Microstructural Observation of Transformed $\hbox {Nb}_{3}\hbox {Al}$ Superconductors Using TEM and Atom Probe Tomography

The pinning mechanism for transformed Nb₃Al is still an open question: the pinning force does not increase monotonically with reciprocal grain size in the case of Nb₃Al, which is remarkably different from the behavior seen in diffusion-processed Nb₃Sn . One of the possible pinning centers is the planer defects stacked in 〈100〉 direction in A15 Nb₃Al phase that are recognized only after massive transformation from BCC Nb-Al to A15 Nb₃Al phases. In this paper, we focus on the stacking faults as possible primary pinning centers for the transformed Nb₃Al phase and discuss the correlation between the pinning characteristics and the stacking faults. Al segregation on the stacking faults was observed using 3-D atom probe tomography. The spacing of the stacking faults and the areal fraction of the stacking fault region were analyzed statistically for the microstructures observed using transmission electron microscopy. Obvious shrinkage of the stacking faults was found in low-pinning-force samples.