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The crystal structure of a long-period stacking-ordered (LPSO) phase of the 14H type formed in a Mg-Al-Gd alloy as a local small part in the intergrowth structure together with that of the 18R type (the majority) has been investigated by scanning transmission and transmission electron microscopy. The LPSO phase of the 14H type in the Mg-Al-Gd system is found to form by stacking structural blocks, each of which consists of seven close-packed atomic planes. In each of structural blocks, a long-range ordering occurs for the constituent Mg, Al and Gd atoms with the enrichment of Gd atoms in the four consecutive atomic planes. The in-plane long-range ordering in the four consecutive atomic planes occurs so as to form Al 6 Gd 8 clusters in a periodic manner. This is exactly the same as what is observed in the LPSO phase of the 18R type. The crystal structure of the 14H-type LPSO phase can thus be described to form simply by adding a Mg layer to the crystal structure of the 18R-type LPSO phase so as to form triple (three consecutive) Mg layers to sandwich the Gd-enriched quadruple layers. The ideal chemical formula of the structural block is Mg 35 Al 3 Gd 4 (Mg-7.1 at.%Al-9.5 at.%Gd). The crystal structure of the LPSO phase can thus be crystallographically described as one of the order-disorder (OD) structures, and the space group of either P6 3 22 or R3c, is assigned when the simplest stacking of structural blocks is assumed.

The most stable crystal structure for an 18R-type order-disorder (OD) intermetallic phase in the Mg-Al-Gd ternary system and its formation processes by annealing at 525 °C have been investigated by means of transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The most energetically favourable polytype at 525 °C is found to be the structurally simplest one, a maximum degree of order (MDO) polytype (monoclinic, 1M, space group: C2/m) that is described with a single stacking vector in stacking sixlayer structural blocks. The formation of this simplest polytype occurs in the sequence of (i) enrichment of Gd and Al in four consecutive close-packed planes while keeping the hcp stacking of the AB-type, (ii) formation of Al 6 Gd 8 clusters in the four consecutive atomic planes, introducing a stacking fault in the middle of the four consecutive atomic planes, (iii) thickening by the formation of Gd and Al-enriched four consecutive planes at a distance of two or three close-packed Mg atomic planes from the pre-existing Gd and Al-enriched four consecutive atomic planes so as to form 6-layer and sometimes 7-layer structural blocks, (iv) in-plane ordering of Al 6 Gd 8 clusters in the four consecutive atomic planes and the stacking of structural blocks in the preferential stacking positions to form the OD structure, and (v) elimination of different structural blocks (other than 6-layer ones) and the long-range ordering in the stacking of structural blocks.

Crystal structures of long-period stacking-ordered (LPSO) phases in the Mg-TM (transition-metal)-RE(rare-earth) systems were investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The 18R-type LPSO phase is constructed by stacking 6-layer structural blocks, each of which contains four consecutive close-packed planes enriched with TM and RE atoms. Formation of the TM6RE8 clusters with the L12 type atomic arrangement is commonly observed in both Mg-Al-Gd and Mg-Zn-Y LPSO phases. The difference between the crystal structures of Mg-Al-Gd and Mg-Zn-Y LPSO phases can be interpreted as the difference in the in-plane ordering of the TM6RE8 clusters in the structural block. The Mg-Al-Gd LPSO phase exhibits a long-range in-plane ordering of Gd and Al, which can be described by the periodic arrangement of the Al6Gd8 clusters with the L12 type atomic arrangement on lattice points of a two-dimensional 2$\sqrt 3 $aMg × 2$\sqrt 3 $aMg primitive hexagonal lattice, although the LPSO phase in the Zn/Y-poor Mg-Zn-Y alloys exhibits a shortrange in-plane ordering of the Zn6Y8 clusters.

Crystal structure of a long period stacking ordered (LPSO) phase newly found in Mg-Al-Gd ternary alloys was investigated by scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM). The Mg-Al-Gd LPSO phase was confirmed to be constructed with 6-layer structural blocks, which is similar to the case of the 18R-type LPSO phases in the other Mg-TM-RE alloys. Atomic resolution high-angle annular dark-field (HAADF) STEM imaging revealed that Gd atoms are enriched in four layers instead of two layers and are ordered in a long range within the 6-layer structural block.