New polytypes of long-period stacking ordered structures in Mg–Co–Y alloys

New types of long-period stacking ordered (LPSO) structures were observed in Mg–Co–Y as-cast alloys by means of transmission electron microscopy and scanning transmission electron microscopy. The new types of LPSO are characterized by a structural unit consisting of either three stacking layers AB′C, or four stacking layers AB′C′A, distributed periodically along the normal of the closely packed planes in the alloy matrix. In the structural units, the B′ and C′ layers are identified as Co/Y-enriched.