Three-dimensional observation of ordered and disordered precipitates using dark-field TEM tomography

For three-dimensional (3D) observation of microstructures in ordering alloys, such as orientation variants and anti-phase domain boundaries, electron tomography using dark-field transmission electron microscopy (DF TEM) may be a potential approach. However, DF TEM tomography has been thought to be impossible because of dynamical diffraction effects in crystals. In this study, we attempted DF TEM tomography of ordered and disordered precipitates in Ni-Mo and Ni-Al-Ti alloys. A tilt series of DF images of the precipitates were obtained by precisely setting diffraction conditions for a superlattice reflection. Owing to a long extinction distance of the superlattice reflection, dynamical diffraction effects, such as thickness fringes, have been effectively suppressed. Preliminary 3D reconstruction from the obtained tilt series clearly shows qualitative morphology and relative positions of the precipitates, which suggests the feasibility of the DF TEM tomography for ordering alloys.