KENBIKYO Vol.50▶No.2 2015
â– Researches Today

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