This study has found that the Z-contrast of aberration-corrected high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) images of solute enriched layers in Mg-TM-RE (TM:Zn, RE:Y, Gd) alloys is sensitive to the imaging conditions: the defocus and the focal depth. Consequently, the depth position of solute enriched layers in the projected direction shows strong effect on the Z-contrast around each layer. The blurring of the Z-contrast is preferentially observed in both sides region along (0001) Mg basal planes. The boundary position between HCP-and FCC-type stacking sequence at edge regions does not change by defocus. The permissible defocus amount is comparable to the focal depth. The aberration-corrected HAADF-STEM has short focal depth less than 10 nm, so that it is critical for interpreting image contrast that the thin foil thickness and the depth position of the nano-size structure such as the solute enriched layers. In turn, it can be presumed, by the blurring of the Z-contrast, that the depth position of the solute enriched layers in the projected direction. Thus, the focal depth of aberration-corrected HAADF-STEM is quite sensitive to the depth position of solute enriched layers in the thin foil of specimens and the defocus of the electron probe, so that the imaging condition of aberration-corrected HAADF-STEM is critical in order to interpret correctly the Z-contrast of the images.

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Three-dimensional (3D) configurations of 14H long-period stacking ordered (LPSO) structures formed in Mg 97 Zn 1 Gd 2 cast alloys at intermediate stages of the formation process have been studied by single tilt-axis electron tomography using high-angle annular dark-field scanning transmission electron microscopy. Lateral morphology of the 14H LPSO is clearly visualized by reconstructing 3D volumes. An existence of "dent-shaped" area was found in a 3D reconstructed volume for the first time. The edge of LPSO shows a characteristic triangular shape with an angle of 60°, which indicates that the growth front is parallel to f11 20g Mg . It is suggested that in-plane irregular or characteristic shapes are related to the lateral growth mechanism of LPSO. Electron tomography has proven to be an indispensable tool to characterize in-plane structural information of LPSO formed in ¡-Mg matrix.

Spatial con gurations and lateral morphology of the 14H long-period stacking ordered (LPSO) phase have been studied by single tilt-axis electron tomography using high-voltage scanning transmission electron microscopy (STEM) operated at 1 MV. A Quonset hut-like lateral shape of the LPSO was found in a tomogram of a specimen as thick as 1.7 μm. The reconstructed volume reveals spatial distribution of residual particulate precipitates of (Mg, Zn) 3 Gd phase 20-30 nm in diameters. The precipitates act as a source of solute elements for the formation and growth processes of 14H LPSO. 1 MV-STEM realizes enough resolution for imaging the morphology of LPSO as well as high electron transmittance (~4.1 μm) without any obvious electron irradiation damages on microstructures.