Мікроскопія нанооб’єктів

Nanostructure of NbTi superconducting alloy
Gordienko Ju.A., Lazareva M.B., Mazilov A.A., Mazilova T.I., Mikhailovskij I.M.
NSC “Kharkov Institute of Physics and Technology” NASU
Akademicheskaya Str., 1, Kharkov, 61108, Ukraine
E-mail: alexey.mazilov@gmail.com
Previous investigations based on ﬁeld ion microscopy revealed that a superconducting wire made of Nb–Ti alloy contains thin and extended regions with a high composition of Nb, which form current-carrying paths with the maximum density. The obtained results conﬁrm that high current-carrying properties of Nb–Ti alloys are due to the presence of nanophases with a high composition Nb as a result of thermomechanical treatment. A superconducting multifilament wire made of Nb-60 at.% Ti alloy was investigated by methods of atom probe/field ion microscopy (APFIM), three-dimensional atom probe (3DAP), high resolution electron microscopy (HREM) and computer simulation. The nanoscale heterogeneities of element concentration in beta-phase were observed. Atom probe analyses of annealed NbTi superconductors indicated that no atomic clusters were formed in alloy prior to the onset of the second phase formation. An atomic structure of interphase boundaries has been determined using HREM and atomistic structure calculations. The boundaries between α- and β-phases reveal the absence of the rigid-body translation and continuity of lattice planes across the interface. The heterogeneity of β-phase chemical structure essentially surpassed alloy heterogeneity revealed by comparison of the element concentration in various samples made from the superconducting wire of the same batch (5 at%). We observed also the fluctuations of a local element concentration inside the β-phase with characteristic linear sizes in an interval of 4 – 10 nm. The statistically significant nanoscale heterogeneity of the structure inside such sites reached 6 at.%. While the β-phase and niobium enriched phase have the configurations extended along the wire axis, the nanoscale heterogeneity inside the β-phases, observed in the present work, was nearly isotropic.