High-angle annular dark-field STEM of partially ordered Ni-19.5at%Mo alloys

Atomic structures of partially ordered Ni-19.5at%Mo alloys are, for the first time, studied by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The small ordered regions with a D1a structure are clearly visualized as the square-arrangement of bright clots corresponding to the Mo-atomic columns in the dark background due to the disordered matrix with a fcc structure. The variation of the image contrast, where the D1a structure is visualized with high contrast in overfocused conditions from the Scherzer defocus, is observed, which is interpreted by using the approximate imaging theory by Jesson et al. and the size-variation of the probe. The imaging theory for the HAADF-STEM of short-range ordered state in binary alloys is formulated, where the dynamical diffraction effect is reduced in comparison with that for transmission electron microscopy (TEM).