Microstructure of NbTi superconducting alloy

Abstract

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 results of the APFIM and HREM analysis suggest, that the specimens contain two phases, a bcc structure (β-phase), and a hcp structure (α-phase). The mesascopic and nanoscale heterogeneities of element concentration in *-phase were observed. Atom probe analyses of annealed NbTi superconductors and a statistical processing of the chains of atoms, consistently registered by mass-analyses, indicated that no atomic clusters were formed in alloy prior to the onset of the second phase formation. The β-phase and niobium enriched phase have the configurations extended along the wire axis, the nanoscale heterogeneity inside the β-phases was nearly isotropic. An atomic structure of interphase boundaries has been determined using HREM and atomistic structure calculations. Interphase boundaries in Nb-Ti alloy are coherent or semi-coherent in spite of the existence of the irregular microprotrusions.The boundaries between α- and β-phases reveal the absence of the rigid-body translation and continuity of lattice planes across the interface. It was demonstrated that the method of simulation in reciprocal space is computationally efficient for investigations of the atomic structure and energy of coherent and non-coherent interphase boundaries. The extremely high critical current density in the optimized Nb-Ti can be due to the coherent structure of interphase boundaries.