HAADF-STEM observation of partially ordered Ni4Mo alloys

Partially ordered Ni4Mo alloys were observed for the first time at an atomi c resolution with high-angle annular dark field scanning transmission elect ron microscopy (HAADF-STEM). The Mo atomic columns in the D1(a) ordered str ucture were clearly visualized as bright square dots in the HAADF-STEM imag e. It was found that the ordered regions were observed with a high contrast in the defocus condition a little overfocused from the Scherzer defocus (D eltaf=rootC(S). lambda) which is the condition for the smallest electron pr obe in STEM. The phenomenon was interpreted successfully by using the appro ximate imaging theory for HAADF-STEM proposed by Jesson et al. Using the pr esent experimental and simulation results, an approximate imaging theory fo r visualizing short range ordered state in binary alloys, such as the Ni4Mo alloys, is proposed.