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

TL;DR

This study investigates the atomic-scale mechanisms behind Zr oxide precipitate formation in internally oxidized Nb3Sn superconductors, revealing nucleation, growth, and microstructural evolution to enhance high-field magnet performance.

Contribution

It provides detailed atomic-scale insights into Zr oxide precipitate nucleation and growth in Nb3Sn, combining experimental analyses and first-principles calculations for the first time.

Findings

High density of Zr oxide nanoprecipitates (~10^23/m^3) in Nb3Sn layers.

Precipitates have a mean diameter less than 10 nm at 625°C and 700°C.

Precipitates serve as pinning centers, refining grain size and improving flux pinning.

Abstract

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 prior to interfacial reactions at Nb/Nb3Sn interfaces, Zr atoms in an unreacted Nb-1Zr-4Ta (at.%) alloy form clusters with O atoms owing to their high affinity for oxygen and are segregated at grain boundaries (GBs) in the Nb grains. Then, nucleation of Zr oxide precipitates occurs in Nb3Sn and at Nb3Sn/Nb interfaces, driven by the small solubility of Zr and O in Nb3Sn compared to Nb. Quantitative APT and TEM analyses of Zr oxide precipitates in Nb3Sn layers demonstrate the nucleation, growth, and coarsening processes of Zr oxide precipitates in Nb3Sn layers. A high number density of Zr oxide nanoprecipitates is observed in the Nb3Sn layers, ~10^23 per m^3, with a mean precipitate diam. <10 nm at 625 oC and 700 oC, which provide pinning centers for grain refinement of Nb3Sn, <100 nm diam., and serve as pinning sites for fluxons. First-principles calculations and classical nucleation theory are employed to describe the nucleation of Zr oxide precipitates in Nb3Sn: energy barriers and the critical radius for nucleation of Zr oxide precipitates in Nb3Sn. Our research yields the kinetic pathways for nucleation and growth of Zr oxide precipitates and the microstructural evolution of Nb3Sn layers, which helps to improve the fabrication process of internally oxidized Nb3Sn wires for high-field superconducting magnets.