A basic understanding of the atomic behavior of alloying elements in Nb3Sn and clarified alloying effects on the diffusional growth of Nb3Sn compounds will contribute to further investigation of optimal alloying strategies.

Impact of Ti-doping position on Nb3Sn layer formation in internal Sn-processed Nb3Sn superconducting wires

Further enhancement of critical current density of Nb3Sn superconductors is still of great interest and challenging towards realization of leading-edge magnets such as the future circular collider (FCC) and demonstration Power Station (DEMO) fusion reactors. In this study, the influence of the Ti-doping position on the Nb3Sn layer formation in internal Sn-processed Nb3Sn superconducting wires, which has not been sufficiently clarified yet, was investigated using the simplest diffusion pair structure that consists of a single Nb/Cu/Sn layer. Three types of specimens were prepared, in which Ti was doped into the Sn core, Nb, and Cu matrix (denoted as ST, NT, and CT, respectively). The NbCuSnTi compound phase appeared as a diffusion barrier at the boundary between the Nb3Sn and Cu–Sn phases in the ST and CT samples and remained during the Nb3Sn layer formation, although it was absent in NT. The electron backscatter diffraction analysis of the formed Nb3Sn grain morphologies showed no significant difference in terms of the average grain size and area fraction of the equiaxed grains regardless of the doping position. However, the average Nb3Sn layer thickness of NT was at least 1.6 times larger than that of ST and CT. The absence of the NbCuSnTi phase promotes Sn diffusion into Nb, which accounts for the thicker Nb3Sn layer and much higher Ic–B characteristics in NT.

Comparative Drawability and Recrystallization Evaluation of Nb4ta and Nb4ta1hf Alloys, and the Beneficial Influence of Hf on Developing Finer Nb3sn Grain Size and Higher Superconducting Critical Current Density

Comparative Drawability and Recrystallization Evaluation of Nb4ta and Nb4ta1hf Alloys, and the Beneficial Influence of Hf on Developing Finer Nb3sn Grain Size and Higher Superconducting Critical Current Density

Origin of the enhanced Nb3Sn performance by combined Hf and Ta doping

In recent years there has been an increasing effort in improving the performance of Nb3Sn for high-field applications, in particular for the fabrication of conductors suitable for the realization of the Future Circular Collider (FCC) at CERN. This challenging task has led to the investigation of new routes to advance the high-field pinning properties, the irreversibility and the upper critical fields (HIrr and Hc2, respectively). The effect of hafnium addition to the standard Nb-4Ta alloy has been recently demonstrated to be particularly promising and, in this paper, we investigate the origins of the observed improvements of the superconducting properties. Electron microscopy, Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) and Atom Probe Tomography (APT) characterization clearly show that, in presence of oxygen, both fine Nb3Sn grains and HfO2 nanoparticles form. Although EXAFS is unable to detect significant amounts of Hf in the A15 structure, APT does indeed reveal some residual intragrain metallic Hf. To investigate the layer properties in more detail, we created a microbridge from a thin lamella extracted by Focused Ion Beam (FIB) and measured the transport properties of Ta-Hf-doped Nb3Sn. Hc2(0) is enhanced to 30.8 T by the introduction of Hf, ~ 1 T higher than those of only Ta-doped Nb3Sn, and, even more importantly the position of the pinning force maximum exceeds 6 T, against the typical ~ 4.5–4.7 T of the only Ta-doped material. These results show that the improvements generated by Hf addition can significantly enhance the high-field performance, bringing Nb3Sn closer to the requirements necessary for FCC realization.

