Diffusional-Displacive Transformation of Synchronized Long Period Stacking Ordered Structure in Mg Alloys

Takanori Kiguchi, Kazuhisa Sato and Toyohiko J. Konno

Abstract: Electron microscopy elucidated on an atomic scale the transformation mechanism of synchronized long-period stacking ordered (LPSO) structure of Mg₉₇Zn₁Y₂ alloys. The irregular stacking sequence of a fragment of 24R-type LPSO acts as a catalyst for the transformation from 18R-type to 14H-type LPSO. The elementary step of transformation from 18R-type to 24R-type LPSO takes place by ledge-pair movement on different (0001)_Mg planes with Shockley partial dislocations. Each ledge has a transition region in front of it. Because the transition regions have HCP-type stacking sequences, the solute elements migrate easily in the region. Therefore, structural modulation occurs by a mechanism resembling diffusional–displacive transformation. Local strain analysis using high-angle annular dark-field scanning transmission electron microscopy (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. This elementary step is valid for the transformation between polytypes 18R-type and 14H-type LPSOs. A pair of mutually neighboring transition regions should move on different basal planes in a cooperative manner. The important point is that they should have single shift and double shift of the basal planes.

Key words: Magunesium alloys, Long-period stacking ordered structure, Diffusional-Displacive transformation, Polytypes, HAADF-STEM, Local strain analysis