No abstract

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.

L1 2 -type TM 6 RE 8 clusters are distributed in enriched layers of LPSO (long period stacking ordered structure), and shrink in the [0001] Mg direction. The growth mechanism of LPSOs was investigated from the viewpoint of a local strain field. The strength of out-of plane normal strain of the enriched layer varied with the average radius of solute elements in the layer. The first principle calculation showed that there is a relationship between the average atomic radius of solute elements and the structural relaxation in the clusters. Zn conclusion, it is suggested that the difference of average atomic radius between Mg and solute elements controls the degree of the cluster shrinkage by the displacement of the solute elements, and that it is the dominant factor giving rise to the out-of plane normal strains of the enrichment layers.

Using conventional transmission electron microscopy (TEM) and aberration-corrected high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), compositional irregularity has been identified around long-period stacking order structures (LPSOs) in aged Mg 97 Zn 1 Y 2 alloys, which have coexistence of LPSO and ¡-Mg matrix. Elemental mappings show that compositional transition layers surround the growing LPSOs. The compositional transition layer includes solute atmosphere in the ¡-Mg matrix and transition layers of LPSOs with lower concentrations of solute elements. The Zn concentration in a transition layer of LPSO is higher than that of Y, which differs from the ¡-Mg matrix and LPSOs. The transition layer is an 18R-type stacking sequence. No transition layer was observed after the transformation from 18R-type to 14H-type LPSOs. These results indicate that the segregation of Zn is faster than that of Y, and that the transition layer is a nonstoichiometric 18R-type LPSO with Zn-rich lower concentration of the solute elements, which connects an 18R-type LPSO and an ¡-Mg matrix.