Very high upper critical fields and enhanced critical current densities in Nb3Sn superconductors based on Nb–Ta–Zr alloys and internal oxidation

The inhibition of Nb3Sn grain growth in the presence of ZrO2 nanoparticles appears to be one of the most promising method for pushing the critical current densities of Nb3Sn superconducting wires to levels that meet the requirements set for the Future Circular Collider. We have investigated the effect of ZrO2 nanoparticles formed by the internal oxidation of Zr on the superconducting properties and microstructure of Nb3Sn formed from Nb-1 wt%Zr, Nb-7.5 wt%Ta, Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt%Zr alloys. A monofilamentary wire configuration was used, with a 0.22 mm outer diameter Nb-alloy tube containing a core of powdered metal oxide (SnO2, CuO or MoO3) as oxygen source and successive deposits of Cu, Sn and Cu on the outer surface. As determined from inductive measurements, the layer critical current densities of the samples based on Nb alloys with internally oxidized Zr were superior to those based on Nb-7.5 wt%Ta. The samples based on Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt%Zr showed higher critical current densities at high magnetic fields (above 10–15 T), and upper critical fields exceeding 28.5 T at 4.2 K (99% normal state resistivity criterion). A record value of 29.2 T of the upper critical field at 4.2 K was obtained on samples based on Nb-7.5 wt%Ta-2 wt%Zr. Hypotheses are proposed and discussed for explaining this unexpected increase of the upper critical field, by considering the possible effects of non-oxidized Zr on the superconducting properties of Nb3Sn and of the oxidized Zr on the formation and microchemistry of Nb3Sn. Regardless of sample type the Nb3Sn grains observed in our samples have an aspect ratio of 1.5–1.7. When compared in the short axis direction, the mean distance between grain boundary intercepts (lineal intercept method) is ∼40% smaller in the samples with internally oxidized Zr than in the reference samples based on Nb-7.5 wt%Ta. In the long axis direction the reduction is of 20%–30%.

The strong influence of Ti, Zr, Hf solutes and their oxidation on microstructure and performance of Nb3Sn superconductors

Over the last few years a new type of Nb3Sn superconducting composite based on the internal oxidation approach has emerged and has demonstrated performance significantly superior to conventional Nb3Sn. It requires a supply of O and the use of a Nb alloy – Nb-X, where X is a solute element that can be selectively oxidized to form oxide particles within the Nb3Sn. Such oxide particles not only refine Nb3Sn grain size, but also have the proper size to act as artificial pinning centers (APC) directly restraining fluxon motion, and thus dramatically improve superconducting properties. In this article we show that the size and volume fraction of the oxide particles determine both the levels of grain refinement and the shift in the peak field of the flux pinning force (Fp-B) curve. We explore the factors influencing the microstructure and properties, which we find include: selection of the solute element X, solute content, O content, and heat treatment. For the selection of X, we searched the periodic table for all promising candidates but focused down on the group-IVB elements (Ti, Zr, Hf) here as the drawability of Nb3Sn wires made from Nb-Ti, Nb-Zr, and Nb-Hf alloys has been demonstrated in the past few decades. We found that while internally oxidizing Nb-1.5 at.%Ti led to negligible grain refinement and Fp-B peak shift, Nb-Zr alloys led to much more dramatic results, and internally oxidizing Nb-Hf alloys led to the strongest grain refinement and Fp-B peak shift. For Hf alloying in particular, we compared our internal oxidation method with another method for grain refinement, which uses Hf alloying itself without oxidation, and found that internal oxidation led to much stronger grain refinement and Fp-B peak shift. We also found that higher solute content and lower reaction temperature led to stronger grain refinement and Fp-B peak shift. We conclude with a discussion of the possible mechanisms for the influence of these factors.

Effect of Nb3Sn Coarse Grains on Critical Current Densities of Internal Tin Nb3Sn Strand

Nb3Sn strands have significant applications in constructing high-field (>10 T) magnets and have great potential value in the next 20 years. Internal-tin (IT) Nb3Sn strand has been developed by many methods for the future fusion reactor after ITER with low hysteresis loss. However, how to improve the current densities of Nb3Sn strand has become very important with the increasing demand for high Jc Nb3Sn strand. In our study, we found that more than double Nb content should be added within Internal Tin Nb3Sn strand and 2 times critical current densities can be enhanced compared with ITER Nb3Sn strand. With this Nb3Sn strand, the phase transition during heat treatment was investigated and new phase (Cu-Nb-Sn) can be formed at 400 °C due to higher ratio of Sn/Cu and more Cu6Sn5 phase decomposition. The coarse Nb3Sn grains were analyzed and a composite structure (Cu and Nb3Sn) can be formed as Nb element diffusion during Cu-Nb-Sn phase formation. We also found that phase-balance (less Cu-Nb-Sn produced), which can control the formation of Nb3Sn coarse during filaments, can be obtained through element ratio optimization. With this method, coarse grains among filaments can be refined from micron size into nano-size. Effect of varisized coarse grains on the critical current densities was also studied and we found that micron sized Nb3Sn (1∼2 μm) grains reduced the critical current densities severely.

