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 fir st lime, studied by high-angle annular dark-field scanning transmission ele ctron microscopy (HAADF-STEM). The small ordered regions with a D1a structu re are clearly visualized as the square-arrangement of bright dots correspo nding to the Mo-atomic columns in the dark background due to the disordered matrix with a fee structure. The variation of the image contrast, where th e D1a structure is visualized with high contrast in overfocused conditions from the Scherzer defocus, is observed, which is interpreted by using the a pproximate imaging theory by Jesson et al.(Proc.Roy.Soc.Lond.,441A('93) 261 ) and the size-variation of the probe. The imaging theory for the HAADF-STE M of short-range ordered state in binary alloys is formulated. where the dy namical diffraction effect is reduced in comparison with that for transmiss ion electron microscopy(TEM).