Structural and Compositional Modulation in Transformation of LPSO Structure in Mg97Zn1Y2 Cast Alloys

This study investigated modulation of the long period stacking order (LPSO) structure in aged Mg97Zn1Y2 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 14-Htype LPSO. The elementary step of the transformation from 18R- to 24R-type takes place by the ledge-pair movement on different (0001)(Mg) 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)(Mg) 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 Mg97Zn1Y2 alloys. A diffusional-displacive type transformation mechanism in LPSO has been proposed.