Three-Dimensional Structural Analysis for Crystal Defects in Phase-Transformed Nb3Al

The 3-D crystal structure of the planar defects formed in the Nb
₃
Al phase via phase transformation was determined. The crystal defects are expected to be the effective pinning centers rather than grain boundaries in the transformation-processed Nb
₃
Al conductors. The atomic arrangement of the Nb
₃
Al phase was determined by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The elemental analysis was performed by energy-dispersive X-ray spectroscopy (EDX). Plotting the atomic positions three-dimensionally considering the distance between the atoms, the 3-D structure of the crystal defect was determined. The planar defect was directly recognized to have the Zr
₄
Al
₃
-type crystalline structure expanding on the {100} plane. The planar defects shift the Nb
₃
Al crystalline phases across the defect plane by half a lattice parameter in the [100] direction. The thickness of the planar defect was 4.684 Å, whereas the lattice parameter of the A15 Nb
₃
Al phase was 5.135 Å in HAADF-STEM images. HAADF-STEM, STEM-EDX, as well as 3-D atom probe tomography (APT) revealed that there was no aluminum segregation at grain boundaries, although Al segregation can be seen at planar defects. On the other hand, slight Ta segregation was detected by APT at the grain boundary in an examined specimen.