Growth of Nb3Sn coating in tin vapor-diffusion process

The potential of Nb3Sn for superconducting radio frequency cavities is widely recognized, and renewed R&amp;D efforts continue to bring new insights about the material's structure and properties. We have systematically coated niobium samples with Nb3Sn for different durations. Several samples were coated over (“overcoat”) multiple times to elucidate the growth kinetics of Nb3Sn during the tin vapor-diffusion process. Analysis of coated samples is consistent with the model in which tin diffuses via grain boundaries to the Nb3Sn-Nb interface, where the growth of Nb3Sn into the niobium bulk takes place. Similar scaling laws are found for Nb3Sn grain growth and layer thickness. Nonparabolic layer growth is consistent with significant grain growth, which reduces the number of Sn transport channels. An examination of patchy regions in Nb3Sn coating, revealing large single-crystalline grains, points to impeded Nb3Sn layer growth due to low grain boundary density, resulting in a significantly thinner coating in those areas.

Microstructure and Superconducting Properties of Brass Matrix Internal Tin Nb$_3$Sn Wire With Ti Doping to Nb Core

Nb 3 Sn superconducting wires are widely used for high field applications. It is also expected to play a vital role for high field application in the future, such as future circular collider (FCC). Further improvement in Jc performance is required to realize FCC. We have been studying the effect of element addition into Cu matrix in the internal tin (IT) processed Nb 3 Sn conductor. In our previous brass method IT wire, where Ti is doped to central Sn cores, Ti is accumulated between the sub-elements and is inhomogeneously distributed across the wire after the heat treatment. To solve this problem, we tried Ti doping to Nb cores on the brass matrix IT wire. We prepared the specimens using Nb-0, 1, 1.54 at%Ti rod as Nb core. Nb 3 Sn grain morphology and diffusion reaction behavior of these specimens were compared and we investigated the correlation between the microstructure and the superconducting properties on the specimens with Ti doping to Nb cores.

The effects of Mg doping on the microstructure and transport properties of internal tin-processed brass matrix Nb3Sn superconductors

The addition of Zn into Cu matrix has been proved to enhance the Sn activity and accelerate the formation of a Nb3Sn layer in internal tin-processed Nb3Sn wires. However, in our samples, Ti is doped in Sn cores, and during the reaction, Ti was observed to accumulate between the inner and outer sub-elements of the 1980-Nb-core multi-filamentary brass matrix wires. With the addition of Mg into the brass matrix, Ti accumulation between the sub-elements was significantly suppressed, which promoted Ti incorporation into the Nb3Sn layers. Meanwhile, the formed Nb3Sn layers exhibited smaller grain sizes. Compared to the brass (Cu-12wt%Zn) matrix wires, an improvement of critical current density (Jc) was found in the Cu-12Zn-0.2Mg matrix wires due to the incorporation of Ti into the Nb3Sn layer and the refinement of the Nb3Sn grain size. The irreversibility field (Birr) and maximum bulk pinning forces (Fp,max) were calculated from the Kramer plot by analyzing the Jc(B) curves. A small amount of Mg (0.2 wt%) doping into the Cu-12Zn matrix wires can increase the Birr value by about 0.5 T and the Fp,max value by about 3 ∼ 4 GN m−3 after the wires were heat treated between 670 °C and 730 °C. The non-Cu Jc of the Cu-12Zn-0.2Mg matrix wires reaches the maximum value of 1533 A mm−2 @12 T after heat treatment at 685 °C and decreases at higher temperatures.

