Direct imaging of local atomic ordering in a Pd-Ni-P bulk metallic glass using C_s-corrected transmission electron microscopy

In amorphous alloys, crystalline atomic clusters as small as 1-2 nm are frequently observed as local lattice fringe images by high-resolution electron microscopy (HREM). These clusters can be understood as local structures of amorphous alloys corresponding to "medium-range-order (MRO)". The MRO structure can be observed only under suitable defocusing conditions of the objective lens in HREM. A clear imaging of the MRO structure is difficult in conventional TEMs, mainly due to the delocalization of the image, caused mainly by the spherical aberration of the objective lens and eventually by the chosen defocus. In the present study, we have examined MRO in a Pd-based bulk metallic glass (Pd₄₀Ni₄₀P₂₀) using a high-resolution TEM (acceleration voltage 200 kV) fitted with a spherical aberration constant corrector (C_s corrector) for aberration correction. We found that when C_s was close to zero and defocus values were near the Gaussian focus, MRO regions with an FCC-Pd structure could be clearly observed with a low image disturbance. Under these conditions, the phase-contrast transfer function was understood to act as an ideal filter function, which distinctly selects specific lattice periods of the FCC-Pd clusters. The obtained atomic images of the glass structure including the FCC-Pd clusters are in good agreement with those expected from image simulation according to our amorphous structure model. In this study, we have demonstrated that the C_s-corrected HREM is a powerful tool to directly image locally ordered structures in metallic glasses. © 2006 Elsevier B.V. All rights reserved.