Structural and Compositional Modulation in Transformation of LPSO Structure in Mg₉₇Zn₁Y₂ Cast Alloys (Special Issue on Long-Period Stacking Ordered Structure and Its Related Materials(1)) Structural and Compositional Modulation in Transformation of LPSO Structure in Mg97Zn1Y2 Cast Alloys
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This study investigated modulation of the long period stacking order (LPSO) structure in aged Mg<sub>97</sub>Zn<sub>1</sub>Y<sub>2</sub> alloys using conventional transmission electron microscopy (TEM) and aberration-corrected high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). The irregular stacking sequence of a fragment of 24R-type LPSO acts as a catalyst for the transformation from 18R- to 14H-type LPSO. The elementary step of the transformation from 18R- to 24R-type takes place by the ledge-pair movement on different (0001)<sub>Mg</sub> planes with Shockley partial dislocations. Each ledge has a transition region in front of it. The transition regions are HCP-type stacking sequence with lower Zn and Y concentrations than those of the FCC-type enrichment layer. The solute elements migrate easily in the region, where solute elements produce a kind of diffusion field. Therefore, structural modulation occurs by a mechanism resembling diffusional–displacive transformation. Local strain analysis using HAADF-STEM images has elucidated that lattice spacing of (0001)<sub>Mg</sub> in the FCC-type enrichment layer is shorter than that in the HCP-type transition region. These structural and compositional irregularities are an elementary step in the transformation of LPSO in Mg<sub>97</sub>Zn<sub>1</sub>Y<sub>2</sub> alloys. A diffusional–displacive type transformation mechanism in LPSO has been proposed.
- MATERIALS TRANSACTIONS
MATERIALS TRANSACTIONS 54(5), 668-674, 2013-05-01
The Japan Institute of Metals and Materials