Atomic-scale analyses of Nb3Sn on Nb prepared by vapor diffusion for superconducting radiofrequency cavity applications: a correlative study

We report on atomic-scale analyses of the microstructure of an Nb3Sn coating on Nb, prepared by a vapor diffusion process for superconducting radiofrequency (SRF) cavity applications using transmission electron microscopy, electron backscatter diffraction and first-principles calculations. Epitaxial growth of Nb3Sn on a Nb substrate is found and four types of orientation relationships (ORs) at the Nb3Sn/Nb interface are identified by electron diffraction or high-resolution scanning transmission electron microscopy (HR-STEM) analyses. Thin Nb3Sn grains are observed in regions with a low Sn flux and they have a specific OR: Nb3Sn //Nb and Nb3Sn //Nb The Nb3Sn/Nb interface of thin grains has a large lattice mismatch, 12.3%, between Nb and Nb3Sn (002) and a high density of misfit dislocations as observed by HR-STEM. Based on our microstructural analyses of the thin grains, we conclude that the thin regions are probably a result of a slow interfacial migration with this particular OR. The Sn-deficient regions are seen to form initially at the Nb3Sn/Nb interface and remain in the grains due to the slow diffusion of Sn in bulk Nb3Sn. The formation of Sn-deficient regions and the effects of interfacial energies on the formation of Sn-deficient regions at different interfaces are estimated by first-principles calculations. The finding of ORs at the Nb3Sn/Nb interface provides important information about the formation of imperfections in Nb3Sn coatings, such as large thin-regions and Sn-deficient regions, which are critical to the performance of Nb3Sn SRF cavities for accelerators.

Effect of Sn Concentration in Bronze Matrix on the Pre-Reaction Formation of Nb3Sn Layers in Bronze-Processed Superconducting Strands of Different Design

The goal of this paper is to study and analyze the effect of Sn concentration (14% or 16%), the mode of Ti-doping and the shape of Nb filaments on the thickness of the “parasitic” layers of prereacted Nb 3Sn grains (formed before the diffusion annealing) in the ITER-type strands and strands for high field applications processed by bronze technology. The use of short (of total duration not more than 1 h) intermediate heat treatments at 500 °С does not allow avoiding the prereaction formation of Nb3Sn in all the samples under study. Sn concentration in bronze decreases by 1.5–2.0% after heat treatments compared to the initial state in all samples. In the samples with Ti-doped bronze matrix and coupled Nb filaments, the average thickness of “parasitic” layers is 112 nm in the case of 14% Sn in bronze and 140 nm for a bronze matrix with 16% Sn. In the sample with 16% Sn in matrix and extended Nb filaments of intricate shape the average thickness of prereacted layers reaches 200 nm. It is concluded that to minimize the parasitic layers formation the strands with high tin bronze matrix should be subjected to short intermediate heat treatments at temperatures lower than 500 °C.

Growth Rate and Morphology of Nb3Sn Layers in ITER-Type Bronze-Processed Wires Under Different Diffusion Annealing Regimes

The Nb3Sn-based bronze-processed multifilamentary composites have been fabricated at the Bochvar Institute of Inorganic Materials, Moscow, Russia, and studied by electron microscopy to determine the relationships between their structural parameters and annealing duration at different temperatures (575 °C, 650 °C, 750 °C). The structure of superconducting layers during their formation is characterized by thickness, average size of equiaxed grains, and by the fraction ratio of columnar and equiaxed grains. It was found that at high diffusion annealing temperatures (above 650 °C), the superconducting layers grow much faster, but the average size of equiaxed Nb3Sn grains even under a short exposure (10 h) is much larger than after a long low-temperature annealing. It is shown that to ensure the best current-carrying capacity of Nb3Sn-based wires in high magnetic fields, it is necessary to take into account different trends: on the one hand, the possibility of approaching the Nb3Sn phase stoichiometry and providing its maximum amount and on the other hand to avoid the undesirable growth of grains resulting in a lower pinning force.