Nanoscale chemistry and Atom Probe Tomography

Atom Probe Tomography (APT) has been used to study the effect of fast neutron irradiation on the local chemistry of Nb₃Sn samples.  Two RRP^(c) wires doped with 2 at% Ti were analysed by the authors of 'Understanding the nanoscale chemistry of as-received and fast neutron irradiated Nb3Sn RRP^(R) wires using atom probe tomography' published in Superconductor Science and Technology.  One of the wires was in the as-received condition and the other irradiated to a neutron fluence (E > 0.1 MeV) of 2.82 x 10²²m^-2 in the TRIGA-II reactor.

The irradiated sample had a reduced T_c, an increase in _p, a shift in the peak of the F_p curve suggesting the introduction of secondary point pinning, and an increase in the estimated scaling field B^*.  APT analysis showed that the polycrystalline Nb₃Sn had three distinct regions of composition, near stoichiometry Nb₃Sn (low Nb), regions with a higher Nb content than expected in equilibrium Nb₃Sn (high Nb) and grain boundaries.  The summed composition of these three regions lay within the Nb₃Sn  phase for both the as-received and irradiated samples. 

The distinct regions of high Nb Nb₃Sn demonstrated incomplete diffusion in the as-received sample, and the reduction in volume of these high Nb regions after irradiation implies significant radiation induced diffusion had occurred.  The presence of other features in the atomic-scale chemistry, such as the extent of Cu segregation at grain boundaries, three types of dislocation array, and unreacted Nb nanoparticles, are compared between samples by the authors.