  The transitional state of ordering in the long-period stacking ordered (LPSO)
structure in Mg-Al-Gd alloy has been investigated by scanning transmission
electron microscopy. It is found that the domains are the origin of observed
transitional state. Three new patterns of clusters at this state can be fully
explained by the superimposition of L12 type clusters located in different
domains. The domains are further verified by different tilted views. Growth of
preferred domains during ageing process will result in final ordered LPSO
structure. Based on the idea of domains, less ordered LPSO structure in other
dilute alloys can be also understood.
